climatic feedbacks and desertiﬁcation: the mediterranean model

8/3/2019 Climatic feedbacks and desertification: the Mediterranean Model

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Climatic Feedbacks and Desertification: The Mediterranean Model

M. M. MILLN,a M. J. ESTRELA,a M. J. SANZ,a E. MANTILLA,a M. MARTN,a F. PASTOR,a R. SALVADOR,a

R. VALLEJO,a L. ALONSO,b G. GANGOITI,b J. L. ILARDIA,b M. NAVAZO,b A. ALBIZURI,c B. ARTANO,d

P. CICCIOLI,e G. KALLOS,f R. A. CARVALHO,g D. ANDRS,h A. HOFF,i J. WERHAHN,j G. SEUFERT,k

AND B. VERSINOk

aFundacin CEAM, Valencia, SpainbUniversidad del Pas Vasco, Bilbao, Spain

cEnvironment & Systems, S.A., Bilbao, SpaindCiemat, Madrid, Spain

e Istituto di Metodologie Chimiche, Area della Ricerca del CNR di Montelibretti, Monterotondo Scalo, Italyf Atmospheric Modelling and Weather Forecasting Group, University of Athens, Athens, Greece

gInstituto de Meteorologia, Lisbon, Portugalh Institut fr Umweltphysik, Universitt Bremen, Bremen, Germany

i Aerodata AG, Braunschweig, Germanyj

Institut fr Meteorologie und Klimaforschung, Bereich Atmosphrische Umweltforschung, Garmisch-Partenkirchen, Germanyk Joint Research Centre, Ispra, Italy

(Manuscript received 29 September 2003, in final form 18 June 2004)

ABSTRACT

Mesometeorological information obtained in several research projects in southern Europe has been usedto analyze perceived changes in the western Mediterranean summer storm regime. A procedure wasdeveloped to disaggregate daily precipitation data into three main components: frontal precipitation, sum-mer storms, and Mediterranean cyclogenesis. Working hypotheses were derived on the likely processesinvolved. The results indicate that the precipitation regime in this Mediterranean region is very sensitive tovariations in surface airmass temperature and moisture. Land-use perturbations that accumulated overhistorical time and greatly accelerated in the last 30 yr may have induced changes from an open, monsoon-type regime with frequent summer storms over the mountains inland to one dominated by closed verticalrecirculations where feedback mechanisms favor the loss of storms over the coastal mountains and addi-

tional heating of the sea surface temperature during summer. This, in turn, favors Mediterranean cyclo-genesis and torrential rains in autumnwinter. Because these intense rains and floods can occur anywherein the basin, perturbations to the hydrological cycle in any part of the basin can propagate to the whole basinand adjacent regions. Furthermore, present levels of air pollutants can produce greenhouse heating, am-plifying the perturbations and pushing the system over critical threshold levels. The questions raised arerelevant for the new European Union (EU) water policies in southern Europe and for other regionsdominated by monsoon-type weather systems.

1. Background and introduction

Around the Mediterranean Sea, deserts and desert-like conditions can be found in close proximity to a verywarm sea and thus to a marine air mass with a high

moisture content, for example, the coasts of easternTunisia, Libya, Egypt, and the province of Almeria inSpain. These regions were covered with vegetation inhistorical times, for example, during the Roman Em-pire (Blle 2003a). In Almeria, the dense oak forests

covering its mountains were cut to fuel lead mines just150 yr ago (Charco 2002), and the area became thedesert-like site for the spaghetti western films. Thequestion is, Did these areas become desertified (or wasthe desertification accelerated) by removing the forests

and desiccating the coastal marshes?According to Claussen (2001), an issue of climate

feedbacks is that predictions of global atmosphericmodels are highly sensitive to prescribed large-scalechanges in vegetation cover and that although avail-able studies illustrate the potential effects of massivevegetation changes on the climate system, they canhardly be validated. We feel, however, that the char-acteristics of the atmospheric circulations in the west-ern Mediterranean basin, resulting from its latitude,

Corresponding author address: Dr. M. M. Milln, FundacinCEAM, Parque Tecnolgico, C/Charles R. Darwin, 14, 46980 Pa-terna (Valencia), Spain.E-mail: [email protected]

684 J O U R N A L O F C L I M A T E VOLUME 18

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orography, and landsea distribution, make this regionan ideal test ground for checking whether or not veg-etation is a passive component of climate and probablyfor other questions related to feedbacks in climatemodeling.

In Spain, the increase in mountain agriculture began

in the late 1500s, and extended grazing began in theearly 1700s. These transformations included cuttingdown forests, clearing large pasture areas, and terracingmountain slopes, all of which increased erosion. Thegrowth of the Ebro River delta began, in fact, in thesixteenth and seventeenth centuries (Puigdefbregasand Mendizabal 1998). The accumulated changes hadalready led to marginal (i.e., subsistence) agriculture bythe time of the Spanish Civil War (193639), which wasfollowed by a massive population exodus from the ruralareas to the larger Spanish cities (Madrid, Barcelona,Valencia, and Bilbao) lasting from the mid-1940s to thelate 1960s. Lands were abandoned, limited grazing re-mained, and, by the time the trend slowed down in the

late 1970s, only the very elderly were left in many vil-lages that have become, de facto, ghost towns. Thesemigrations have contributed to desertification in Spain(Puigdefbregas and Mendizabal 2003) and, throughsimilar mechanisms, in other Mediterranean countries(Bchler 1995).

