Isotopic composition of single rain events in the central Mediterranean

Marcello Liotta,1 Serafino Bellissimo,2 Rocco Favara,1 and Mariano Valenza2

Received 20 February 2008; revised 11 April 2008; accepted 20 May 2008; published 20 August 2008.

[1] The ratios of stable isotopes of single rain events were investigated during the periodOctober 2005 to September 2006 in the central Mediterranean. Clear seasonal trendswere identified in both oxygen isotope ratios and the deuterium-excess parameter, andthese were ascribed to the dominant circulation systems during both cold and hotintraannual periods. Rain events were classified on the basis of the origin of rain-bearingsystems. Air masses coming from the south usually give rise to rainwater with a lowdeuterium excess. Air masses coming from the north and the northeast are often dry andcold, and are associated with high evaporation from the Mediterranean Sea that occursunder isotopic nonequilibrium conditions. Kinetic fractionation enhances lighterisotopomers in the vapor phase, increasing the deuterium excess. During cold periodslarge vapor fluxes from the Mediterranean Sea, as estimated by the ECMWF (EuropeanCentre for Medium-Range Weather Forecasts), usually precede rain events with a highdeuterium excess. However, the isotope signatures of the Mediterranean moisturecontribution may be masked by the original isotope content of the circulating air massesand/or by secondary evaporation effects.

Citation: Liotta, M., S. Bellissimo, R. Favara, and M. Valenza (2008), Isotopic composition of single rain events in the central

Mediterranean, J. Geophys. Res., 113, D16304, doi:10.1029/2008JD009996.

1. Introduction

[2] The stable isotope content of meteoric water providesthe best natural tracer for studying the hydrological cycle.The isotopic composition of precipitation is controlled byseveral fractionation processes occurring during phasechanges in the hydrological cycle. The first process is theformation of water vapor at the sea surface. When under-saturated air travels over the sea, rapid evaporation produceswater vapor that is not in isotopic equilibrium with seawater.This process, described in detail for the Mediterranean byGat and Carmi [1970] and Gat et al. [2003], is responsiblefor the high values of the deuterium-excess parameter foundin meteoric waters of the Mediterranean. Moreover, Gat etal. [2003] suggested that high moisture values can resultfrom the evaporation of sea-spray droplets in the absence ofsignificant isotope fractionation. The water vapor travelsuntil a supersaturated state is reached and condensationoccurs. After nucleation, droplets usually grow in isotopicequilibrium with vapor. During the resulting rain event, theinteraction between the falling drops and the surroundingatmosphere may change the original isotope content [Penget al., 2007]. If the atmosphere between the cloud base andthe ground is significantly undersaturated, the evaporationof falling drops produces an isotopic enrichment of theliquid phase occurring under nonequilibrium conditions,

and consequently the deuterium excess decreases rapidly.On the other hand, when raindrops pass through lower levelclouds (e.g., orographic clouds, seeder-feeder mechanism),the final isotopic composition is due to the mixing of twowater sources: falling raindrops and lower cloud droplets[Liotta et al., 2006a]. The morphological, climatic, andgeographical features of the Mediterranean make it a uniquenatural laboratory for studying isotopes. Several publishedstudies have investigated the isotopic composition of pre-cipitation in the central Mediterranean, at Mount Etna[D’Alessandro et al., 2004], Stromboli [Liotta et al.,2006b], southeastern Sicily [Grassa et al., 2006], andnorthwest Sicily [Liotta et al., 2006a]. Although all of thesestudies were based on monthly sampling, it was stillpossible to recognize some relationship between isotopiccomposition and the origin of the air masses [Liotta et al.,2006a]. From a climatic point of view, the MediterraneanSea is located in a transitional zone where competitionbetween midlatitude and tropical variabilities is important[Lionello et al., 2006a]. Under these conditions, studiesbased on single rain events provide the best opportunity torelate atmospheric processes to the isotopic composition ofrainwater. Rindsberger et al. [1983] found a relation be-tween air trajectories and the isotopic composition ofindividual storms in the eastern Mediterranean. Single rainevents in southeastern Spain were sampled and analyzed byCruz-San Julian et al. [1992], who highlighted the presenceof Mediterranean precipitation during springtime. Rainwatersamples of this type were found to be high in both deuteriumand 18O, and characterized by high deuterium-excess values.Celle-Jeanton et al. [2001] investigated the isotopic com-position of precipitation in the western Mediterranean and,on the basis of daily surveys, found that the origins and

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1Sezione di Palermo, Istituto Nazionale di Geofisica e Vulcanologia,Palermo, Italy.

