The General Circulation and Robust Relative Humidity

The sensitivity of free-tropospheric relative humidity to cloud microphysics and dynamics is explored using a simple 2-D humidity model and various configurations of the NCAR CAM3 Atmospheric General Circulation Model (AGCM). In one configuration the imposed surface temperatures and radiative perturbations effectively eliminated the Hadley and Walker circulations and the main westerly jet, creating instead a homogeneous “boiling kettle” world in lowand mid-latitudes. A similarly homogeneous state was created in the 2-D model by rapid horizontal mixing. Relative humidity R simulated by the AGCM was insensitive to surface warming. Doubling a parameter governing cloud water re-evaporation increased tropical mean R near mid-troposphere by about 4% with a realistic circulation but by more than 10% in the horizontally homogeneous states. This was consistent in both models. AGCM microphysical sensitivity decreased in the upper troposphere, and vanished outside the Tropics. Convective organization by the general circulation evidently makes relative humidity much more robust to microphysical details by concentrating the rainfall in moist environments. Models that fail to capture this will overestimate the microphysical sensitivity of humidity. Based on these results, the uncertainty in the strength of the water vapor feedback associated with cloud microphysical processes seems unlikely to exceed a few percent. This does not include uncertainties associated with large-scale dynamics or cloud-radiative effects, which we cannot quantify, although radical CAM3 circulation changes reported here had surprisingly little impact on simulated relative humidity.