Comment on acp-2022-149

Abstract

In this study, we investigate trends in total column water vapour (TCWV) retrieved from measurements of the Ozone Monitoring Instrument (OMI) for the time range between January 2005 to December 2020. The trend analysis reveals on global average an annual increase in the TCWV amount of approximately +0.056 kg m−2 y−1 or +0.24 % y−1. After the application of a Z-test (to the significance level of 5 %) and a false discovery rate test to the results of the trend analysis, mainly positive trends remain, in particular over the Northern subtropics in the East Pacific. Combining the relative TCWV trends with trends in air temperature, we also analyze trends in relative humidity (RH) on local scale. This analysis reveals that the assumption of temporally invariant RH is not always fulfilled: we obtain increasing and decreasing RH trends over large areas of the ocean and land surface and also observe that these trends are not limited to arid and humid regions, respectively. For instance, we find decreasing RH trends over the (humid) tropical Pacific ocean in the region of the intertropical convergence zone. Interestingly, these decreasing RH trends in the tropical Pacific ocean coincide well to decreasing trends in precipitation. Additional investigations of the global response of TCWV to changes in (surface) air temperature show that the relative TCWV trends do not follow a Clausius-Clapeyron response (i.e. 6–7 % K−1) and are about 2 to 3 times higher even for the case of global averages. Moreover, by combining the trends of TCWV, surface temperature, and precipitation we derive trends for the global water vapour turnover time (TUT) of approximately +0.02 d y−1. Also, we obtain a TUT rate of change of around 11 % K−1 which is 2 to 4 times higher than the values obtained in previous studies.