2Dipartimento di Chimica e Fisica della Terra, Universita degli Studi diPalermo, Palermo, Italy.

Copyright 2008 by the American Geophysical Union.0148-0227/08/2008JD009996$09.00

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trajectories of air masses determine the amount and oxygen-isotope ratio of the rain. They found Mediterranean precip-itation to be isotopically enriched and most abundantwhereas Atlantic precipitation depleted and less abundant.Argiriou and Lykoudis [2006] observed a marked seasonalityin deuterium-excess values of rainwater event-based samplescollected at Athens (eastern Mediterranean), being maximalduring winter and minimal during summer. A similarbehavior was observed in the northeastern Mediterraneanby Vreca et al. [2007]. The overall link between the isotopiccomposition of precipitation and the Mediterranean climateraises the question of how future climate change [Piervitaliet al., 1998] could affect the isotope ratios of precipitationand, as consequence, of groundwater. In the MediterraneanBasin, the promising role of the isotopic approach as anadditional parameter in monitoring changes in circulationpatterns has been highlighted by the International AtomicEnergy Agency [2005]. The aim of this study was toelucidate the relationships between the isotopic composi-tions of single rain events and the surrounding weatherconditions in order to provide a useful tool for understand-ing possible future climate change. Despite the significanceof air mass trajectories, direct observation of cloud forma-tion and movement (from satellite imaging) and quantitativeestimation of vapor fluxes from the Mediterranean Seamight be closely related to the isotopic composition ofsingle rain events. The present study analyzed data onclimatic and isotopic features on all rainy days during a1-year observation period (from October 2005 to September2006).

2. Climatic Setting

[3] The Mediterranean area is characterized by a homon-ymous climate. In summer (from May to August), subtrop-ical high-pressure cells drift toward the NorthernHemisphere. During the winter, the high-pressure cells driftback toward the equator and the weather is dominated bycyclonic storms [Bolle, 2003]. The Mediterranean Searepresents an important source of energy and moisture forcyclone development [Lionello et al., 2006b]. Severalauthors have provided detailed descriptions of climaticfeatures and cyclogenesis in the Mediterranean, and related

them to orographic effects [Buzzi and Tibaldi, 1978; Buzziet al., 2003] and to the large-scale circulation [Lionello andSanna, 2005; Chaboureau and Claud, 2006]. In addition,climate change is believed to be responsible for the in-creased intensity of cyclones over the Mediterranean Sea[Gaertner et al., 2007]. The hydrological cycle in theMediterranean has been described by Mariotti et al.[2002]. These authors have shown that evaporation fromthe Mediterranean Sea is greatest during winter and in itseastern part.

3. Sampling and Methods

[4] Rainwater samples were collected after every rainevent at Palermo (Figure 1) from October 2005 to September2006, with three samples being collected during one ofthese events in order to evaluate the temporal evolution ofthe isotopic composition (event E14a,b,c; 13–15 December,2005). The rainwater samples were analyzed for theiroxygen and hydrogen isotopic composition using isotoperatio mass spectrometers (Analytical Precision AP 2003 andFinnigan MAT Delta Plus, respectively). The isotope ratiosare expressed as the deviation per mil (d%) from thereference V-SMOW (Vienna Standard Mean Ocean Water).The uncertainties are ±0.1% for d18O and ±1% for dD (onestandard deviation). Data on atmospheric dynamics wereobtained from the BOLAM meteorological model [Buzzi etal., 2003] and EUMETSAT satellite images. Meteorologicaldata such as temperature and relative humidity wererecorded by balloon sounding at Birgi (16429 WMOstation) (http://www.weather.uwyo.edu) and surface obser-vations at Boccadifalco (16410 WMO station). In addition,evaporation-flux forecasts were obtained from the ECMWF(European Centre for Medium-Range Weather Forecasts).

4. Results

[5] The isotopic composition of samples related to 48 rainevents were analyzed. Values related to event E14a,b,c werecomputed as the weighted average of samples E14a, E14b,and E14c. The isotope ratios covered wide ranges, withd18O% values from –10.3 to 1.2 and dD% values from –67 to 11 (Figure 2). Descriptive statistic data are reported in

Figure 1. Base map. The sampling site was located in the central Mediterranean (circled cross). Boxesindicate areas used for evaporation computation (see section 4).

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Table 1. The linear regression equation for the relationbetween dD% and d18O% was

dD ¼ 6:49� 0:32ð Þ � d18O þ 5:28� 1:75ð Þr2 ¼ 0:90 p < 0:00001; t � testð Þ:

In order to facilitate rapid comparisons with data in theexisting literature, most of which are based on monthlysamples, we also considered the weighted monthly values,

dD ¼ 6:81� 0:48ð Þ � d18O þ 6:18� 2:97ð Þr2 ¼ 0:96 p < 0:00001; t � testð Þ:

These two equations do not differ significantly.