For the past 2000 yr, and especially during the nine-teenth and twentieth centuries, coastal marshes aroundthe Mediterranean Sea have been drained and desic-cated, initially for health reasons, that is, to avoid ma-laria, and later for agricultural use (Bolle 2003a; b). Asthe coastal areas have also been at the receiving end ofthe migrations mentioned (three of the cities above arecoastal), they have become increasingly urbanized and

industrialized since the late 1950s. The rate of changeincreased greatly after the 1974 Middle East war andwas followed by extended demands for housing (resi-dential, vocational, and tourist). As a result, much ofthe coastal agricultural land used in the 1950s has beenurbanized, and few (or none) of the virgin coastlandsremain. Finally, along with the increased anthropogenicpressures, other disturbances in this region, for ex-ample, air pollution and forest fires, have also increasedsignificantly in the last three decades (Milln et al. 1998;Moreno and Oechel 1994; Moreno 1998).

The study of atmospheric processes in the Mediter-ranean basin has been part of the European Commis-sions Research Programs, and authors of this paper

have participated (since 1974) in projects related to at-mospheric chemistry (air pollution meteorology), de-sertification and carbon balances (see appendix). Infact, it was while collecting field information for plan-ning measurement campaigns (in 198687) that mem-bers of the team were first told that storms in the moun-tains surrounding the Mediterranean Sea did not reachthe mature-storm stage as frequently as in the past.And, according to local senior residents, the creekswhere they had often fished in their childhood were

now dry most of the year. Similar information has beenconveyed to us since then, regarding the loss of summerstorms in the Italian provinces of Basilicata and Cala-bria, as well as in Sardinia, Sicily, and other Mediter-ranean islands.

The purpose of this paper is to combine experimental

and modeling results from the aforementioned projectswith precipitation data to derive working hypothesesabout the possible processes and feedbacks involved inthe changes observed in the summer storm regime. Sec-tion 2 describes the precipitation types and presents thecriteria used to disaggregate the main components fromthe daily rainfall data series in the study region. Section3 reviews the most relevant findings regarding the seabreezes and other mesoscale winds that lead to the ver-tical recirculations and long residence times of airmasses within the western Mediterranean basin. Sec-tion 4 uses experimental data and modeling results toestimate the temperature and the moisture gained bythe sea breeze on its way inland, as well as the addi-tional temperature increases resulting from the radia-tive effects of air pollution. The individual and com-bined effects of these factors on the cloud condensationlevel (CCL) are also evaluated in section 4, while sec-tion 5 addresses the possible feedbacks. Finally, section6 presents the conclusions.

2. Precipitation types and data analysis

a. Precipitation types

The study area considered (Fig. 1) includes the Va-lencia region and the neighboring inland areas reached

by the combined sea breeze and upslope winds, that is,the combined breeze (section 3). Extended use of dailyrecording precipitation gauges in rural Spain began inthe 1950s, and the number of stations in the study areahas increased from 60 in 1950 to 509 at present. Ofthese, 60 have climatologically homogeneous records(i.e., continuous data for 30 yr) since 1950, and 250stations now cover the last 30 yr (to 2001). For thiswork, 116 stations were selected with a 97% data con-fidence level. However, even though the dataset is cli-matologically homogeneous, the spatial distribution ofthe stations reflects the site criteria of more than 30 yrago.

The average precipitation series (yearly precipitation

total collected by selected stations divided by numberof selected stations each year) is shown in Fig. 1. Thelinear regression shows a decreasing trend and, in spiteof the increase in the number of stations over the years,the average series becomes more spiky toward the end.The 5-yr running average for the same data series (Fig.2a) smoothes out the individual peaks but shows adeepening of the large cycles with the maxima increas-ing and the minima decreasing during the period con-sidered.

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FIG. 1. Location of the study area within Europe, the Mediterranean basin, and area-averaged precipitations for the Valencia regionfor the period 19502001. Stars mark sites where meteorological measurements, including simultaneous vertical sounding, wereperformed during the measurement campaigns.



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This picture changes when the data are separatedaccording to the distance from the station to the coast(Fig. 2b). For the stations in the coastal plains located at40 km from the coast, the linear regression shows an

increase in precipitation and the 5-yr running averageaccentuates the increase in the maxima (see below).For the stations in the mountains inland at 40 kmfrom the coast, the regression shows a marked decreas-

FIG. 2. The 5-yr running averages of the area-averaged precipitations for the Valencia region for the period19502001 and for stations located at 40 and at 40 km from the coast.

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ing trend, and it is the minima that become smaller inthe 5-yr running average.

A meteorological analysis of the precipitation eventsshows that the rain regimes in this region are clearlydistinct spatially and temporally (Estrela et al. 2000;Pearrocha et al. 2002). The last orographic barrier en-

countered by Atlantic fronts before moving into theMediterranean is the coastal mountain chain. Precipi-tation from these fronts occurs mainly on the windward(west) side of the mountains (Fig. 3a) and is followed bya Fhn-type effect over the Spanish east coast (Millnet al. 1998). Summer storms, on the other hand, areassociated with the final stages in the development ofthe combined breeze, and the precipitation occursmainly in the mid- to late afternoon on the east side ofthe same mountain ranges (Fig. 3b).

