Factors affecting isotopic composition of the rainwater in the Negev Desert, Israel

Factors affecting isotopic composition of the rainwaterin the Negev Desert, Israel

Avraham Dody1,2 and Baruch Ziv3

Received 5 December 2012; revised 17 June 2013; accepted 20 June 2013; published 6 August 2013.

[1] The study examines the δ18O of rain observed in the middle of the Negev Desert, Israel,based on seven rainstorms, associated with convective clouds resulting from midlatitudecyclones found over Trans-Jordan. The analysis is based on synoptic maps, verticalatmospheric cross sections, and air back trajectories. The rainwater sampling was done intemporal scale of hours. Four known factors were addressed here: the temperature effect, theamount effect, and two geographical effects: the marine and continental effects. Thetemperature effect is expressed by a positive relation between the lower level temperatureand the isotopic composition. The amount effect, related to rainfall, is negative. The marineeffect, expressed by a relation between the time the rain-producing air spends above theMediterranean Sea, is positive and the continental effect, associated with the passage overland is negative. The linear relationships found in this study met the expectations, althoughonly the amount effect was statistically significant. A fifth effect, entitled cloud top effectproposed here, relates to the height of the convective cloud layer top. It assumes that lowertop prevents entrainment of upper cold air into the clouds, so the isotopic composition isenriched. The cloud top effect was found most statistically significant.

Citation: Dody, A., and B. Ziv (2013), Factors affecting isotopic composition of the rainwater in the Negev Desert, Israel,J. Geophys. Res. Atmos., 118, 8274–8284, doi:10.1002/jgrd.50592.

1. Introduction

[2] Stable isotopes within rainwater are used as tracers inatmospheric water to improve our understanding of the pro-cesses involved in moisture transport and rain production.There are a large number of observations of stable isotopesin precipitation with monthly time resolution all over theglobe [e.g., Schootterer et al., 1996]. Others like, Araguas-Araguas et al. [2000], Gat [1996], and Lawrence et al.[2004], dealt with stable isotopes in atmospheric water vapor.[3] It is well known that temperature is the main factor

determining the isotopic composition; i.e., the higher thetemperature in the moisture source, the heavier (enriched)the isotopic composition. The temperature depends on alti-tude, latitude, seasonality, and the geographic factors (mainlythe identity of the source region as sea or land) [e.g., Clarkand Fritz, 1999]. The path of air parcels from the sourceregion on their way to the target area also influences the iso-topic composition. An example for such an influence is thecontinental effect, expressed in a depletion in the isotopic

composition of the cloud water (and the resulting rain), whilea maritime air mass moves inland [Clark and Fritz, 1999].[4] Another effect is the amount effect; i.e., the larger the

rain amount, the more depleted the isotopic compositionbecomes. This effect was first reported by Dansgaard[1964], who attributed it to evaporation from the raindropsto the surrounding air because the lighter water moleculesevaporate more easily. He proposed that in the beginning ofa rain, the surrounding air is still dry so that evaporation isrelatively intense, and subtracts large part of the lighterisotopes. After considerable rain, the air becomes almostsaturated with water vapor so that the evaporation effectbecomes much smaller. Later, others like Clark and Fritz[1999], showed the amount effect by comparing the isotopiccomposition of rain below and above 20 mm in a dry climatein Bahrain. Lee and Fung, 2007 explained the amount effectby the interaction between raindrop size, rain intensity, andwater vapor near the planetary boundary layer (PBL). Dodyet al. [2010] analyzed the relation between the isotopiccomposition (of the database used in the present study) andthe instantaneous rain intensity and found no correlationbetween them. Miyake et al. [1968] reported that the ex-change between raindrops and water vapor is more effectivethan the evaporation in enriching the isotopic composition.[5] Previous studies, e.g., Cappa et al. [2003] and Gat

et al. [2003], added specific parameters, such as evaporation,relative humidity (RH) and sea surface temperature (SST),which influence the isotopic composition of the water vaporand, consequently, the rainwater. Evaporation from waterbodies includes mainly depleted water molecules, causingthe remaining water body to become more enriched,

1NRCN P.O.B. 9001 Beer Sheva, Israel.2Department of Geography and Environmental Studies, Ben Gurion

University of the Negev, Beer Sheva, Israel.3Department of Natural Sciences, The Open University of Israel,

Raanna, Israel.

