ABSTRACT:The interannual variations and the spatial distribution of rainfall in the Mediterranean and semi-arid regions of Israel are analysed with respect to variations in the occurrence of the typical synoptic systems of the Eastern Mediterranean. The synoptic analysis is based on a daily, semi-objective synoptic classification (Alpert et al., 2004a). The study covers the months November-March, in which 90% of the annual rainfall is obtained, mostly resulting from Cyprus lows. The interannual variations of the rainfall are well explained by the synoptic types, and the occurrences of Cyprus lows are highly correlated with the rainfall. It was found that the daily and seasonal rainfall are highly dependent on the depth of the cyclone. Moreover, deep lows are more effective for the mountainous regions, due both to the enhanced orographic effect and to the fact that stronger winds, associated with deep lows, are more efficient in transporting rain-producing clouds from the Mediterranean Sea inland. The location of the cyclone determines the spatial distribution of the rain it produces over Israel. The cyclones located east of Cyprus were found productive mainly for the southern parts of the study region, while those located to the west and north of Israel were found productive for the north of the country. The high sensitivity of the rainfall to the location of the surface cyclones emphasizes the major role that lower level moisture transport plays in rain formation.

Trends in the orographic rainfall ratio R 0 over Israel are reevaluated. It is shown that the rainfall has not changed significantly over most of the mountainous stations, with some significant increases over the central mountains. The overall evaluation of R 0 for all potential station pairs, calculating the ratio of each mountain station separately over each coastal or seashore station, indicates that about 50% of all pairs show a positive trend in R 0 . The high spatial variability, especially over the mountains, allows for finding orographic rainfall ratio trends that are significant in both the positive and negative directions. The correct definition of R 0 in the Israeli case requires the use of a seashore cluster of stations. If some of the seashore stations are replaced by inland stations, and in particular stations that are right over the region of maximum positive rainfall urban enhancement due to the thermal heat island or other urban effects, a seemingly decreasing "orographic ratio" is unavoidable. In such a case, urban dynamical positive effects on coastal rainfall can be erroneously interpreted as pollution suppression of orographic rainfall. When seashore stations are selected as required by a proper definition of the orographic ratio, increasing R 0 is obtained over central Israel and an insignificant trend over the north is found. Furthermore, evaluation of the ratio of rainfall for the upwind in comparison with the downwind side of the Galilee Mountains exhibits an increasing trend, opposite to the recent findings of Givati and Rosenfeld. The rainfall analysis shows no evidence of any suppression of rainfall over the mountains due to pollution, and at least in Israel other factors besides aerosols are predominant in defining the trends in the orographic rainfall ratio.

In this paper we analyze spatial variations of the annual rainfall that have taken place in the non-arid regions of Israel (annual rainfall > 200 mm) during the years 1952-2006, incorporating all available data. The results of the present study over the research area as a whole indicate that no significant temporal change of the annual rainfall occurred in any region of the study area. However, focusing on spatial rainfall fluctuations between sub-regions in the study area, a significant increase was observed between the stations located downwind and those upwind of the Greater Tel Aviv region.This increase supports previous reports showing that rainfall enhancement is observed downwind (and close) to urban centers. In contrast to a few previous reports, no decrease in the ratio between the mountain precipitation to that over the coastal region was found. Over the period of the present study, the rainfall ratio between the upwind slopes and the seashore remained unchanged, with a slight increase in the central part of the country.The only hilly place where a slight decrease in annual rainfall was observed is the lee side (eastern slopes) of the Galilee Mountains. This result is important because the eastern slopes of the Galilee Mountains have for years been part of the target area for Israeli artificial cloud seeding for rain enhancement. The results therefore suggest that unless there was a pronounced change in the synoptic conditions during rain spells, seeding in Israel had no positive effect on rainfall amounts.

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The primary objective here is the study of the urban rainfall anomaly patterns, particularly the positive/negative dipole reported in the literature as well as their temporal/spatial evolution due to rapid urban development. The spatial changes in the annual rainfall distribution, eastward and downwind of the largest coastal urban area of Israel, i.e., the Greater Tel Aviv region, in relation to the rapid expansion of the urban area are analyzed. This provides a unique opportunity, as shown here, to study the effect of a most rapid urban expansion on the potential for urban rainfall anomalies. Tel-Aviv probably serves as a case study for other fast-growing Mediterranean cities. Rain gauges’ data (over 100) collected over a period of 70 years (1948–2018), are divided into six sub-periods of 20 years and plotted on top of the urban area growth in those years. The average precipitation spatial distributions and their anomalies are both calculated for each sub-period. The results were examined along three geographic lines of particularly rapid urban expansion over the area, towards the northeast, east, and southeast. Increases of the precipitation downwind of the urban area are noticed when progressing along with the urban development. In addition, an upwind decrease over the coastal region is found. These findings are well correlated with the expansion of the urban area and the rainfall urban anomalies, P d e v , are of the order of 50–100 mm/y. Other potential explanations to these anomalies are discussed and suggested to be less feasible.