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The Red Sea trough (RST) is a low‐pressure trough extending from south towards the Levant. Unlike previous synoptic classifications covering all systems that affect the region, our algorithm focuses on the RST alone. It uses sea‐level pressure (SLP) and relative geostrophic vorticity for identifying the existence of an RST and classifying it to one of three types, according to the location of the trough axis with respect to 35°E longitude. The following conditions were imposed to assure the existence of an RST: (a) north to south SLP drop across the Levant, (b) average positive sea‐level relative vorticity over the region of interest, (c) existence of a distinct and continuous trough axis from the south towards the region of interest and (d) absence of any pronounced closed cyclone near the Levant. The algorithm was applied on the NCEP/NCAR reanalysis, 2.5° × 2.5° resolution and the ERA‐Interim, 2.5° × 2.5° and 0.75° × 0.75° resolutions. An evaluation of the algorithm against subjective identification, based on the NCEP reanalysis, showed an agreement of 93% for RST identification and 79% for correct classification. The use of fine resolution data may insert noise that reduce the identification rate of RSTs but improves the axis locating. The autumn is the main season of RST, with a maximum in November and a consistent decrease towards the July minimum. The annual frequency varies among the data sources between 17.6 and 24.6%. The trough axis is shown to have a diurnal oscillation; towards the eastern coast of the Mediterranean at nighttime and eastward, inland, at noontime. No consistent long‐term trend was found for the period 1979–2016, during which the global warming was persistent. This automated algorithm is flexible in the sense that it is not confined to any predetermined spatial resolution and is applicable to operational forecast model as well as to climate model outputs. [ABSTRACT FROM AUTHOR]

ABSTRACT Prolonged dry spells (PDSs) during the rainy season have severe environmental implications, including water shortage, damage to agriculture and increased potential for forest fires. This holds in particular for vulnerable regions, such as the Levant, already subjected to decrease in rainfall and lengthening of dry spells, in agreement with predictions of climatic models for the coming decades. This is the first comprehensive study which identifies atmospheric patterns responsible for PDS occurrence on thousands of kilometres scale. A total of 178 PDSs, of >7 days, were found within the 62 seasons studied. A subjective inspection of upper-level geopotential height (GPH), sea-level pressure (SLP) and lower-level temperature anomalies point at three types, each associated with a definite climatic regime. The 'subtropical' type is associated with an expansion of the subtropical high over the majority of the Mediterranean, accompanied by northward migration of the Mediterranean cyclone track. The 'baroclinic', the most frequent type, is induced by a pronounced stagnant ridge over the eastern Mediterranean, being a part of Rossby wave, accompanied by a pronounced trough/cut-off low over the western Mediterranean. The 'polar' type results from intrusion of lower-level continental polar air associated with upper-level trough east of the Levant and blocking high over central Europe. Quantitative indices were derived for objective classification of the types, based on the climatic regimes defined subjectively, and the centers of action representing each. Composite maps for each type indicate substantial differences in the synoptic configuration and the factors explaining absence of rain. For the subtropical type, the dynamic factor of subsidence is dominant. For the polar, the thermodynamic factor of continental dry advection is dominant and for the baroclinic, both dynamic and thermodynamic factors are important. Classification of PDSs according to synoptic scenarios enables analysis of future changes in the occurrence and duration pattern of PDSs, using output of climate models. [ABSTRACT FROM AUTHOR]

The article analyzes the relation between dust in Negev desert, in southern Israel and its general impact to global-scale weather systems. It indicates that the high dust season is between December and May wherein more than 89% of the total annual dust is collected. The study uses two indicators in evaluating the amount of dust such as dust observations and accumulated dust. It depicts that there is a correlation between rainfall in northern Israel and the dust in Negev desert showing that the rainy years tend to coincide with rich-dust years. Correlation maps manifest that the cyclonic activity over the Mediterranean is high during dust-rich years and it is in contrast during poor-dust years.

