ESA Water Vapour Climate Change Initiative - 2nd User workshop

14 Oct 2024, 14:00 → 16 Oct 2024, 12:30 Europe/Berlin

Room 4001b (FZ Jülich; Building 15.9 (INM))

Background

The Water Vapour Climate Change Initiative (WV_cci), is a project of the European Space Agency (ESA) with the overall goal to generate climate data records (CDRs) of atmospheric water vapour for use in climate applications. The project develops, validates, and releases quality-controlled, long-term CDRs of total column water vapour (TCWV) and water vapour profile across troposphere and stratosphere (2D, 3D).

More details are available on https://climate.esa.int/en/projects/water-vapour

Objectives

Following the success of the first User Workshop, the aim of this second User Workshop was to bring together the broader water vapour community, including those interested in the generation of water vapour CDRs and data users (such as climate modellers and NWP researchers) in order to discuss the most recent scientific applications and challenges in processing and using water vapour CDRs. 

Topics of the workshop include:

• Discuss challenges related to the generation of water vapour CDRs.
• Show-case climate applications of water vapour CDRs.
• Collect and update user requirements for atmospheric water vapour.
• Present and discuss results from climate analysis, climate applications, and process studies using water vapour CDRs.
• The effect of the Hunga Tonga-Hunga Ha'apai eruption on the stratosphere and resulting impacts on the troposphere.
• Water vapour in NWP models.

Presented material

Slides from individual presentations or collected by session can be downloaded here. Video recordings are available to registered participants.

Format

The workshop was held from Monday 14 Oct (14:00 CEST) to Wednesday 16 Oct (12:30 CEST) 2024 in a hybrid mode, at Forschungszentrum Jülich (Germany) and online.

Invited presentations provideed elements of discussion for the workshop: gaps in knowledge, needs for Earth Observations, etc

 

 

WVCCI-UW_202410_Opening.zip

WVCCI-UW_202410_Strato.zip

WVCCI-UW_202410_Tropo.zip

WVCCI-UW_202410_UTLS.zip

Monday, 14 October

14:00 → 14:30 Opening: Welcome

Convener: Michaela Hegglin (U. Reading / FZ Jülich)

14:00 Welcome

Speaker: Prof. Michaela Hegglin (Forschungszentrum Julich)

14:10 Introduction to ESA CCI

Speaker: Michael Eisinger (ESA)

14:15 Introduction to WV_cci

Speaker: Prof. Michaela Hegglin (Forschungszentrum Julich)

WVCCI-UW_202410_Opening_Hegglin.pptx

14:30 → 15:30 Tropospheric Water Vapour

Convener: Michaela Hegglin (U. Reading / FZ Jülich)

14:30 (Invited) Large uncertainties in near-surface humidity over oceans

Over land we find the signal of increasing near-surface water vapour while saturation levels have decreased reasonably robust. Drivers of this, at large scales at least, are reasonably well understood. Over oceans however, uncertainty remains very large with very little agreement between in situ, reanalyses and modelled estimates, very limited spatial coverage of in situ observations, and challenges in producing comparable estimates from remotely sensed platforms. Here I will present recent work exploring differences and potential sources of error between the in situ HadISDH near-surface humidity product, ERA5 reanalysis and a selection of CMIP6 models.

Speaker: Kate Willett (Met Office Hadley Centre)

WVCCI-UW_202410_Tropo_Willett.pptx

15:00 A combined total column water vapour data record from microwave and near-infrared imager observations: new developments and results from validation and applications

Water vapour is the single most important natural greenhouse gas in the atmosphere, thereby constraining the Earth’s energy balance, directly and indirectly through the water vapour feedback mechanism. In addition, water vapour is a key component of the water cycle. There is consequently the need to consolidate our knowledge of natural variability and past changes in water vapour and to establish climate data records (CDRs) of both total column and vertically resolved water vapour for use in climate research. This is the objective of the ESA Water Vapour Climate Change Initiative (WV_cci).

Within WV_cci a global total column water vapour (TCWV) data record was generated by combining microwave over the ice-free ocean with near-infrared imager based TCWV over land, coastal ocean and sea-ice. The data record relies on microwave imager observations, partly based on a fundamental climate data record from EUMETSAT CM SAF and on near-infrared observations from MERIS, MODIS and OLCI. Both, the microwave and near-infrared data streams are processed independently and combined afterwards by not changing the individual TCWV values and their uncertainties. The latest version of the data record is freely available to the public via 10.5676/EUM_SAF_CM/COMBI/V001. A new version will be released in 2025 which relies on more sensors, improved stability and uncertainty characterisation and a high-resolution regional product will be generated as well.

This presentation will briefly introduce WV_cci and the data records and then focuses on: new developments and improvements, results from validation and comparison to various other data records, results from variability, temporal change analysis and example applications.

Speaker: Marc Schroeder (DWD)

WVCCI-UW_202410_Tropo_Schroeder.pdf

15:15 Evaluation of CMIP6 models using ESA satellite data over tropical region : a case study of the atmospheric water vapor and clouds

Water vapor and clouds are among the fundamental atmospheric components, and as such, are part of the Essential Climate Variable (ECV) monitored by the Global Climate Observing System (GCOS). In this work, the global water vapor Climate Data Record (CDR) generated within the ESA Water Vapor climate change initiative project (WV_cci) is used as reference (daily, 0.1°, 2003-2014) to evaluate a sample of the Coupled Model Intercomparison Project phase 6 (CMIP6) as well as the fifth generation ECMWF reanalysis (ERA5), with a focus on temporal signal decomposition. This temporal decomposition is performed using multi-resolution analysis (MRA). MRA is a mathematical tool which consists of decomposing a signal into its subcomponents on different time scales. Using this tool, the representation of the total column water vapor (tcwv) and total cloud coverage (tcc) over the tropics in the CMIP6 models and ERA5 can be assessed separately from daily to annual and decadal time scales, including monthly and seasonal time scales. Hence, at the global-tropical scale, CMIP6 models produce reliable evolution of water vapour and cloud coverage at seasonal (2 - 8 months) and interannual (1 - 1.4 year) time scales. However, most of the CMIP6 models underestimate the trend observed in the evolution of water vapour over the ocean. While over the land, ESA shows an increase of ~2 kg/m²/dec which is very high and probably due to the instrumentation used. Using a linear regression, we attempt to reconstruct the WV_cci signal using the CMIP6 models as explanatory variables based on the correlation found between the models and WV_cci at each level of decomposition. Such reconstruction highlights the scales of variability that are closest to the observed one. The reconstructed signal of (water vapour / cloud coverage) shows higher correlation with respect to ESA over the ocean (0.7/0.55) than land (0.63/0.28). This can be explained by the use of AMIP simulations which are forced by the Sea Surface Temperatures (SST) leading to good correlation over ocean. The representation of cloud coverage in CMIP6 remains challenging except for the IPSL model over the ocean.

Speaker: Cedric Langue (LATMOS / UVSQ)

WVCCI-UW_202410_Tropo_Langue.pptx

15:30 → 16:00 Coffee

16:00 → 17:30 Tropospheric Water Vapour

Convener: Helene Brogniez (LSCE/UVSQ)

16:00 Examining Water Vapour Residency Times from Observational and Model Ensembles

The hydrological cycle is a key component of the Earth system, constituting the largest movement of any substance on Earth. Water vapour, while only accounting for 0.001% of all water mass, is an important natural greenhouse gas influencing the radiative balance of the Earth as well as surface and soil moisture fluxes. The time moisture spends in the atmosphere (i.e. the time between evaporation and precipitation) is referred to as the Water Vapour Residency Time (WVRT) and is related to the rate of energy transformations and water mass turnover (sources and sinks) as well as providing essential insight into the time lags and linkages between processes (e.g. precipitation event and contributing evaporation sources). While WVRT is a key diagnostic for hydrological sensitivity, it is a variable we cannot directly observe. This study uses the long-established turnover time (TUT) method to estimate WVRT from observational, reanalysis, and climate model ensembles as part of a comparative analysis. We start by introducing the data ensembles used along with the TUT methodology and considerations needed for estimating WVRT. We will then present a global and large-scale regional analysis of TUT between 1988 and 2014 from these ensembles. We will contextualise our findings using the Precipitation Driver and Response Model Intercomparison Project (PDRMIP) results.

Additionally, we use tools from the GEWEX Water Vapor Assessment (G-VAP) to characterise these ensembles and provide trend estimates of total column water vapour (TCWV), precipitation, and TUT from recent rises in global temperatures. To better understand the impacts of future warming, we also present the results of changes to TUT due to potential future warming from the CMIP6 Shared Socioeconomic Pathways (SSP) scenarios and relate these changes to regional freshwater fluxes. Finally, we will introduce how improved capacity for the remote sensing of stable water vapour isotopologues can provide an observational constraint on atmospheric moisture pathways, reducing uncertainty around WVRT.

