Journal of Hydrology - rainforclimate.com · Mediterranean coast to the headwaters of the Mijares...

Journal of Hydrology 518 (2014) 206–224

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Journal of Hydrology

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Extreme hydrometeorological events and climate change predictionsin Europe

0022-1694/$ - see front matter � 2014 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.jhydrol.2013.12.041

⇑ Address: CEAM, C/Charles Darwin 14, Parque Tecnológico, 46980 Paterna,Valencia, Spain. Tel.: +34 96 131 8227, mobile: +34 699 433 276; fax: +34 96 1318190.

E-mail address: [email protected]

Millán M. Millán ⇑Fundación CEAM, Valencia, Spain

a r t i c l e i n f o

Article history:Available online 8 January 2014

Keywords:Hydrological cycleTorrential rains in EuropeMediterranean mesometeorologyClimate feedbacks in Europe

s u m m a r y

Field meteorological data collected in several European Commission projects (from 1974 to 2011) werere-analysed in the context of a perceived reduction in summer storms around the Western MediterraneanBasin (WMB). The findings reveal some hitherto overlooked processes that raise questions about directimpacts on European hydrological cycles, e.g., extreme hydrometeorological events, and about the roleof feedbacks on climate models and climate predictions. For instance, the summer storms are affectedby land-use changes along the coasts and mountain slopes. Their loss triggers a chain of events that leadsto an Accumulation Mode (AM) where water vapour and air pollutants (ozone) become stacked in layers,up to 4000(+) m, over the WMB. The AM cycle can last 3–5 consecutive days, and recur several times eachmonth from mid May to late August. At the end of each cycle the accumulated water vapour can feed Vb

track events and generate intense rainfall and summer floods in Central Europe. Venting out of the watervapour that should have precipitated within the WMB increases the salinity of the sea and affects theAtlantic-Mediterranean Salinity valve at Gibraltar. This, in turn, can alter the tracks of Atlantic Depres-sions and their frontal systems over Atlantic Europe. Another effect is the greenhouse heating by watervapour and photo-oxidants (e.g., O3) when layered over the Basin during the AM cycle. This increases theSea Surface Temperature (SST), and the higher SST intensifies torrential rain events over the Mediterra-nean coasts in autumn. All these processes raise research questions that must be addressed to improvethe meteorological forecasting of extreme events, as well as climate model predictions.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

After the 1972 UN Stockholm Conference on the Environment, anumber of actions were initiated worldwide following the recom-mendations of the Conference (MIT, 1970, 1971). The EuropeanCommission initiated its programme in Environment and Climatein 1973, by launching several large projects, with intensive fieldcampaigns in the areas of Atmospheric Chemistry, i.e., Air Pollution(Guillot, 1985) and Desertification (Mairota et al., 1998). Whilesearching for measurement sites, and during instrumental deploy-ments, scientists were alerted by locals regarding a decrease in thenumber of summer storms. An oddity at the time was that thecomments came mainly from areas surrounding the Western Med-iterranean Basin (WMB), except from Central Italy.

In December 1991, the EC’s Unit Head for Environment andClimate (Dr. Heinrich Ott) and I met to review data from the MECA-PIP and RECAPMA projects (Appendix A). The subject of thesummer storms came up, and he inquired about using the field

information and meso-meteorological data collected in the EC pro-jects to find an answer to 18 years of comments about storm lossaround the Mediterranean. Moreover, the query of the storms, asin this Special Issue, offered a chance to challenge conventionalwisdom, raise questions, and postulate mechanisms that could beused to select topics for EC research programmes. This paper, fol-lowing the initial objective, describes the analysis sequence,including a review of the available field data, the interpretations,the questions arising, and the likely answers and hypotheses de-rived at each stage.

2. The precipitation types

In 1992 we talked again about initiating the precipitation studyin areas where large field campaigns had taken place, and the com-ments about the storms had arisen (Appendix A). For example, inMarseille-Fos Berre (France), in the Po Valley (Italy), in the MijaresValley, Valencia-Teruel (Spain). We also considered the availabilityof the data and, above all, its spatial extent. In the MECAPIP themeasurements covered the north-eastern quadrant of the IberianPeninsula. They included meteorological towers, tethered balloonsand instrumented aircraft measurements, all the way from the

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M.M. Millán / Journal of Hydrology 518 (2014) 206–224 207

Mediterranean coast to the headwaters of the Mijares Valley, andbeyond, i.e., to Madrid, and Bilbao in North-Atlantic Spain, andalong the Ebro Valley (Millán et al., 1991, 1992). The RECAPMAproject had further extended the experimental coverage from theAtlantic coast of Portugal, to southern France and Italy. It includedinstrumented flights over the WMB in July 1991. In fact, we werereviewing those results when the query about the storms came up.

From 1992 to 1994, other projects (SECAP, BEMA I) werelaunched, extending the spatial scale from Portugal to Greece, Tur-

Fig. 1. Spatial and temporal disaggregation of the precipitation types for the Valencia RPrecipitation contours in mm. Yearly average series for the whole domain in mm.

key and Israel (Millán et al., 1997). In all, the Mijares Valley hadbeen used in five campaigns, which put this area at the head ofthe list for the precipitation study. The issue of the storms was fi-nally addressed in 1993 – at first, riding piggy-back on the budgetof on-going EC projects and funds from the Spanish National Re-search Programme (SNRP). It became a full project in 1995, afterfunding was procured from the SNRP to purchase high spatial res-olution, daily precipitation data. The data series from 497 stationsfor the period (1950–1996) in the area outlined in Fig. 1, cost us

egion and neighbouring areas in Spain (domain maps) for the period 1950–2000.

208 M.M. Millán / Journal of Hydrology 518 (2014) 206–224

45,000(+) €. This was the final deciding factor for limiting the studyto a domain that includes the Valencia Region, and adjacent areasin Spain.

Because the comments pertained to summer storms the firststep was to disaggregate the precipitation data by weather types,using criteria derived from the MECAPIP, RECAPMA, and SECAPprojects. The detailed procedure, including the co-analysis of therain series with the daily synoptic maps, and the correlations withthe North Atlantic Oscillation (NAO) index was published by Millánet al. (2005a,b). At the end of 1997, we had identified three sourcesof precipitation in this domain:

(A) Classic Atlantic Fronts. Their most frequent occurrence isfrom early autumn to late spring (Fig. 1A, series A). The mainfoci occur over the west-facing slopes of the Iberian Cordil-lera System. There are also weaker, secondary, foci oversome of its ranges extending towards the sea. We alsoincluded in this type the convergences of westerly Atlanticflows with Mediterranean seabreezes along the Iberian Cor-dillera System in late summer. These may be accompaniedby a cold trough at the 500 hPa level. In these cases, the rainscan be very intense, the maxima occur directly over the Ibe-rian System, and the collected amounts taper down to zeroat the coast. This type contributes <20% of the total precipi-tation in the study domain. Its correlation with the (NAO)index is negative.

(B) Summer Storms in this area are associated with the diurnaldevelopment of the Iberian Thermal Low in summer (Barryand Chorley, 1987). This type includes the storms drivenby the combined seabreeze and up-slope winds (Sections 4and 7), which develop in the afternoon over the mountainranges at 60–100 km from the coast. They are more frequentfrom late April to September (Fig. 1B, series B). During thelate evening and night the storms can migrate easterly(i.e., towards the coast) before dissipating. Rain amountswere attributed to this type whenever the Iberian ThermalLow was observed on the synoptic maps at 12:00 UTC and/or at 18:00 UTC on the day of the event. Their main focioccur over the east-facing slopes of the Iberian CordilleraSystem, i.e., facing the Mediterranean Sea. They contribute11–16% of the total precipitation in the domain and showno correlation with the NAO index.

