Identification and classification of the wet Red Sea Trough over Israel

The Red-Sea Trough (RST) extends toward the Levant from the south and is mostly associated with dry conditions. This study analysed the rarer type, rain-producing RSTs, referred to as ‘wet RSTs’ (WRST), using data from 1958 to 2019. A WRST archive was established, based on rain data from a study region covering the arid part of Israel, the region where this system is most active. The study also aimed to find an atmospheric proxy for rainfall, to replace precipitation estimates obtained from climate models. A WRST day was defined as a day when the RST was identified and rainfall of ≥0.1 mm was observed by at least two of the 39 rain stations used. The annual occurrence of WRSTs is 16.4 days, ~16% of all RSTs, peaking in the winter. The relative contribution of WRSTs to the annual rainfall ranges from 16% in the northern part of the study region to 50% in the south. The tropical air intrusion was quantified through trajectory analysis and the results revealed that in most WRST days, 72%, the fraction of air originating from the tropics was less than 10% and only in 3.7% of the days did it exceed 50%. Attempts to extract an atmospheric proxy which identifies a WRST did not yield significant separation between dry and wet RSTs, because of significant differences among several WRST types. Therefore, identification of future changes in WRSTs will still require the use of rainfall data offered by climate models. We automatically classified the WRST days into four types. Two types are associated with 850 hPa negative temperature anomalies and the other two with higher-than-normal temperatures, resembling ‘tropical-like’ RSTs, which are known as ‘active RSTs’ and tropical plumes. A predictive equation was extracted to quantify the tropical air intrusion and replace the use of trajectory analysis in future studies.