Acknowledgements: Daniel Watters1, Marc Schroeder2, Richard Allen3, Hartmut Boesch4, Matthias Schneider5, Farahnaz Khosrawi5, Amelie Röhling5, Christopher Diekmann6, Harald Sodemann7, and Iris Thurnherr8

(1) NASA, (2) DWD/CM SAF, (3) University of Reading, (4) University of Bremen, (5) Karlsruhe Institute of Technology, (6) EUMETSAT, (7) University of Bergen, (8) ETH Zurich

Speaker: Tim Trent (University of Leicester)

WVCCI-UW_202410_Tropo_Trent.pdf

16:15 Long-Term Water Vapor Observations at the Arctic Research Station AWIPEV, Ny-Ålesund (Svalbard)

Water vapor is an essential component in the water and energy cycles of the Arctic. Especially in the light of Arctic amplification, changes in water vapor are of high interest but are difficult to observe due to the sparsity of data in that region. This makes the long-term observations of water vapor at the Arctic research station AWIPEV at Ny-Ålesund, Svalbard, even more valuable: in addition to an enhanced process understanding, they can also serve as a reference for models and satellite products. At Ny-Ålesund, climate change is strongly pronounced, i.e., a significant increase of annual mean 2 m temperature of +1.2 ±0.6 K/decade is observed for the time period 1994-2023, with the largest increase in winter (+2.1 ±1.6 K/decade). Radiosonde measurements date back to the beginning of the 1990s and allow for the detection of WV trends: radiosonde time series of vertically integrated water vapor (IWV) reveal a significant moistening trend for 1993-2022, in particular in autumn (+0.8 ±0.6 kg m-2 /decade). The daily radiosonde IWV data is complemented by temporally highly resolved (2-3 s) ground-based microwave radiometer observations (since 2012), which allow for a more detailed assessment of the temporal water vapor variability at that site with values ranging from low as 0.5 kg m-2 to high as 33.8 kg m-2.

In addition to the column amount of water vapor, we will also analyze its vertical distribution, particularly water vapor inversions. These are a common feature of the Arctic atmosphere impacting e.g., cloud formation/persistence and surface radiation. Statistics, e.g., on their occurrence and strengths, as well as potential trends, will be discussed.

Speaker: Dr Kerstin Ebell (University of Cologne)

WVCCI-UW_202410_Tropo_Ebell.pdf

16:30 Evaluating water vapour products of state-of-the-art models and satellite products in the Arctic Ocean

Water vapour is a critical component of the surface radiation budget in the Arctic Ocean due to the strong direct and indirect (clouds) greenhouse effect. The amplified warming of the Arctic and changes in the atmospheric circulation led to positive trends in water vapour. However, due to the sparsity of ground stations and challenges in satellite remote sensing, water vapour estimates and trends are uncertain in the Arctic Ocean. To analyze the quality of state-of-the-art water vapour products from models (reanalyses ERA5 and MERRA-2; forecast systems ICON and CAFS) and satellites (IASI combined sounding product), we use reference observations from the Multidisciplinary drifting Observatory for Study of the Arctic Climate (MOSAiC) expedition. The MOSAiC expedition took place in the Arctic Ocean, where the German icebreaker Polarstern drifted along with the sea ice for almost an entire year (October 2019 – September 2020). Radiosonde observations during the MOSAiC expedition provide detailed humidity profiles with a coarse temporal resolution. Water vapour profiles and integrated water vapour (IWV) derived from the combination of two microwave radiometers with different moisture sensitivities (HATPRO, 22 – 58 GHz and MiRAC-P, 175 – 340 GHz) complement the radiosonde observations due to their high temporal resolution. We use the high quality IWV product from the microwave radiometers and the humidity profiles from the radiosondes as reference for the evaluation.

The IWV comparison revealed a strong negative bias (15 %) for the satellite product in moist conditions and a negative bias of 10 % for MERRA-2 in dry conditions. In the cold seasons, most models underestimated the specific humidity in the lowest 2 km, except for a shallow surface layer. Further, we analyze the representation of the frequently observed humidity inversions, which affect the downwelling longwave radiation and the formation and maintenance of clouds, in all data sets in comparison to radiosondes. The presence of surface inversions is generally well detected by the models but underestimated by the microwave radiometers and satellite product. All data sets miss 10 – 30 % of the elevated inversions, especially in summer. Inversions tend to be smoother (overestimated depth, underestimated strength) in the models and remote sensing observations than in the radiosondes due to the differences in vertical resolution.

Speaker: Andreas Walbröl (Institute for Geophysics and Meteorology, University of Cologne)

WVCCI-UW_202410_Tropo_Walbroel.pptx

16:45 A comparative study of the atmospheric water vapor, stratocumulus, and rainfall between the Atacama and Namib desert

The importance of water vapor in arid and hyperarid regions motivates a comparative study in the Atacama and Namib deserts, a natural laboratory to test the role of topography and circulation in present-day climate. Based on ERA5 reanalysis evaluated by satellite and ground-based instruments, we analyzed the total column water vapor (TCWV) from seasonal to interannual scales offshore from these deserts. Separating TCWV into the contributions by the marine boundary layer (MBL) and the free troposphere, we found that along the Namib coast, moisture variability is strongly influenced by air mass transport aloft. During austral fall and winter, warm, dry air masses from the continent lower the MBL to just a few hundred meters, drying the region and reducing stratocumulus cloud cover by 30-60% compared to the Atacama coast. This occurs despite the stronger lower troposphere stability and colder sea surface temperatures on the South Atlantic coast relative to the Southeast Pacific. In contrast, the Atacama coast is largely shielded from continental air due to the Andes, resulting in a moister MBL, more frequent stratocumulus clouds, but a drier free troposphere enhancing the hyperaridity. However, the long-term dry conditions are occasionally interrupted by rainfall in Atacama’s hyperarid core. Of particular interest are the summer rainfall episodes, which have received less attention than the winter ones. To study precipitation, we complemented ERA5 with high-resolution simulations and surface observations. We found that ~75% of rain episodes in the hyperarid core between 1960—2020 are triggered by the transport of moisture from the tropical Pacific along the west coast of South America (Moist Northerlies). As moisture is transported above the MBL, the daily heating of the west slope of the Andes pumps the moist enriched air inland, leading to cloud formation, widespread rainfall, and embedded convection. Our results also show that this synoptic pattern has become more frequent, increasing TCWV summer mean ~1 kg m-2 decade-1, with a notorious increase in daily extreme water vapor values between 2011—2020. We hypothesize that this is linked to the summer Hadley cell expansion increasing the occurrence of Moist Northerlies via an upper-troposphere mechanism. High-quality water vapor information, ideally profiles, is needed to better determine long-term trends in moisture and to identify their causes and effects, e.g., the role of circulation and relation to changes in the stratocumulus clouds. We believe that analyzing the water cycle in two regions with similar climatological features but also distinct differences (Namib and Atacama) helps to better understand the response of the Hadley cell to climate change.

Speaker: José Miguel Vicencio Veloso (Universität zu Köln)

WVCCI-UW_202410_Tropo_Vicencio.pdf

17:00 (online) Water vapour isotopologue observations from space and their scientific potential

Tropospheric water vapour isotopologue ratios (expressed as δD) give unique insight into moisture sources and cloud processes, in particular if analysed together with the water vapour concentration.

In this presentation we briefly introduce the theoretical framework of generating {H2O,δD}-pair distributions and we present the MUSICA IASI {H2O,δD}-pair data set (1.5 billion individual data points, offering twice daily global coverage for 10/2014 to 12/2020, [1]). We present results from recent studies that document the potential of the MUSICA isotopologue data for validating the representation of cloud processes in atmospheric models [2-4].

Furthermore, we discuss the promising opportunities of the upcoming Metop-SG-A missions for generating tropospheric water vapour isotopologue profile data by combining the two sensors IASI-NG and Sentinel-5.

[1] Diekmann, C. J., Schneider, M., Ertl, B., Hase, F., García, O., Khosrawi, F., Sepúlveda, E., Knippertz, P., and Braesicke, P.: The global and multi-annual MUSICA IASI {H2O, δD} pair dataset, Earth Syst. Sci. Data, 13, 5273–5292, https://doi.org/10.5194/essd-13-5273-2021, 2021.

[2] Galewsky, J., Schneider, M., Diekmann, C., Semie, A., Bony, S., Risi, C., et al. (2023). The influence of convective aggregation on the stable isotopic composition of water vapor. AGU Advances, 4, e2023AV000877. https://doi.org/10.1029/2023AV000877

[3] Schneider, M., Toride, K., Khosrawi, F., Hase, F., Ertl, B., Diekmann, C. J., and Yoshimura, K.: Assessing the potential of free tropospheric water vapour isotopologue satellite observations for improving the analyses of convective events, accepted for Atmos. Meas. Tech., June 2024, AMTD version: https://egusphere.copernicus.org/preprints/2023/egusphere-2023-1121/

[4] Diekmann, C. J., Schneider, M., Knippertz, P., Trent, T., Boesch, H., Roehling. A. J., Worden, J., Ertl, B., Khosrawi, F., and Hase, F.: Water vapour isotopes over West Africa as observed from space: which processes control tropospheric H2O/HDO pair distributions?, submitted to Atmos. Chem. Phys., May 2024.

