Satellite-Derived Warm Rain Fraction As a Constraint on the Cloud Lifetime Effect

We present a global climatology of warm-rain occurrence fraction derived from CALIPSO-CloudSat satellite observations. (Warm rain is defined as liquid precipitation at the surface originating from pure liquid-water clouds.) This climatology shows that warm rain is rare over land compared to ocean, especially in the extratropics. In a subset of AeroCom and CMIP5 models, we can diagnose the warm-rain fraction and compare it to the satellite climatology. We find that the modeled land--sea contrast is smaller and the warm-rain fraction is larger by an order of magnitude in comparison to the satellite data.

Our satellite climatology may provide a powerful observational constraint on the magnitude of the cloud lifetime effect. The more precipitating warm clouds are simulated in a model, the more opportunity aerosols have to influence the precipitation microphysics, since the aerosol influence is mainly implemented for autoconversion in liquid-water clouds in current models. We hypothesize that the strength of the cloud lifetime effect in models is therefore related to the warm-rain fraction. Here we present a test of this hypothesis based on sensitivity studies in ECHAM-HAM where we tune the warm-rain fraction to match the satellite climatology.