Evidence for the importance of precipitation in closed to open cell cumulus transitions

Precipitation has been hypothesized to play an important role in the transition of low clouds from closed to open cell cumulus in regions of large scale subsidence. This transition may be one of reasons these cloud regimes show such a large cloud radiative forcing sensitivity to temperature. At the cloud scale, the presence of precipitation may increase the cloud liquid water path (LWP), optical depth, and drop sizes; however, the loss of water content and reduction in cloud fraction due to the precipitation process may also be important for the formation of pockets of open cells and in the transition from closed to open cell cumulus. A synthesis of A-Train satellite measurements is used to examine the cloud-scale and mesoscale impacts of precipitation on the cloud and radiative properties across the transition from closed to open cells in the southeastern Pacific. Results show that at the cloud pixel scale, even clouds with very light drizzle have considerably larger LWP, optical depth, and particle sizes than non-precipitating clouds. At the mesoscale, cloud fraction decreases more rapidly in areas surrounding precipitating low clouds than around non-precipitating low clouds. The area of transition from closed to open cells appears especially sensitive to the presence of precipitation. There is also dependence on precipitation rate and LWP, with higher precipitation rates or lower cloud LWP leading to a more rapid reduction in surrounding cloud fraction. Analysis of the radiative impacts associated with the changes in cloud-scale and mesoscale cloud properties is also planned.