Clear-sky biases in satellite infrared estimates of upper tropospheric humidity and its trends

Abstract

[1] We use microwave retrievals of upper tropospheric humidity (UTH) to estimate the impact of clear-sky-only sampling by infrared instruments on the distribution, variability, and trends in UTH. Our method isolates the impact of the clear-sky-only sampling, without convolving errors from other sources. On daily time scales, IR-sampled UTH contains large data gaps in convectively active areas, with only about 20–30 % of the tropics (30°S–30°N) being sampled. This results in a dry bias of about −9 %RH in the area-weighted tropical daily UTH time series. On monthly scales, maximum clear-sky bias (CSB) is up to −30 %RH over convectively active areas. The magnitude of CSB shows significant correlations with UTH itself (−0.5) and also with the variability in UTH (−0.6). We also show that IR-sampled UTH time series have higher interannual variability and smaller trends compared to microwave sampling. We argue that a significant part of the smaller trend results from the contrasting influence of diurnal drift in the satellite measurements on the wet and dry regions of the tropics.