Corresponding author: A. Dody, NRCN P.O.B. 9001 Beer Sheva, Israel.([email protected])

©2013. American Geophysical Union. All Rights Reserved.2169-897X/13/10.1002/jgrd.50592

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especially over small lakes, where the RH is relatively lowand the evaporation rate is larger. As for the SST, the higherthe SST, the more intense is the evaporation due to the higherkinetic energy of the individual molecules so that the water va-por is more enriched and so are the resulting raindrops. Anothergeographical effect that is relevant to the study region is the ma-rine effect, i.e., the time the rain-producing air mass spends overthe Mediterranean Sea before reaching the Negev Desert. Wesuggest that while moving over the sea surface the otherwisecontinental air (as is reflected by the air trajectories shown insection 3) would gain more heavy isotopes so that it becomesmore enriched. This is because the Mediterranean Sea containsmainly heavy stable isotopes [Gat et al., 2003].[6] The isotopic composition of the rain in Israel has been

previously studied by Rindsberger et al. [1983], who analyzedrain samples from 10 rainstorms in its northern part. They

found a statistically significant positive linear relationshipbetween the 800 hPa (~2000 m above sea level) temperatureat Mid Israel (where soundings station is located) and the isoto-pic composition, deducing that the latter may be regarded as anindicator for the source latitude of the cold air that produces thewinter rains in Israel. Leguy et al. [1983] and Rindsberger et al.[1983] were the first to depict air back trajectory as a basis foridentifying the sources and paths of the rain-producing air massin Israel. The back trajectories derived by Rindsberger et al.[1983] were based on 800 hPa level alone. This approach islimited by its implied neglecting the vertical displacement ofair masses, especially in the presence of cyclones, in whichthe rain is produced. Moreover, the choice of such a midleveldisplacement may miss the contribution of the PBL, which isnormally confined to the lower levels.[7] Dody et al. [2010] claimed that to improve our under-

standing of the fractionation process under atmospheric con-ditions, sampling in high temporal and spatial resolution isneeded. Pfahl and Wernli [2008] discussed the effect of airtrajectory on stable isotopes in water vapor in the easternMediterranean. They estimated the synoptic scale trajectory,based on the RH, as measured in Rehovot (Israel).[8] Several studies concentrated on the Negev Desert located

at southern Israel. Levin et al. [1980] stated that the isotopiccomposition is relatively uniform over a wide area on any par-ticular day but differs substantially from one storm to another.Dody [1995] and Adar et al. [1998] showed, for the same geo-graphical area, that the isotopic composition varies sometimesdramatically even within individual rainstorms.[9] Climatic and synoptic studies have pointed at several

sources of air from which the rain over the Negev Desert isassociated. Kahana et al. [2002] showed that half of the majorfloods in the northern part of theNegev are associatedwithmoistair flowing from the Mediterranean and the other half by intru-sions of tropical air masses from south. Dayan et al. [2001]and Ziv et al. [2004] found that tropical air masses are involvedin autumnal severe floods that occur in south Israel under the in-fluence of a Red Sea trough. Ziv [2001] and Rubin et al. [2007]showed that the moisture in some winter rainstorms transporteddevelopment produced by tropical plumes [McGuirk et al.,1987, 1988] originating from tropical western Africa.[10] The linear relationship between the source latitude of the

air mass and the isotopic composition of the rain it produces[Rindsberger et al., 1983] suggests that an analysis of the isoto-pic composition of underground water in North Israel can pointat the dominating source of air masses that produced rain in thepast. The results of such analysis can therefore serve as a proxyfor past climatic conditions in that region. One of our goals is toexamine whether this also holds for the central Negev Desertcovering the southern part of Israel.[11] The aim of this study is to isolate the effects expected

to influence the isotopic composition of the rain in southIsrael based on sampling the rainwater in high temporal res-olution, in an order of hours. The factors analyzed are thetemperature, amount, marine and continental effects, andan effect related to the height of the convective top proposedhere (cloud top effect, see section 3.2). The analysis uses airback trajectories, synoptic maps, and vertical atmosphericcross sections. Section 2 specifies the data sources andmethods used. Section 3 elaborates the functional relation-ships between the isotopic composition and its governingfactors and presents three case studies to demonstrate the

Figure 1. Long-term mean annual rainfall map of Israel.The study region is denoted by a six-pointed star.