Speaker: Matthias Schneider (Karlsruhe Institute of Technology)

WVCCI-UW_202410_Tropo_Schneider.pdf

17:15 A geostationary satellite-based estimate of convective mass flux and its implications for water vapor studies

Convective mass flux is crucial for understanding convective clouds and their impacts on the ambient environment. A large number of GCM cumulus parameterization schemes rely on this concept, but measuring convective mass flux remains challenging due to the difficulty of capturing vertical air motion within intense convective clouds. No current satellite can measure these motions directly (with the notable exception of EarthCARE, although w band signals attenuate quickly inside convective cores). To address this, Masunaga and Luo (2016, hereinafter ML16) developed a novel satellite-based method using plume model computations constrained by satellite observations. Initially applied to A-Train satellite data, this approach showed good agreement with collocated ground-based radar wind profiler observations. We have since adapted it to GEO observations, allowing us to capture convective life cycles and diurnal variations. Using this data, we revisited the “hot tower” hypothesis, comparing our mass flux estimates and hot tower counts with those of Riehl and Malkus (1958) and Riehl and Simpson (1979). Finally, we discuss the implications of convective mass flux data for water vapor studies, as it controls the amount of atmospheric heating and drying.

Speaker: Dr Zhengzhao Johnny Luo (City University of New York)

WVCCI-UW_202410_Tropo_Luo.pptx

17:30 → 18:00 User requirements survey

Conveners: Helene Brogniez (LSCE/UVSQ), Tim Trent (Universiy of Leicester), Daan Hubert (BIRA-IASB)

Tuesday, 15 October

09:00 → 10:30 Tropospheric Water Vapour

Convener: Marc Schröder (DWD)

09:00 (Invited, online) Creating a Long-Term TCWV Climate Data Record Using Ground-Based GNSS Observations

Water vapor is a key driver of the Earth’s hydrological cycle and plays a critical role in both weather patterns and long-term climate processes. Ground-based GNSS TCWV observations have consistently proven to be accurate and reliable over extended periods. Their consistency with the most precise radiosonde and microwave products is currently within 0.5 – 0.7 kg m-2 in absolute value, with an RMS deviation of approximately 1 kg m-2. However, small biases and changes up to 1 kg m-2 have been observed, often linked to station instrumentation updates, changes in data processing methods, and potential alterations in the measurement environment. Fortunately, such discontinuities can be adjusted by a data homogenization procedure.

Building on this foundation, this paper introduces the data processing and post-processing steps, including homogenization, involved in creating a new global, long-term GNSS TCWV data record spanning from 1994 to 2023. The data set encompasses over 6,000 stations, each with more than a decade of observations. So far, more than 600 stations with over 20 years of data have been segmented and homogenized using a new statistical method proposed by Nguyen et al., 2024. This work will continue to include more stations, and the resulting data set will be made available to the scientific community for a range of applications, including climate change and atmospheric process monitoring, as well as the validation of reanalyses, satellite products, and climate models. The paper will also present some preliminary results of long-term linear trend analysis from GNSS and reanalyses, both at global and regional scales.

Nguyen, K. N., Bock, O., & Lebarbier, E. (2024). A statistical method for the attribution of change-points in segmented Integrated Water Vapor difference time series. International Journal of Climatology, 44(6), 2069–2086. https://doi.org/10.1002/joc.8441

Speaker: Olivier Bock (IPGP-IGN)

WVCCI-UW_202410_Tropo_Bock.pdf

09:30 Comparison Between GNSS Radio Occultation and RAL IMS Water Vapour Profiles

Since 2006, the Metop satellites have orbited the Earth equipped with different instruments to collect atmospheric data. Among them, the GRAS receiver providing GNSS Radio Occultation (GNSS-RO) data, the Infrared Atmospheric Sounding Interferometer (IASI), the Advanced Microwave Sounding Unit (AMSU) and the Microwave Humidity Sounder (MHS).

These sensors operate in different frequency bands and different geometries. GNSS-RO is a limb-view technique, whereas the others aforementioned are nadir systems sensing within a span of zenith angles. Common among them is the capability of providing vertical profiles of water vapour, which is one of the essential variables to model the weather and track climate change.

Given the independence in the data acquisition, we present an assessment of tropospheric differences between the RO-GRAS water vapour (WV) product processed by Radio Occultation Meteorology Satellite Application Facility (ROM SAF) and the Rutherford Appleton Laboratory (RAL) Infrared Microwave Sounding (IMS) data set, which is composed by WV profiles jointly retrieved from IASI, AMSU and MHS measurements.

Profiles used in this study are collocated assuming the maximum time interval of 3 hours and distance of 300 km. The matchups are grouped in three different latitude bands (low, mid, and high latitude) during 2006 and 2016, and the comparison assumes ERA5 forecasted WV profiles as the reference.

The results are presented as two data sets. One uses GRAS-RO and ERA-5 profiles simply interpolated to IMS pressure levels in the analysis, and the other assumes filtered profiles by IMS averaging kernels (AKs). Besides the primary goal of this study, i.e., comparison between the data sets, the results motivate a discussion about the relevance of AKs filtering to equalize the difference in footprints and vertical representativeness among the data.

These are preliminary results of the RO contribution to the ESA Water Vapour Climate Change Initiative (ESA WV_cci) project. They should lead to a better understanding of the differences between RO and IMS WV products and their uncertainties.

Speaker: Vinícius Ludwig Barbosa (Danish Meterological Institute)

WVCCI-UW_202410_Tropo_Barbosa.pdf

09:45 (online) Global and Regional Atmospheric Water Vapor Trends from STAR Multi-Radio Occultation Mission Data and Their Relationship with Surface Temperature Warming

Atmospheric water vapor is vital in regulating the global energy balance and the hydrological cycle. Accurate estimates of atmospheric water vapor profiles in the troposphere are essential for understanding the mechanisms that control Earth's climate. The temperature and water vapor retrieved from global GNSS radio occultation (RO) data are invaluable for climate studies due to their accurate, all-sky global observations over both land and ocean. Consistency in multi-RO mission water vapor data is particularly important for long-term global climate studies, which require reliable and consistent data to identify trends, patterns, and changes in atmospheric water vapor. In this study, we use water vapor data from multiple RO missions (e.g., COSMIC-1, COSMIC-2, MetOp A/B/C, KOMPSAT-5, GeoOptics, PlanetiQ, and Spire), consistently retrieved with the same NOAA Center for Satellite Applications and Research (STAR) 1DVAR retrieval model, to establish long-term climate data records (CDRs) of tropospheric water vapor and further study water vapor trends. Since different missions have varying penetration depths, calculating total column water vapor (TCWV) involves filling in the water vapor gap between the RO penetration depth and the surface. This process can introduce uncertainties in RO water vapor trends. To address this issue and reduce the uncertainties, we instead calculated the partial column water vapor (PCWV) from RO data by integrating the water vapor from 850 hPa to upper troposphere. We constructed the time series of PCWV from multiple RO missions, removed sampling errors, de-seasonalized the time series by removing annual cycles, and then estimated global and regional PCWV trends. We quantitatively evaluated the spatiotemporal variabilities of STAR RO PCWV. The RO PCWV trends are compared with those in both PCWV and TCWV derived from ERA5 water vapor data to understand the consistency and difference between RO and reanalysis models over global land+ocean, land, and ocean, and on regional scales. The difference between ERA5 PCWV and TCWV trends (%/decade) is further assessed to understand the effects of water vapor below and above 850 hPa on the overall trends in TCWV. We also identified anomalous water vapor variations related to El Niño or La Niña events and assessed their regional extents in RO and reanalysis data. Furthermore, the relationship between surface temperature and the atmosphere's capacity to hold water vapor, governed by the Clausius-Clapeyron equation (approximately 7% increase in atmospheric water vapor per 1 K global surface temperature rise when negligible changes in relative humidity are assumed), was assessed with RO and ERA5 data. We quantitatively evaluated the relation between global and regional RO and ERA5 water vapor trends and ERA5 surface temperature changes in the context of climate change.