(C) Mediterranean Cyclogenesis events, ‘‘Backdoor Cold Fronts’’(Huschke, 1986) or ‘‘Levanters’’ (Meteorological Office,1962). These develop during the migratory period of the

Fig. 2. Drainage Basins of the World (Source United Nations, en.wikipedia

anticyclones from the Atlantic to Siberia from early fall untillate winter-spring (Fig. 1C, series C). Thus, they are alwaysassociated with an anticyclone over Central Europe drivingan easterly flow of cold continental air over a warmer Med-iterranean sea. They can be very intense, and they tend tooccur mainly at night and be localised spatially. They canlast from one to three days. If a cut-off low develops south-west of the Iberian Peninsula (Gulf of Cadiz), and migratesvia Gibraltar and the Sea of Alboran towards the WMB, theeasterly flow is reinforced. The area affected can be muchlarger and the precipitation can occur day and night. Theycan also last longer (i.e., of the order of one week). This typecontributes >65% of the total precipitation in the studydomain, and the focal areas are right over the coastal areas.The correlation with the NAO index is positive.

A first question concerns the generalised use of the NAO indexas a simple climatic indicator for precipitation in this area of com-plex terrain which extends from the sea to the European Continen-tal Divide (Figs. 2 and 3). Other findings are that the contributionfrom Atlantic depressions (A) has been decreasing for the last50 years in this region. The summer storms over the mountainssurrounding this part of the Mediterranean Basin have nearly dis-appeared in the last 50 years, while extreme precipitation eventsassociated with Mediterranean Cyclogenesis (C) appear to beincreasing rapidly, becoming more torrential in nature, continuingwell into the spring period (Peñarrocha et al., 2002; Millán et al.,2005b), and also beginning as early as late summer.

Finally, the sum of rain types (B) and (C) amounts to some 75–80% of the total precipitation in this area. Both occur with easterlywinds and, thus, from water evaporated within the MediterraneanBasin. This is a recurring theme in this work, since it would suggestthat the rain water from these two types may simply be recycledwithin the Mediterranean Basin itself. Thus, the initial questionsregard the influence of orography, and the source(s) of the watervapour involved in each precipitation type.

3. The lay of the land: the role of the European continentaldivide

The spatial distribution of components (A) and (C) in Fig. 1raises the question of which precipitation is Atlantic, i.e., fromwater evaporated from the Atlantic, and which is Mediterranean.To start with, Fig. 2 shows that the real, and ‘‘whole’’,

.org). Grey areas are endorheic basins that do not drain to the ocean.

Fig. 3. Colour-coded altitude map of Europe (Source USGS and the European Soil Bureau). The dark-blue line marks the European Continental-Water Divide. (Forinterpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)


Mediterranean Catchment Basin extends to the sources of the NileRiver, and includes the Black Sea as well as other parts of Europethat are not usually considered Mediterranean (e.g., Austria, Hun-gary, Romania, etc.). A closer look over Europe, in Fig. 3, shows thatthe European Continental-Water Divide follows the high groundand the peaks of the major mountain ranges, including the IberianCordillera System on the western border of the study area. To theright and south of the Divide all waters drain into the Mediterra-nean, and to the left and north they drain into the Atlantic.

Depending on the meteorological conditions, e.g., advectionacross the Divide, Föhn type effects tend to keep a large fraction(i.e., up to 1/3) of the water vapour carried by the airmasses onthe upwind side of the Divide. Thus, this mechanism limits theamount of water vapour transported from one side of the Divideto the other. In other meteorological conditions, e.g., Vb track typeevents (van Bebber, 1891), the Divide favours surface convergenceof airmasses from the Atlantic and the Mediterranean (type A,above), and can become the focus for intense precipitation andrunoff to either side of the Divide (Ulbrich et al., 2003; Gangoitiet al., 2011a,b).

There are also mountain passes on the Divide that provide di-rect paths for airmasses into and out of the Mediterranean Basin,and are notorious for generating intense ‘‘Gap Winds’’, e.g., the Tra-muntana through the Carcassone Gap, and the Tarifa winds atGibraltar (Scorer, 1952). This fact, in turn, raises more questionsregarding the source and transport of water vapour, and the triggermechanisms for some Mediterranean Cyclogenesis events. Forexample, the formation of Genoa Depressions: are they concurrentwith, or are they a result of, intense cold Mistral winds over warmwaters in the Gulf of Genoa? Thus, we could ask how much of therunoff on each side of the Divide proceeds from water vapour evap-orated within, and converging from, the same side? what condi-tions favour the net transfer of water vapour from one side ofthe Divide to the other? and where along the Divide? and howmuch?

Finally, if the concept of Atlantic Frontal Systems is to be up-held, e.g., for rain in places like the Italian Peninsula, the questionsare: how much additional water vapour? and evaporated fromwhere? is required to re-activate the Atlantic Fronts, or any uppertrough, after they cross the Divide into the Mediterranean Catch-ment Basin. An answer to these questions may require re-defining

some terms. For example, the southerly migration of an upper coldtrough, or a pool of cold air aloft from higher latitudes can act as atrigger for intense Mediterranean Cyclogenesis. Though, only ifthere is enough moisture in the lower atmosphere, or if the processis accompanied by a mechanism to recharge moisture, e.g., cold airadvection over warm water, as in a Backdoor Cold Front (Huschke,1986).

Previous work indicates that the water vapour involved in pre-cipitation types (B) and (C), regardless of the trigger mechanism,originates from within the Mediterranean Basin (Millán et al.,1995; Pastor et al., 2001). This will be reconsidered in Section 8by introducing the concepts of a carrier component and a triggercomponent for the water vapour involved in the precipitation. Anextensive validation of this hypothesis and of the recurring ques-tions raised above requires a combination of in-depth meso-mete-orological analysis and the disaggregation of precipitation byweather types across the Basin, rather than just modelling (Sec-tion 6), as well as extensive studies of isotopic rain compositionalong judiciously selected transects across the Divide.

4. Meso-meteorological processes: The combined up-slope andsea-breeze

The comments from locals indicated that summer storms form(or used to form) in the afternoon over the mountains surroundingthe WMB, at distances ranging from 60 to 100(+) km from the sea.By 1995, we also knew that the storms developed as the final stagein a coastal wind system that develops on summer days and com-bines the up-slope winds and the sea breeze, henceforth called the‘‘combined breeze’’. The specific characteristics of this wind sys-tem have been documented in ten EC projects since 1986 (Appen-dix A, Millán et al., 1987, 1992, 1997, 2000, 2002) and derive fromthe nature of the WBM. That is, a large and deep sea totally sur-rounded by high mountains in the subtropical latitudes. The devel-opment of the combined breeze is strongly influenced byorography (Mahrer and Pielke, 1977; Miao et al., 2003), and it isquite different from a ‘‘classic seabreeze’’ (Munn, 1966; Stull,1988), since the marine air mass becomes strongly modified alongits path. Fig. 4 illustrates its evolution during the day with model-ling results (RAMS, Pielke et al., 1992). This is the key to under-standing the feedback processes described in this work.

t = 02:00 UTC t = 10:00 UTC

a b

t = 12:00 UTC t = 14:00 UTC

t = 22:00 UTCt = 18:00 UTC

dc

e f

MECAPIP 27 - July -1989 (ω Component)

Fig. 4. RAMS simulated vertical (x) wind component on July 27, 1989, along �40�N Latitude, crossing the coastal city of Castellón towards the Javalambre massif (map inFig. 11 and text). Solid lines mark the ascent component of the wind, and dotted lines the descent. Red arrows mark the surface component of the seabreeze and the upslopewind cells, and blue arrows the compensatory subsidence aloft, and the drainage winds.

1 For interpretation of color in Fig. 4, the reader is referred to the web version ofis article.


(1) Upslope wind cells develop early in the morning, i.e., rightafter sunrise, over the east-and-south facing slopes of thecoastal mountains (Munn, 1966; Millán et al., 1991, 1992;Salvador et al., 1997).

(2) The seabreeze develops over the coastal plains, and hence-forth its progress inland occurs in a stepwise manner.Namely, it goes up the slopes by incorporating one afteranother the up-slope wind cells formed earlier that morning.This sequence is illustrated in Fig. 4 (graphs b, c, d, e), dis-cussed further below.

(3) After advancing each step, the leading edge (or front) of thecombined breeze rests, and remains stationary over thatposition, from �1/2 to 1(+) h, see orographic chimneysbelow.

(4) During July and August, the combined breeze can last from12 to 14 h at the coast, and its average wind run can reach160 km, as shown in Fig. 5 (Millán et al., 2000).