DODY AND ZIV: ISOTOPIC COMPOSITION OF RAINWATER

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dynamics involved. Section 4 summarizes results anddiscusses their implications.

2. Materials

[12] The study region is the central part of the NegevDesert, Israel, having an average annual precipitation of~100 mm, distributed during 20 rain days during thewinter season (on average) [Zangvil and Druian, 1989].The rain recording and the isotopic sampling were taken fromthe Sde Boqer station (30.5°N, 34.8°E, see Figure 1). Theresearch covers eight rainstorms that occurred during theyears 1990–1993 and described in detail by Dody et al.[2010]. All of the rain samples were analyzed at theInternational Atomic Energy Agency (IAEA), Vienna.Samples of three rainstorms were also analyzed at theNiedersachsisches Landesmat for Forchung (NLFB)Laboratory in Hannover, Germany. The standard errors forthe sampling are ±1‰ for δD and ±0.1‰ for δ18O, whichare acceptable for such analysis. No differences were foundbetween the results obtained in the two labs (Dody et al.2010). In this study, we present only the δ18O values.Details concerning the eight storms analyzed are specifiedin Table 1.[13] A mechanical continuous sequential rain sampler with

high resolution, developed by Adar et al. [1991], was used.This sampler collects 250 to 500 mL of water, equivalent to1 and 2 mm of rain, respectively. The novelty of this methodis that once an assigned volume of water is collected, theweight of the water load in the container is used as a mechan-ical seal that isolates it as a separate rain sample. The impli-cation is that the higher the rain intensity, the higher thetemporal resolution. The sampler has 20 rain bags, whichallow the collection of rainwater from a storm with 20 to40 mm.[14] Air back trajectories were extracted using the Hybrid

Single Particle Lagrangian Integrated Trajectory model ofthe National Oceanic and Atmospheric Administration(NOAA) Air Resources Laboratory (http://ready.arl.noaa.gov/HYSPLIT.php). The displacement of an air parcel iscomputed based on the 3-D wind field of the NationalCenters for Environmental Prediction (NCEP)-National

Center for Atmospheric Research (NCAR) reanalysis data[Kalnay et al., 1996 and Kistler et al., 2001], given at2.5° × 2.5° resolution, in 6 h time increments. In the analysis,the levels from which air back trajectories were derived are500 and 1000 m above ground level (AGL), assuming thatthese heights represent the PBL from which the rainclouds develop.[15] The impacts of various effects were examined: the

amount effect, the air temperature, the continental and marineeffect, and the cloud top effect (described in section 3.2). Foreach effect, a linear regression with the isotopic depletionwas derived, and its statistical significance was assessedthrough Pearson correlation. The amount effect was analyzedfor rain segments each separated by >4 h from each other.The air temperature at the study region was taken from the850 hPa level, a standard level where data are given.According to Rozanski et al. [1993], this effect holds whenthe temperature is <15°C. The temperatures observed in thisstudy are<7°C. Taking into account that the typical height ofthe 850 hPa level is 1500 m, the cloud base is ~1000 m andthat the atmospheric lapse rate is <1°C/100 m, the abovecondition is met in the study area and period.[16] The continental and the marine effects were estimated

by the time the air spent over land and theMediterranean Sea,respectively, before entering to the study region at 500 and1000 m AGL (using back trajectories derived from theHYSPLIT software, see above). The top of the convectivecloud layer (as an estimate of the cloud top effect, seesection 3.2) was estimated by the uppermost pressure levelat which the RH exceeded 60%, based on verticalcross sections derived from the NCEP-NCAR reanalyzeddata. All above relationships were analyzed applyinglinear regression.[17] Two factors were not included in this study. One is the

determination of the air origin, which depends on an arbitrarychoice of the duration of the back trajectory. Hence, the airorigin is treated subjectively and is estimated through thetemperature effect (as done by Rindsberger et al. [1983]).The second factor excluded from this study is that of theMediterranean SST. This is due to its being stable duringthe months included in this study, January to April, andvaries in the small range of 17–19°C.