Intense precipitation in the western Mediterraneanoccurs mainly in autumnwinter when cold(er) mP orcP air masses are advected over a warmer sea, and themoisture recharge is governed by the differences in

temperature between the sea surface and the air mass.This mechanism is highly sensitive to changes in theMediterranean sea surface temperature (SST; Milln etal. 1995; Pastor et al. 2001), but the areas affected bythe precipitation depend on the direction of the ad-vected air mass (Meteorological Office 1962; Barry andChorley 1987).

Advections from northwest to north along the Car-cassone Gap and/or the Rhne valley reinforce cyclo-genesis to the lee of the Alps (Genoa depressions),which affects mainly Italy and parts of northern Africa(Tunisia). The western coasts of Mediterranean NorthAfrica, the Spanish east coast, and southern France areaffected by northeasterly to southeasterly advection,

which we consider as examples of backdoor (cold)fronts (Huschke 1959; Bluestein 1993). Figure 3c showsthe resulting spatially averaged pattern.

b. Precipitation data analysis

Table 1 includes the main characteristics and the me-teorological criteria used by our group to disaggregatethe daily precipitation data into the three main compo-nents. However, because the generalized availability ofsynoptic weather maps for Spain dates from 1959, thedetailed treatment had to be limited to the period 19592001. For each day in this period, the criteria were ap-plied to the joint analysis of the daily precipitation data;

the surface pressure maps at 0000, 0600, 1200, and 1800UTC; and the 500- and 300-hPa maps at 1200 UTC.Figure 4 shows the results for the subset of stations onthe coastal strip (40 km from the coast) and for thestations in the inland mountains (40 km from thecoast). And thus, bearing in mind the caveat about thelength of the records, the disaggregated data indicatethat in the last 41 yr, the following conditions are true:

The average annual precipitation over the coastalstrip has not changed significantly.

On the other hand, the average annual precipitationover the stations inland shows a decreasing trend.

At present, Atlantic fronts contribute approximately20% of total precipitations. Their contribution showsa decreasing trend in both subareas, though morepronounced inland.

Summer storms contribute approximately 11% of thetotal precipitation, and their contribution shows a de-creasing trend over the whole area (coastal strip andinland).

Backdoor fronts contribute approximately 65% of to-tal. The average for this component has remainedessentially unchanged over the mountain areas butshows an increasing trend over the coastal strip. Themajor torrential events in this region, with largefloods in 1959, 1972, 1980, and 1989 appear in thisseries.

Finally, since summer storms and intense precipita-tion originate mainly from water evaporated from theMediterranean Sea, their sum, that is, 75%80%,

could be considered an estimate of the precipitationgenerated by (self-) evaporation within the region.

The perceived change in the storms inland, therefore,seems to be the result of a decreased contribution fromAtlantic (cold) fronts and summer storms, while on thecoastal strip the precipitation losses from summerstorms and Atlantic fronts appear to be balanced by anincrease in the contribution from backdoor (cold)fronts. This situation has been further analyzed, and theresults in Fig. 5 confirm a trend suggested by Figs. 1, 2,and 4b, that is, that precipitation in the region has alsochanged in character, becoming more torrential, that is,more precipitation in a smaller number of larger events

occurring closer to the coast. Figure 5 also shows anincrease in large events during the first part of the year.The decreases in the contributions by frontal systems

probably derive from changes in the conditions over theAtlantic and are not analyzed here. Finally, calculationsusing climatological data and experimental results fromthe European Commission (EC) projects (Table 2)show that the air mass in the combined breeze trans-ports enough water vapor to feed the equivalent of sev-eral summer storms each day. The questions then are asfollows: why do summer storms appear to be diminish-ing, and why are torrential rains increasing?

3. Mesoscale atmospheric circulations: Theexperimental and modeling evidence

Atmospheric processes in the Mediterranean regionvary with the season and specific basin (MeteorologicalOffice 1962; Barry and Chorley 1987). The easternMediterranean (sub)basin is limited to the south by amountainless desert. In summer, it falls under the in-fluence of the Asian monsoon system, and advectiondominates (Kallos et al. 1998). At the same time, thewestern Mediterranean (sub)basin, totally surrounded



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FIG. 3. Averaged spatial distribution of precipitation types for the period 1959 2001. Contributions from (a)Atlantic fronts, (b) summer storms, and (c) backdoor (cold) fronts (Mediterranean cyclogenesis). Crosses mark thestations used.

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by high mountains, falls under the influence of thesemipermanent Azores anticyclone. Clear skies prevailunder a generalized level of subsidence aloft, and meso-meteorological processes with marked diurnal cyclesdominate (Milln et al. 1997).

a. Processes at the meso-and meso- scales

In the western basin, the EC research projects docu-mented that the upslope winds reinforce the seabreezes along the coasts and create combinedbreezes, known to be stronger than the sum of theircomponents (Mahrer and Pielke 1977; Miao et al.

2003). In these circulations the sea breeze progressesinland during the morning and early afternoon in astepwise fashion by incorporating weaker upslope cir-culatory cells (Milln et al. 1992; Milln et al. 1998;Salvador et al. 1997). By late afternoon, the sea-breezefront tends to become locked along the ridges of themountains surrounding the basin (Milln et al. 2000;Milln et al. 2002).

