Further Results on the Sensitivity of Simulated Storm Precipitation Efficiency to Environmental Temperature

A method is devised for diagnosing the condensation rate in simulations using the Regional Atmospheric Modeling System (RAMS) model, where ice-liquid water potential temperature is a prognostic variable and an iterative procedure must be used to diagnose the temperature and water vapor mixing ratio from ice-liquid water potential temperature. The condensation rate is then used to compute the microphysical precipitation efficiency (PE), which is defined as the ratio of the precipitation rate at the ground to the sum of the condensation and deposition rates. Precipitation efficiency is compared for pairs of numerical simulations, initialized with soundings having all key environmental parameters identical except for their temperature. The authors' previous study showed that with a colder initial sounding, the conversion of cloud water to precipitation is relatively inefficient, but updrafts are stronger and there is relatively less evaporation of precipitation, with the net result being a larger climatological PE in the colder environment. Here, the authors consider the time lag between condensation and precipitation and demonstrate that in calculating a properly lagged microphysical PE, the combined effect of the decreased production of precipitation and the decreased evaporation is that the temperature of the initial soundings has no significant influence on the microphysical PE. To the authors' knowledge, this is the first time that the lag has been used to compute PE. These results concerning PE are relevant only to deep convection.