The meteorological importance of sea-breezes in the Levant region of Spain

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The meteorological importance of sea-breezes in the Levant region of Spain 282 Jorge Olcina Cantos César Azorín Molina Climatology Laboratory, University of Alicante, Spain Sea-breezes are the main

1 The meteorological importance of sea-breezes in the Levant region of Spain 282 Jorge Olcina Cantos César Azorín Molina Climatology Laboratory, University of Alicante, Spain Sea-breezes are the main wind circulation during the hot season of the year (April September) in the Levant region of Spain (Fig. 1). This article reviews the atmospheric features and the main characteristics of these sea-breezes. It also highlights the role that these mesoscale wind circulations play in developing unstable sea-breeze fronts in the coastal hinterland and mountainous areas close to the Mediterranean coast. Atmospheric features of sea-breezes on the Mediterranean shore of Spain Sea-breeze circulations play an especially important role during the summer, when the wind circulation is determined by the formation of sea- and land-breezes on approximately 9 out of 10 days. Less well known are the different atmospheric circulations that trigger this system, including the appearance of sea-breeze fronts with diverse structures and dynamic behaviour. The genesis of unstable situations related to breezes and the development within them of precipitation is a major feature of the summer half of the year in the Levant. Unstable sea-breeze fronts provide 70% of the precipitation during June, July and August in the Valencia region. Although seabreezes occur with greater regularity and for longer periods during the hot months of the year, they can occur throughout the year along the Mediterranean coast of the Iberian peninsula. For example, in January, and especially in February, breezes are triggered on a large number of days when the westerly circulation is replaced by blocking anticyclones over western Europe. Generally speaking, we can establish a clear division of the wind circulation so that, while regional-scale winds (driven by the regional pressure distribution) are predominant in autumn and winter, in spring, and especially in summer, the leading role is played by local, periodic sea-breeze winds (see Table 1 and Figs. 2(a) and (b)). For the sea-breeze phenomenon to become significant, surface pressure gradients must be small. Quereda Sala and Montón Chiva (1994) pointed out that the sea-breeze circulation develops when the horizontal pressure gradient does not exceed 3 mbar 100 km 1, the normal situation during the hot season. In the Levant the sea-breeze is the most important wind, due Table 1 Utiel LLíria Villena Novelda Fig. 1 to the frequency with which it blows throughout the year (71% of total days).* In the coastal cities it is also the main climatic comfort mechanism because the inrush of the sea-breeze around midday arrests the Main direction and average speed of all winds in summer in the Levant region of Spain June July Aug.Sept. Alicante, Ciudad Jardín SE (10.4)* SE (9.9) SE (10.4) SE (10.6) Valencia, Els Vivers SE (8.7) SE (9.0) SE (9.2) SE (7.8) Castellón SE (7.2) SE (7.7) SE (7.2) SE (7.1) * Average wind speed (km h 1 ). Source: Pérez Cueva (1994). Morella Adzaneta VALENCIA ALICANTE CASTELLÓN Km *The average annual number of days in which the sea-breeze is triggered over the Campo de Alicante reaches 260 (based on observations from 2000 to 2002), this being 71% of the days of the year. Source: The doctoral thesis Breeze circulations: Causes and effects on the weather and climate in the Region of Alicante, by César Azorín Molina. MEDITERRANEAN SEA Location of the area studied the Spanish Levant region Altitude (m) < >1500