This paper examines the precision of location and top height of mesoscale convective systems, as forecast by the Aviation Weather Center (AWC). The examination was motivated by the Mediterranean Israeli Dust Experiment (MEIDEX) on the space shuttle Columbia, aimed to image transient luminous events (TLEs), such as sprites, jets, and elves, from orbit. Mesoscale convective systems offer a high probability for the occurrence of TLEs above active thunderstorms. Because the operational methodology was planned around a 24-h cycle, there was a need for a global forecast of areas with a high probability of massive thunderstorms that are prone to exhibit TLE activity. The forecast was based on the high-level significant weather (SIGWX) maps, commonly used for civil aviation, provided by the AWC on the Internet. To estimate the operational skill of this forecast for successfully detecting clouds with a high probability for producing TLEs, predictions for selected dates were compared with satellite observations. The locations of 66 mesoscale cloud systems on Significant Weather Maps, produced for eight different dates in August 2001, were compared with satellite global IR images for these dates. Operational skill was determined as the percentage of observed cloud systems found within a 5° range in the regions that appeared on the forecast maps as having the potential to contain thunderclouds and was found to be 92%. No consistent error was found in location. The predicted size of the convective system was typically larger than the observed size. Cloud-top heights of 53 systems were examined on four dates in October–November 2001, using IR radiances converted to brightness temperatures. For each convective system, the coldest cloud-top temperature was converted to height, using the NCEP–NCAR reanalysis data for the respective location and time. The standard error in the forecast heights was 2516 m. Because the purpose was to get true alerts of potential TLE occurrences, operational forecast skill was defined as the percentage of forecasts that were accurate within 1000 m or higher than observed. The 1000-m tolerance was allowed because of inevitable uncertainties underlying this method of analysis. Operational skill was found to be only 43%. During the “STS-107” mission flown in January 2003, the forecasted areas of main convective centers were transmitted daily to the crew and helped them in pointing the cameras and targeting thunderstorms. This ensured the success of the MEIDEX sprite observations that recorded numerous events in many different geographical locations. [ABSTRACT FROM AUTHOR]

The Dead Sea is a terminal lake of one of the largest hydrological systems in the Levant and may thus be viewed as a large rain gauge for the region. Variations of its level are indicative of the climate variations in the region. Here, we present the decadal- to centennial-resolution Holocene lake-level curve of the Dead Sea. Then we determine the regional hydroclimatology that affected level variations. To achieve this goal we compare modern natural lake-level variations and instrumental rainfall records and quantify the hydrology relative to lake-level rise, fall, or stability. To quantify that relationship under natural conditions, rainfall data pre-dating the artificial Dead Sea level drop since the 1960s are used. In this respect, Jerusalem station offers the longest uninterrupted pre-1960s rainfall record and Jerusalem rains serve as an adequate proxy for the Dead Sea headwaters rainfall. Principal component analysis indicates that temporal variations of annual precipitation in all stations in Israel north of the current 200 mm yr−1 average isohyet during 1940–1990 are largely synchronous and in phase (∼70% of the total variance explained by PC1). This station also represents well northern Jordan and the area all the way to Beirut, Lebanon, especially during extreme drought and wet spells. We (a) determine the modern, and propose the past regional hydrology and Eastern Mediterranean (EM) climatology that affected the severity and length of droughts/wet spells associated with multiyear episodes of Dead Sea level falls/rises and (b) determine that EM cyclone tracks were different in average number and latitude in wet and dry years in Jerusalem. The mean composite sea level pressure and 500-mb height anomalies indicate that the potential causes for wet and dry episodes span the entire EM and are rooted in the larger-scale northern hemisphere atmospheric circulation. We also identified remarkably close association (within radiocarbon resolution) between climatic changes in the Levant, reflected by level changes, and culture shifts in this region. [Copyright &y& Elsevier]

This paper presents a study of the characteristics of lightning activity during the Cyprus low winter storms over the eastern coast of the Mediterranean. The focus is on changes in the nature of thunderstorms crossing the coastline from the sea into the northern and central parts of Israel, as manifested in their electrical activity. It is based on the Lightning Position and Tracking System (LPATS) measurements of lightning ground strikes during four winter seasons between 1995 and 1999. The spatial distribution shows a maximum of lightning ground strikes over Mount Carmel, possibly due to its topographical forcing. The annual variation shows a major maximum in January with two minor peaks, one in November and another in March, which can be explained by changes in the static instability of the atmosphere throughout the rainy period. The average fraction of positive ground flashes was found to be 6% and their average peak current +41 kA. The average peak current of negative ground flashes was -27 kA. Larger frequencies of ground flashes were detected over the sea than over land during the study period. This is probably due to the large heat and humidity fluxes from the sea surface, which destabilize the colder air above and drive cloud convection. The annual distribution shows that during midwinter (December–January–February) there is higher flash density over the sea, while during autumn and spring the flash density is similar above the two regions. The diurnal variation shows that the maximum in maritime lightning activity was at 0500 LST and over land at 1300 LST. The mean peak current of positive ground flashes was higher over land and of negative ground flashes, over the sea. [ABSTRACT FROM AUTHOR]

