Trends in the upper tropospheric humidity from 40 years of HIRS brightness temperature measurements on NOAA satellites

Kurzfassung

We use brightness temperature data from the High-Resolution Infrared Radiation Sounder (HIRS) onboard the NOAA series of satellites, to produce a 40-year data set of Upper Tropospheric Humidity with respect to Ice (UTHi). UTHi values are derived from brightness temperatures in channels 12 and 6 (water vapour channel and temperature channel, respectively) using the retrieval method developed by Gierens and Eleftheratos (2019) combined with the 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, channel 6 brightness temperature records were negatively affected by about 2K 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 in 2000, which was accompanied by a change in the central wavelength of channel 12, there was a break in the time series of T12 data and subsequently to 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 the 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 2.5° × 2.5° data grid for 60°S-60°N at daily, monthly, and decadal time scales. We present the temporal evolution of UTHi in different latitudinal zones across the globe and estimate long-term UTHi trends from 1980 to 2020. The results show that the upper tropospheric humidity has increased in the past 40 years in the midlatitudes but not in the tropics. Estimated trends are about 0.4 to 0.5% per decade in the northern midlatitudes, between 0.1 and 0.5% per decade in the southern midlatitudes, and no or negative trends in the tropics (Figure 1). For the globe, we estimate a near-global trend of about 0.15% per decade which means an overall increase from 1980 to 2020 of about 0.6%. The findings are discussed.