Speaker: Xi Shao (ESSIC/CISESS, University of Maryland, College Park)

WVCCI-UW_202410_Tropo_Shao.pptx

10:00 Global total column water vapor 1991 until current from ERS-1 to Sentinel-3

The Microwave Radiometer (MWR) flown on Envisat, ERS-1 and ERS-2, and Sentinel-3 provides a nearly uninterrupted time series of microwave observations between 1991 and 2024. This dataset complements other microwave datasets, e.g. the SSM/I. Despite its nadir-only coverage it provides an opportunity to independently provide estimates on total column water vapor (TCWV) and cloud liquid water path (LWP). We report on our efforts towards a fully inter-calibrated and validated physical retrieval of TCWV and LWP for MWR. We address issues related to satellite inter-calibration, homogeneity of the time series of brightness temperatures, observation-simulation biases, and provide results of physical optimal estimation-based time series of TCWV.

Speaker: Ralf Bennartz (Vanderbilt University)

WVCCI-UW_202410_Tropo_Bennartz.pptx

10:30 → 11:00 Coffee

11:00 → 12:30 Tropospheric Water Vapour

Convener: Marc Schröder (DWD)

11:00 Integrated water vapor content retrieval above clouds from SWIR satellite observations in the context of C³IEL mission

The launch of the C³IEL (Cluster for Cloud evolution, ClImatE and Lightning) mission, dedicated to convective clouds, is scheduled for 2027 (Rosenfeld et al., 2022). This mission consists in a train of two nano-satellites, which will acquire, at a high spatial and temporal resolution, 4 sequences per day orbit of 11 observations every 20s of 2 simultaneous images. Among the various imagers, some are dedicated to the retrieval of the atmospheric water vapor content above and around convective clouds, at a horizontal resolution of about 150 meters, using radiances in three Short-Wave InfraRed (SWIR) channels, centered in a non-water vapor-absorbing band (1.04µm), a moderately water vapor-absorbing band (1.13µm) and a strongly water vapor-absorbing band (1.37µm).

In this context, an algorithm is currently being developed to obtain the integrated water vapor content above clouds or above continental land using an optimal estimation method.
The retrieval of water vapour content above clouds takes advantage of the knowledge of cloud top altitude, stereo-retrieved from the CLOUD part of the mission (Dandini et al., 2022). In this first version, a plane-parallel atmosphere and cloud for each pixel are assumed in the model used in the algorithm. The algorithm is evaluated using realistic profiles from the ECMWF-IFS database and shows good agreement between target and retrieved values with an RMSE of few kg/m2.

The presentation will introduce the C3IEL mission, the principle of the algorithm currently under development, and discuss the first results and their current limitations.

References :
Rosenfeld, D., Cornet, C., Aviad, S., Binet, R., Crebassol, P., Dandini, P, Defer E. Deschamps A., Fenouil L. Frid A., Holodovky V. Kaidar A., Peroni R., Pierangelo C., Price C., Ricard D., Schechner Y. and Yair, Y. (2022). C3IEL: Cluster for Cloud Evolution, ClImatE and Lightning. arXiv preprint arXiv:2202.03182. (https://arxiv.org/abs/2202.03182)

Dandini, P., Cornet, C., Binet, R., Fenouil, L., Holodovsky, V., Y. Schechner, Y., Ricard, D. & Rosenfeld, D. (2022). 3D cloud envelope and cloud development velocity from simulated CLOUD (C3IEL) stereo images. Atmos. Meas. Tech., 15(20), 6221-6242. https://doi.org/10.5194/amt-15-6221-2022

Speaker: Céline Cornet (Université Lille, CNRS)

WVCCI-UW_202410_Tropo_Cornet.pdf

11:15 (online) Total column water vapour operational products for Sentinel-4, -5 and 5P

In this contribution, an algorithm is presented that derives water vapor content in the atmosphere from TROPOMI (on Sentinel-5P) measurements in the near-ultraviolet spectral band. The algorithm, based on a classical differential optical absorption spectroscopy (DOAS) approach, contains some specificities. They concern the use of surface reflectance co-fitted, at pixel level, with the water vapor slant column, the statistical use of water vapor profiles to minimize the reliance on model climatologies as a-priori assumption, and a strategy for correcting the water vapor column in the presence of clouds. Looking forward, the development of this algorithm will allow the creation of a homogeneous dataset together with measurements from future Sentinel-4 (geostationary) and Sentinel-5 (polar orbiting) platforms, both equipped with the same spectral window placed between 430 and 450 nm.

Speaker: Luca Lelli (German Aerospace Centre (DLR))

WVCCI-UW_202410_Tropo_Lelli.pdf

11:30 Generative NWP as a Tool for Predicting Extreme Precipitation Events

In a warming climate, the expectation is that most regions will experience an increase in extreme precipitation (10 and 50 years) events. The latest IPCC (AR6) report finds that the frequency and intensity of heavy rainfall events “have likely increased at the global scale over a majority of land regions with good observational coverage”. In regions with sparse or no in situ measurements, this presents a real challenge, especially if the area is already vulnerable to extreme events. In these cases, accurate forecasts are essential to mitigate the impacts of heavy rainfall. Additionally, these areas are often remote or lack the local infrastructure to provide the decision-makers on the ground with regular regional forecasts.

In this study, we will utilise a data assimilation framework to assess whether a state-of-the-art generative weather forecast model can provide actionable information for decision-makers. Our case study will focus on the 2022 flooding in Pakistan (June to October), which killed 1,739 people and caused $30 billion of damage and economic losses. We will investigate the impact of assimilating high-resolution total column water vapour from ESA CCI on precipitation skill for these experiments, benchmarking the forecasts against NASAs Integrated Multi-satellitE Retrievals for GPM (IMERG) product. Finally, we discuss how the ability of these models to produce ensembles can be used to provide actionable information.

Speaker: Antonio Giovanni Bruno (University of Leicester - NCEO)

WVCCI-UW_202410_Tropo_Bruno.pptx

11:45 (online) Extending and Reprocessing the NASA NVAP-M Water Vapour Dataset

Our team at the Cooperative Institute for Research in the Atmosphere (CIRA) has begun a multiyear effort to reprocess and extend the NVAP-M global water vapour dataset to span the 40 years from 1988-2027. Satellite microwave observations provide the core data for NVAP. Hypersectral infrared instruments such as AIRS, IASI and CrIS provide a foundation for the 21st century portion of the time series. Examples of analyses for both climate and weather / climate interface studies will be presented. Plans for reprocessing and extending the record in light of sampling limitations will described. Increased use of passive microwave soundings using both Gaussian and non-Gaussian frameworks will be discussed.

Speaker: John Forsythe (CIRA / Colorado State University)

WVCCI-UW_202410_Tropo_Forsythe.pptx

12:00 Characterization of the specific humidity product in ROM SAF reprocessed data records

The ROM SAF second reprocessing (RE2) specific humidity data set is based on radio occultation (RO) refractivity, retrieved by the Metop, COSMIC-1, GRACE, and CHAMP missions. The estimated uncertainties and error correlations of both RO observations and background temperature and specific humidity are determining for the quality of the specific humidity product derived through a one-dimensional variational (1D-Var) procedure. Hence, close attention must be given to ensure that the used error covariance matrices reflect the true uncertainty and error correlations of these input data. We apply new procedures, based on a generalized Three Cornered Hat (3CH) method, for empirical estimation of uncertainties and error covariance matrices for both refractivity profiles and background temperature and specific humidity profiles. The error covariance matrices are estimated by combining GRUAN radiosonde relative humidity and temperature with RO refractivities and ERA5 temperature and humidity forecast. It is also demonstrated how positive biases are reduced by allowing negative specific humidity in the 1D-Var solution.

The characteristics of the ROM SAF RE2 specific humidity product are evaluated by comparison to GRUAN radiosondes. The tropospheric water vapour and information content is discussed in terms of prior fractions, averaging kernels, and cumulative degrees of freedom in different latitude bands.

