The Weight of Rain in Climate Models Impacts Cloud Evolution
As the spatial resolution used in climate simulations becomes finer, the models become capable of representing more intense convective rain events (showery rain). However, the mass of precipitation in a cloud is not accounted for in many global atmospheric models (AGCMs), including the Community Atmosphere Model (CAM), but it may have an important impact on cloud evolution. DOE-funded researchers examined results from a cloud resolving model (CRM) that uses extremely high horizontal resolution and explicitly resolves atmospheric convection. They found that the weight of precipitation can increase atmospheric pressure by significant amounts over areas as large as 25km x 25km—an area similar to the grid box sizes in the coming generation of AGCMs. A simple representation of the pressure perturbation caused by precipitation mass was constructed and introduced into the latest version of CAM v5. Effects on both the intensity spectrum of precipitation and its mean distribution were found. This additional pressure tended to reduce the strength of the most intense small-scale upward motion and the frequency of intense precipitation in the model.
Bacmeister, J. T., P. H. Lauritzen, A. Dai, and J. E. Truesdale. 2012. “Assessing Possible Dynamical Effects of Condensate in High-Resolution Climate Simulations,” Geophysical Research Letters 39, L04806, DOI: 10.1029/2011GL050533.