Deep vertical injections take place at the front of thecombined breeze, and the mountain slopes can be con-sidered to act as convectiveorographic chimneys thatlink the surface flows directly with their return flowsaloft. By the time the circulations are fully developed,several return layers can be observed (Fig. 6), and ver-

tical injections higher than 34 km over the ridges ofthe coastal mountains 60 to 100 km inland have beendocumented (Milln et al. 1992) and modeled (Salva-dor et al. 1997; Salvador et al. 1999; Milln et al. 2002).The return flows produce a system of stacked layersover the sea (Fig. 7), which leads to vertical recircula-tions of pollutants emitted along the coasts. Experi-ments on the Spanish east coast during the MECAPIPproject (see appendix for definitions of EC projectshereafter) showed that a tracer emitted into the sea

breeze (i.e., inland bound) reentered inland comingfrom the sea 2 days later (Milln et al. 1992).

Figure 7 also shows the temperature profile mea-sured along leg 1 of the flight (highlighted). The layerfrom2000 to 3000 m follows, essentially, a dry adiabatwith a potential temperature of315 K, and the profilecan be compared with two Regional Atmospheric Mod-eling System (RAMS) simulations (). In Fig. 7b, themonthly averaged western Mediterranean basin SSTdistribution was used, and in Fig. 7c, the SST was ob-tained from National Oceanic and Atmospheric Ad-ministration (NOAA) satellite measurements on theday previous to the measurements. The simulated pro-files compare well with observations but do not repro-duce the finer temperature structure associated withthe (air pollution) layers.

b. Processes at the mesoscale

Another finding during the RECAPMA project wasthat the combined breezes, their return flows aloft, andtheir compensatory subsidence over the sea becomeself-organized by late morning at a scale that encom-passes the whole western basin. The main features ofthis flow are illustrated in Fig. 8, that is, the westernbasin becomes like a large cauldron that boils from theedges toward the center. The final result is that a re-

gionally linked vertical recirculation system is gener-ated (Milln et al. 1997). Similar effects have beendocumented in other areas of the Mediterranean (Kal-los 1997; Kallos et al. 1998).

When the coastal recirculations (12-day returntimes; see above) grow and become part of the largerone (regional scale) the results are increased stratifica-tion and longer recirculatory times. The motion of mul-tiple particles has been simulated using the Hybrid Par-ticle and Concentration Transport Model (HYPACT;

TABLE 1. Precipitation types in the Valencian region.

Component Characteristics Criteria

Precipitation fromclassic (Atlantic)fronts

Their most frequent occurrence is from earlyautumn to late spring.

Collected rain amounts were assigned to this categorywhenever a clearly noticeable frontal passage hadoccurred during the precipitation event.

Summer storms In this category we consider the storms driven

by the combined sea breeze and upslope winds.

Precipitation was attributed to this category whenever

the Iberian Thermal Low was observed at 1200and/or at 1800 UTC on the day of the event.They tend to develop during the afternoon over

the top of the coastal mountain ranges, some6080 km inland, and are more frequent fromlate Apr to Sep.

During the late evening and night, the stormsmigrate easterly (toward the coast) beforethey dissipate.

Backdoor coldfronts

Precipitations associated with Mediterraneancyclogenesis. They can be very intense andoccur mainly over the sea and coastal areasand near mountain slopes in autumn andwinter.

To be included in this category, the precipitation eventmust coincide with easterly advection over the westernMediterranean, that is, a well-established anticycloneover central Europe, accompanied but not necessarilylimited by the development of a low-pressure area overnorthern Africa and/or a pool of cold air aloft over theIberian Peninsula.



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FIG. 4. Time series of the area-averaged precipitation for the subsets of stations located at 40 and 40 km from the coast, forthe period 19592001: (a) precipitation total, (b) Atlantic fronts, (c) summer storms, and (d) backdoor cold fronts.

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Tremback et al. 1993) coupled to the meteorological

fields provided by either RAMS or the fifth-generationPennsylvania State University (PSU)National Centerfor Atmospheric Research (NCAR) Mesoscale Model(MM5; Gangoiti et al. 2001). The results indicate thatthe time period for renovating 80% of the air mass

below approximately 3500 m in the western Mediterra-

nean basin in summer is of the order of 7 to 10 days,depending on the point of release. In this process, aparticle can enter inland with the sea breeze, be liftedinto the return flows along the orographic chimneys inthe breeze front, sink over the sea, reenter inland with

TABLE 2. Precipitation estimates based on experimental and climatological data.

Applicable results from European researchprojects (western Mediterranean basin) Average dewpoint (Dp) temperatures

at coastline: 18.2C (Jul), 20.2C (Aug). Average sea surface temperature: 26C. Average distance from coast to top of

coastal mountain ranges: 80 km.

Average distance traveled by combinedbreeze: 160 km day1. Final depth of surface boundary layer:250 m.

Observed range of dewpointtemperatures at 78 km inland:2228C.

