The Sensitivity of Stratocumulus to Shallow Cumulus Transitions to Precipitation

English Abstract:

In regions of large-scale subsidence, broad areas of marine stratocumulus clouds transition to shallow cumulus convection. Previous studies suggest that precipitation may play a role in these transitions. A synthesis of CloudSat, CALIPSO, and MODIS satellite measurements is used to examine the relationship between the presence of precipitation and changes in cloud fraction that occur across the transition from stratocumulus to shallow cumulus convection in the southeastern Pacific. The mesosocale gradient of cloud fraction for precipitating and non-precipitating scenes is quantified and the sensitivity of the results to cloud water content and precipitation intensity is examined. For a given location, the mesoscale cloud fraction in areas surrounding precipitation decreases more rapidly than around non-precipitating convection. Areas of predominantly stratocumulus show little sensitivity in cloud fraction to the presence of precipitation, while cloud fraction in areas dominated by shallow cumulus actually tend to increase when precipitation is present. However, the area of transition from stratocumulus to shallow cumulus appears especially sensitive to the presence of precipitation, with large mesosocale cloud fraction gradients when precipitation is present. The results are also sensitive to precipitation intensity and cloud liquid water path, with higher precipitation rates or lower cloud liquid water path showing larger gradients in mesoscale cloud fraction. Analysis of the sensitivity to precipitation cell spacing is also planned.