Speaker: Johannes K. Nielsen (DMI)

WVCCI-UW_202410_Tropo_Nielsen.pdf

12:30 → 13:30 Lunch

13:30 → 15:00 UTLS Water Vapour

Convener: Daan Hubert (BIRA-IASB)

13:30 (Invited, online) Climatology and variability of water vapour in the Asian monsoon tropopause layer from satellite observations and reanalysis products

Variations in water vapor and other constituents in the upper troposphere and lower stratosphere (UTLS) have important radiative and chemical impacts on climate. Here we use gridded data from Aura Microwave Limb Sounder (MLS) satellite observations and five meteorological and composition-focused reanalyses (ERA5, MERRA-2, JRA-3Q, M2-SCREAM, and CAMSRA) to evaluate reanalysis representations of the mean state of water vapor, ozone, carbon monoxide (CO), and dynamical and thermodynamic fields in the tropopause layer (147–68 hPa) above the Asian summer monsoon (ASM) during the warm seasons (May–September) of 2005–2021. All reanalyses largely capture the climatological distributions and seasonal cycles of water vapor, but with systematic moist biases relative to Aura MLS. Among the five reanalyses evaluated here, M2-SCREAM (which assimilates Aura MLS) and JRA-3Q are in best agreement with Aura MLS for total water vapor; however, good quantitative agreement between JRA-3Q and Aura MLS masks compensating biases between different levels and a different seasonal cycle near the tropopause. We further describe three leading modes of deseasonalized water vapor variability in the tropopause layer (147–68 hPa) above the Asian summer monsoon (ASM). The first mode, which consists of regional-scale moist or dry anomalies on the interannual scale, is decomposed into a linear trend over 2005–2021 and detrended interannual variability. Despite strong agreement among the reanalysis products, the spatial pattern and sign of the linear trend in tropopause-layer water vapor over this period differ between Aura MLS and the reanalyses and we view the reanalysis-based trend with skepticism. Signatures of interannual variability are otherwise largely consistent, with detrended interannual variability in water vapor attributable mainly to the pre-monsoon influence of the quasi-biennial oscillation. The second mode features dry or moist anomalies centered in the northeastern and southwestern quadrants of the anticyclone coupled with weaker opposing anomalies in the southeast, while the third mode features a horizontal dipole oriented east-to-west. The second and third modes vary on subseasonal scales and often occur in quadrature, representing the propagation of quasi-biweekly waves across the monsoon domain. Although questions remain regarding the linear trend, mean biases, and a lack of direct data assimilation constraints, the overall consistency between Aura MLS and reanalysis-derived modes of variability in UTLS water vapor in this region is a promising sign that reanalyses are increasingly able to capture the processes controlling water vapor near the tropopause.

Speaker: Jonathon Wright (Tsinghua University)

WVCCI-UW_202410_UTLS_Wright.pdf

14:00 (online) Trends in the upper tropospheric humidity and ice saturation from HIRS satellite measurements in the period 1979-2020

We present trends in upper tropospheric humidity and ice saturation based on NOAA/HIRS brightness temperature measurements over the past 40 years (1979-2020). The analysis is based on data of upper tropospheric humidity with respect to ice (UTHi) derived from measurements in HIRS channels 12 and 6 (T12 and T6; water vapour channel and temperature channel, respectively) using the retrieval method developed by Gierens and Eleftheratos (2019), combined with application of intercalibration coefficients from Shi and Bates (2011).

As channel 6 is in the CO2 absorption band and CO2 concentrations have increased since 1980, T6 records were negatively affected by about 2oK over the 40-year period (Shi et al., 2016). We correct the T6 data by applying a correction formula based on the global CO2-related bias decrease in brightness temperatures over the 40-year period and the month-to-month change in CO2 with a reference CO2 value of 370 ppm. Due to the change of the HIRS instrument from HIRS/2 to HIRS/3 in 2000, which was accompanied by a change in the central wavelength of channel 12, there was a break in the time series of the T12 data and subsequently in the UTHi data. We test the severity and consistency of the break across multiple latitude zones by calculating the external variance between segments in the time series before and after the year 2000. We then perform a bias correction and eliminate this break in the time series during application of the retrieval equation. According to the breakpoint detection algorithm using the external variance as a criterion, the UTHi time series no longer shows a consistent and severe break around 2000.

After application of the bias corrections described before, we compute the UTHi data on a 2.5x2.5 grid for 60°N-60°S at daily, monthly, and decadal time scales and estimate long-term trends in UTHi and ice saturation for the period 1979-2020 in different latitudinal zones. Results show that UTHi increased in the northern midlatitudes by about 0.4-0.5%/decade, between 0.1-0.5%/decade in the southern midlatitudes, and no or negative trends are computed for the tropics. The trends in the extratropics are statistically significant on a 5% level according to Mann-Kendall trend test. For the near-global mean (60°N-60°S), we estimate a trend of about 0.15%/decade. Maps of long-term trends in the mean UTHi and in ice saturation, as given from UTHi values exceeding 70%, are presented.

Speaker: Charis Benetatos (Department of Geology and Geoenvironment, National and Kapodistrian University of Athens, Greece)

WVCCI-UW_202410_UTLS_Benetatos.pptx

14:15 Airborne lidar observations of tropospheric and lower stratospheric water vapor demonstrate the potential of a space-borne DIAL to create future CDRs

The four-wavelengths differential absorption lidar (DIAL) WALES has been operated onboard the research aircraft HALO over the past 15 years to measure water vapor (WV) profiles from the Arctic to the Tropics. Here, we present a study on the quantification of the lower-stratospheric moist bias in the ERA5 reanalysis using the multi-campaign WALES dataset. The applied 33000 mid-latitude profiles from six campaigns (41 flights) allow the strong vertical gradients at the tropopause and the vertical structure of the moist bias to be better characterized and quantified. The transport of WV across the tropopause, which is shaping WV in the upper-troposphere and lower-stratosphere (UTLS), and the relation of lower tropospheric humidity to oceanic surface fluxes and large-scale dynamics are upcoming science topics of the North Atlantic Waveguide, DI, and Downstream Impact Campaign (NAWDIC) in 2026. We illustrate plans to combine WALES with a wind lidar during NAWDIC to derive vertical and horizontal WV fluxes.

Although the deployment of an airborne DIAL is limited to individual campaigns and only allows sporadic events and particular processes to be studied, applications in atmospheric dynamics, cloud research and model verification demonstrate the merit and potential of the DIAL technique. Through technological innovations in the past decades and the recent success with operating lidars from space (EarthCARE, Aeolus), we believe it is the right time to promote the implementation of a space-borne DIAL for the future generation of range-resolved WV climate data records (CDRs) in the stratosphere and troposphere. We present a suggestion for such a space-borne DIAL for the monitoring of global WV, which is based on the experience with the airborne DIAL WALES and highlight related challenges and opportunities for climate applications and to improve numerical weather prediction.

Speaker: Andreas Schäfler (Deutsches Zentrum für Luft- und Raumfahrt (DLR))

WVCCI-UW_202410_UTLS_Schafler.pdf

14:30 Water Vapor Transport to the UTLS via Lightning-Intense Deep Convection: ICON-CLM k-Scale Simulation for the Third Pole

Lightning can be considered a signature of deep convection over the observed region. Such deep convective systems can transport significant water vapor to the upper troposphere and lower stratosphere (UTLS) region. We used the Icosahedral Nonhydrostatic Weather and Climate Model (ICON) in Climate Limited-area Mode (CLM) at the k-scale to investigate how lightning-associated deep convective systems affect the movement of water vapor and trace gases in the UTLS region. A one-year simulation shows increased water vapor concentration for lightning events at 100 hPa with some time lag, which ERA5 and AIRS further support with some differences. Noticeably, ERA5 overestimates the water vapor increase at 100 hPa and 200 hPa during the monsoon period, while AIRS underestimates it at 200 hPa compared to the k-scale simulation. Possibly, deep convection parametrization is one of the reasons for the additional water vapor transport to the UTLS in ERA5. Several high-intensity lightning cases produced by the ICON-CLM simulation were analyzed in detail over different seasons. Winter-isolated events show a much higher and distinct rise in water vapor concentration in the UTLS region over the Third Pole. Meanwhile, pre-monsoon and monsoon periods show variations in results. A Lagrangian analysis was used to understand the transport paths over the Third Pole during such events.

Speaker: Prashant Singh (Goethe University Frankfurt)

WVCCI-UW_202410_UTLS_Singh.pdf

14:45 (online) Water vapor observations in the UTLS with Raman Lidar

Measuring water vapor concentration in the Upper Troposphere/Lower Stratosphere (UTLS) region presents significant challenges due to complex atmospheric conditions and low densities. The current study sheds light on several Raman lidar systems with promising capabilities for providing high-resolution and accurate water vapor profiles, namely: the IPRAL Raman lidar (25 km south of Paris) and the Lid1200 Raman lidar (La Réunion Island in the Indian Ocean). Many years of nocturnal atmospheric observations are analyzed from both sites. This research represents the first comprehensive effort to evaluate the IPRAL lidar system, focusing particularly on the Raman channels used to derive hourly Water Vapor Mixing Ratio (WVMR) profiles at aircraft cruising altitudes. On the other hand, we show recent efforts to retrieve near-monthly Lid1200 water vapor densities up to lower stratospheric altitudes. The Lid1200 lidar system, being coaxial, is calibrated using GNSS Total Column Water Vapor measurements at 5-minute intervals (Vérèmes et al., 2019). Meanwhile, we have calibrated 6 years of biaxial IPRAL WVMR using co-located ERA5 water vapor profiles between 4 and 6 km altitude. A unique IPRAL WVMR calibration factor on an hourly basis is calculated, and a full-night calibration coefficient is obtained as the median of hourly factors over the night. Daily calibrations are inspected to detect any instrumental changes, and final generalized scaling factors are applied for quasi-stationary periods.