Typical storm 5 108 liters rainwater, that is,

20 L m2 over a footprint of 2.5km wide by 10 km long.

Air volume displaced by combined

breeze

Slab of air 10 km wide along thecoastline ( to length of stormtrack considered), times

Average depth for the combinedbreeze 250 m, gives 2.5 106 m2, times

Average traveled distance160 km, yields

Displaced air volume:4 1011 m3 (4.78 1011 kg)

Available water vapor at beginningof breeze run

Assume an average value of 19C Dp,that is, a mixing ratio 13.6 g kg1

(11.4 cm3 m3) of water vapor atthe coast

Further assumptions Only half of the displaced air mass(i.e., 80 km long) participates in theprecipitation,

Only 5 g kg1 of the water vaporinitially available at the coast iseffectively rained out,

Potential precipitation 1012 cm3 of water ( twice the

5 1011cm3 required for a typicalstorm).

FIG. 5. Decadal variation in the number of intense rain events during the first and second parts of the year for the period19512000, for amounts (a) 80, (b) 125, (c) 150 mm day1.



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the sea breeze several days

FIG. 6. Development of vertical recirculations on the Spanish east coast, as documented by the ozone and relative humidity profilesobtained by an instrumented aircraft at 14491535 UTC on 20 Jul 1989. The actual flight path (seesaw pattern) is shown with a dottedline, and the ground track relative to the Iberian Peninsula appears at left.

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later, and return aloft toward the sea and do this severaltimes before escaping out of the basin. Just for com-parison purposes, during summer the air masses below

3500 m over the United Kingdom, under mainly ad-vective conditions, can be renovated once or twice aday.

c. The effects of compensatory subsidence onsea-breeze development

Finally, as the local-to-regional circulations grow andbecome self-organized at the regional level during theday, their compensatory subsidence intensifies over thesea (Figs. 8 and 9) and over the coastal areas. This

generates a lower inversion along the coasts, which hasbeen observed to sink during the afternoon and confinethe inland-bound leg of the surface flows to a depth of

150 to 300 m (Milln et al. 1992; Milln et al. 1997;Milln et al. 2002). Thus, by the time the regional flowsare fully developed, this lower inversion caps the com-bined breeze and extends most of the way from thecoast to the mountains surrounding the basin. A verysignificant finding, therefore, is that per unit distancealong the coasts, the combined sea breezes and upslopewinds end up displacing a volume of air that is very long(160 km) but rather shallow (on the average 250 mdeep in eastern Spain).

FIG. 7. (a) Vertical ozone distribution along 450 km of the Spanish east coast at 05270621 UTC on 20 Jul 1989, showing the layeredstructure resulting from the recirculations on the previous day(s). The aircraft flew in a seesaw pattern over the sea (track A BC atleft) at 4060 km from the coast. (b), (c) Measured, and simulated (), temperature profiles along leg 1 of the flight (highlighted); seetext.



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FIG. 8. RAMS simulation for the Mediterranean at 1600 UTC on 19 Jul 1991. The western basin shows ananticyclonic-like field of surface winds (14.8-m height) flowing toward convergence lines on the coastal mountainranges (red lines). (b) Vertical speeds over a parallel (39.5N) passing near the base of the spiral flight (red triangle)for Fig. 9; ascent is represented by solid line and sinking is represented by dotted line. The simulated subsidencenear the Balearic Islands (at 4 cm s1) greatly underestimates the measured values (see Fig. 9).

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The grid sizes in the mesometeorological modelsused (RAMS and MM5) have been optimized to incor-porate the effects of the complex terrain surroundingthe Mediterranean basin (Salvador et al. 1999). Addi-tional methods have also been developed to incorpo-rate the real Mediterranean sea surface temperaturedistribution derived from NOAA satellite measure-ments (Bdenas et al. 1997). With these, we have beenable to simulate the main features of the observed windfields, including the deep vertical exchanges along themountains, and also the precipitation fields producedby torrential rains (Pastor et al. 2001). However, evenusing the finest vertical resolution at the limit of nu-merical instability, we still have not been able to repro-duce the amount of compensatory subsidence observed(cf. Figs. 7, and 8, and 9), nor the fine structure of theresulting vertical stratification (Milln et al. 2000;Milln et al. 2002) nor the inversion capping the seabreezes. Thus, we feel that the real residence recircu-lation times could be even longer than those mentionedabove.

d. Implications for climatological studies

The key characteristics of the summer atmosphericcirculations in the western Mediterranean basin are 1)the narrow and deep vertical exchanges along themountain ranges surrounding the basin, which producestratified layers over the sea; 2) the self-organization ofthe coastal circulations at the regional scale; and 3) theresulting compensatory subsidence of these layers overthe sea, during the day, as part of a basin-scale vertical

recirculation. And, for the purpose of this work, themost important aspects are 4) the vertically confinednature of the surface flows in the coastal recirculations,5) the long residence times of the air masses involved,and 6) the resulting accumulation of pollutants (ozone),as shown in Figs. 7 and 9.

A relevant aspect relating to climate is that airmasses involved in the vertical recirculations can ac-quire a memory of their previous interactions with thesurface (land and sea), for example, heating, moisture-content gains. This suggests that feedback processeswithin the surfaceatmospherehydrological systemcould manifest themselves much earlier in the westernMediterranean than in regions where air masses are intransit most of the time.

