Simulation of precipitation formation in the Eastern Mediterranean coastal zone using a spectral microphysics cloud ensemble model

Abstract

The rain event at the end of November 1991 in the Eastern Mediterranean was simulated using a 2-D cloud ensemble model. The description of microphysical processes is based on solving kinetic equations for size distribution functions of water droplets and ice particles of six types: ice crystals (columnar, plate-like and dendrites), snowflakes, graupel and frozen drops. Each type is described by a special size distribution function containing 33 categories. Nucleation (activation) processes are described using the size distribution function for cloud condensation nuclei (33 size categories). The cloud model includes processes of ice nucleation from water vapor as well as by drop freezing. Ice processes, such as ice crystal diffusional growth, riming, aggregation, melting, etc. are included into the model based on the spectral approach. The main question we are addressing in the 2-D simulations is the impact of processes connected with ice generation on precipitation formation and distribution. The formation of ice particles does not lead to any significant dynamical changes in the location of persistent cloud generation or in the velocity fields. Moreover, the inclusion of the ice phase has little effect on the accumulated rain integrated over the whole computational area. On the other hand, ice formation was found to lead to a time delay in precipitation formation and a significant spatial redistribution of precipitation. Low density ice particles are transported inland by the background wind, so that the precipitation over the land at the distance of a few tens of kilometers from the sea shore is determined by these particles (mainly snowflakes). Because of the large size of melted raindrops, radar reflectivity turns out to be large (up to 35 dBZ) over the land under a comparably small rain rate of 1 mm/hour. In the experiment with the warm-rain microphysics only (no ice) precipitation forms faster and is concentrated mostly over the sea and within the coastal area.