The impact of the choice of the entire drop size distribution function on Cumulonimbus characteristics

Abstract

The study compares two different ways to represent the size distributions of cloud droplets and raindrops in bulk microphysical schemes in the scope of cloud-resolving mesoscale modeling of cumulonimbus clouds. A unified Khrgian-Mazin size distribution for the entire liquid water is systematically compared to a widely applied approach using a Marshall-Palmer size distribution for rain and a fitted monodisperse distribution for cloud droplets. The impact of the distribution function on cloud microphysics, precipitation characteristics, cloud appearance and dynamics of one simulated cumulonimbus case is investigated. The agreement of the model values with typical observed cloud characteristics is discussed. The results of our study reveal that there are considerable differences between the two approaches, both with respect to the microphysical production terms and with respect to the cloud appearance while the differences are less pronounced in cloud dynamics. An important result is that the Khrgian-Mazin size distribution leads to more cumulative rain compared to the one composed of a monodisperse and a Marshall-Palmer distribution. There is observational evidence that a storm splitting, hail field characteristics and cumulative total precipitations are simulated more accurately by the Khrgian-Mazin distribution function.