Given the limitations of current mesoscale models, ahelpful situation for analyzing feedbacks derives fromthe vertically confined nature of the surface flows andthe almost laterally homogeneous layered structureover the sea resulting from the vertical recirculations

(Fig. 7). The air mass in the combined breeze can thusbe assumed to maintain its integrity along its path overthe surface; that is, lateral exchanges are compensatedby air with similar characteristics, while the surface airmass remains essentially free from vertical exchangesuntil it reaches its orographic chimney inland. Thismeans that the simplest approach to evaluate possiblefeedbacks involving exchanges of the surface flows is anatmospheric thermodynamic diagram.

4. Surface exchanges and air pollution effects

a. Surface heating

The temperature gained by the surface air mass aftertraveling over the sun-heated coastal plains and slopescan be obtained from the temperature data in the upperpart of the observed return flows. To exclude additionalgains due to latent heat exchanges from condensation,the profile in Fig. 7 was used because no cloud forma-tion had been observed over the mountains on the pre-vious days. The potential temperature of the upperlayer, that is, 2000 to 3000 m, is 315 K and indicatesthat by the time the air mass that leaves the coast at26C (299 K) becomes injected into the return flows,it has gained 16 K.

Gains of this order are consistent with most of theexperimental temperature profiles obtained during the

measurement campaigns in this region in summer, andthe 312315-K potential temperature range appears tobe clearly associated with the upper part of the returnflows in the coastal recirculations (see also Fig. 9). Ifthere are no other processes involved, Fig. 10 indicatesthat the cloud condensation level (CCL) resulting fromthe interception of the 315-K potential temperature lineand the mixing ratio line corresponding to a dewpointof 19C at the coast (the average value for July andAugust) would occur above 2740 m, which is substan-

FIG. 9. Experimental profiles of ozone, temperature, and verti-cal speed obtained over the Balearic basin (marked point in Fig.8) from 1557 to 1639 on 19 Jul 1991. The data show an average O3concentration in the order of 70 ppbv (140 g m3) through the

sounding. The measured sinking speeds near the top (3500 masl) are in the order of 15 to 25 cm s1. The structure of thetemperature profile, with long and essentially adiabatic sections, isindicative of sustained sinking of air masses that have acquiredequal potential temperature.



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tially higher than the coastal mountains in this region(1800 to 2060 m).

b. Evapotranspiration and land-use changes

To calculate the water vapor contributed by vegeta-tion to the combined breeze we assume 1) that thecombined breeze displaces a volume of air 160 km longby 250 m high, per unit length along the coast; 2) thatevapotranspiration takes place only along the first 80km of its path, as a typical distance from the coast to themountain tops; and 3) that evapotranspiration takesplace during the breeze period. Finally, estimates of theactual evapotranspiration for this region in summervary from 5 to 7 () Lm2 day1 over the irrigated

coastal plains, to 13 Lm2 day1 over the drier areas(pines and maquia) in the mountains inland, and wehave further assumed a value of 6 lm2 day1 along thefirst 20 km and 2 Lm2 day1 along the remaining60 km.

We find that vegetation contributes 5 g kg

1 ofadditional water vapor to the volume of air displaced bythe combined breeze. Thus, by the time the air parcelreaches the mountain ridges, its water vapor mixingratio has increased to 19 g kg1 (a dewpoint of24C), from the initial 13.8 g kg1 (19C dewpoint) atthe coast. This value compares favorably with the 2228C range of dewpoint temperatures measured duringthe MECAPIP (Milln et al. 1992) and follow-up pro-

jects at the Valbona-Mora site located 78 km inland.

FIG. 10. Emagram showing possible cloud condensation levels for the conditions and processes observed in theregion.

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Figure 10 indicates that the CCL for the estimatedvalue of the mixing ratio and the 315-K potential tem-perature drops by some 600 m, bringing it down toapproximately 2150 m, that is, closer to the mountainpeaks. The CCL drops to 1700 m if the highest observedvalue of the dewpoint temperatures (28C) is consid-

ered, and even further, to 1400 m, if the 312-K po-tential temperature is assumed as a lower estimate forheating the surface air mass.

The range of possible heights for the CCL, that is,from 1400 to 2750 m, indicates that both heating andevaporation along the surface play major roles in theformation of summer storms in these circulatory sys-tems; the range also shows how sensitive the system isto variations in these two components. The moistureadded by evaporation over the coastal wetlands and bythe vegetation along the breeze path lowers the CCLand can be considered to act as a priming/triggermechanism that favors the onset of condensation/precipitation, tending to flush away part of the water

vapor already available in the marine air mass. Thus,increasing this moisture contribution would propitiatethe development of a feedback mechanism towardmore storms and more vegetation.

On the other hand, any perturbation in the surfacecharacteristics that further heats the surface air mass,for example, increasing the surface dryness, would liftthe CCL and lower the probability of storms develop-ing. The same result would occur by diminishing thesources of evaporation and evapotranspiration, for ex-ample, drainage of coastal marshes and/or changesfrom flooding to drip irrigation procedures, increaseddeforestation, and/or forest fires, or combinations ofthese.

c. Air pollution effects

Under strong summer insolation, the coastal recircu-lations become large natural photochemical reactorswhere coastal emissions of NOx and other precursorsare transformed into oxidants, acidic compounds, aero-sols, and ozone. Observations on rural mountain sites inthe region show average daily O3 values within the 125160 g m3 range almost every day from May to Au-gust (Milln et al. 2000; Milln et al. 2002), and aircraftmeasurements over the Mediterranean Sea (Figs. 7 and9) also yield average values of 70 ppb (140 g m3)within the lower 3500 m.