Both lidar WVMR profiles are compared to the most spatially and temporally coincident ERA5 profiles. The IPRAL/ERA5 comparison aims to evaluate ERA5's ability to detect supersaturation events in the upper Troposphere. ERA5 water vapor concentrations are found to be about 20% lower than those measured by IPRAL at cruising altitudes (9-11 km). An assessment and potential correction of ERA5 data were also performed and examined.

The Lid1200/ERA5 comparison provides an opportunity to examine ERA5's capacities and limitations at higher altitudes (LS domain up to 20 km) following the Lid1200 capacities in near-monthly screenings.

The newly developed upper tropospheric midnight-calibrated IPRAL WVMR profiles are also compared to midnight-launched radiosonde data, including Meteomodem M10 and GRUAN-corrected M10. Results indicate a high degree of agreement between data sources, with significant correlation coefficients exceeding 90%. Notably, there is excellent concordance with GRUAN-corrected M10 radiosondes up to 10.5 km altitude. Below 8 km, a negative bias of approximately 10% is observed when comparing IPRAL with the standard M10. This bias is thought to be linked to the radiosondes' time lag effect.

This comprehensive study aims to enhance the accuracy of UT water vapor measurements by IPRAL. The newly developed software from this study is intended to improve atmospheric models and, consequently, better understand contrail contributions in future air traffic regulations, as part of the European project BeCoM.

Speaker: Dunya Alraddawi (LATMOS/CNRS)

WVCCI-UW_202410_UTLS_Alraddawi.pptx

15:00 → 15:30 Coffee

15:30 → 16:45 UTLS Water Vapour

Convener: Christian Rolf (Forschungszentrum Jülich)

15:30 Long-term trends of IAGOS water vapour, temperature, and RH_ice in the extratropical upper troposphere (Ex‐UT) of the northern mid-latitudes

The unique characteristics of the combined data set from MOZAIC and IAGOS (http://www.iagos.org) with its global-scale sampling on air traffic routes make it ideally suited for long-term characterizations in the extratropical UT, namely at mid-latitudes with the highest flight densities.

We will present trend analyses from 1996 to 2019 for absolute humidity, temperature, and relative humidity with respect to ice (RH_ice). Our focus lies on different altitude levels in the UT over the regions Eastern North America, North Atlantic and Europe. We believe that these trends are very well suited for comparison with trends calculated from the CDR-4 data product.

Speaker: Susanne Rohs (Forschungszentrum Jülich GmbH)

WVCCI-UW_202410_UTLS_Rohs.pdf

15:45 Water Vapor climatologies in the extra-tropical Upper Troposphere and Lower Stratosphere derived from a Synthesis of Passenger and Research Aircraft Measurements

Water vapor (H$_2$O) is a key trace gas in the upper troposphere (UT) and lowermost stratosphere (LMS), as it significantly influences the Earth's climate system through its roles in radiative forcing and cloud formation. However, accurate knowledge of the amount of H$_2$O in this atmospheric region is still insufficient due to the difficulty and lack of precise in-situ and space-borne measurements. This study presents a new methodology to derive adjusted H$_2$O climatologies for the extra-tropical UT/LMS from regular measurements aboard passenger aircraft between 1994 and 2022 within the IAGOS (In-service Aircraft for a Global Observing System) research infrastructure.
To this end, a synthesis of mean H$_2$O is performed by sampling air mass bins of similar origin and thermodynamic conditions relative to the tropopause between a dataset from ~60.000 flights applying the IAGOS-MOZAIC and -CORE compact hygrometer(ICH) and a data set of ~500 flights using the more sophisticated IAGOS-CARIBIC hygrometer. % Maybe clarify that JULIA was also use: First, campaign measurements summarized in the JULIA dataset are compared to CARIBIC to varifiy the measurement quality of CARIBIC. Even for low LMS H$_2$O, CARIBIC and JULIA are in good agreement within the known uncertainties.

The analysis is, in combination with ECMWF ERA5 meteorological data, accomplished for the extra-tropical northern hemisphere, where the datasets have the largest common coverage. We find very good agreement in the UT, but a systematic positive humidity bias in the ICH measurements for the LMS. To account for this bias, mean H$_2$O of the ICH are adjusted to the IAGOS-CARIBIC measurements based on a new mapping and adjustment approach. After applying this new method, the LMS H$_2$O measurements are in good agreement between all investigated platforms. The extensive H$_2$O data set from the compact IAGOS sensor can now be used to produce highly resolved H$_2$O climatologies for the climatically sensitive LMS region.

Speaker: Patrick Konjari (Forschungsentrum Jülich)

WVCCI-UW_202410_UTLS_Konjari.pdf

16:00 A new Upper Tropospheric Humidity (UTH) dataset based on passive microwave (MW) sounders

A new global UTH dataset, produced within the framework of the EUMETSAT Satellite Application Facility for Climate Monitoring (CM SAF), is presented. The dataset is based on data from 12 polar-orbiting MW sounders operating at 183 GHz that are combined into a single time series covering the period 06/07/94 to 31/12/18. The data are provided on hourly and daily time steps at 1°x1° lat-long. Uncertainty components that capture sources of random, locally correlated and systematic errors in the data are also provided for each grid cell. These uncertainties have been propagated from the input MW top of atmosphere observations and through the UTH retrieval; uncertainties from the retrieval and the incomplete spatial sampling are also included.

Speaker: Elizabeth Good (Met Office, UK)

WVCCI-UW_202410_UTLS_Good.pptx

16:15 Understanding Boreal Summer UTLS Water Vapor Variations in Monsoon Regions: A Lagrangian Perspective

Water vapor in the Upper Troposphere and Lower Stratosphere (UTLS) is critical to climate feedback mechanisms through its influence on radiation, chemistry, and atmospheric dynamics. The amount of water vapor entering the stratosphere is sensitive to the cold point temperature (CPT), which makes the Northern Hemisphere summer monsoons more favorable for transporting high-water vapor mixing ratios into the lower stratosphere during boreal summer. In this study, we use Lagrangian methods to reconstruct water vapor over the Asian summer monsoon (ASM) and North American monsoon (NAM), aiming to understand their contributions to stratospheric water vapor. The Lagrangian method, which tracks individual air parcels and focuses on large-scale flow behaviors, identifies the relevant CPT based on the coldest temperature encountered along each parcel's trajectory, contrasting with the Eulerian approach that identifies the CPT along local vertical temperature profiles.We validate the reconstructed water vapor fields against the satellite observations from the Stratospheric Aerosol and Gas Experiment III on the International Space Station (SAGE III/ISS) and NASA’s Aura Microwave Limb Sounder (MLS), while also comparing the SAGE and MLS observations with each other. Observations from SAGE III/ISS reveal stratospheric water vapor anomalies within the ASM and NAM anticyclones that are largely consistent with MLS data but exhibit stronger moisture enhancements. The Lagrangian trajectory-based advection-condensation approach, although systematically dry-biased compared to SAGE III/ISS and MLS observations, effectively reconstructs the water vapor concentrations in the UTLS (correlation coefficient about 0.75) and captures the moist anomalies in the ASM, but less well in the NAM. Our analysis reveals that UTLS water vapor mixing ratios in the ASM are influenced by large-scale tropopause temperatures (Lagrangian CPTs), similarly to the deep tropics. Furthermore, UTLS water vapor mixing ratios in the NAM are found to be significantly affected by long-range transport from Asia. However, some large-scale effects of convection, such as the drying and moistening effects of east-west shifts within the ASM, are not captured by the trajectory-based method, likely due to unresolved temperature variability and ice microphysics within the deep convection in the ERA5 meteorology. Despite limitations such as dry biases and computational demands, the Lagrangian method offers valuable insights into atmospheric water vapor transport processes, providing a robust tool for analyzing UTLS water vapor distribution.

Speaker: Hongyue Wang (Institute for Climate and Energy Systems, Stratosphere (ICE-4))

WVCCI-UW_202410_UTLS_WangH.pdf

16:30 Investigating observations, climatology, trends, and variability of upper tropospheric humidity using satellite observations, reanalysis, and models.

Upper tropospheric humidity (UTH) is a crucial diagnostic of the atmospheric water cycle and a significant contributor to climate sensitivity, this highlights the importance of understanding its variability and how it is represented in global climate models.

Initially, using a radiative transfer code, the sensitivity of satellite observations of UTH to temperature and specific humidity is investigated. While these competing changes are minimal enough for the observations to be considered a valid proxy for relative humidity, a spurious increase in UTH of 1% has been identified for a 1K increase in profile temperature when relative humidity remains constant.

Next, we compare the infrared and microwave brightness temperature satellite observations with satellite simulations from the ECMWF Reanalysis v5 (ERA5) reanalysis and the Hadley Centre Global Environment Model version 3 (HadGEM3). These four datasets are utilised to evaluate and characterise UTH climatology and variability from 1979 to the present.