2 rise in the air temperature and improves ventilation. The main characteristics of sea-breeze calculations in the region are shown in Table 2. Figure 3 shows the influence of sea-breezes on daily air temperature and relative humidity in Alicante from 4 to 11 August 2003; the relationship with temperature, wind speed and direction, atmospheric pressure and synoptic situation is shown in more detail in Table 3. Main atmospheric effects of mesoscale breeze circulations (a) (b) Fig. 2 Frequency of wind direction in (a) May October and (b) November April. We highlight the importance of calms (C) in both May October and November April, and note the seasonal nature of wind direction between sea-breeze winds from the east and south-east (May October) and synoptic winds from the north-west, west and south-west (November April). Source: Pérez Cueva (1994). An important aspect of the sea-breeze is the extent to which it penetrates inland and its influence on the thermal and hygrometric features of inland areas. The friction and bending effects of the Coriolis force limit the capacity of the sea-breeze to penetrate inland. On the Mediterranean coast, this extension is facilitated by the existence of flat terrain close to the coast. In summer, the inland penetration of sea-breezes along the Mediterranean coast can reach over 100 km, due to the channelling of these circulations along valleys (Quereda Sala and Montón Chiva 1994). Between April and September it is not unusual for sea-breezes to enhance the development of valley-wind circulations. The latter help to reinforce the stable breeze fronts with their associated cloud formations (cumulus, altocumulus and stratocumulus). In general, sea-breeze circulations are associated with stable atmospheric conditions. However, the genesis of sea-breeze cells is, by its very nature, a thermoconvective process of conditional instability. This means that the genesis of cloud and breeze fronts is the result of a thermodynamic process of pseudo-diabatic evolution of surface flow up to the first m where the most important factor is the anticyclonic stability of the medium and high layers of the troposphere. This explains the appearance of the stratified medium- and low-level clouds typical of sea-breeze fronts, which remain on the horizon for a few hours and then disappear when surface feeding of the sea-breeze ceases to operate. At the beginning of the twentieth century, Fontserè (1917) spoke of summer convection winds when referring to the intense circulation of sea-breezes along the Catalonian coast. Recently, the importance of sea-breezes as significant elements of the regional atmospheric dynamics on the Spanish Mediterranean coast has been reviewed. Estrela and Millán (1994) underlined the importance of the seabreeze land-breeze mechanism in explaining changes in the general circulation of this geographical area. Their proposed revision of the so-called Iberian monsoon is that the breezes direct the circulatory system within the summer Iberian thermal low pressure system. They also underline the fact that study of the intensity and components of the breezes would help to identify the capacity for dispersing pollutants on the Mediterranean coast. This fact was also noted by Quereda Sala and Montón Chiva (1998) in their analysis of the atmospheric scenario of pollutants on the Mediterranean coast. As well as the amelioration of heat during fine weather, precipitation can be triggered in sea-breeze fronts if instability persists in the medium and high layers of the troposphere. The importance of unstable sea-breeze fronts in summer Despite the dominance of anticyclonic subsidence and, hence, atmospheric stability over the Iberian peninsula and the adjacent Mediterranean, summer rains do occur in the Levant, on occasions very heavily, particularly in mountainous inland regions. It does not therefore seem to be opportune to maintain the idea, widespread among those dealing with Spanish climatology, of the settled nature of the Iberian summer. Summer storms, perhaps incorrectly called heat storms, always require atmospheric instability, as well as a particular amount of sensible heat (varying from one weather situation to another) for their formation. Local topography can also favour their genesis and intensity. The Mediterranean coast is no exception and, as has already been mentioned, the formation of unstable seabreeze fronts plays an important role. In the Levant in July and August, 70% of the precipitation is due to the genesis of unstable sea-breeze fronts and the development of active storms within them. This percentage rises to almost 90% at some observatories in inland areas of the Valencia region, particularly in the Alicante sector, where all the precipitation of July and August is derived from these features (see Table 4). The fact that the percentage of summer precipitation attributable to unstable seabreeze fronts is higher in inland areas of the Sea-breezes in the Levant region of Spain 283

Sea-breezes in the Levant region of Spain Table 2 Atmospheric features of breeze circulations in the Levant region of Spain Timing of sea-breeze Mean daily duration of sea-breeze Timing of

