Jonathan Wright, Postdoctoral Fellow, Columbia University

The Influence of Condensate Evaporation on Atmospheric Humidity: Observations and Model Results

The success of simple advection-condensation models indicates that, to leading order, atmospheric water vapor is controlled by the large-scale temperature and circulation. This suggests that the influence of condensate evaporation is small - but how small? In particular, can it safely be neglected, and in what circumstances? Here, we provide additional constraints on this problem using both observations and model results. First, we examine the interactions between tropical deep convection and upper tropospheric water vapor using data from the Tropical Rainfall Measuring Mission Precipitation Radar and Atmospheric Infrared Sounder satellite instruments. Convective events detraining larger amounts of ice are associated with enhanced upper tropospheric moistening in both absolute and relative terms. In particular, an increase in ice water content of approximately 400% corresponds to a 10?90% increase in the likelihood of moistening and a 30-50% increase in the magnitude of moistening. This influence can be observed up to 48 hours and to order 1000 km from the convective source. Second, we assess the direct effect of condensate evaporation on atmospheric water vapor and its isotopic composition in a climate model. The model contains two parallel hydrologic cycles, an active one which influences the model physics and dynamics and a passive one which does not. We perform two model simulations, one in which passive cloud and precipitation can evaporate and one in which they cannot. The active hydrologic cycles, and thus the simulated circulations and temperatures, are identical in both simulations. Eliminating passive condensate evaporation reduces the specific humidity in the passive cycle by around 5%; this reduction varies from a few percent to 25% of the control value, depending on location.