Table 1. List of the Eight Rainstorms Analyzed and Their Synoptic Features

PeriodNo. ofδ18O

No. of RainSegments

No. of AtmosphericSamples Storm Type Air Trajectory

25–26 January1990

12 Not analyzed Not analyzed Tropical easterly trough Originated from Equatorial Africa at the mid levels

1–2 February1990

6 2 2 Midlatitude low over south Israel,moving eastward

Mediterranean

11–13 March1990

15 3 3 Midlatitude low over Trans-Jordan Mediterranean

1–2 April1990

15 4 4 Midlatitude low over Trans-Jordan Mediterranean at the beginning, later from Europe(through the Mediterranean)

24–25 January1991

18 2 3 Midlatitude low over Trans-Jordan Europe (through the Mediterranean)

7–8 February1991


5–6 March1991

14 4 4 Midlatitude low over Trans-Jordan Mediterranean at the beginning, later from Europe(through the Mediterranean)

23–24 March1991


Total 110 19 21


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http://ready.arl.noaa.gov/HYSPLIT.php

http://ready.arl.noaa.gov/HYSPLIT.php

3. Results

[18] The synoptic features of the eight rainstorms analyzedare specified in Table 1. Seven of them were associated withmidlatitude cyclones located over Trans-Jordan, and one,occurred at 25–26 January 1990, was associated with tropicallike easterly trough. The rains resulting from the midlatitudecyclones were formed under northwesterly maritime flowthat entered the study region from the Mediterranean.Following Kahana et al. [2002] and Saaroni et al. [2010],the clouds in these storms are convective and emanate fromthe PBL. These rains are enhanced by orography along thenorthern slopes of the Negev Highland. In contrast to thesestorms, the rain of 25–26 January 1990 developed withinstratified cloud layer centered at 2–3 km height. Because ofits exceptional type, source of air, and type of clouds andrain, this storm was excluded from the present study.

3.1. Evaluation of the Factors AffectingIsotopic Composition

[19] The numerical values of the δ18O and the governingfactors for the seven storms associated with midlatitudecyclones are specified in Table 2. The linear relationshipsderived for these factors are presented in Table 3. The rela-tionships for the temperature and the rain amount are shownin Figures 2a and 2b, respectively. Both relations are inagreement with the theoretical expectations (see section 1),

but only that of the rain amount is statistically significant(at the 0.9 level, using one directional assumption, althoughbeing low, R = 0.38). The continental and the marine effectswere found to meet the expected relation but are statisticallyinsignificant (see Table 3). We suggest that the reason for themarginal influence of the marine effect for the study regionstems from the fact that in all the samples analyzed, the min-imum time spent over the Mediterranean was 15 h, whichmay be sufficient to gain considerable isotopic enriched com-position. It should be noted that when the eighth storm wasincluded in the sample, the temperature isotopic compositionrelation was inversed (still, statistically insignificant).

3.2. Hypothesized Additional Effect—The CloudTop Effect

[20] Here we hypothesize an additional effect related to theheight of the convective clouds tops. Since cold air containsdepleted composition, the lower top of the clouds impliesheavier composition. This idea comes from inspection of in-dividual cases, in which sharp changes in the isotopic com-position could not be explained by any of the alreadyknown factors. This effect is called hereafter the cloud top ef-fect. The cloud top effect is estimated here by the pressurelevel separating high values of RH below and low valuesabove (note that higher values of pressure level means loweraltitude). The threshold value used here is 60% because inone case, the maximum was <70%. The higher this level

Table 2. List of Samples Taken From the Seven Rainstorms Associated With Midlatitude Cyclones and the Values of the Relevant Factors

Serial Number Date Hour (UTC) δ18O T (°C) Time Over Med. (h) Time Over Land (h)Top of Convective

