Abstract
Using the National Center for Atmospheric Research Community Climate Model, version 3, radiation transfer
model and a realistic tropospheric environment including the International Satellite Cloud Climatology Project
cloud fields, all-sky radiative sensitivity to water vapor is assessed. The analysis improves upon previous clearsky
and model-based studies by using observed clouds, assessing realistic vertically varying perturbations, and
considering spatial gradients in sensitivity through the Tropics and subtropics. The linearity of sensitivity is also
explored. The dry zones of the subtropics and the eastern Pacific Ocean are found to be particularly sensitive
to the water vapor distribution, especially for variations in the upper troposphere. The cloud field is instrumental
in determining spatial gradients in sensitivity both at the top of the atmosphere and the surface. Throughout the
Tropics, outgoing longwave radiation is most sensitive to water vapor in the upper troposphere, especially when
perturbations characteristic of either natural variations or measurement uncertainties are considered. In contrast,
surface radiative fluxes are everywhere most sensitive to specific humidity variations in the lower troposphere.