[6] The isotopic composition was inversely related to therainfall (Figure 3). The heaviest rain events were usuallydepleted in both oxygen and deuterium, and the deuteriumexcess appeared to be unrelated to the rainfall. Severalsamples showed high values of deuterium excess (definedas those higher than the mean value plus 1s as given inTable 1; i.e., higher than 19%) and a high precipitation, butalso some samples involving only a few millimeters ofprecipitation were characterized by a very high deuteriumexcess.[7] There were seasonal variations in the isotopic com-

position (Figure 4), which were previously observed in thecentral Mediterranean [Liotta et al., 2006a] and are partic-ularly evident when deuterium excess is taken into account.During the cold period (October to March), the deuteriumexcess was usually high or very high (up to 29). Incontrast, during the hot period most of the collectedsamples showed a low deuterium excess (defined as thoselower than the mean value minus 1s as given in Table 1;i.e., lower than 5%). On average, deuterium and oxygenwere both depleted during cold months and enriched duringthe hot months. Nevertheless, they showed high-frequencyvariations that cannot be ascribed to seasonal climatevariations.[8] In order to explain the observed variations, relation-

ships between the origin of rain events and their isotopic

Figure 2. Diagram of dD% versus d18O%. The blue dots are values for all single rain events. The solidred line is the regression line of all values. The solid and dashed black lines are the global meteoric waterline (GMWL) [Craig, 1961] and the eastern Mediterranean meteoric water line (EMMWL) [Gat andCarmi, 1970], respectively.

Table 1. Descriptive Statistic Data of Analyzed Samples and

Weighted Mean Values

dD d18O d Millimeters

Valid Number 48 48 48 48Minimum �67 �10.3 �4 0.1Maximum 11 1.2 29 100.1Mean �25 �4.7 12 12.3Standard Deviation 19 2.8 7 17.6WMVa �41 �7.0 15

aWeighted mean values.

%

%

%

%

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composition were investigated. We first classified rainevents on the basis of the cloud motion direction as inferredfrom infrared satellite images (IR 10.8 Meteosat 8 and 9).We defined five groups: A, clouds coming from the south-west (events E23, E24, E26, E34, E35, E44, and E45); B,clouds coming from west-northwest (events E2, E3, E4, E5,E9, E10, E11, E12, E15, E18, E28, E30, E33, E38, E41,E42, E43, E46, and E47); C, clouds coming from the north(events E6, E16, E17, and E40); D, clouds coming from thenortheast (events E7, E20, E21, E22, E25, E32, and E39);and E, cyclones, corresponding to inward spiraling windsthat collect air masses from multiple directions (events E1,E8, E13, E14a,b,c, E19, E27, E29, E31, E36, E37, andE48).

[9] Figure 5 plots samples in each group in diagrams ofdD% versus d18O%. Except for sample E23, rain eventsgenerated by air masses coming from the southwest werecharacterized by a low deuterium excess (d � 11; i.e., mostof them falling below the global meteoric water line(GMWL)) and covering a wide range (Figure 5a). Thisgroup included the most enriched samples of the entiremonitoring period (E35 and E44). Samples whose origincould be ascribed to perturbations coming from west-north-west represented about 40% of the total collected samples.They covered a wide range and mainly fell around theGMWL (Figure 5b). Rain events attributable to air massescoming directly from the north fell within a restricted range,with all of them being enriched (Figure 5c), and alsoshowed very different values of deuterium excess (–2 �d � 29). Rain events generated by air masses coming fromthe northeast were usually depleted and characterized by ahigh deuterium excess. Cyclones produced only 30% of thetotal rain events but included the periods with heaviestprecipitation (representing 49.4% of the total precipitation).Cyclones covered a wide range and comprised the mostdepleted samples (E13 and E14a,b,c).[10] Figure 5 illustrates the specific feature in each group,