Clouds (hPa)

1 1.2.90 21 –2.5 3.5 36 3 6902 2.2.90 6 –3.5 2.5 48 6 7403 11.3.90 6 –1.7 4 48 6 7104 12.3.90 18 –8 4.5 48 29 4905 13.3.90 9 –2.5 4.5 26 46 5806 1.4.90 3 –4.7 6 46 26 3907 1.4.90 12 –3 7 60 12 6808 2.4.90 3 –5 3 15 7 5409 2.4.90 12 –3.5 4 15 6 56010 25.1.91 12 –5 1.5 33 3 77011 25.1.91 18 –8.3 0.5 30 4 50012 26.1.91 12 –9.5 0 20 3 55013 7.2.91 12 –7 5 32 4 40014 8.2.91 12 –5 3.5 70 4 77015 5.3.91 0 –5 8 60 8 43016 5.3.91 12 –9 4 28 8 38017 6.3.91 3 –5 1 22 6 61518 6.3.91 12 –4.5 2 18 3 78019 23.3.91 12 –3.8 7.7 20 5 53020 24.3.91 3 –9.5 6.8 18 9 65521 24.3.91 18 –3 9 20 9 700

Table 3. Linear Relations Between δ18O and its Governing Factors and Their Statistical Significancea

Factor Sample Size Linear Relationship Fit to Theoretical Expectations Statistical Significance Reference

Temperature 21 y = 0.26x – 6.26 Yes No Clark and Fritz [1999]Amount effect 19 y = –0.15x – 4.3 Yes At the 0.9 level "Marine effect 20 y = 0.03x – 6.18 Yes No Gat et al. [2003]Continental effect 21 y = –0.03x – 5.02 Yes No Clark and Fritz [1999]Cloud top effect 21 y = 0.009x – 10.8 Yes At the 0.95 level –

aNote that the cloud top effect is estimated by the pressure level of the cloud tops, so that the positive coefficient means a negative relation of the isotopiccomposition with the height (in meters).


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is, less portion of cold air entrained with the clouds. The hy-pothesized relation was examined for our data (21 samples)and found to meet this hypothesis (Figure 3 and Tables 2and 3) and even found the most statistically significant effect,at the 0.95 level. The correlation between the isotopic com-position and the cloud top effect was found to be 0.55.Since the isotopic composition is affected by several factors,this correlation can be regarded rather high.

3.3. Case Studies

[21] This section analyzes three cases that exemplify theimpact of the various governing factors on the variations inthe isotopic composition. Each case offers a clear examplethat demonstrates the contribution of specific factors. Foreach case, the sea level pressure (SLP) maps, the air backtrajectories and the rainfall chart of the study region areshown, together with the corresponding δ18O. Note that thethree cases are fromMarch and April. This may impose a biassince they represent the end of the rainy season. The onlyvariable that has a seasonal course in the study region is theMediterranean SST, which remains stable through January–April (see also section 2), from which the rain samples weretaken (Table 1).3.3.1. Case 1: 11–13 March 1990[22] This case lasted two and half days and yielded 17 mm of

rain. The isotopic composition varied between –1.7 and –9.0‰.The dominating synoptic systemwas a cyclone that was formedin north Saudi Arabia in the morning of 11 March 1990 and

remained at that vicinity during the following days. In part ofthe time, a secondary small-scale cyclone was formed overthe Nile Delta, but its effect on the wind over the study areawas minor. The resulting wind over the study area undulatedaround the northwesterly-westerly direction, as is manifestedby the air back trajectories noted at three different times duringthe storm (Figures 4b–4d).[23] The storm is divided to three phases according to

the rain segments and the origin of the air that enteredthe study area. At the first phase, the air originated fromTurkey (Figure 4b), at the second it originated fromGreece (Figure 4c), and at the third from the EasternMediterranean itself (Figure 4d). However, the 850 hPatemperature (Figure 4f) was stable during the storm andremained around 4°C, suggesting that the differencesamong the sources of the air mass affecting the study areawere small. In spite of the steady temperature, the δ18Ovaried considerably, from –1.7‰ at the first phase, to –8‰at the second and back to –3‰ at the third (Figure 4f). Sincethe temperature itself did not change during the storm, thechanges in the δ18O should be explained by changes inother factors.[24] At the first phase, the air entered the study region from