(5) It can take from �4 to 6 (+) h for the front of the combinedbreeze to reach all the way from the coast to the top of themountains 60–100 km inland (Figs. 4 and 5).

(6) The experimental data and the modelling results also showthat once the front reaches the highest mountain tops, ittends to become locked onto that position, i.e., this becomesthe last step, and it can remain in that location for 4–6 h, asshown in Fig. 4 (graphs d, e), i.e., until the end of the breezeperiod (Salvador et al., 1997).

(7) During the evening and night, drainage flows developtowards the sea, i.e., the land breeze (Fig. 4, graphs a andf), and convective activity develops over the coast-sea inter-phase as the colder drainage winds meet the warmer waters(e.g., red1 arrow in Fig. 4 graph a).

The graphs in Fig. 4 show that the up-slope wind cells remain inplace until incorporated into the growing (larger) wind system. Forexample, by 10:00 UTC the combined breeze has progressed 30 kminland, while three up-slope wind cells remain further up theslopes. At 12:00 UTC, it has progressed 40 km inland, and appears

th

Fig. 5. Averages of the wind direction and speed at Castellón (CS-SUR, Fig. 11). Dataobtained during the last two weeks of July in the years 1989, 1990, 1991, 1994,1995. The seabreeze period lasts from �07:00 to �21:00, and its wind run is�160 km each day.


to be in the process of incorporating another cell (two still remainin place). By 1400 UTC, it has incorporated all the upslope windcells, and reached 80 km inland, while the vertical injection at itsfront reaches �3500 m high. At 1800 UTC, the front of the com-bined breeze is still locked at the mountain ridges 80(+) km inland,but its injection is weaker (to �3000 m). By 2200 UTC, the com-bined breeze has ceased and the surface drainage flows are becom-ing established and flowing towards the sea (see also Fig. 4 graph a,at 02:00 UTC).

This kind of step-like progress was first documented during theMECAPIP project (in 1989) by photographically tracking the evolu-tion of small cumulus clouds forming at the leading edge of thebreeze (Millán et al., 1992). It was then observed that the upslopewind cells tend to develop over the same orographic features, eachday, under similar weather conditions. This has also been repro-duced by modelling. Thus, in the ‘‘combined breeze’’ the positionsof its front are conditioned by the orography, and the injection overeach of those points resembles the mechanism creating chimneyclouds (Huschke, 1986). Therefore, the term convective-orographic‘‘chimney’’ will be used henceforth to refer to ‘‘a sustained verticalinjection of surface air at a specific front position’’. In addition, thealtitude of the injections into the return flows increases at eachchimney, both because: the airmass gains more potential temper-ature, as it follows a longer path along the heated surface, and thebases of successive chimneys occur at ever increasing heights upthe mountain slopes.

Since each chimney remains stationary for extended periods oftime, see (3) above, the surface airmass that reaches its base be-comes injected upwards, directly into the return flows aloft. There,the injected airmasses move seaward and sink under compensa-tory subsidence along their way. The total amount of sinking sus-tained by the return flows seems to be comparable to thealtitude of the feet of the chimneys over which they were injected(Salvador et al., 1997). Sinking also increases the stability of the re-turn flows, and favours the formation of layers over the coastalareas and the sea (Millán et al., 2000).

5. Boundary layer folding and the formation of strata over theMediterranean Sea

As a result of the processes described, the part of the surfaceboundary layer that arrives at each new chimney is injected into

the return flows aloft, stratified, and folded on top of the (older)part that had been injected previously at a lower height and closerto the coast. The length of the strata formed in each step can alsobe comparable to the distance between the foot of the currentchimney and that of the previous one, or longer, as the return flowsunder increasing compensatory subsidence (Section 6) may ac-quire jet-like characteristics. Finally, the compensatory subsidenceover the sea completes the atmospheric circulation. Thus, thesemechanisms form part of a ‘‘closed’’ vertical circulation that growsduring the day (see Section 7), reaches its maximum developmentin the mid-to-late afternoon, and ceases by evening, only to beginanew the following day.

An example of the pollutants (ozone) and the water vapour in-jected into the return flows is shown in Fig. 6. The measurementswere contracted by the JRC-ISPRA and taken by an aircraft instru-mented by FhG-IFU and Aerodata during the MECAPIP project. Atthe time of the measurements, three layers can be seen outlinedby arrow lines. The two vertical arrows in Fig. 6 show how a verti-cal-looking satellite, i.e., the NASA MODIS-Terra (King et al., 2003),would see a large column value for water vapour when lookingdown the developing orographic chimneys, and through the vari-ous layers formed over the slopes of the coastal mountains. Thiswould explain the comment by Gao and Kaufman (2003) aboutthe MODIS signal over the Mediterranean Basin in summer ‘‘beinglargest above the coastal side of the mountains’’. In Section 9 thewater vapour signal will be used as a tracer of opportunity to char-acterise the dynamics of the airmasses recirculated vertically bythe coastal wind systems in the Western Mediterranean Basin.

Examples of the resulting temperature structure over the seaare shown in Figs. 7, 12 and 13. The inversions mark the differentlayers formed over the sea, and Fig. 7 also illustrates how repetitivethe coastal circulations can be over the same part of the WesternMediterranean Basin in summer. Common features in all the pro-files are: a nearly adiabatic layer in the upper 1500(+) m of thesoundings, i.e., from �2000 m to �3500(+) m, and a serrated pro-file with multiple temperature inversions in the lower 2000 m.The former is indicative of a very well mixed airmass, or of the sus-tained subsidence of a well-mixed airmass, or both. The upper lay-ers are formed at the last stage of the combined breeze, over themountain tops, after the surface air has become well mixed duringits travel over the heated land, and the layer formation time is alsothe longest (4–6 h). The thickness of the upper nearly-adiabaticlayers (suggested in Fig. 4e and c) is comparable to the altitudeof the ridges over which their injection took place (e.g., 1500–2000 m).

When the MECAPIP flights were being planned (1987), it wasnot really known how high the layers could reach, or how far theywould extend over the sea (Millán et al., 1987). The early morningflights over the sea were intended to document the layers formedthe previous day. The vertical flight plane was over the sea at�40 km from the coast, and the top of the sawtooth flight patternwas fixed at 3500 m. However, the height of most of the coastalflights was arbitrarily limited to less than 1200 m by the flightcrew, and to less than 3000 m over land (Fig. 6). Only in the lastpart of the project, i.e., after 22 of the 32 contracted flight hourshad been used up, and after studying the lone flight leg thatreached 3000 m over the coast (Fig. 7, black profile), did the flightsreach 3000 m, or up to 3500 m on a few occasions.

This illustrates a problem of credibility between the project’sPrincipal Investigator (the author) and the scientists performingthe flights. Above all, it shows the enduring difficulties in convey-ing processes of this nature to people used to boundary layers ofless than 1000 m high, over flat grassy terrain in higher latitudes,and under, so-called, prevailing winds that do not change directionby almost 180�, from day to night, every day. The following RECAP-MA project was intended to document how far, and how high, the

Fig. 6. Ozone and water vapour on a vertical flight plane across 350 km of the Iberian Peninsula at 1449–1559 UTC, July 20, 1989 (transect H ? G, in the map). Note thecolumn of cleaner and drier air sinking at the left of the combined breeze front (see Fig. 4, graphs d and e), for the ? MODELLED PART see Section 6, and Figs. 8 and 9.


layers reached over the Western Basin. These flights were carriedout by partner CEA (FR), all of them reached 3500 m high, andsome 4000 m; however, they were still not high enough.

The MECAPIP and RECAPMA results were mentioned in the Re-search Result Reports of the EC (Le Bras, 1993; Le Bras and Angeletti,1995), and presented at the 5th and 6th EC Symposia on PHYSICO-CHEMICAL BEHAVIOUR OF ATMOSPHERIC POLLUTANTS (Restelli andAngeletti, 1990; Angeletti and Restelli, 1994), EUROTRAC Symposia,NATO ITM meetings, and in the Atmospheric Environment Service ofCanada in Toronto in 1991 and 1993. Following this, the formation of(ozone) layers was also documented over the Adriatic Sea (Fortezzaet al., 1993; Orciari et al., 1993; Georgiadis et al., 1994), and in thearea of Vancouver, BC, Canada (McKendry and Lundgren, 2000).More recently, nocturnal temperature soundings (2009–2011) fromcruise ships in the Western Mediterranean, conducted by JRC-ISPRA,show that the upper nearly adiabatic layers can reach as high as5000 m.

