Summer rain episodes in a Mediterranean climate, the case of Israel: Climatological-dynamical analysis

Summer in the eastern Mediterranean basin and the Levant region is characte rized by an absence of rain. A persistent subsidence aloft, combined with a lower-level cool marine flow, results in a semi permanent inversion which suppresses vertical growth of low clouds, hence inhibiting rain production. However, rain episodes are observed over the northern half of Israel almos t every year. This paper examines the regime of summer rainfall in Israel, based on measu rements taken over the past 50 years. The rain episodes are found to be rat her mild, producing only several millimetres of rainfall, each episode with a mode duration of 1 day. The rain is highly localized. The synoptic conditions during rain episodes are studied from the European Centre of Medium Weather Forecasting (ECMWF) data sets. These show that the prevailing summer synoptic system over the region remains unchanged during rain episodes, that the synoptic-scale subsidence exists throughout, but t hat the prevailing upper-inversion rises, weakens or even collapses as a re sult of cooling in the lower-mid-levels, presumably due to the enhanced col d advection within these levels. These findings, together with the observed locality of the rain, suggest th at the cause of rain is not a synoptic-scale forcing, but rather a weakenin g of the rain-suppressive thermodynamic conditions over the Levant region. Copyright (C) 2000 Royal Meteorological Society.