the sea and then moved ~50 km over land within 6 h(Figure 4b). At the second phase, the continental path ofthe air was much longer, ~400 km (~24 h span, Figure 4c),presumably due to a secondary cyclone that was formed overthe Nile delta (Figure 4a). This can explain why the isotopiccomposition was more depleted than in phase 1. At the thirdphase, the air path could not be determined because of thehigh differences between the back trajectories entering thestudy region at 500 and 1000 AGL (Figure 4d).[25] Two additional factors can better explain the δ18O

sharp changes of the isotopic composition during the storm:the amount effect and cloud top effect. The rainfall in the firstphase was 4 mm; at the second, 12 mm; and at the third,2 mm, in agreement with the changes in the isotopic compo-sition, –1.7, –8.0, and –2.5‰, respectively. As for the top ofthe convective layer, it rose from 710 hPa (~2900 m) in the

Figure 2. δ18O as a function of (a) temperature and (b) rainamount. Each graph contains the linear relationship and R2.

Figure 3. δ18O as a function of the level representing thetop of the convective layer. The units are hPa. Lower valuesimplies higher level; e.g., 500 hPa is ~5500 m, 700 hPa is~3000 m, etc.


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first phase to 490 hPa (~5700 m) in the second and subsidedback to 580 hPa (~4400 m) at the third (for more details, seeTable 2). Note the positive relation between the pressurelevel of the convective layer top and the δ18O (Figure 3).This case demonstrates the contribution of the continental,amount, and the cloud top effect.

3.3.2. Case 2: 1–2 April 1990[26] This case lasted two days and yielded 20 mm of rain.

Figure 5a shows the synoptic situation as represented bythe SLP of 2 April 1990 00 UTC, showing a pronouncedcyclone centered over northwest Iraq. The resulting flowover the study area was northwesterly during the entire

Figure 4. (a) SLP for 12 March 1990 18 UTC, with the notation L at the centers of the cyclones; air backtrajectories arriving at the study region at 500 (blue) and 1000 m (red) AGL for (b) 11March 1990 04 UTC,(c) 12 March 1990 20 UTC, and (d) 13 March 1990 13 UTC; (e) rain chart Sde Boqer station and (f) thecorresponding isotopic composition with notations (red) of the 850 hPa temperature at the study regionfor selected times.


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storm. The back trajectories, taken at two different times(1 April 1990 14 UTC and 2 April 04 UTC, Figures 5band 5c, respectively) show that the air that entered thestudy region at the lower levels, represented by theheights of 500 and 1000 m AGL, approached from thenorthwest. Figure 5d indicates that the rain was con-centrated in four distinct segments, and 70% of it wasobtained in the second and the third of them, 8 and 6mm, respectively.

[27] Comparison between the first phase of the storm,including the first two rain segments, and the second, includ-ing the last two segments (see Figures 5d and 5e), indicatesthat two factors contributed to a depletion of the isotopiccomposition, while two others contributed to its enrichment.One depleting contribution was a change in the source of theair producing rain (compare Figures 5b and 5c). The backtrajectories corresponding to the first phase (Figure 5b)originated from the Mediterranean region, south of Greece,

Figure 5. (a) SLP for 2 April 1990 00 UTC; air back trajectories arriving at the study region at 500 (blue)and 1000 m (red) AGL for (b) 1 April 1990 14 UTC and (c) 2 April 1990 04 UTC; (d) rain chart Sde Boqerstation and (e) the corresponding isotopic composition with notations (red) of the 850 hPa temperature atthe study region for selected times.


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Figure 6. (a) SLP for 6March 1991 00 UTC; air back trajectories arriving at the study region at 500 (blue)and 1000 m (red) AGL for (b) 5 March 1991 01 UTC and (c) 5 March 1991 12 UTC; (d) rain chart SdeBoqer station and (e) the corresponding isotopic composition with notations (red) of the 850 hPa temper-ature at the study region for selected times.


