Search

A unique chain connects the flow over the North Atlantic and the development of cyclones within the Mediterranean basin. One typical mechanism includes several successive processes: upper-level flow perturbations upstream cause Rossby wave breaking (RWB) events along the jet stream, which in turn develop into potential vorticity streamers. These streamers reach the Mediterranean, and through increased baroclinicity they enhance cyclonic activity in the region. Using ERA5 reanalysis data and rain gauge measurements, we provide a systematic analysis connecting wintertime North Atlantic storm track regimes and Eastern Mediterranean cyclones and rainfall. To do so, we use different detection algorithms for each element in the chain (RWBs, streamers and cyclones). A cluster analysis of upper tropospheric eddy kinetic energy reveals a favorable configuration of the storm track where North Atlantic storms are able to propagate farther northeast. This results in upper-level potential vorticity streamers forming more eastward alongside above-average precipitation over the Levant. Meanwhile, other latitudinal positions of the storm track (southward or northward) were found to hinder cyclonic activity in the region and reduce rainfall there. The intense rainy winter of 1991–1992 is brought as a test case to exemplify this mechanism in its extreme. We show that the rain-enhancing storm track regime was prominent throughout most of this season, alongside frequent streamers in the Eastern Mediterranean. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

ABSTRACT: The Levant region is dominated in the summer by the persistent Persian Trough. Thus, the current synoptic classifications lack the ability to reflect inter‐diurnal variations in the local weather conditions. This study aims to overcome this weakness and has two objectives: to develop operative prediction equations for the local weather conditions based on synoptic‐scale data, and to propose a new synoptic classification, based on the environment to climate approach, to better reflect the local climatic stress. Prediction equations were derived for the national heat‐stress (NHS) and the height of the persistent marine inversion (inversion base, IB). The potential predictors are comprised of atmospheric variables found correlated with these weather attributes together with indices representing synoptic to large‐scale features. The indices were derived through a composite maps analysis of days with extreme values of these attributes. The Climatic Stress Index (CSI) developed is a combination of the heat‐stress and the inversion height, for which a separate prediction equation was derived. The spectrum of the calculated daily CSI values was divided into three parts of equal size, used as the basis of the new synoptic types, defined as: ‘comfort’, ‘medium’, and ‘discomfort’. The attribution of a certain day to one of these types is determined according to its calculated CSI. Our proposed classification was found to better delineate the NHS and the height of the IB in Israel. The downscaling schemes and the new synoptic classification are applicable for both operative weather prediction and climate prediction. The latter enables the assessment of future trends in the climatic stress, in general, and heat‐stress, in particular. This is due to being compatible with the output of operative weather prediction models as well as for the format of the disseminated output of climate models. [ABSTRACT FROM AUTHOR]

This study examines the synoptic conditions controlling NO x pollution in the metropolitan area of Tel Aviv, using a semi-objective synoptic classification for the eastern Mediterranean. A day in which NO x concentration exceeded the Israeli standard in ≥1 of the seven monitoring stations was defined an “exceeding day” and in ≥5 as an “extensive exceeding day”. For 1998–2004, 19% and 3% of the days were found exceeding and extensive exceeding days, respectively, over 85% of them in the winter months, November–March. The inter-annual variation in the occurrence of the synoptic types was found to explain 72% of the variations in the number of exceeding days. A significant negative trend in the occurrence of types with high pollution potential explained the decrease of 10% per year in the number of exceeding days during 1998–2004. The Red Sea Trough, though being cyclonic system, contributed 51% of the exceeding days, while highs, though being more frequent, contributed only 35%. The “pollution potential” of a synoptic type was defined as the percentage of exceeding days belonging to this type. The majority of synoptic types with the highest pollution potential were cyclonic, most being the Red Sea Trough with western axis, with 82% potential. Our findings indicate that the identity of the synoptic system as cyclonic or anticyclonic is not the key factor for the pollution potential in the study region, but rather, the ambient atmospheric conditions they induce, i.e., high temperatures, static stability, and weak easterly offshore flow. Local processes are the direct cause of the pollution and that the role of the synoptic conditions is to enable, or even to reinforce, the supportive meso-scale processes. This study is a first step in downscaling synoptic features to local NO x pollution potential, constituting a basis for alarming against pollution events, based on the predicted synoptic conditions. [ABSTRACT FROM AUTHOR]

Abstract: We report synchronized optical observations of sprites in Israel during the winters of 2006/7–2007/8. Based on several events, we suggest that the elements of columniform sprites are organized in spaced intervals on the circumference of a circle centered directly above, or a little offset, to the vertical direction from the parent lightning. In 2D images most of the cases show columns to be arranged in highly eccentric elliptical forms or in straight rows. The analysis of the optical images provided the geometrical dimensions of the columns and their spatial organization. We used an electrostatic model of the QE field with reasonable assumptions on the location and magnitude of the cloud charge center, constrained by ELF evaluation of the Charge Moment Change in the parent flash, to show that the observed diameter of the columnar arrangement closely matches the conventional breakdown field line contour at the same altitude. [Copyright &y& Elsevier]