3 Sea-breezes in the Levant region of Spain Table 2 Atmospheric features of breeze circulations in the Levant region of Spain Timing of sea-breeze Mean daily duration of sea-breeze Timing of land-breeze Transition periods Horizontal temperature gradient Trigger temperature for breeze initiation Trigger direction Rotation of sea-breeze In summer: active from approximately 0930 GMT until GMT In winter: active from GMT until GMT In spring and October: active from GMT until GMT Around 12 hours in summer Between 4 and 6 hours in winter From 8 to 10 hours in spring and autumn In winter: blows during the night for much longer than the sea-breeze In summer: normally weak and only appears for a few hours Brief periods of time between the start-up of the two breeze circuits (the sea-breeze land-breeze cycle) In summer: the land-breeze normally stops at between 0700 and 0800 GMT, while the sea-breeze is not triggered until 0930 GMT Necessarily positive. In general, more than 3 degc between the surface temperature of the Mediterranean Sea and the mainland. Varies from C in January and February to C in July and August In summer: the sea-breeze penetrates from ENE, E or SE In winter: the sea-breeze penetrates from S or SSW Salvador and Millán (2003) concluded that in Castellón the sea-breezes showed cyclonic rotation during the day in winter (from SSE to NE, contrary to typical breeze behaviour, which should adopt anticyclonic rotation due to the Coriolis force), whilst from May to August they acquire an anticyclonic daily rotation. Other researchers have pointed out that for Barcelona the daily evolution of the flow changes clockwise, so that the breeze is roughly parallel to the coast (Redaño et al. 1991). Average speed of sea-breeze In summer (April September): 10 km h 1 In winter: 8 km h 1 Maximum sea-breeze gust speed Between 20 and 35 km h 1, strongest in spring Maximum land-breeze gust speed Between 10 and 15 km h 1 in winter Thickness of circulation cell Reaches a thickness of between 1500 and 2000 m in summer Inland penetration Average: 50 km. However, with the help of the river valleys parallel to the sea-breeze flow, it can penetrate more than 100 km. Quereda Sala and Montón Chiva (1994) pointed out the possibility that intense sea-breeze circulations can penetrate 100 to 150 km inland, with vertical thicknesses of 4 to 5 km and average speeds between 3 and 4 m s (a) (b) Fig. 3 (a) Air temperature ( C) and (b) relative humidity (%) at the Meteorological Observatory, Climatology Laboratory of the University of Alicante, 4 to 11 August The inrush of the sea-breeze at midday arrests the rise in the air temperature (daily maxima 33 to 34 C) and brings with it a rise in humidity (minimum daily relative humidity 35 to 40% around 1200 local time). Note that the relative humidity scale is inverted. Valencia region, and above all in Alicante, than in inland areas of the Castellón region (Maestrazgo, Els Ports) is related to the influence of westerly circulations in Castellón. Indeed, tail ends of cold fronts that cross the north of Iberia may give some rain in the northern areas of the Valencia region. These fronts become active here because of the topographic configuration and the accumulation of sensible heat that occurs here in midsummer. Indeed, in inland areas of Castellón (such as at Morella), the summer half-year from April to September is wetter on average than the winter half-year. Generally speaking, unstable sea-breeze fronts provide most of the precipitation in inland parts of the Valencia region during the hot summer months. Days having rain in summer due to sea-breeze fronts tend to be characterised by the presence of small troughs with a sea-breeze regime on the surface related to topography, with little horizontal pressure gradient. In these cases, the formation of unstable sea-breeze fronts leads to the genesis of active storm cells in

4 Table 3 Main characteristics of the sea-breeze in Alicante, 4 11 August 2003 Time (GMT) Temperature ( C) Direction Speed (km h 1 ) Day Sunrise Trigger Stop Lag (1) (2) Duration Duration Breeze SST* Gradient Breeze Land- Breeze Max.Time of (1) (2) (hours: breeze land-breeze trigger (4) (3) (4) trigger breeze trigger gust max. gust min) (hours:min) (hours:min) (3) (degc) (GMT) :25 13:00 8: E WNW :54 10:30 6: ENE WNW :53 10:30 7: ESE WNW :52 10:30 5: SE WNW :51 10:00 6: SSE WNW :20 10:30 8: ESE WNW :19 11:30 9: E WNW :18 8:30 5: SSE WNW Component Atmospheric pressure (mbar) Humidity (%) Day Direction Type Breeze Min.Min. Lag Max.Max.Min.Min.Synoptic situation (surface analysis) trigger (5) (6) time (5) (6) time time (GMT) (GMT) (GMT) 4 E ESE ENE Mixed Relatively low pressures 5 ENE Static Relatively high pressures 6 ENE Static Relatively high pressures 7 SE NE Cyclonic Flat barometer 8 SSE ENE Cyclonic Flat barometer 9 ESE ENE Cyclonic Flat barometer 10 E NE Cyclonic Flat barometer 11 SSE NE Cyclonic Relatively high pressures Sea-breezes in the Levant region of Spain *Sea surface temperature. Source: Data extracted from the climate database of the University of Alicante Climatology Laboratory meteorological observatory. Based on the doctoral thesis Breeze circulations:causes and effects on the weather and climate in the Region of Alicante, by César Azorín Molina. Research funded by the Ministry of Education, Culture and Sport. Table 4 Summer precipitation and stormy days at meteorological observatories in the Valencia region Average summer Observatory precipitation, Average No. of days July Aug. (mm) with storms in summer Coast Alicante Valencia Castellón Pre-coastal Novelda Llíria Atzeneta del Maestrat Inland Villena Utiel Morella Source: Created from data obtained from the National Institute of Meteorology and Pérez Cueva (1994). their cores that cause occasionally significant rainfall in the early afternoon. The onshore wind within the breeze cell helps to move the warm, moist air into inland areas, but it is the presence of advection sectors with high vorticity in the upper troposphere that establishes the storms and their movement towards the north-east. One point of interest is the the behaviour of the maximum temperatures recorded during the days prior to the triggering of the rains; the accumulation of sensible heat plays an important part in the build-up to major convective storms. This is typically related to the arrival of tongues of warm, often tropical, continental air of Saharan origin. This enhances the absolute instability of the troposphere by increasing the thermal gradient between the surface and the mid-troposphere. Conclusion a classification of unstable sea-breeze fronts Two types of unstable sea-breeze front can be identified: (i) Ordinary active sea-breeze fronts: those giving rise to very intense, short-lived showers rarely exceeding 30 mm of precipitation in an hour. This is the most common type of front in mountainous areas of the Levant during summer, leading to downpours which also contribute towards local cooling. (ii) Extraordinary active sea-breeze fronts: these are truly significant meteorological episodes, as rainfall can exceed 150 mm and can even approach 200 mm in two or three hours. These situations are comparable with the intense cloudbursts 285

