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Uri Dayan

Abstract

The Air Resources Laboratory's trajectory model was used to calculate five-day back trajectories from Israel at the 850 mb pressure surface for five years (January 1978–December 1982). These air flow patterns were evaluated in conjunction with seasonal weather types for which a detailed synoptic analysis and classification of the weather situations prevailing during these air flow patterns has been performed. A significant difference between the typical seasonal trajectories is pointed out. Trajectories reach Israel from five major source regions: 1) northwest Europe; 2) eastern Europe; 3) Saudi Arabia and Jordan; 4a) North African coast; and 4b) inland North Africa.

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Uri Dayan
and
Jean Koch

Abstract

This paper analyzes some measurements of the Southern California Air Quality Study, which collected a comprehensive air quality, meteorological, and emissions database in the Los Angeles Basin. This analysis emphasizes the interaction of the enriched ozone layer existing aloft with the top of the convective boundary layer (CBL) in the early afternoon of warm summer days, leading to downward mixing (fumigation) of the ozone cloud toward the ground. This process was shown to contribute to the high ozone concentrations measured at inland elevated sites. It is suggested that this mechanism also exists in Israel and contributes to the elevated concentrations observed in the summer on the slopes of the Judean Hills. This analogy is based on the similarity between the Los Angeles Basin and central Israel regarding the climate, the local circulation (sea breeze), the orientation of the coast, and the upwind location of ozone precursor sources. The temporal fluctuations of the synoptic configuration persisting over Israel during the summer cause rapid variations in the depth of the CBL inland and its subsequent interaction with an ozone layer aloft.

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Uri Dayan
and
Jacob Rodnizki

Abstract

Upper-air measurements collected for three consecutive years (1987–89) from the Israel Meteorological Service permanent sounding site, in Beit-Dagan, Israel, enabled the temporal behavior of the atmospheric boundary layer over Israel to be characterized. Data analyzed consisted of the layer depth, the thermal gradient within the layer, and occurrence frequency of radiative and elevated inversions. To adequately represent the multiyear seasonal and diurnal behavior, the 3-yr databases were merged based on the tested hypothesis that the month sample in each individual year comes from the same population. The analysis shows that the depth of the radiative ground-based inversion, its frequency, as well as its thermal profile are maximal during spring and early summer. The upper-inversion layer is well defined during the summer, its lowest base (0.5–1 km MSL) indicating a sharp interface layer formed between the marine turbulent boundary layer at the shallow layer of the atmosphere and the subsiding downward motion caused by the subtropical high pressure system. During the other three seasons a significant temporal variation of the upper-inversion base is observed as a result of the frequent larger-scale synoptic weather systems. The diurnal variation of the mixed-layer depth is most evident during the summer because it is mainly governed by heat fluxes and the daily sea-breeze cycle that are most intensive then. Henceforth, the layer minimal depth, along the coast, usually occurs during late afternoon hours when the wind speed of the cool sea breeze reaches its minimal rate and heat fluxes dissipate rapidly, leading to a decrease of the marine turbulent boundary layer.

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Ronit Nirel
and
Uri Dayan

Abstract

The ratio of sulfur dioxide to nitrogen oxides (RSN = SO2/NOx) is one indicator of air pollution sources. The role of this ratio in source attribution is illustrated here for the Ashdod area, located in the southern coastal plain of Israel. The main sources of pollution in the area are the tall stacks of the Eshkol power plant, the stacks of oil refineries, and areal sources (stationary and mobile). The factors that affect RSN are studied using four regression models: a binary regression tree in original scale, a tree in logarithmic scale, a data partition produced by a combination of the two trees, and a linear regression model. All models have similar relative prediction error, with the combined partition best highlighting the sources of variability in RSN: (a) very low values (interquartile range of [0.12, 0.48]) are associated with traffic, (b) low values ([0.43, 1.00]) are attributed to the power plant and to daytime emissions of local industry, (c) medium values ([0.74, 1.90]) are associated with local industry emissions during cooler hours of the day and refinery emissions mainly on slow wind episodes, and (d) high values ([1.07, 4.30]) are attributed to refinery emissions during moderate to fast wind episodes. Analysis of the number of episodes of increased concentrations indicates that, during 1996 and 1997, about 42% of SO2 episodes are attributable to the power plant and 33% to the refineries. Increased-NOx episodes are mainly contributed by traffic (91%) and power plant (4.5%) emissions.