Satellite data (Fishman et al. 1990) yield vertically

integrated O3 values over the western Mediterraneanof the order of 5055 Dobson units during JuneAugust, that is, an average of 125150 g m3 within an8000-m-high column. Finally, modeling of the ozonesurface concentrations (Bastrup-Birk et al. 1997;Lelieveld and Dentener 2000) yields comparable aver-ages (120140 g m3), thus placing observations andmodeling results within the same overall values.

The observed tropospheric ozone, however, is alwaysassociated with haze, and a number of research projects

have been proposed to the EC (e.g., PHAMA) to de-termine their origin, composition, vertical structure, ra-diative effects, and fate. Some of the relevant questionsregarding this matter are whether aerosols reflect solarradiation and cause cooling (Lelieveld et al. 2002) orwhether, as in the case of contrails (Travis et al. 2002),

the combined radiative effect of aerosols mixed withpollutants (and/or water vapor) showing greenhouseproperties is to produce additional heating.

Actual indications of the net radiative effects in thewestern basin can be obtained by comparing the tem-perature profiles derived from high-resolution meso-scale modeling with the measured temperature profilesassociated with the layers of ozone and aerosols, asshown in Fig. 7. The two simulations run with RAMSwere intended to explore a range of possibilities sincethe net radiative effect could, in fact, be sea surfaceheating (see below).

In both cases, the model matches the upper part ofthe observed temperature profile but shows lower val-

ues in the lowest 2200 m. In the first case, the differ-ences increase toward the bottom and reach an averagevalue of approximately 23 K throughout the lower1500 m. In the second, the lowest boundary valuesmatch, and the differences between simulation andmeasurements are most marked at the peaks associatedwith the pollution layers. These differences increasefrom 0.7 K at 1750 m to more than 3 K between 500and 750 m.

Given the observed temperature range in the returnflows, the 312-K line can then be considered the resultof surface heating alone, and the 315-K line can beconsidered the present limit of accumulated surfaceheating plus (possible) radiative effects from air pollu-

tion. Thus, if the net radiative effect of present tropo-spheric ozone and aerosol levels is to heat the loweratmosphere by an additional 13 K, the CCL of the airmass in the combined breeze would rise some 100 to300 m by the time it reaches the mountain ridges. Andraising the CCL higher over the mountain tops wouldlower the probability of the storms maturing.

5. Feedbacks and runaway effects

When storms mature, some of the water vapor in thecombined breeze is rained out, part of the released la-tent heat is used to feed deep convection, and anotherpart is advected away at the tropopause level. In these

conditions, the surface air mass becomes entrainedthroughout the troposphere, and the coastal circula-tions can be considered open. This would be the situ-ation in a monsoon-type circulation. In the westernMediterranean, the transition from a vertical recircula-tion in the morning to an open-type circulation with thedevelopment of moist convection, cumulus, and stormsin the afternoon still occurs frequently on summer daysalong the Italian Appenine Mountains (Cant andGandino 1977; Milln et al. 1997).



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If the water vapor is not condensed, Fig. 6 shows thatit follows the ozone into the return flows of the com-bined breeze. Thus, while this situation lasts, the coastalwind system remains closed, that is, dominated byorographically aided convection that favors the verticalrecirculations. Raising the CCL of the surface air mass

in this region, therefore, provides a mechanism forkeeping the coastal circulations closed and for loadingthe troposphere with additional moisture to the depthreached by the coastal recirculations, that is, to approxi-mately 25003500 m over the sea.

Finally, the reevaporation of droplets from the un-matured cumulus clouds also increases the water vaporcontent in the layers above 3500 m. How this contri-bution compares to the moisture reevaporated duringthe dissipating phase of a matured storm and the re-sulting temperature profiles through the tropospherefor either case are two additional questions. It is sig-nificant that through the mechanisms described, mois-ture loading and stability changes can take placethrough the troposphere under anticyclonic conditionsof subsidence aloft.

It is at this point that the self-organizing nature of thecoastal recirculations and the resulting long residencetimes within the western basin come into play. Theadditional moisture could contribute to the creation ofan anomalously deep, moist, and potentially unstabletroposphere awaiting the right mechanism to triggerprecipitation, for example, destabilization by advectionof cold air aloft. Whether or not this can explain theincrease in large storms over the coasts and sea inspring is a question that must still be determined. In anycase, the more humid air mass may also be advected

away and participate in stronger precipitation else-where [e.g., 2002 summer floods over Central EuropeUlbrich et al. 2003a,b)].

Moreover, water vapor is an important (the most im-portant?) greenhouse gas, and increasing its amount inthe midtroposphere over this region would add to anyradiative heating effects caused by air pollutants, lead-ing to additional rises in land and sea surface tempera-tures and, ultimately, in the temperature of the air nearthe surface. An increase in evaporation from the seawould tend to lower the CCL of the airmass in thecombined breeze. Increased heating by the land surface(with lower heat capacity than the water) would tend toraise the CCL. Which process dominates, and in which

order, are two more questions that must be answered.Additional heating of the sea surface is cumulative

and can lead to delayed effects. For example, it mayresult in higher SST in the western Mediterranean basinby early autumn and winter and favor more intenserains (Milln et al. 1995; Pastor et al. 2001). Radiativeheat trapping may also result in delayed cooling of thesea surface in winter and faster warming in early springand may explain the shift in the observed frequency oftorrential rains shown in Fig. 5.

