Synoptic climatological analysis of Red Sea Trough and non-Red Sea Trough rain situations over Israel

摘要

Winter (October to April) rainfall in Israel is caused mostly by migratingMediterranean cyclones but certain rain situations are accompanied by a lowpressure trough extending northward from the southern Red Sea towards theEastern Mediterranean (EM) and the Levant. This system, the Red Sea Trough(RST) is one of the most frequent surface atmospheric circulation patternsover the southeastern Mediterranean, but most of the RSTs are notaccompanied by rain. This paper presents a synoptic climatologicalcomparative analysis between rain situations associated with RSTs, and thosenot connected with RSTs (non-RST rain, NRR). The RST situations wereidentified objectively with the aid of a new algorithm applied to the GEOS-1reanalysis gridded data set from NASA, for the period of March 1985–November 1995.Results show that RST rain is accompanied by a relatively narrow 500-hPatrough, located west of the Israeli coast-line and characterized by a deepsouthward penetration, while Non-RST-associated rain (NRR) is accompanied bya wider upper trough, located over the Israeli coast-line with a shallowersouthward penetration. We found a south-southwesterly wind vector anomaly at200 hPa over Israel during RST rains, while during NRRs a similar windvector anomaly pattern is observed east of Israel. There is a divergencecenter over, or a few degrees east of Israel during RST rains, while NRR isassociated with a divergence value of nearly zero over Israel and a maximumdivergence center located east of Israel. The moisture flux during NRR at700 and 900 hPa is from the Mediterranean, while during RST rain there is asouth-westerly moisture flux at 700 hPa from equatorial Africa to Israel andvicinity. A steeper temperature lapse-rate between 950–500 hPa was foundduring RST rain compared with NRR, resulting from a combination of coolingaloft together with heating near the surface.