6. Modelling and the self organisation of the coastal circulations

Fig. 8 shows a vertical section of a RAMS model (Pielke et al.,1992) simulation of the wind field over the last 180 km of the flight

plane in Fig. 6 (marked by ? MODELLED PART), at 1600 UTC. Thegrid size used was 2 km � 2 km, and in order to emphasise thestructure, the vertical component of the wind has been multipliedby ten. At the leading edge of the combined breeze, at �90 km in-land at that time, it shows an injection to �5000 m, which can becompared with those in Fig. 4. The lines mark the approximate ver-tical boundaries of the flight path in Fig. 6, making it obvious (now)that the flight did not reach high enough to fully capture the depthof the vertical injections.

If ‘‘closed-loop’’ vertical circulations develop in other coastalareas of the WMB, they can merge and become self-organised dur-ing the day to originate a meso-a scale circulation extending to thewhole Basin. In this case, the chimneys at the leading edges of thecombined breezes join and form well-defined convergence linesalong the mountains surrounding the basin, i.e., some 60–100 kminland from the coasts, as illustrated in Fig. 9 for the Iberian Penin-sula, and in Fig. 10 for the main Mediterranean Basin.

The upper part of Fig. 10 shows that the surface winds emergefrom the centre of the Western Basin, and increase in speed whileflowing anticyclonically (clockwise) towards convergence linesover the main mountain ranges surrounding the basin (in grey).Their resolution can be compared with those in Fig. 9. An outcome


of the self-organisation is that the compensatory subsidence be-comes generalised and more intense over the coastal areas andthe sea, as illustrated at the bottom of Fig. 10. It shows that toreplenish the surface air moving towards the coasts, continuity re-quires sinking of the air, i.e., compensatory subsidence (dottedlines) over the seas.

This is important because the subsidence can keep the surfacelayer confined below a height of �200 m, as illustrated in Fig. 11.The series show the average height to the first observed tempera-ture inversion in black, and the average plus one standard devia-tion (in light grey) to emphasise the amplitude of theoscillations. In Valbona, the soundings were performed at selectedtime periods instead of the half-hour cadence used on the othersites; hence, the gaps in the series. The confinement and the oscil-lations can be considered ‘‘unusual’’, or even as faulty data, if inter-preted in the light of conventional Boundary Layer (BL) theories(Delbarre et al., 2005).

Fig. 11 shows that the BL at the coast grows initially and thenoscillates around the 200 m level, and that the oscillations occur,or propagate, along its path up the slopes. Similar behaviour,including the vertical confinement of the BL, and the oscillationspropagating downwind, had been observed in 1978 during theNanticoke Power Plant Experiment on the northern shores of LakeErie, Ontario, Canada (Portelli et al., 1982; Kerman et al., 1982). Inthe WMB, the strong confinement of the boundary layer plays akey role in the development of storms (Section 8) but it is not wellsimulated by the model which, for the same boundary conditions,yields BL values in the 900–2000 m range (Salvador et al., 1999).

In the WMB, the shallow vertical confinement of the BL hasbeen observed to extend all the way from the coast to each ofthe orographic chimneys along the path of the combined breezeand, eventually, to the last chimney that forms in the afternoon

Conc. O3 (ppb).

500

1000

1500

2000

2500

3000

0

3500

0 10 20 30 40 50 60

10 15 20Temperature (C).

HEI

GH

T (m

)

Fig. 7. Temperature (red) and ozone (blue) profiles over the Gulf of Valencia in the Baleover the triangles in the inserted maps at right. The thick black temperature profile wasproject (from 0527 to 0541 UTC on July 20, 1989) two years earlier. The black triangle l

as far as 80–100 km inland. This is shown in Fig. 11 for this studyarea, and it was also documented by soundings during the RECAP-MA project from the Italian coast (Castel Porciano), past Rome tothe mountains. However, being considered anomalous, or faulty,at the time by the partner in charge (CNRS, IT), the data somehowvanished. In the large meso-a circulation the confinement of theBLs due to compensatory subsidences can also occur over inlandareas, and yield surprisingly low mixing heights in the most unex-pected places. Some of these areas have been outlined with ellipsesin Fig. 9. This could explain the ‘‘highly anomalous’’ temperaturesoundings, i.e., yielding less than 350 m mixing heights on summerafternoons, as observed by Bölle et al. (1993) during the EFEDAproject over the parched dry lands of Albacete (ellipse A inFig. 9), data which have also disappeared.

To simulate the development of the sea breeze, mesoscalemeteorological models require the use of grids smaller than9 km � 9 km (Salvador et al., 1999). With larger grids the breezesare not simulated unless the domain extends to the whole basin(Fig. 10). But even when using smaller grids (i.e., 5 km � 5 km)and vertical resolutions at the limit of numerical instability (e.g.,36 m), current models are not able to reproduce all the structuresdocumented experimentally over the Western Mediterranean(Figs. 7, 12 and 13). The first recurring and non-trivial problem isthe interplay between the size of the model domain and the gridsize used. For example, the wrong domain can eliminate compen-satory subsidence over the sea and overland areas (Millán, 2008),and/or play dramatically with the simulated injection heights, asindicated in the comments to Figs. 4, 6, 8 and 10. The second prob-lem is the prevailing tendency to interpret experimental data ‘‘toolocally’’, as if nothing else is happening outside the experimentalarea (Bölle et al., 1993; Delbarre et al., 2005). The third problemis that current BL parametrisations in meteorological models, for

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aric Basin, obtained by spiral flights during the RECAPMA project (on 16 July 1991)obtained over the same area (short trace in the middle map) during the MECAPIP

ine shows the modelled temperature profile for the MECAPIP flight.

Fig. 8. RAMS simulation of the vertical wind field over the last 180 km of the flight,marked by ? MODELLED PART in Fig. 6. The lines mark the vertical limits of theinstrumented flight on that day.

Fig. 9. RAMS simulation of the surface wind field (h = 14.8 m) over the IberianPeninsula at 1600 h, 20 July 1989 (5 km � 5 km grid). The intersect of the flightplane in Fig. 6, marked ? MODELLED PART, for Fig. 8 is shown by a red line. Greyoutlines mark the lines of convergence of the surface flows, i.e., the locus of thehighest injections (Fig. 8) where summer storms used to develop. The ellipses markareas where streamlines appear, signalling strong compensatory subsidence tomaintain the 3D continuity of the flows.


flat grassy terrains under moderate to strong winds, do not reallyapply in complex-coastal-terrains (Stull, 1988).

Although the meso-scale model can reproduce the main fea-tures of the flows, it does not capture the details, i.e., the fine tem-perature structure that defines all the layers formed in the returnflows (black triangle line in Fig. 7). Nor does the model simulatethe amount of compensatory subsidence that laminates the layersand keeps the surface boundary layer so strongly confined verti-cally. All these issues were first noticed when trying to simulatethe MECAPIP and RECAPMA temperature soundings in 1992–1993. An example of two extreme cases encountered is shown inFig. 12. The conclusions are that modelling requires a modellerwith a very good insight into the possible meteorological processesinvolved, i.e., from the local to regional scales (meso-scales) includ-ing how (very) far their effects can reach, including their compen-satory mechanisms. It also requires a critical attitude towards theresults, and a judicious iterative play with simulations using differ-ent grid sizes and domain combinations (Millán, 2008). For

example, by comparing details about stream lines, and conver-gence lines over the Iberian Peninsula in Figs. 9 and 10.

