Simulation of Coastal Circulation in the Eastern Mediterranean Using a Spectral Microphysics Cloud Ensemble Model

Abstract

The interaction of the cold season land breeze with the background flow in the Eastern Mediterranean and its influence on the climatic distribution of convective precipitation is studied using a 2D nonhydrostatic cloud ensemble model with the spectral approach in the description of cloud microphysics. The model microphysics is based on solving two kinetic equations for the size distribution functions for water droplets and ice particles. Each function is described using 33 mass categories. The model takes into account the following microphysical processes: nucleation of cloud condensation nuclei; nucleation of ice nuclei., condensational growth/evaporation of drops; growth/sublimation of ice due to accretion; freezing of droplets; melting of ice particles; and coalescence of drops, drops and ice, and ice particles themselves. The computational domain (200 km by 12 km) is covered by a finite-difference grid consisting of 129 ? 31 grid points. It is shown that the model is able to reproduce wind velocity and the distribution and intensity of precipitation. Results indicate that the interaction of the winter land breeze and the background flow determine to a great extent the climatic distribution of convective-type precipitation in the Eastern Mediterranean. The background wind substantially influences both the amount and distribution of precipitation. It determines the width of the zone of convective activity and its location relative to the seashore. It is also shown that latent heat release greatly increases both the intensity of thermally induced circulation and its vertical and horizontal spreading. It is indicated that deep convection triggered by the boundary layer circulation not only increases the intensity of breeze circulation but changes the thermodynamic structure through an increase of the temperature gradients between the areas of intense convection and surrounding areas. These gradients seem to maintain the breeze front location over the sea in case of moderate onshore winds. The results show that relative humidity over the land is one of the main factors determining the difference in the precipitation amount in the northern and southern regions of Israel.