Examines the extent to which the floods in the Negev Desert of Israel are not the outcome of purely local weather conditions but are the result of distinct synoptic-scale events. How this was done through compiling and analysing a hydro-climatological database of all the major floods in the Negev; Data indicating that distinct extreme synoptic patterns are associated with 42 of the 52 floods in the region; Findings suggesting that the major floods in the Negev can be considered as signatures of exceptional synoptic-scale evolutions, and that major floods reflect extreme climatic events; Data indicating that it is possible to use a set of dynamic and thermodynamic variables for predicting the occurrence and location of major flash floods.

Abstract: Modern-day synoptic-scale eastern Mediterranean climatology provides a useful context to synthesize the diverse late Pleistocene (60–12 ka) paleohydrologic and paleoenvironmental indicators of past climatic conditions in the Levant and the deserts to its south and east. We first critically evaluate, extract, and summarize paleoenvironmental and paleohydrologic records. Then, we propose a framework of eastern Mediterranean atmospheric circulation features interacting with the morphology and location of the southeast Mediterranean coast. Together they strongly control the spatial distribution of rainfall and wind pattern. This cyclone–physiography interaction enforces the observed rainfall patterns by hampering rainfall generation south and southeast of the latitude of the north Sinai coast, currently at 31°15′. The proposed framework explains the much-increased rains in Lebanon and northern Israel and Jordan as deduced from pollen, rise and maintenance of Lake Lisan, and speleothem formation in areas currently arid and semiarid. The proposed framework also accounts for the southward and eastward transition into semiarid, arid, and hyperarid deserts as expressed in thick loess accumulation at the deserts'' margins, dune migration from west to east in the Sinai and the western Negev, and the formation of hyperarid (<80 mm yr−1) gypsic–salic soils in the southern Negev and Sinai. Our climatic synthesis explains the hyperarid condition in the southern Negev, located only 200–250 km south of the much-increased rains in the north, probably reflecting a steeper rainfall gradient than the present-day gradient from the wetter Levant into its bordering southern and eastern deserts. At present, the rainiest winter seasons in Lebanon and northern and central Israel are associated with more frequent (+20%), deeper Cyprus Lows traversing the eastern Mediterranean at approximately the latitude of southern Turkey. Even these wettest years in northern Israel do not yield above average annual rainfall amounts in the hyperarid southern Negev. This region is mainly influenced by the Active Red Sea Troughs that produce only localized rains. The eastern Mediterranean Cyprus Lows also produce more dust storms and transport higher amounts of suspended dust to the loess area than any other atmospheric pattern. Concurrent rainfall and dust are essential to the late Pleistocene formation of the elongated thick loess zone along the desert northern margin. Even with existing dust storms, the lack of rain and very sparse vegetation account for the absence of late Pleistocene loess sequences from the southern Negev and the formation of hyperarid soils. When the north Sinai coast shifted 30–70 km northwest due to last glacial global sea level lowering, the newly exposed coastal areas supplied the sand and dust to these active eastern Mediterranean cyclones. This enforced the latitude of the northern boundary of the loess zone to be directly due east of the LGM shoreline. This shift of coast to the northwest inhibited rainfall in the southern Levant deserts and maintained their hyperaridity. Concurrently, frequent deep eastern Mediterranean Cyprus Lows were funneled along the northern Mediterranean increasing (probably doubling) the rains in central and northern Israel, Lebanon, southwestern Syria and northern Jordan. These storms and rains formed lakes, forests, and speleothems only a short distance north of the deserts in the southern Levant. [Copyright &y& Elsevier]