Speaker
Description
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.
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