Clear-sky biases in satellite infrared estimates of upper tropospheric humidity and its trends
John, V.O., Holl, G., Allan, R., Buehler, S.A., Parker, D.E. and Soden, B.J. (2011) Clear-sky biases in satellite infrared estimates of upper tropospheric humidity and its trends. Journal of Geophysical Research, 116. D14108. ISSN 0148-0227
To link to this article DOI: 10.1029/2010JD015355
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.