and indicates that the deuterium excess is probably theparameter that is most influenced by the origin of a rainevent. This strongly depends on the evaporation conditionsat the air–sea interface. In the Mediterranean, high values ofdeuterium excess have been attributed to kinetic fraction-ation processes that occur when cold and dry air massestravel on the sea [Gat and Carmi, 1970]. Figure 6 comparesdeuterium excess with the evaporation rate from the Med-iterranean computed by the ECMWF in several regionsindicated by the boxes in Figure 1. According to thescenario described by Mariotti et al. [2002], the intensityof Mediterranean evaporation is greatest during wintermonths, especially in November and December, and islower from April to July. Several events characterized bya high deuterium excess (events E6, E7, E8, E22, E25, andE32) were preceded by significant evaporation (>6 mm/d),indicating that the Mediterranean Sea contributes a largeamount of moisture. In contrast, other events with a highdeuterium excess were not accompanied by significantmoisture contributions (events E10, E13, E17, and E23).In August and September the evaporation was often higherthan 6 mm/d, whereas the deuterium excess was always lowor intermediate (–4 < d < 14). Event E17 occurred at 0700h UTC on 22 December 2005 and was associated with avery small rainfall (just 1.4 mm). Satellite imagery showedair masses coming from the north, and revealed the forma-tion of scattered orographic clouds (MODIS channels 1, 3,and 4; http://www.sat.dundee.ac.uk). The event was char-acterized by a very high deuterium excess (29%). As shownin Figure 6, the event was not associated with a highlyevaporative period. Nevertheless, the vertical profile ofrelative humidity recorded at Birgi (not shown; providedas auxiliary material1: E17_RH%_profiles) indicate that airmasses traveling over the Mediterranean during the 30h before the rain event were often characterized by a lower

Figure 3. Rainfall versus oxygen (a) d%, (b) deuteriumd%, and (c) deuterium excess. Case numbers are specifiedin the histograms on the right axis.

1Auxiliary material data sets are available at ftp://ftp.agu.org/apend/jd/2008jd009996. Other auxiliary material files are in the HTML.

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layer (0–500 m a.s.l.) with a relative humidity lower than60%. This could have favored vapor formation underkinetic fractionation conditions.

5. Event E14a, b, c

[11] Event E14a, b, c is the only one in which sampleswere taken at multiple times during the rainy period (13,14, and 15 December 2005). The total rainfall was about100 mm. Samples were collected from 0000 to 1700 h UTCon 13 December (E14a), from 0000 to 1400 h UTC on 14December (E14b), and from 1500 to 1700 h UTC on 15December (E14c). The rain event was associated with a

cyclonic circulation system. The MODIS satellite imageshown in Figure 7 depicts a well-developed cyclonicsystem with a prominent eye-like feature centered betweenSicily and Tunisia; while EUMETSAT stationary satelliteimages (available as auxiliary material) show the evolutionof the system. The cyclone formed over southern Italy on13 December, moved between Sicily and Tunisia on 14December, and then moved southeastward. As shown inFigure 7, the samples exhibited homogeneous deuterium-excess values with very different isotope ratios (Table 2).In addition, the temporal evolution of the isotope ratios isopposite that of a normal Rayleigh-type fractionationprocess. In fact, the isotopic composition became more

Figure 4. Temporal variation of the isotopic composition: (a) oxygen, (b) deuterium, and (c) deuteriumexcess and temperature. Solid gray lines indicate the mean values of dD%, d18O%, and deuteriumexcess, while dashed gray lines delineate variations from the mean value of ±1s.

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positive as the rain event evolved. The samples were verywell fitted by the equation dD% = 7.50 � d18O% + 10.4(r = 0.999).

6. Discussion

[12] The linear regression line of dD% versus d18O%computed using weighted values was similar to thatcomputed by Longinelli and Selmo [2003] for southernItaly: dD% = 7.0 � d18O% + 7.3 (r2 = 0.95). The twoequations also cover the same compositional range, indi-cating that large-scale processes governing the isotopiccomposition of precipitation in southern Italy are homo-geneous. The isotope ratios for single rain events areinversely related to the rainfall. We found several samplesthat were characterized by only a few millimeters ofprecipitation and a high deuterium excess, which weassume were not affected by evaporative processes occur-ring during falling, as evaporation should rapidly reducethe deuterium excess. Seasonal climatic conditions strong-ly influence the isotopic composition. During cold months,when the Mediterranean climate is dominated by subpolarlows, rainwater is on average depleted and shows highvalues of deuterium excess. During the summer high-pressure belts shift northward, and rain samples are usuallyenriched and show low deuterium-excess values. Thedeuterium excess is often maintained at that in pristinemoisture, and provides useful information on vapor for-mation conditions; while isotope ratios show a largevariability that mainly depends on the fraction removedfrom the original vapor. The seasonal trends identified inthe isotopic composition appear to be related to tempera-ture variations. However, the isotope content is more likelyto be affected by many other factors that influence thecirculation systems. Explaining the isotopic composition inany stage of the rain-production process is very complex,but some general rules emerge when cloud movementdirections are compared with the isotopic composition.Cloud coming from North Africa and moving northeast-ward usually generates rain that is characterized by a lowdeuterium excess. Wet and warm air masses do not causekinetic fractionation at the air-sea interface, whereas dryand cold air masses coming from north-northeast stronglyinteract with the Mediterranean Sea to produce rain eventscharacterized by a high deuterium excess. Therefore,evaporation over the Mediterranean Sea provides a usefultool for evaluating regional vapor contributions to rain-bearing systems. The findings of Gat et al. [2003] high-light that intensive air–sea interactions near the coastresult in a large deuterium excess both when cold air from