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Uri Dayan
and
Ilan Levy

Abstract

A 3-yr dataset (February 2000–November 2002) of 30-min averages for particulates, visibility, and meteorological data were analyzed to examine the dependence of particulate matter of less than 10 μm in diameter (PM10) mass concentrations and visibility in Tel Aviv, Israel, on seasonal meteorological conditions, synoptic weather patterns, and airmass history. The persistent nature of the summer synoptic weather pattern in the east Mediterranean (EM) region yielded the highest mean visibility (35 km) and lowest variability in PM10 concentrations, as compared with the higher variance for the other seasons, and spring in particular. The frequent passage of the “Sharav” cyclones during spring causes natural dust outbreaks with extreme values that result in a much higher PM10 annual mean (57 μg m−3) as compared with other larger cities. The history of the air mass dictating the physical and chemical properties of the particulate matter reaching Israel was assessed by back-trajectory analysis from predetermined transport sectors. The west and central European sectors, representing mostly the summer flow regime of European clean air masses that are advected over the Mediterranean, differ significantly in their reduced PM10 loading and high visual range from the Arabian Peninsula and North African sectors, originating from the surrounding deserts. Because the most frequent direction of air masses reaching the EM is from European origins, Tel Aviv experiences most of the time satisfying visibility conditions. Applying the definition “very good visibility” for visibility exceeding 19 km shows that about 90% of the time these conditions were met for this 3-yr period.

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Ram Hashmonay
,
Ariel Cohen
, and
Uri Dayan

Abstract

The temporal variation of the atmospheric boundary layer (ABL) over Jerusalem is accurately measured by means of a lidar system. The findings are explained and discussed based on the specific synoptic situation of typical summer days in the Middle East. The different behavior of the ABL near the seashore and inland is stressed. The measurement technique is also used to detect the entrainment zone and its development over Jerusalem.

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Uri Dayan
,
Roni Shenhav
, and
Michael Graber

Abstract

A detailed study to determine the spatial and temporal behavior of the coastal and inland mixed layer in Israel was undertaken. Over 300 upper air measurements (minisonde and airplane) from the central and southern parts of Israel were compiled and analyzed utilizing a statistical methodology developed for this purpose.

The degree to which the upper air measurements taken at the permanent site at Beit Dagan represent five satellite sites in the central and southern part of the country was established. It was found that in cases of unstable atmospheric conditions accompanied by strong winds, the spatial correlation was higher than in cases of weak surface pressure gradients and stable atmospheric conditions. Significant differences in the mixing depth values were found for all synoptic categories at the permanent site, whereas the satellite sites exhibited significant differences only for the two most frequent summer synoptic conditions.

The inland variance of the thickness of the mixed layer over complex terrain did not show a parabolic profile as suggested by other investigators for a flat terrain on theoretical and experimental grounds. For the rough terrain dealt with in this work, the top of the mixed layer tends to rise at a slower rate than the rise in the topographical relief; thus the thickness of the mixed layer decreases with the distance from the shoreline.

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Uri Dayan
and
Richard Abramski

A major and fatal flooding event illustrative of rare similar cases in the Middle East was related to unusual characteristics of the jet stream. Contrary to the usual conditions, the jet stream had anticyclonic curvature and sloped northward with increasing altitude.

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Itamar M. Lensky
and
Uri Dayan

No Abstract available.

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Uri Dayan
,
Batia Lifshitz-Goldreich
, and
Karel Pick

Abstract

An opportunity to improve understanding related to the structure of the atmospheric boundary layer (ABL) in Israel along the coastal region environs emerged in April 1997 when the Israel Electric Corporation, Ltd. (IEC), installed and began the operation of a 1290-MHz radar profiler and radio acoustic sounding system (RASS) midway between Haifa and Tel Aviv. This instrument is, in addition to the upper-air measurements carried out routinely by the Israel Meteorological Service (IMS) in Beit-Dagan, at the central coastal plain of Israel. In order to better characterize the spatial variation of the turbulent mixed layer and the stable layer capping it within the ABL, the upper-air measurements from the profiler were compared with the concurrent rawinsonde data from the permanent sounding site in Beit-Dagan. The analysis and simultaneous comparison of both instruments was done for noontime of two consecutive summers (1997–98) and included the following ABL parameters: depth and lapse rate of the mixed layer, depth and virtual temperature gradient of the inversion layer, wind speed and direction at base and top of the inversion layer, and wind speed and direction shears within the inversion. All the sampling days were classified into three modes of the predominant synoptic situation typifying most of the summer days in Israel (i.e., the “Persian trough” synoptic system). Means, standard deviation, and relative standard deviation were calculated and compared between both sites for each of the assigned synoptic category. The analysis shows that (a) a consistently deeper and less unstable mixed layer is observed in the northern coastal site; (b) the interface separating the mixed layer and its capping inversion is sharper at the farther southern and inland site—a feature caused by stronger subsiding conditions above this site; (c) variation in the depth of the mixed layer for the different mode of the Persian trough is more pronounced at the northern site because of the proximity of this site to the cyclonic center of this trough; (d) an anticyclonic veering in horizontal wind direction and speed weakening is observed along the whole ABL profile going inland and south—a finding significant for all synoptic modes except for the shallow Persian trough mode; (e) at the northern coastal site, an anticyclonic shear in horizontal wind direction of 6°–12° (100 m)−1 is observed as ascending from the elevated inversion bottom to its top along the different synoptic modes of the Persian trough; and (f) differences in ABL properties are more affected by distance from shore than by the different modes of the synoptic pattern. This conclusion holds for wind speed and lapse rate within the mixed layer and virtual temperature gradient within the stable layer but not for mixed layer depths that were found to be more affected by the synoptic conditions prevailing over the region.

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