6. Discussion

Available experimental results suggest that, becauseof the nature of the atmospheric circulations along itscoasts, the hydrological cycle in the western Mediter-ranean basin has been drifting toward a critical thresh-old level, that is, when the cloud condensation levels ofthe surface air masses rise above the coastal mountainridges, leading to the loss of summer storms. Any ad-ditional perturbations to the surface air masses, includ-ing increasing their temperature (by whatever means)or decreasing evaporation and evapotranspirationalong their paths, or any combinations of these, couldtip the present situation further toward the loss ofmountain storms and desertification at the localregional scale.

The question of the end radiative effect of air pollu-tion in the Mediterranean basin, that is, is it cooling orwarming the sea and surface air mass, is still open, eventhough our results indicate that warming dominates inthe western Mediterranean at this stage. The hypoth-esis that the radiative effects of particulates cool theMediterranean (Lelieveld et al. 2002) would indeedlead to less evaporation and fewer summer storms, butit would also lead to fewer torrential rains in autumnwinterspring, which is contrary to observations in thewestern basin.

Our working hypotheses on the possible reasons forthe observed precipitation changes in the westernMediterranean basin relate to 1) land-use changes, asthe main factor drying the surface and 2) atmosphericcomposition changes, that is, an increase in tropo-spheric ozone and aerosols producing additional green-house heating. The hypotheses are as follows:

Land use that results in surface drying leads to in-creased heating of the surface air mass, lower evapo-ration, higher cloud condensation levels, and thusfewer summer storms in the mountains inland.

The radiative effects of air pollution could be furtherheating the surface air mass and lifting the CCL; thiscould be perturbing (present) local balances justenough to tip the equilibrium toward decreased sum-mer storms.

In turn, the recirculatory nature and the long resi-dence times of the air masses in this region lead to theaccumulation of ozone and water vapor (both actingas greenhouse gases) in the lower-to-upper tropo-sphere over the western Mediterranean, adding to

the heating of the sea surface during the summer andto an increase in torrential rains in autumn and win-ter at the regional (basin) scale.

In relation to climatevegetation feedbacks we can con-sider the following:

1) long-term effects, that is, fewer storms in the moun-tains, positive feedback through vegetation losses(less evapotranspiration), and increased surfaceheating (drier soil) toward desertification;

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2) delayed effects, that is, a warmer Mediterranean bythe end of summer and early autumn leading tomore frequent and intense torrential rains, by posi-tive feedback. These rains can occur anywhere in thebasin and result in flash floods over the coasts andnearby mountain slopes; and

3) if the slope vegetation response to these flash floodsis increased soil erosion, then the positive feedbacktoward desertification would be further reinforced.

Finally, another far-reaching conclusion is that any per-turbation to the hydrological cycle in any part of thewestern basin can propagate to other parts of the basinand neighboring regions. This derives from the fact thatthe warmer water pools move (i.e., rotate) within thebasin, and, thus, any increase in sea surface tempera-ture can result in torrential rain and flash floods any-where in the basin.

Acknowledgments. The experimental data and mod-eling results used here were obtained under the Euro-pean Commission projects MECAPIP, RECAPMA,SECAP, T-TRAPEM, BEMA, MEDEFLU, RECAB,ADIOS, and CARBOMONT. The ECs Joint Re-search Centre (JRC), Ispra, provided additional fund-ing for the MECAPIP instrumented aircraft; the Com-misariat lEnergie Atomique (CEA), Saclay, France,supported the RECAPMA flights in the western basin;and the Israeli Air Force supported the SECAP flightsin the eastern basin. Complementary financing to theEC funding was provided by the National ResearchPrograms in the countries of the participating teams.CEAM co-financing of these projects was provided bythe government of the Valencia region, BANCAJA,and the Spanish National Research Program. Finally,the authors would like to dedicate this work to thememory of Dr. Heinrich (Heinz) Ott (2004) for hisencouragement and his support of this research duringthe last 20 yr.

APPENDIX

Definitions of EC Project Acronyms

MECAPIP Meso-Meteorological Cycles of AirPollution in the Iberian Peninsula(198891)

RECAPMA Regional Cycles of Air Pollution inthe Western Mediterranean Area

(199092)SECAP South European Cycles of Air Pollu-

tion (199295)T-TRAPEM Transport and Transformation of Air

Pollutants on East Mediterranean(199295)

BEMA Biogenic Emissions in the Mediterra-nean Area, Phase I (199395) andPhase II (199698)

MEDEFLU Carbon and Water Fluxes of Mediter-

ranean Forest and Impacts of LandUse/Cover Changes (19982000)

RECAB Regional Assessment and Modellingof the Carbon Balance within Europe(200003)

ADIOS Atmospheric Deposition and Impact

of Pollutants, Key Elements and Nu-trients on the Open MediterraneanSea (200003)

CARBOMONT Effects of Land Use Changes onSources, Sinks and Fluxes of Carbonin European Mountain Areas (200104)

PHAMA Photo-Oxidant and Aerosols in theMediterranean Area (1999)

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