The observed temperature profiles show increases from 13 K to19 K in the return flows (Fig. 13) and a higher water content in theupper layers than in the layers directly above the sea (Fig. 6). Thatis, the perturbed Relative Humidity (RH) of the IPCC (2001, 2007).This is the result of the water vapour added during their path overland (albeit not enough to trigger storms, as mentioned below).The observed temperature and moisture profiles can thus be con-sidered ‘‘anomalous’’ with respect to current advection-dominatedmodel simulations, which assume that the water vapour is evapo-rated directly from the surface (or comes from upstream). And be-cause the models are unable to simulate the systematic folding ofthe surface boundary layer over the sea for several consecutivedays, they cannot simulate the resulting accumulation of atmo-spheric properties (i.e., temperature, pollutants and water vapour)in layers over the coastal areas and over this large interior sea.

The self-organisation of the local circulations from the meso-cscale (to �20 km), through the meso-b scale (to �200 km), to afull-fledged meso-a scale (to �2000 km) circulation during theday can be considered a good example of atmospheric propertiesand processes spilling up and down the meteorological scales, typ-ical of sub-tropical latitudes with complex coastal terrains. Thetemperature and humidity profiles observed over the coastal areasand the sea are the result of the systematic folding of the surfaceboundary layers for several consecutive days, along with other pro-cesses that may dominate during the night. For example, the relax-ation of the compensatory subsidences, the interaction of thedrainage flows with the layer system near the coasts (Fig. 4 grapha, at 02:00 UTC), and the vertical re-stacking of the layers accord-ing to their potential temperatures in a Margulis 2nd type circula-tion (Iribarne and Godson, 1981). These can modify the structure,and pump upwards the whole layer system during the night, andthey also require specific experimental research.

7. Open or closed atmospheric circulations in the WMB, and thefirst climatic tipping point

The number of steps that the combined breeze takes to reachthe mountain ridges inland and, thus, the number of chimneys,and layers, formed during its development depend on the lay ofthe slopes around the WMB basin (Figs. 4 and 6) During this pro-cess, storms can develop whenever the Convective (-orographic)Condensation Level (CCL) of the incoming air mass is reached with-in the chimney at the leading edge of the combined breeze. Thiscan trigger deeper (moist) convection and the development of con-vective showers or a storm. If storms do develop, the air mass be-comes mixed all the way up to the tropopause, and the closed-loopcoastal wind system becomes ‘‘open’’. That is, it ends up behavinglike a small monsoon. This is illustrated in Fig. 14A.

However, the airmass coming inland from the sea sustains heat-ing as it moves inland along the warm surface. Thus, the formationof a storm requires the addition of water vapour to offset its heat-ing, and keep the CCL of the incoming airmass below its height ofinjection into the return flows aloft. For example, by evaporationfrom: the surface, coastal wetlands, vegetation, forests, irrigatedcrops, etc. Otherwise, heating prevails and the CCL of the incomingairmass will keep on rising, the combined breeze will keep pro-gressing inland, and we can consider that a ‘‘first critical threshold,or tipping point’’ in this system will be crossed when the CCL be-comes higher than the highest injections over the mountaintops. This is represented by the first loop in Fig. 15 (EC, 2007)which presents a synthesis of our results and postulates. The otherloops in this figure will be discussed along the text.

Fig. 10. Top: RAMS simulation of the surface wind field (at a height of 14.8 m) over the Mediterranean at 16:00 UTC on 19 July 1991 (40 km � 40 km grid). Bottom: Thevertical component of the wind field along the 39.5 North Parallel (grey trace in the upper graph) shows deep orographic-convective injections (solid traces) over EasternSpain and, to the right, over Sardinia and the west-facing coasts of Italy, Greece and Turkey.


Under these conditions the coastal circulations remain‘‘closed’’. That is, if storms do not develop all the return flowsformed will continue moving seaward, under compensatorysubsidence during the entire day, forming layers that contain thenon-precipitated water vapour and the pollutants carried by thecombined breeze, and piling them up to 4000(+) m over the Wes-tern Mediterranean Basin (Figs. 7, 12 and 13). This was the situa-tion prevailing during the EC projects when their objective wasto validate the postulated mechanisms (vertical recirculationsand the formation of reservoir layers) responsible for the high lev-els of surface ozone, and other photo-oxidants, measured in theWMB. In fact, at the time of preparing the MECAPIP proposal in1986, the last three weeks of July and the first week of August wereselected by the author, after studying 20 years of synoptic weathermaps, as the best time window for the instrumented aircraft toavoid storms (Millán et al., 1987, 1992). The temporal histogramin Fig. 1B, prepared independently, using a different data set (dailyrain values) more than 10 years later, seems to validate the earlymeteorological choice.

Moreover, because summer precipitation will decrease over themountain slopes if storms do not develop, the conditions thatmaintain the combined breeze as a ‘‘closed’’ wind system (viz.,too much heating and insufficient evaporation along its surfacepath) can be considered part of a first feedback loop that tips thelocal climate towards increased drought inland, shown inFig. 14B. The change from a ‘‘closed’’ circulation in the morning

to an ‘‘open’’ system (storm/or shower) in the afternoon still oc-curs almost daily over Italy in summer. This is due to the conver-gence of the combined breezes from the Ionic and the AdriaticSeas over the Apennine Mountains (Section 8) and is suggestedby the model results in Fig. 10.

In the Western Mediterranean Basin, the ‘‘closed loop’’ condi-tions last from 3 to 5 consecutive days and recur several times amonth. That is, for a total of 12–24(+) days per month, from latespring to late summer (i.e., May–September). This was determinedinitially from the ozone accumulation cycles recorded by air qual-ity monitoring networks since 1994 (MIMAM, 2009). During theseperiods the Western Mediterranean Basin behaves like a ‘‘largecauldron’’ (Section 8) where the airmasses ‘‘boil’’ from the edgestowards the centre, favouring photochemical reactions betweennew coastal emissions and the ‘‘aged’’ recirculating airmasses.Using satellite (MODIS-terra) data, it has been estimated that thecoastal winds recirculate vertically each day from �1/4 to 1/2 ofthe depth of the layers accumulated over the sea on the previousdays (Millán, 2008).

8. Summer storms and land-use changes

The main problem with the condensation level is that the tem-perature/moisture relationship is exponential (Iribarne and God-son, 1981), and the consequences of this can now be explored

Surface Layer Evolution (CAST-PUERTO)

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Fig. 11. Top left: One experimental deployment area used in nine EC projects in the Mediterranean; arrows mark the three tethered balloon sounding sites for the othergraphs. These show the hourly evolution of the mixed layer height (m), the experimental period (years), and the number of soundings used for the statistics at each site (seetext).


using the experimental parameters measured during the EC pro-jects. In the first place, Fig. 13 shows the profiles with the highestand the lowest potential temperatures measured during theRECAPMA project in the Balearic Basin. The values span the312 K (39 �C) to 318 K (45 �C) range in the upper parts of theflights, and 24 �C (297 K) to 26 �C (299 K) at the surface, and sug-gest that the marine airmasses that enter the coastline gain from13 K to 19 K by the time they become injected into the uppermostreturn flows aloft.

During the EC projects, the climatic values (1966–1995) for thewater vapour mixing ratio and the temperature at the coast were�14 g/km and 26 �C (299 K), respectively. These yield a Lifting Con-densation Level (LCL) at �780 m for the marine airmass at thecoastline (Iribarne and Godson, 1981). However, if the combinedbreeze gains an average of 16 K along its land path (see Fig. 13),the modified airmass needs to increase its water vapour mixing ra-tio to P21 g/kg to keep its Convective (-orographic) CondensationLevel (CCL) below the approximate height of the coastal mountainranges, i.e., 2000 m altitude.

The one benefit of the closed vertical circulations is that the dis-placed air volumes, per unit width along the coast, are reasonablywell defined, and this allows for tentative calculations. For exam-ple, the average boundary layer depth along the path of the com-bined breeze can be defined from Fig. 11 as 200 m. The averagerun of the combined breeze at the coast (from Fig. 5) is 160 km(Millán et al., 2000). Typical evaporation rates over irrigated areason the coastal plains are from 5 to 7 L/m2 day, and over the moun-tain maquia from 1 to 3 L/m2 day. From these values, the water

vapour mixing ratio contributed by surface evaporation can be cal-culated to be of the order of 5–6 g/kg under the best conditions(Millán, 2008). This would yield a sum value 620 g/kg, whichwould be just marginal to produce condensation at 2000 m, asindicated in Fig. 14.