Figure 5. Diagrams of dD% versus d18O%. Samples havebeen grouped on the basis of directions of cloud motionfrom the (a) southwest, (b) west-northwest, (c) north, and(d) northeast, and (e) as cyclones. Symbols in the lowerright corner indicate the direction quadrant. Double circlesindicate cyclones. The GMWL and EMMWL are alsoplotted for comparison.

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Figure 6. Comparison of deuterium excess with evaporation from three areas of the Mediterranean Sea(boxes in Figure 1). Symbols over each label indicate the direction quadrant. Double circles indicatecyclones.

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Figure 6. (continued)

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the European continent moves over the sea as well aswhen warm and dry air from North Africa moves over theMediterranean Sea.[13] Our data set suggests that evaporative periods are

not always associated with precipitation exhibiting a highdeuterium excess. As shown in Figure 6, only coldevaporative periods were followed by rain events with ahigh deuterium excess. This was particularly evident forair masses coming from the northeast (events E7, E22,E25, and E32) and confirms the findings of Gat andCarmi [1970]. In August and September we found severalevaporative periods followed by rain events with anintermediate to a very low deuterium excess. We cannotexclude that evaporation during rain falling decreased thedeuterium excess. However, if collected samples exhibitvalues of deuterium excess associated with the pristinecloud moisture, we can assume that sea evaporationoccurring during the summer does not exert significantkinetic effects on the isotope fractionation during evapo-ration. A possible explanation is that the specific humid-ity over the Mediterranean Sea shows seasonal trends,being higher in the summer. Under these conditions, theMediterranean contribution to the total moisture wouldbecome negligible in summer and very important duringwinter.

7. Conclusions

[14] The sampling of single rain events provides usefulinformation for tracing the contributions of water vapor to

precipitation events. Our results suggest that rain amount inthe central Mediterranean cannot be used to identify rainevents affected by evaporation, owing to many samplesbeing characterized by very small rainfalls and a highdeuterium excess. Seasonal trends are attributable to themain circulation system prevailing during cold and hotperiods. Air masses coming from the Atlantic Ocean oftenexhibit a deuterium excess (d 10) associated with pristinevapor, while northern air masses strongly interact with theMediterranean Sea to enhance the local vapor contribution.Such interactions are evident in rain samples with a highdeuterium excess. Cyclonic systems collect air masses fromseveral directions, and hence produce rain events with anintermediate deuterium excess. Moreover, they contributeabout 50% of the total annual rainfall. In the presence ofclimate change, knowledge of the relationships between theisotopic composition of rainfall and the origin of rain eventsis crucial to interpreting possible isotope changes in mete-oric water.

Figure 7. (a) Diagram of dD% versus d18O% for samples collected during event E14a, b, c. (b) Rain wasproduced by a cyclonic system in the central Mediterranean (image courtesy of MODIS Rapid ResponseProject at NASA/GSFC, acquired on 14 December 2005, 1220 UTC, and is available at http://rapidfire.sci.gsfc.nasa.gov/realtime/single.php?2005348/crefl2_143.A2005348122001-2005348122500.2km.jpg).

Table 2. Isotopic Composition and Rain Amount Relative to E14

Event

Sample dD d18O d Millimeters

E14a �69 �10.6 16 87.1E14b �52 �8.3 14 12.7E14c �35 �6.1 14 0.3

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[15] Acknowledgments. Evaporation forecasts were obtained fromthe ECMWF (European Centre for Medium-Range Weather Forecasts)MARS archive under license from the Italian Air Force MeteorologicalService. Thanks go to EUMETSAT dissemination service and MODISRapid Response Project at NASA/GSFC, which provided satellite images.

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�����������������������S. Bellissimo and M. Valenza, Dipartimento di Chimica e Fisica della

Terra, Universita degli Studi di Palermo, I-90123 Palermo, Italy.R. Favara and M. Liotta, Istituto Nazionale di Geofisica e Vulcanologia,

Sezione di Palermo, Via U. La Malfa, 153, I-90146 Palermo, Italy.(m.liotta@pa.ingv.it)

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