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and that in the second phase originated farther north, from theBlack Sea (Figure 5c). The temperature dropped, accord-ingly, from 6–7°C in the first phase to 3–4°C in the second.Another depleting contribution originated from the continen-tal effect; the span of the air over land in the first phase was12–26 h and dropped to 7 h in the second (Table 2). At thesame time, other factors acted at the opposite sense, i.e., to en-rich the isotopic composition. One is the cloud top effect,expressed by a drop of the top of the convective layer from390 hPa (~7500 m) to around 600 hPa (~4000 m, Table 2).The second is the marine effect; the time the air spun overthe Mediterranean was ~50 h at the first phase but only 15 hin the second.[28] This case demonstrates how two effects, the temperature

and the continental, acting at one direction, are counteracted bythe cloud top and the marine effects. It should be noted that be-side the small changes found among the average values of theisotopic composition for the different rain segments the differ-ences among the individual samples were larger (in the range of–2.2‰ to –7‰) but were smoothed out due to the limited res-olution of the synoptic data.3.3.3. Case 3: 5–6 March 1991[29] This case lasted two days and a half and yielded 17 mm

of rain. The synoptic dominant feature was a midlatitudecyclone that was formed in north Iraq on the morning of5 March 1991 and remained there for the rest of the storm(exemplified by Figure 6a), with only slight changes. A north-westerly flow dominated the lower levels in the study region,as is reflected by the air back trajectories (Figures 6b and6c). The origin of the air that entered the study area at the

beginning of the storm was the Mediterranean Sea(Figure 6b), then shifted to the Black Sea (Figure 6c) and laterto the Balkans (not shown).[30] An examination of the amount effect (compare

Figures 6d and 6f) shows that during the most intensiverain segment (the second, with 7 mm of rain), the isotopiccomposition was the depleted, in agreement with theexpected relation. On the first day of this storm, the tem-perature isotopic composition relationship is clearly seen.While the 850 hPa temperature dropped from 8 to 4°C,the δ18O became depleted from –4 to –8‰. From the be-ginning of 6 March, while both the back trajectories andthe temperature remained unchanged (for the temperature,see Figure 6e), the δ18O became considerably enriched,from –9 to –4‰. The only factor that changed consider-ably during 6 March was the cloud top effect. This is rep-resented in Figure 7, showing a vertical cross section ofthe RH along 35°E. It shows that at 5 March 18 UTCthe convective layer, within which the RH exceeded60% at 30.5°N (where the study area is located), extendedup to ~400 hPa, implying that the cloudiness could have adepth of ~7 km. Figures 7b–7d show that the moist layerbecame thinner gradually; until at 6 March 12 UTC, theend of the rainstorm (Figure 6d), its depth was <3 km(Figure 7d). The decrease in the depth of the convectivelayer during 6 March explains the gradual enrichment ofthe isotopic composition during the second day of thestorm, in spite of the steadiness of the other atmosphericfactors. This case clearly demonstrates the contributionof the cloud top effect.

Figure 7. Vertical cross section through the 35°E longitude of the relative humidity for (a) 5 March 18UTC, (b) 6 March 00 UTC, (c) 6 March 06 UTC, and (d) 6 March 12 UTC. The vertical line denotes thelatitude of Sde Boqer.


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4. Discussion and Summary

[31] The study examines the δ18O of rain observed in thecenter of the Negev Desert, Israel. The results of sevenrainstorms, all characterized by convective clouds associ-ated with midlatitude cyclones found over Trans-Jordan,were analyzed using synoptic maps, vertical atmosphericcross sections, and air back trajectories, to identify themajor factors determining the isotopic composition of therainwater. The rainwater sampling was done in hightemporal resolution, i.e., a time scale of hours. The recon-struction of air back trajectories is based on the HYSPLITmodel.[32] Five factors were addressed here (Table 3). The first is

the temperature effect, which is a proxy for the origin of theair mass within which the rain clouds are formed; the secondis the amount effect; the third is the marine effect, representedby the time the air spent over the Mediterranean Sea beforereaching its southeastern coast; the fourth is the continentaleffect imparted by the passage of the rain-producing air overland before reaching the study area. The fifth effect is pro-posed here, entitled the cloud top effect, related to the verticaldepth of the convective cloud layer.[33] The temperature effect, expressed by a positive rela-