Heatings of 19 K were also observed (Fig. 13), which would re-quire a mixing ratio of P25 g/kg to keep the CCL at 2000 m. That is,an additional amount of 11 g/kg, which is nearly impossible toreach under present evapotranspitration conditions. With the low-er heating measured (13 K in Fig. 13), the airmass requires only anadditional 4 g/kg to reach condensation at 2000 m. It seems thatconditions in the past, with extensive forests and large coastalmarshes, limited the surface heating and provided enough mois-ture (Fig. 14A) to generate precipitation almost every summerday at the �1000 m altitude. In fact, the remains of extensive in-land marshes, and the long drainage channels that desiccatedthem, can still be seen in Barracas (Castellón, Spain) at the 980 maltitude, and 60 km inland. That situation kept a large amount ofwater recycling within the coastal system, i.e., including the evap-oration, the precipitation, the surface water and the acquifers.

The fact that summer storms did not seem to falter as muchover Italy (Section 1) was brought to the authors attention by Dr.S. Sandroni (of JRC-ISPRA) in 1979, during the EC Remote SensingCampaign in Turbigo, Italy (Sandroni and de Groot, 1980), andthe convergence of coastal airmasses towards the Apennines(Cantú and Gandino, 1977) was mentioned while planning theMECAPIP flights in 1987. In Italy the coastal circulations at eitherside of the mountains begin as a ‘‘closed’’ loop during the morning,

Fig. 12. Comparison between temperature profiles over the places with triangles at right. Measured (solid black line, time in brackets), and simulated (small circles line, timefull hour). These are two extreme cases where the modelling results clearly underestimate the temperature structure measured by the instrumented aircraft.

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Fig. 13. Range of temperature soundings measured over the Balearic Basin duringthe EC’s RECAPMA project in July 1991. This overlay includes the profiles with thehighest and the lowest potential temperatures observed. The dashed 303 Kadiabatic profile (30 �C from 0 m to 2000 m) is included as a reference. Weak bluelines are ozone concentrations.


until the two combined breezes converge over the Apennines inthe afternoon, or earlier if the added moisture is sufficient to reachthe CCL. This is different from the coastal mountains of EasternSpain, Southern France, and Northern Africa where the leadingedges of the combined breezes tend to encounter drier, more con-tinental air on the inland side (Figs. 4, 6 and 8). More frequentstorms keep the surface wetter, and this, in turn, favours the devel-opment of yet more storms on the following days (i.e., by generat-ing more evaporation and less heating).

It seems interesting to explore a concept from chemical engi-neering, i.e. the terms: ‘‘carrier’’, and reacting or ‘‘trigger’’ compo-nents for the water vapour in the combined breeze. For example, ifit is assumed that the maximum water precipitated from a moistairmass is 1/3 of the water vapour it contains. Then, the values inFig. 14 take a different meaning. That is, if the airmass requires amixing ratio of 21 g/kg to condense at 2000 m, the water vapourat the beginning of the breeze, i.e., 14 g/kg, can be considered asthe ‘‘carrier component’’, which is necessary to obtain the rightpercentages but is not sufficient for the condensation to occur.The additional 7 g/kg contributed by evapo-transpiration alongits path becomes the ‘‘trigger component’’ required for condensa-tion and precipitation to occur.

In this study area, the 7 g/kg value also happens to be theupper limit (1/3) of the amount of water that could be recov-ered by precipitation, i.e., from the �21 g/kg required for con-densation. This yields another interesting concept, since itimplies that the precipitated water is the same amount (or less)as the water evapo-transpirated along the path of the combinedbreeze. This would further suggest that this amount of water issimply recycled by this wind system under the right conditions.Finally, this raises yet another question concerning the conceptof water resources. For example, whether the 120 L/m2 per yearcontributed by the storms could be considered the product of15 L/m2 per storm-event, times 8 storm-events/year. Are we see-ing the same 15 L eight times? and, do we require the evapo-transpiration from the surface to see that amount of waterthose 8 times?

The most probable answer is yes, and if so, the concept of a free(God-given) water resource, which only needs to be managedproperly, appears to be one of the great fallacies of conventionalwisdom. In this case, it seems that the storms must be ‘‘cultivated’’

(A)

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Fig. 14. The summer shower/storm cycle on Western Mediterranean coasts. Note that the water evaporated at the coasts precipitates in the interior (see text) and, thus, thatthe real water cycle includes the moisture in the soil, the marshes, the river flow and the acquifer that feeds the coastal marshes; i.e., they are all part of the same ‘‘waterbody’’.

Fig. 15. Postulated feedback loops between land-use perturbations in the Western Mediterranean basin and the climatic-hydrological system from the local through theregional to the global scales. Beginning at the blocks Land-Use and Evaporation From Sea, the path of the water vapour is marked by dark blue arrows, the directly relatedeffects by black arrows, and the indirect effects by other colours. Critical thresholds are squared in red and indicate when the system tips to a different state. The various loopsare discussed in the text.



(Millán, 2012) in order to keep that amount of water recyclingwithin this system (Fig. 14). Similarly, in response to some of thequestions raised initially, one could also consider the amount ofwater vapour that crosses the European Divide as a carrier compo-nent that requires the additional water evaporated from within theMediterranean Catchment basin as the trigger component for therain events to occur. If this is so, how much of the water evapo-rated within the basin is returned by the precipitation? This couldbe another interesting topic for research and isotopic studies ofrainfall.

Finally, it seems that ‘‘closed’’ conditions in various parts ofthe Mediterranean Basin may have begun with the desiccationof coastal marches carried out by the Romans in Spain andnorthern Africa (Bölle, 2003a,b) to avoid malaria. This was fol-lowed by centuries of agricultural activities in the desiccatedareas, and deforestation and forest fires over the mountainslopes. More recently, explosive urbanisation along the coastshave sealed the soil with houses, asphalt, and cement, thusincreasing the heating of the airmass in the combined breezes.All of these have contributed to the expansion of the areas withclosed vertical circulations. The dramatic increase in tropo-spheric ozone in the late 1970s affecting crops in the Mediterra-nean (Naveh et al., 1978; Lorenzini and Panattoni, 1986)followed the intense industrialisation of the coasts (Refineries,Power Plants) after the 1974 Middle East war.

The atmospheric circulations becoming closed in more areas,through land-use changes, explains the accumulation of oxidantsand the ozone levels detected (Millán et al., 2000, 2002). Finally,other significant changes in the rain runoff records detected atthe end of the 1970s, i.e., the ‘‘1980 runoff anomaly’’ (Cabezas,2007), illustrated in Fig. 16, suggest that the atmospheric circula-tions over the coasts of the Western Mediterranean Basin in sum-mer had gone from being mostly open in the past (as they still areover Central Italy) to being closed on the majority of the coastsnowadays. This transition to a more closed system, i.e., to ‘‘therecirculating cauldron’’ (Section 7), leads to the accumulationmode for pollutants and water vapour, now detected by satellites,which probably occurred at the end of the 1970s. As a result, the1980 water runoff anomaly may just reflect the combined loss ofprecipitation from the Atlantic frontal systems and from the sum-mer storms, and that the focal areas for rains from Mediterraneancyclogenesis got closer to the coast (Section 2). This should also bestudied further.

Fig. 16. The 1980 water runoff anomaly as recorded by the Segura Watershed Aut(Universidad Politécnica de Cartagena, UPCT).

9. The accumulation mode and satellite water vapour data

As mentioned above, current numerical simulation (meso-scale) models, even when using 2 km � 2 km grids, do not captureall the details of the observed processes; e.g., they cannot repro-duce either the vertical recirculations or the layering over thesea. Nevertheless, the outcome of these processes, i.e., the develop-ment of an accumulation mode for water vapour (and pollutants)over the sea, can be validated with water column data from theNASA-MODIS satellite (Gao and Kaufman, 2003; King et al., 2003)since the year 2000. The concept is shown in Fig. 6, and was intro-duced in Section 5.

