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.
Highlights
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 data set is based on measurements of the Ozone Monitoring Instrument OMI (Levelt et al, 2006, 2018) which are analyzed by means of Differential Optical Absorption Spectroscopy (DOAS; Platt and Stutz, 2008) in the visible blue spectral range using the TROPOMI TCWV 70 retrieval of Borger et al (2020): First, a spectral analysis is performed in a fit window of 430–450 nm taking into account the specific instrumental properties of OMI
The response of TCWV and the water vapour turnover time to changes in surface air temperature were investigated under consideration of theoretically expected TCWV responses based on the Clausius-Clapeyron (CC) equation
Summary
Water vapour is the most abundant greenhouse gas in the Earth’s atmosphere and is involved in several atmospheric processes across all atmospheric scales: starting from phenomena like cloud droplet growth on the microscale, to thunderstorms on the 20 mesoscale, to hurricanes on the synoptic scale and on the climate or global scale by influencing the Earth’s energy balance via the greenhouse effect and cloud, lapse rate, and water vapour feedback mechanisms (Kiehl and Trenberth, 1997; Randall et al, 2007). Trenberth et al (2005) analyzed trends for the time period of 1988 to 2003 from a TCWV data set of merged microwave satellite sensors and found generally positive trends that are consistent with assumption of fairly constant relative humidity. Wang et al (2016) investigated TCWV trends for the time period from 1995 to 2011 for a TCWV data set combining measurements from radiosondes, GPS radio occultation, and microwave satellite instruments. They found positive but slightly weaker TCWV trends which they attributed to the slowdown in the global warming rate since 2000.
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