tion between the lower level temperature and the δ18O (foundfor north Israel by Rindsberger et al. [1983]), was also foundin this study, although not statistically significant, presum-ably due to the interference with other factors. The amounteffect, expressed in a negative relation between the rainfallat a certain rain segment and the isotopic composition, wasfound here to have low correlation (0.38) and statisticallysignificant, at the 0.90 level. The marine effect, expressedin a positive relation with the isotopic composition, and thecontinental effect, expressed in a negative relation with theisotopic composition, were both identified in this study,although being statistically insignificant.[34] The cloud top effect, proposed here, assumes that the

lowering of the top of the convective layer prevents the en-trainment of upper level cold air, containing depleted isotopiccomposition, into the clouds. In other words, the truncationof the cloud tops implies a smaller portion of light isotopes,i.e., enriched δ18O. The cloud top effect is exemplified heremostly by case 3, above, in which a substantial enrichmentof the δ18O was observed while minimal change in the othereffects was observed, except for a considerable lowering ofthe top of the convective layer. The statistical significanceof this effect was found to be the highest (at the 0.95 level),and the correlation R was 0.55.[35] The cloud top effect and the amount effect are

expected to be correlated since as the cloud tops becamehigher, they tend to produce more rain. Unfortunately, thesetwo effects could not be correlated due to the different rainsampling methodologies (see section 2).[36] It should be noted that this study covers only part of

the rain-producing systems, i.e., the midlatitude cycloneswhile positioned over Trans-Jordan. These produce rainshowers associated with convective clouds that developwithin the PBL. For the sake of homogeneity, we selectedout one additional storm (No. 8) associated with mediumstratified clouds that developed within tropical air mass.Storms associated with tropical air masses, which contributeconsiderable rain in the Negev Desert (e.g., Kahana et al.

[2002]) should be studied separately, after a considerableamount of rain samples is accumulated.[37] Our results (Table 3) indicate that for the Negev

Desert of Israel, the monotonic relationship between the iso-topic composition and source latitude of the rain-producing airmass, as found by Rindsberger et al. [1983], exists. However,this relation is not statistically significant, presumably due tothe interference with the other effects, such as the cloud top ef-fect, whichwas found highly significant. It is possible that in theNegev Desert, the changes in the top of the convective layer areassociated with the proximity of this region to the descendingbranch of the Hadley Cell, which may lower it more frequentlythan in North Israel, where the temperature effect was found sta-tistically significant. Moreover, the northern part of Israel ob-tains its rain mainly under southwesterly winds [Saaroniet al., 2010] entering from the Mediterranean Sea. This impliesthat the probability of the rain-producing air to cross a consider-able continental area (and hence to be subjected to the continen-tal effect) is low (Goldreich, 2003).[38] An attempt to evaluate the variability of the isotopic

contribution explained by the above effects was done byapplying a multi regression analysis. The analysis does notinclude the amount effect due to the inability to unify thelatter data set, based on the spatial distribution of the rainsegments, with the other samples, based on the availabilityof atmospheric data (in 6 h intervals). The multiregressionanalysis (using the “stepwise” option) identified only thecloud top effect as significant.[39] The effect found most significant in this study, the

cloud top effect, together with the temperature effect (statis-tically significant at the 0.85 level), reflect similar atmo-spheric situation, i.e., a southward intrusion of polar air andan equatorial retreat of the descending branch of the HadleyCell. If the rain over the study area is found to be contributedmainly by midlatitude cyclones, then the isotopic composi-tion of δ18O in the underground water in that region wouldserve as a proxy for past climate as it reflects the degree atwhich the midlatitude systems affect the Negev Desert.

[40] Acknowledgments. This study was supported by the IsraeliScience Foundation (ISF grant 108/10). Thanks also are given to Mrs.Cathy Kelly for reviewing the paper.

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Factors affecting isotopic composition of the rainwater in the Negev Desert, Israel

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