Contribution of water vapor to observational estimates of longwave cloud radiative forcing



[1] It has been long recognized that cloud-free regions are typically associated with drier air, whereas cloudy regions typically are associated with more moisture. Using Advanced Microwave Scanning Radiometer (AMSR-E) passive microwave observations, we estimated the bias between water vapor path (WVP) of clear-sky and all-sky conditions. Furthermore, we assessed the impact of these biases on longwave cloud radiative forcing (CRF). Results indicate that, in a zonal mean sense, the difference between all-sky and clear-sky WVP estimates is around 2 mm, with highest values in the midlatitudes summer, where differences up to 7 mm occur. The corresponding CRF change forced by these WVP changes is about 2 W m−2 in a zonal mean sense. Highest values occur in the midlatitudes of the northern hemisphere in which a magnitude up to 6 W m−2 is shown. Over the convectively active regions, the WVP contributed CRF appears to be smaller than 5 W m−2, substantially smaller than the CRF contributed by upper tropospheric water vapor only, suggesting that a significant portion of satellite-estimated CRF over the tropics is contributed by the redistribution of water vapor associated with convection development.