An investigation of the climatology of UTH shows that ERA5 and HadGEM3 produced higher UTH values compared to satellite observations, with mean global UTH being 9.13% and 10.81% higher than the microwave observations, respectively. Further trend evaluations indicate regional increases in UTH over the Indian Ocean while decreases are observed over South America and Indonesia. Variability in UTH has been analysed using Principal Component Analysis (PCA) which shows that the first two main modes of variance in UTH are attributed to the El Niño Southern Oscillation (ENSO).

Speaker: Thea Stevens (University of Reading)

WVCCI-UW_202410_UTLS_Stevens.pptx

16:45 → 17:30 Stratospheric Water Vapour

Convener: Christian Rolf (Forschungszentrum Jülich)

16:45 (Invited, online) The evolution of the Hunga hydration over the 2.5 years since the eruption

Hunga, a submarine volcano in the South Pacific, reached an eruption climax on 15 January 2022. This eruption not only led to the largest perturbation in the stratospheric aerosol loading in the last 30 years but also injected around 150 Tg of water directly into the stratosphere, increasing the stratospheric water vapor mass by ∼10% instantaneously. The Hunga eruption has impacted stratospheric chemistry, and the stratospheric and mesospheric temperature and circulation. In this presentation, we use MLS data to investigate the 2.5-year evolution of the water vapor plume, examine the dehydration in 2023 caused by polar stratospheric clouds, and discuss potential future scenarios for the stratospheric water vapor burden. The Hunga eruption significantly altered the middle-atmosphere, leaving it in an unprecedented anomalous state.

Speaker: Luis Millan (NASA Jet Propulsion Laboratory)

WVCCI-UW_202410_Strato_Millan.pptx

17:15 (online) SWOOSH water vapor updates, their implications for variability and trends, and preparing for the loss of the Aura Microwave Limb Sounder

The Stratospheric Water and Ozone Satellite Homogenized (SWOOSH) database is a monthly mean merged data set of vertically resolved ozone and water vapor data from a subset of limb profiling instruments operating since the 1980s. In this presentation, we summarize recent updates and improvements to SWOOSH water vapor data that were made as part of the current version 2.7. Changes include updated versions of existing data sets, new sources of data, and ongoing efforts at improvements for better quantifying uncertainties.
One of the major changes in SWOOSH v2.7 is the switch to using version 5 of Aura MLS data, whereas in previous versions of SWOOSH Aura MLS version 4.2 data were used. We discuss the impact on interannual variability and trends due to the Aura MLS version change. Finally, given the recent reduction in Aura MLS water vapor data sampling and impending loss of Aura MLS in the coming years, we discuss the impacts to the SWOOSH record and future plans to mitigate the loss of this critical data set.

Speaker: Sean Davis (NOAA Earth System Research Laboratory)

WVCCI-UW_202410_Strato_Davis.pptx

18:30 → 21:30 Workshop dinner (tbd)

Wednesday, 16 October

09:00 → 10:30 Stratospheric Water Vapour

Convener: Daan Hubert (BIRA-IASB)

09:00 (Invited, online) In-situ stratospheric water vapour measurements using balloon-borne frost-point hygrometers: A GRUAN perspective

Balloon-borne frost-point hygrometers have been used for process studies, trend analysis, and validation of satellite and radiosonde measurements for water vapor in the upper troposphere and lower stratosphere. They use a small mirror, which is cooled (or heated) appropriately, so that the condensate layer on the mirror surface is kept constant throughout the balloon sounding. The mirror temperature is measured as the frost (or dew) point temperature of the ambient air. The NOAA FPH and CFH instruments have been using a cryogen material which is trifluoromethane (R23) and electric heater to control the mirror temperature, while Meisei SKYDEW and some other instruments use a Peltier thermoelectric cooling device.

The cryogen R23 was covered by the 2016 Kigali Amendment to the Montreal Protocol because of its very large global warming potential, leading to a ban on its sale in several countries. Since then, versions with alternative coolants such as dry ice plus ethanol or liquid nitrogen have been developed for NOAA FPH and CFH.

The GRUAN stands for Global Climate Observing System (GCOS) Reference Upper Air Network (https://www.gruan.org/). It is an international ground-based reference observing network of sites measuring essential climate variables above Earth's surface, including stratospheric water vapour. For several radiosonde types, GRUAN has produced their GRUAN Data Products (GDPs), with fully quantified uncertainty budget for each data point. Currently, GRUAN is working on producing GDPs for NOAA FPH, CFH, and SKYDEW measurements.

In the presentation, we will discuss (i) brief technological introduction to balloon-borne frost-point hygrometers including the solutions for the R23 issue, (ii) some examples of past satellite validation activities using these hygrometers, and (iii) plans and issues toward the GDPs for NOAA FPH, CFH, and SKYDEW.

Speaker: Masatomo Fujiwara (Hokkaido University)

WVCCI-UW_202410_Strato_Fujiwara.pptx

09:30 Machine learning for improvement of upper tropospheric relative humidity in ERA5 weather model data

Knowledge of humidity in the upper troposphere and lower stratosphere (UTLS) is of special interest due to its importance for cirrus cloud formation and the resulting climate impact. However, the accurate description of the UTLS water vapor distribution in current weather models is subject to large uncertainties. Here, we develop a dynamic-based humidity correction method using an artificial neural network (ANN) to improve the relative humidity over ice (RHi) of numerical weather predictions from the European Center for Medium-Range Weather Forecast (ECMWF). The ANN model is trained over Europe, Africa and the East Atlantic with measured humidity data from the In-service Aircraft for a Global Observing System (IAGOS) in 2020 and with 8 time-dependent thermodynamic and dynamical variables from ECMWF ERA5. Previous (timesteps at -6 h and -2 h) and current atmospheric variables within ±2 ERA5 pressure layers around the IAGOS flight altitude between 400 and 200 hPa are used for the ANN training. Amongst them, RHi, temperature and geopotential exhibit the highest impact on ANN results, while other dynamic variables are of minor importance.

The ANN shows excellent model skills and the predicted RHi in the UT has a mean absolute error MAE of 6.6% and a coefficient of determination R² of 0.93, which is significantly improved compared to ERA5 RHi (MAE of 15.7%; R² of 0.66). The ANN model also improves the prediction skill for ice supersaturation in the all sky LS, as well as the cloudy and the clear sky UTLS, with MAE between 4.33% and 6.77% and R² of 0.92 to 0.95. Also, the erroneous peak at 100% RHi in the ERA5 data sets is corrected by the ANN. For a specific contrail cirrus region over the East Atlantic on 14 April 2021, the contrail predictions using the ANN are in better agreement with MSG satellite observations of ice optical thickness than the results without the ANN humidity correction. Our results suggest that the ANN method can be applied to other weather models to improve humidity predictions and to support climate research applications.

Reference
Wang, Z., Bugliaro, L., Gierens, K., Hegglin, M. I., Rohs, S., Petzold, A., Kaufmann, S., and Voigt, C.: Machine learning for improvement of upper tropospheric relative humidity in ERA5 weather model data, EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-2012, 2024.

Speaker: Ziming Wang (German Aerospace Center (DLR) / JGU Mainz)

WVCCI-UW_202410_Strato_WangZ.pptx

09:45 Long-term response in stratospheric water vapour and radiative forcing from the Hunga Tonga-Hunga Ha’apai eruption

The climactic Hunga Tonga-Hunga Ha’apai (HT-HH) eruption injected an unprecedented amount of water vapour into the stratosphere. As part of the ESA Water Vapour Climate Change Initiative (WV_cci), new vertically resolved climate data records (CDRs) of water vapour in the stratosphere and upper troposphere are used to study the evolution of water vapour injected into the stratosphere. The local stratospheric water vapour increased dramatically after the eruption, and the moist signal was transported upwards and then horizontally to high latitudes in the stratosphere by the Brewer-Dobson Circulation (BDC). The stratospheric water vapour increases by 1.1 ppmv globally between 100 — 0.1 hPa in December 2023, a signal from the eruption still seen after nearly two years. At the same time, this increase in stratospheric water vapour has an impact on the local dynamic circulation, including a slowdown in the BDC. Using an off-line radiative transfer model, we estimate the radiative forcings from the injected water vapour in the upper troposphere and stratosphere.

Speaker: Hao Ye (University of Reading)

10:00 Stratospheric water vapor affecting atmospheric circulation

Water vapor plays an important role in many aspects of the climate system, by affecting radiation, cloud formation, atmospheric chemistry and dynamics. Even the low stratospheric water vapor content provides an important climate feedback, but current climate models and NWP models show a substantial moist bias in the lowermost stratosphere. Here we report crucial sensitivity of the atmospheric circulation in the stratosphere and troposphere to the abundance of water vapor in the lowermost stratosphere. We show from a mechanistic climate model experiment and inter-model variability that lowermost stratospheric water vapor decreases local temperatures, and thereby causes an upward and poleward shift of subtropical jets, a strengthening of the stratospheric circulation, a poleward shift of the tropospheric eddy-driven jet and regional climate impacts. The mechanistic model experiment in combination with atmospheric observations further shows that the prevailing moist bias in current models is likely caused by the transport scheme, and can be alleviated by employing a less diffusive Lagrangian scheme. The related effects on atmospheric circulation are of similar magnitude as climate change effects. Hence, lowermost stratospheric water vapor exerts a first order effect on atmospheric circulation and improving its representation in models offers promising prospects for future research aiming at reducing biases in projections. Such model improvements strongly rely on reliable observational water vapor datasets, in particular in the extratropical lowermost stratosphere region.

