A modified precipitation efficiency and its application on warm rain processes

Precipitation is directly produced by cloud microphysical processes but the occurrence of precipitation is associated with environmental dynamics and thermodynamics of weather and climate events. Therefore, the measure of how efficiently a convective system produces precipitation, termed precipitation efficiency (PE), has been estimated in the past by different ways. PE has been defined as the ratio of surface rain rate to the sum of vapor condensation and deposition rates, or cloud-microphysics precipitation efficiency (CMPE). The CMPE, is examined mainly in cloud modeling studies. PE has also been defined as the ratio of surface rain rate to the sum of vapor convergence and surface evaporation rates, or large-scale precipitation efficiency (LSPE). The LSPE has been used mostly in observational studies. While both PEs appear physically reasonable, they are not always well correlated. Our analysis indicates that a more complete definition of PE can be obtained by modifying the existing CMPE and LSPE. These modified PEs are examined based on cloud-resolving model simulations. The properly defined PEs include all moisture and hydrometeor sources associated with surface rainfall processes so that they range from 0 to 100 %. Furthermore, the modified PEs are highly correlated. Their linear correlation coefficient and root-mean-squared difference are insensitive to the spatial scales of averaged data, and are moderately sensitive to the time period of averaged data.

The modified PE is applied to estimate the efficiency of warm rain processes in the tropics as a function of sea surface temperature to evaluate whether there is a substantial increase in precipitation efficiency of light warm rain as the sea surface temperature increases.