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

Abstract. Winter (October to April) rainfall in Israel is caused mostly by migrating Mediterranean cyclones but certain rain situations are accompanied by a low pressure trough extending northward from the southern Red Sea towards the Eastern Mediterranean (EM) and the Levant. This system, the Red Sea Trough (RST) is one of the most frequent surface atmospheric circulation patterns over the southeastern Mediterranean, but most of the RSTs are not accompanied by rain. This paper presents a synoptic climatological comparative analysis between rain situations associated with RSTs, and those not connected with RSTs (non-RST rain, NRR). The RST situations were identified objectively with the aid of a new algorithm applied to the GEOS-1 reanalysis 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-hPa trough, located west of the Israeli coast-line and characterized by a deep southward penetration, while Non-RST-associated rain (NRR) is accompanied by a wider upper trough, located over the Israeli coast-line with a shallower southward penetration. We found a south-southwesterly wind vector anomaly at 200 hPa over Israel during RST rains, while during NRRs a similar wind vector anomaly pattern is observed east of Israel. There is a divergence center over, or a few degrees east of Israel during RST rains, while NRR is associated with a divergence value of nearly zero over Israel and a maximum divergence center located east of Israel. The moisture flux during NRR at 700 and 900 hPa is from the Mediterranean, while during RST rain there is a south-westerly moisture flux at 700 hPa from equatorial Africa to Israel and vicinity. A steeper temperature lapse-rate between 950–500 hPa was found during RST rain compared with NRR, resulting from a combination of cooling aloft together with heating near the surface.