Dry bias in satellite-derived clear-sky water vapor and its contribution to longwave cloud radiative forcing

We assessed how much of the satellite-derived longwave cloud radiative forcing (CRF) is due to an increase in upper tropospheric water vapor associated with the evolution from clear-sky to the observed all-sky condition. This is important because the satellite-derived clear-sky outgoing radiative fluxes needed for the CRF determination are from cloud-free areas away from the cloudy regions in order to avoid cloud contamination of the clear sky fluxes. However, avoidance of cloud contamination implies a sampling problem as the clear-sky fluxes represent an area drier than the hypothetical clear-sky humidity in cloudy regions. While this issue has been recognized in earlier works this study makes an attempt to quantitatively estimate the bias in the clear sky longwave CRF. Water vapor amounts in the 200-500mb layer corresponding to all-sky condition are derived from microwave measurements with the Special Sensor for Microwave/Temperature-2 (SSM/T2) and are used in combination with cloud data for determining the clear-sky water vapor distribution of that layer. The obtained water vapor information is then used to constrain the humidity profiles for calculating clear-sky longwave fluxes at the top of the atmosphere (TOA). It is shown that the clear-sky moisture bias in the upper troposphere can be up to 40-50% drier over convectively active regions. Results indicate that up to 12 Wm-2 corresponding to about 15% of the satellite-derived longwave CRF in tropical regions can be attributed to the water vapor changes associated with cloud development.