Speaker: Felix Plöger (FZ Juelich)

WVCCI-UW_202410_Strato_Ploeger.pdf

10:15 Stratospheric Water Vapor Trends: Satellite versus In-Situ Observations

The significant climate feedback of stratospheric water vapor (SWV) necessitates accurate estimates of SWV budget changes, with the strongest impacts expected in the tropical and extratropical lower stratosphere, often referred to as the lowermost stratosphere (LMS). Common data sources to quantify SWV variability over the past 3-4 decades in this critical region include satellite observations (e.g., MLS, SWOOSH), long-term in-situ measurements (e.g., Boulder FPH, JULIA), and Lagrangian models driven by meteorological reanalyses (e.g., CLaMS driven by ERA5). To estimate SWV trends and disentangle anthropogenic contributions, it is essential to first quantify natural variability, particularly on decadal time scales.

Based on our previous studies, which primarily employed satellite and CLaMS data, we demonstrate the presence of a robust multi-decadal variation in SWV that distinctly separates the past 40 years into two wet periods (1986–1997; 2010–2020) and one dry period (1998–2009). This multi-decadal variability complicates the detection of long-term anthropogenic trends relevant for assessing surface climate impacts, especially in the near term. In the extratropical LMS, the current quality of satellite data and model simulations appears insufficient for reliable long-term trend assessments, underscoring the importance of long-term in-situ observations, such as those from JULIA. We present preliminary results comparing climatologies in the LMS derived from JULIA, CLaMS, and ERA5.

Speaker: Paul Konopka (Forschungszentrum Juelich)

WVCCI-UW_202410_Strato_Konopka.pdf

10:30 → 11:00 Coffee

11:00 → 12:15 Stratospheric Water Vapour

Convener: Michaela Hegglin (U. Reading / FZ Jülich)

11:00 Stratospheric humidity analysis

The ECMWF stratospheric humidity analysis is currently unaffected by assimilation, in both the Numerical Weather Prediction (NWP) configuration and the Atmospheric Composition configuration that is used by the Copernicus Atmosphere Monitoring Service (CAMS), because the analysis increments are forced to be zero above a diagnosed model tropopause. The humidity near the tropopause is incremented only if is necessary to remove supersaturation caused by a temperature increment. The ECMWF forecasts exhibit an extratropical lower stratospheric moist bias that is co-located with a cold bias. The temperature is constrained by observations in the analysis, but the cold bias then grows with forecast time above the tropopause due to an excessive long-wave radiative cooling from the additional moisture.

As developed in the Horizon Europe CAMEO project, we allow stratospheric humidity increments and conjointly assimilate EOS-Aura Microwave Limb Sounder water vapour retrievals. This new development successfully reduces the moist bias in the analysis and improves humidity and temperature forecasts. This work is needed as preparation for the development of a weak constraint 4D-Var framework for stratospheric humidity in the IFS.

Speaker: Noureddine Semane (ECMWF)

WVCCI-UW_202410_Strato_Semane.pdf

11:15 (online) Version 8 IMK/IAA MIPAS water vapour

The latest KIT-IMK/CSIC-IAA MIPAS V8 water vapour data set is presented. We discuss the major changes with respect to preceding data versions, and show data characterization and a thorough error analysis. One achievement of the V8 data set is the improved agreement between different MIPAS measurement modes, as well as a consistency improvement between the MIPAS nominal mode measurements with full and reduced spectral resolution. By consideration of Non-LTE effects in nominal mode retrievals, those profiles have become far more realistic in the upper stratosphere/mesophere range. Preliminary validation results show: The overall accuracy of the data product can be expected to be better than 10 % over a major part of the middle atmosphere.

Speaker: Michael Kiefer (Karlsruhe Institute of Technology)

WVCCI-UW_202410_Strato_Kiefer.pdf

11:30 (online) Investigating possible contributions of IASI/IASI-NG for bridging the upcoming gap of limb sounding observation capabilities

Limb missions like MIPAS, ACE-FTS, or Aura/MLS have already ended or are expected to end several years before respective next-generation missions will be operative (e.g. the current ESA Earth Explorer 11 candidate CAIRT).

Here we present our updated MUSICA IASI processor and investigate the anomalies of H2O, N2O and SO2 as observed in the UTLS and the stratosphere during the Hunga Tonga-Hunga Ha’apai event. We document the agreement to collocated observation of the limb-sounders Aura/MLS and ACE-FTS.

We conclude, that due to their long-term availability from the 2000s to the 2040s, the IASI/IASI-NG missions can bridge the limb-sounding observation gap, thus reducing the scientific impact of missing limb data products in the next decade.

Speaker: Matthias Schneider (Karlsruhe Institute of Technology)

WVCCI-UW_202410_Strato_Schneider.pdf

11:45 The Changing-Atmosphere Infra-Red Tomography Explorer CAIRT – a candidate mission for ESA’s Earth Explorer 11

The Changing-Atmosphere Infra-Red Tomography Explorer (CAIRT) is one of the two remaining candidate missions competing for implementation as ESA’s Earth Explorer 11. CAIRT aims to reveal, resolve, and unravel the complex coupling between composition, circulation, and climate in our middle atmosphere, by improving our knowledge of the chemical-dynamic-radiative interactions that govern our climate system. CAIRT, a Fourier Transform Spectrometer for infrared limb hyperspectral imaging, would provide continuous limb radiance measurements from the mid-troposphere to the lower thermosphere, at high spectral resolution and with unprecedented horizontal and vertical sampling. Leveraging an innovative tomographic retrieval approach, CAIRT would produce a unique three-dimensional dataset of numerous trace gases, temperature and aerosols across the entire middle atmosphere to the edge of space. With this, CAIRT would provide critical information on: (a) atmospheric gravity waves, circulation and mixing; (b) coupling with the upper atmosphere, solar variability and space weather and; (c) aerosols and pollutants in the upper troposphere and lower stratosphere. CAIRT would thoroughly explore and elucidate the role of the middle atmosphere in climate dynamics, forcing and feedbacks. It would firmly anchor this knowledge in a more holistic understanding of fundamental processes within the Earth system.

Speaker: Peter Preusse (Forschungszentrum Juelich)

WVCCI-UW_202410_Strato_Preusse.pptx

12:00 Observations of mesoscale water vapour distributions in the UT/LMS by the infrared limb-imager GLORIA and comparison with high-resolution short-term deterministic forecasts by the ECMWF IFS

Mesoscale distributions of water vapour can be detected by means of the infrared limb-imaging technique. The Gimballed Limb Observer for Radiance Imaging of the Atmosphere (GLORIA) has been deployed onboard different aircraft and balloon platforms during measurement campaigns focusing on regions from the Arctic to the Antarctic. Here, we present airborne GLORIA observations of mesoscale water vapour distributions in the upper troposphere and lowermost stratosphere (UT/LMS) with a focus the Arctic winter and spring 2015/16. 2-dimensional vertical cross sections of water vapour derived from the observations of GLORIA onboard the German High Altitude and Long Range Research Aircraft (HALO) on 12 January 2016 resolve the mesoscale fine structure of tropopause fold over mountains and provide insight into an active mixing region. Comparisons with high-resolution deterministic forecasts by the ECMWF IFS (European Centre for Medium-Range Weather Forecasts – Integrated Forecasting System) show that the model reproduces the location, shape and depth of the tropopause fold very well. They also confirm a moist bias, which is known to be inherent to current numerical weather prediction systems and causes a systematic cold bias there. Using the GLORIA observations, the moist bias in the ECMWF IFS data is analysed for five Arctic flights. The observations confirm the that the moist bias is already present in the forecast initialisation. They furthermore suggest that the moist bias is the consequence of a model bias and a lack of water vapor observations suitable for assimilation above the tropopause. GLORIA is the airborne demonstrator for the ESA Earth Explorer 11 candidate CAIRT (Changing-Atmosphere Infra-Red Tomography explorer), which has been selected for the Phase A study. A successful mission implementation would allow global observations of water vapour in the UT and particularly the LMS, a region, where dense and continuous observations of this important species are lacking.

Speaker: Wolfgang Woiwode (Karlsruhe Institute of Technology)

WVCCI-UW_202410_Strato_Woiwode.pdf

12:15 → 12:30 Summary