Fig. 17 shows the monthly averages of the NASA MODIS – Terrameasurements for July 2000 and 2005. The Day product derivedfrom the morning pass at 10:30 UTC emphasises the areas wherethe satellite looks down the deep orographic-convection chimneysdeveloping at the fronts of the combined breezes, i.e., around theedges of the basin (Figs. 6, 8 and 10). It also outlines the deep in-land penetration of the seabreezes over the desert areas of Tunisia,Libya and Egypt. The Day + Night product emphasises the areasover which water vapour accumulation occurs at the end of thediurnal vertical circulation cycle. That is, the water vapour thatdid not precipitate over the coastal mountains in the afternoon fol-lows the return flows aloft and fills in the Western MediterraneanBasin (to 4500 + m) with layers of water vapour (and pollutants).The same appears to occur over the Adriatic and Black seas.

This produces an ‘‘Accumulation Mode’’ in summer and, with-out requiring the high evaporation rates of more tropical latitudes,e.g., the Gulf of Mexico (Ulbrich et al., 2003), the mechanisms de-scribed in the text are able, in just a few days, to generate a verylarge, deep and polluted air mass that increases both in moisturecontent and in potential instability with each passing day. Assum-ing that the vertical circulation accumulation cycles last 3–5 days,and that they recur several times a month, e.g., for an average of 21recirculating days during July–August, it can be estimated that themonthly averages obtained from the MODIS-Terra Day + Nightproducts (e.g., Figs. 17 and 19) are comparable to the column val-ues of water vapour accumulated after approximately 3–4 days ofvertical recirculations.

The upper part of Fig. 18 shows two areas for which the dailyMODIS-Terra Day + Night product has been averaged for everyday of the year from 2000 to 2008. The results at the bottom partof the Figure show that there is an intense and prolonged accumu-

hority (Confederación Hidrográfica del Segura). Provided by Prof. Sandra García

Fig. 17. Monthly averages of the NASA MODIS-Terra water vapour data for July 2001 and 2005. The units are total precipitable cm.


lation mode over the Western Basin and the Black Sea in summer,and two weaker accumulation modes over the Eastern Basin inspring and autumn. This also brings out the point that meteorolog-ical processes are different in the various Mediterranean sub-basins.

10. What happens to the unprecipitated water vapour?

There are several significant consequences from the accumula-tion mode. The first effect can occur every few days, when themoist and polluted air masses are vented out of the area by anupper atmospheric perturbation. Under certain conditions theaccumulated water vapour can contribute to intense Vb precipita-tion events and summer floods in Central and Eastern Europe, asillustrated in Fig. 19 (Ulbrich et al., 2003) and by modelling inGangoiti et al. (2011a,b). The response time from the accumulationmode to the precipitation is of the order of days (to a week)(Gangoiti et al., 2011a). Along with Fig. 3, they illustrate the inter-connection between hydrological processes in Europe. That is, howa local loss of summer storms around the Western Mediterranean,caused by land use changes, leads to a local-to-regional verticalrecirculation-accumulation mode of water vapour over the sea,and how the accumulated water vapour can be advected out ofthe WMB and participate in major precipitation events, and floods,in other parts of Europe.

The second effect occurs during the vertical recirculation-accu-mulation periods. The greenhouse heating of the water vapour(Figs. 17 and 18), the photo-oxidants (ozone) and the aerosolsaccumulated in layers over the sea increases the Sea Surface Tem-perature during summer. This propitiates a second feedback loop,shown in Fig. 15, where Mediterranean Cyclogenesis fed by a war-m(er) sea (Millán et al., 1995; Pastor et al., 2001) contributes to an

increase in torrential rains and floods in Mediterranean coastalareas and islands, from autumn to spring. These effects have a de-lay of a few months with respect to the closing of the first loop thatleads to the accumulation mode.

In this second feedback loop, another ‘‘second critical thresh-old’’ or tipping point may then be crossed during intense Mediter-ranean Cyclogenesis events, if torrential rains set off flash floodsand/or mud flows over the mountains already affected by the firstfeedback loop, i.e., over drier and/or vegetation-deprived slopes.This can increase erosion and/or produce massive soil losses which,in turn, can further reinforce the first feedback loop, and the accu-mulation mode. In this context, the Accumulation Mode could beconsidered as ‘‘the memory of the system to land-use perturba-tions’’, since it can propagate its effects (desertification) throughthe increase in torrential rains to the same area, or (randomly, likein a Russian roulette) to other parts of the Mediterranean basin.

For example, around the Mediterranean sea, deserts and desert-like conditions are now found in proximity to a warm sea and,thus, to a marine airmass with a high moisture content, e.g., thecoasts of Algeria, Tunisia, Libya, and Almeria in Southeastern Spain.These regions were covered with vegetation in historical times,e.g., during the Roman Empire (Bölle, 2003a). In Almeria, the oakforests covering the mountains were cut down to fuel mines some200 years ago (Fletcher, 1991; Charco, 2002), mud flows followedin the early 1800s, leaving it the most desert-like area in Europe(Grove and Rackham, 2001). Thus, the question is: did these areasrun the two feedback loops towards drought and desertification asa consequence of desiccating the coastal marshes and removingthe forests?

The important point here is that the storms in the coastal sys-tems could be recovered, i.e., to stabilise the local-to-regional com-ponent of the hydrological cycle, but only if the second criticalthreshold, i.e., the massive loss of soil, has not been crossed. This

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Fig. 18. Top: Two areas in the Western (WMB) and Eastern Mediterranean Basin(EMB) used to average MODIS-Terra data. Bottom: Ordinate, average of the dailyDay + Night product for the years 2000 to 2008. It shows the differences betweenthe accumulation modes for water vapour in the East and West Basins.


would require judicious reforestation programs (Millán, 2012) to-gether with appropriate management of the water cycle at thesame scale, e.g., to cultivate the storms, and could be regarded asadaptation measures to climate change.

Fig. 15 shows a ‘‘third Atlantic-global feedback loop’’ with twobranches that could affect the North Atlantic Oscillation (NAO). Theoceanic component derives from losing the moisture accumulatedover the Western Mediterranean to feed summer floods in Central-Eastern Europe. It would favour the output of saltier water to theAtlantic, which has been considered an important tipping pointfor world climate (Kemp, 2005). The other path involves perturba-tions to the extra-tropical depressions and hurricanes in the Gulf of

Fig. 19. Left: Monthly average of the MODIS water vapour column; Day + Night product ffed rains in Germany and the Czech Republic on 11–13 August, 2002 (Ulbrich et al., 200

Mexico caused by changing the characteristics of the Saharan dusttransported across the Atlantic (Prospero, 1996), and this will bediscussed now.

During the vertical recirculating periods, new Atlantic air entersthe WMB every day (mostly at night), mainly through the Carcas-sone gap. Part of the vertically recirculated and aged airmass exitsto the Atlantic, either directly through Gibraltar and/or indirectlythrough the Sicilian Channel (mostly during the day). In the lattercase, the airmasses can follow the Southern Atlas Corridor towardsthe Canary Islands, and further to the Middle Atlantic, as illustratedwith satellite data in Fig. 20. This possibility was already postu-lated in 1995 (Millán et al., 1997) and was considered by the ECfor his research programmes (EC, 2001). This transport mechanism,and its continuity towards the Caribbean, has been simulated bymodelling (Gangoiti et al., 2006).

The airmasses that exit through the Sicilian Channel travelalong the southern Atlas corridor (Fig. 20) and provide a moist,and very polluted, background airmass. Moreover, the upslopewinds on the south-facing slopes of the Atlas also generate verticalrecirculations in which the added moisture would favour the for-mation of shallow clouds. NASA has detected dust layers of up to�7 km in height, topped by shallow clouds, over the southern Atlascorridor as early in the year as March (Winker et al., 1996). Theseclouds would provide the right environment for heterogeneousreactions between the Saharan dust and the pollutants carried bythe Mediterranean airmass, i.e., for the sulphatation and nitrifica-tion of the Saharan dust, which can be transported across the Cen-tral Atlantic towards the Caribbean, and could explain availabledata (Hamelin et al., 1989; Savoie et al., 1992, 2002).

It is also interesting that the southern Atlas corridor lies alongthe headwaters of one of the endorheic basins shown in Fig. 2. Thisappears to be another area where deforestation played a key role inthe loss of precipitation in the recent past. Fig. 20 shows largeamounts of water vapour following this path from the Mediterra-nean to the Atlantic, e.g., up to 40 mm over Southern Algiers. Thequestions are: did the massive deforestation of the Southern Atlas,to fabricate railroad ties in the 1800s, have anything to do withthis? For example, by eliminating or diminishing the evaporativecomponent required to trigger storms, as in Almeria, and has itgone through the second critical threshold of mud flows?

11. Comments concerning climate modelling and extremeevents

‘‘It is very likely that hot extremes, heat waves and heavy pre-cipitation events will continue to become more frequent’’. This is

or August 2002. The average is 3 cm (30 L/m2). Right: Back trajectories (type Vb) that3). This event took place over the European Continental Divide (Fig. 3).

Fig. 20. Averages of the NASA MODIS-Terra measurements for September 2002 and 2005. The Day product shows large amounts of water vapour along the Southern AtlasCorridor crossing from the Mediterranean to the mid Atlantic. This is now an endorheic basin (Fig. 2) where a series of ephemeral lakes remain witnesses to times of moreprecipitation.


a conclusion common to all IPCC Assessment Reports. However,‘‘although the ability of Atmosphere–Ocean General CirculationModels (AOGCMs) to simulate extreme events, especially hot andcold spells, has improved, the frequency and amount of precipita-tion falling in intense events are underestimated’’ (IPCC, 2007).Nevertheless, some of the observed anomalies and results of theseprocesses were indeed addressed both in the Fourth AssessmentReport AR4 (IPCC, 2007) and in the earlier Third Assessment ReportTAR (IPCC, 2001), as information resulting from EC research pro-jects filtered into the system. This can be considered relevant asAR4 Section 8.2.2.2 Horizontal and Vertical Resolution states that‘‘Changes around continental margins are very important for regio-nal climate change’’.

The development of large meso-scale circulations around large-enclosed (Mediterranean) or semi-enclosed (e.g., Sea of Japan) seascould underscore two other aspects mentioned in AR4 when refer-ring to Continental edge effects and lapse rates (8.6.3.1 Water Va-pour and Lapse Rate). This document states that ‘‘In the planetaryboundary layer, humidity is controlled by strong coupling withthe surface, and a broad-scale quasi-unchanged Relative Humidity(RH) response is uncontroversial’’. The uncontroversial part of thisstatement is questionable for any region outside flat terrains inmid latitudes.

The reason: if the edges of the continents wrap around a largeinterior sea like the Mediterranean, or a large semi-enclosed sealike the Sea of Japan, Continental Edge Effects blend together tocreate their own large meso-meteorological circulation and

dominate the weather patterns in these regions for months. Final-ly, the Continental Edge Effect becomes particularly important ifthe continental edges are backed by complex coastal terrains, thatis, coasts backed by high mountains up to 80(+) km from the sea.The reason for this, as related above, is that the combined breezesmake the coastal boundary layers fold over themselves, creating amultitude of ‘‘residual boundary layers’’ piled-up several km highover the enclosed seas. This accumulation mode could explainthe observed temperature lapse rates (Figs. 7, 12 and 13) and thewater vapour profiles, i.e., the perturbed temperature and RelativeHumidity (RH), observed over some coastal areas, and over largeinterior seas in sub-tropical latitudes.

12. Conclusions

To finalise, the following points should be considered regardingextreme hydrometeorological events and possible climaticimplications:

(a) Drought and torrential rains in areas around enclosed seas inthe subtropical latitudes (e.g. Mediterranean, Sea of Japan,South China Sea) are the result of a series of concatenatedmeteorological processes involving water vapour accumula-tion modes over the interior seas.

(b) These result from atmosphere–land–oceanic feedbacks andthe folding of the boundary layers over the seas, and, in par-ticular over the WMB.


(c) The same processes can also lead to intense precipitationevents and summer floods in other parts of Europe (i.e.,points along the European Continental Divide, or withinthe Mediterranean Catchment side of the Divide).

(d) Moreover, through the intensification of the Atlantic-Medi-terranean Salinity Valve at Gibraltar, the North AtlanticOscillation could be perturbed and affect precipitationregimes on the Atlantic side of the European ContinentalDivide.

(e) Therefore, the local-to-regional perturbations initiated byland-use changes at the local level may propagate theireffects to the Global Climate System.

(f) The basic atmosphere–land–ocean exchange governingthese processes, through BL parametrisations, is not (andprobably cannot be) included in current Atmosphere–OceanGeneral Circulation Models (AOGCMs).

(g) Thus, the feedback processes in the hydrological cycle,which govern the partitioning of precipitation and the recy-cling of water vapour, as well as the development of extremehydro-meteorological events, cannot currently be simulatedin the AOGC Models used to assess extreme events in sub-tropical latitudes.

This situation now presents the research community with veryimportant challenges to improve: first, meteorological forecastingmethods adapted to these processes, and second, Atmosphere–Land–Oceanic parametrisations in Atmosphere–Ocean General Cir-culation Models. It alerts that hasty decisions on the future of thewater cycle using current models could be wrong and have cata-strophic consequences not only for Southern Europe, but also forthe rest of Europe.

Acknowledgements

This paper was supported by the Generalitat Valenciana, andthe projects CIRCE (EC), GRACCIE (Consolider-Ingenio 2010, SNRP)and FEEDBACKS (Prometeo – Generalitat Valenciana).

The MODIS images used in this study were acquired with theGES-DISC Interactive Online Visualization and Analysis Infrastruc-ture (Giovanni) as part of NASA Goddard Earth Sciences (GES)‘‘Data and Information Services Center (DISC)’’. The author wouldlike to acknowledge Prof. Lucio Alonso at UPV-EHU (Bilbao) forpreparing all the satellite water vapour averages shown, as wellas other colleagues who have cooperated with the preparation ofthe Figures: R. Salvador (Figs. 4, 8–10 and 12), E. Mantilla (Figs. 5,7, 8, 11 and 13), M.J. Estrela (Fig. 1), J.L. Palau (Fig. 18), I. Gómez-Domenech, and G.Gangoiti (Fig. 12).

The author would like to dedicate this work to the memories ofDr. Heinrich (Heinz) Ott (�2004), for his encouragement and sup-port at the beginning of this work, and to Dr. Anver Ghazi (�2005), who continued his support in difficult times.

Appendix A

The first experimental data used for this work were obtainedduring the European Commission Campaigns on the Remote Sens-ing of Air Pollution in: LACQ (France, 1975), TURBIGO (Po Valley,Italy, 1979), and FOS-BERRE (Marseille, France, 1983). Additionalexperimental and modelling results come from the European Com-mission research projects: MECAPIP (1988–1991), RECAPMA(1990–1992), SECAP (1992–1995), T-TRAPEM (1992–1995), MED-CAPHOT-TRACE (1993–1995), BEMA I (1994–1995), VOTALP I(1996–1998), BEMA II (Phase II/1996–1998), VOTALP II (1998–2000), MEDEFLU (1998–2000), RECAB (2000–2003), ADIOS

(2001–2003), MICE (2001–2003), CARBOMONT (2001–2004),FUMAPEX (2002–2005), and CIRCE (2007–2011).

The author participated directly in all the projects underlinedand, through his membership in the EC Project Cluster EvaluationGroups, had first-hand access to the Progress Reports of the otherprojects listed. The ten projects in bold print used the Valencia-Castellón experimental area. The source of information and com-ments from projects on Desertification was Dr. Heinrich Ott (EC).The EC’s Joint Research Centre JRC-Ispra provided the funding forthe MECAPIP instrumented aircraft, the Commisariat à l’EnergieAtomique (CEA), Saclay, France, supported the RECAPMA flightsin the Western Basin, and the Israeli Air Force the SECAP flightsin the Eastern Mediterranean Basin. Complementary financing tothe EC funding was provided by the National Research Programsin the countries of the participating teams. In the case of FundaciónCEAM, co-financing of these projects was provided by Spanish Na-tional Research Program (SNRP, for the daily precipitation data),the Generalitat Valenciana, and BANCAJA.

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