5 Sea-breezes in the Levant region of Spain that affect many sectors of the coast of the Levant in the late summer due to the formation of cold pools in the upper troposphere with an easterly circulation on the surface. Both types of unstable sea-breeze front have common features; they involve the formation of storm nuclei that precipitate heavy rain, sometimes accompanied by hail. In addition, the developing clouds form around midday, reach maturity during the afternoon, when the precipitation occurs, and dissipate at nightfall due to the disappearance of the sea-breeze cell. It is therefore during the afternoon when the sea-breeze really takes centre stage, as shortly after midday it becomes stronger and penetrates inland, which involves moist sea air overflowing into the natural inland depressions and valleys. A process of interaction then takes place between the seabreeze and the anabatic circulations that operate on the intensely heated slopes facing the sun an important mechanism for generating rainfall over the inland and mountainous areas of the Levant region. References Estrela, M. J. and Millán, M. (1994) Manual práctico de introducción a la meteorología. Centro de Estudios Ambientales del Mediterráneo, Valencia Fontserè, E. (1917) Sobre els vents estivals de convecció a la costa catalana. Arxius de l Institut de Ciències, 3, pp Pérez Cueva, A. J. (Ed.) (1994) Atlas Climático de la Comunidad Valenciana. Consellería de Obras Públicas, Urbanismo y Transportes de la Generalitat Valenciana, Valencia Quereda Sala, J. and Montón Chiva, E. (1994) Los vientos de superficie en el litoral de Castellón. Ediciones de la Caja Rural Credicoop, Castellón (1998) El escenario atmosférico de los contaminantes sobre el litoral mediterráneo. In: Férnandez García, F., Galán Gallego, E. and Cañada Torrecilla, R.(Eds.) Clima y ambiente urbano en ciudades ibéricas e iberoamericanas, Editorial Parteluz, Madrid, pp Redaño, A., Cruz, J. and Lorente, J. (1991) Main features of the sea-breeze in Barcelona. Meteorol. Atmos. Phys., 46, pp Salvador, R. and Millán, M. (2003) Análisis histórico de las brisas en Castellón. Thethys, Asociación Catalana de Meteorología, pp Correspondence to: César Azorín Molina, Laboratorio de Climatología, Instituto Universitario de Geografía, Universidad de Alicante, Campus de San Vicente del Raspeig, Alicante, Spain. cesar.azorin@ua.es Royal Meteorological Society, doi: /wea Meeting report Water land atmosphere interactions It may seem surprising to the layman that the academic disciplines of hydrology and meteorology are not inextricably intermingled, but a joint British Hydrological Society and Royal Meteorological Society (RMetS) discussion meeting, entitled Water land atmosphere interactions, held on 17 March, demonstrated that they are still disconcertingly far apart. The meeting was opened and chaired by Chris Collier (University of Salford), Vice- President of the RMetS, and then introduced by Nick Chappell (University of Lancaster) with a talk entitled An introduction to coupling meteorology and hydrology, which was designed to show how research in meteorology can further hydrological research and vice versa. Nick identified four areas where new meteorological research is assisting hydrology. Firstly, there is value in improving understanding of rainfall, as its spatial and temporal distributions are the key control for river discharges; hence more accurate rainfall estimates lead to better flood prediction. Secondly, the latest understanding of climate dynamics is vital (e.g. ENSO), as this can mask or magnify the effects of land-use changes on the hydrological system. Thirdly, the meteorological data available to the hydrologist have been vastly improved (such as improvements in satellite and radar data), whilst merged precipitation datasets (e.g. the Global Precipitation Climatology Project) have improved the description of global precipitation. These new data are a useful tool for general circulation model (GCM) evaluation and for continental-scale hydrological analysis (e.g. the Global Energy and Water Cycle Experiment). Finally, the frequent use of GCMs is forcing hydrologists to think about the scaling issues involved with describing processes at much larger scales than previously. Hydrologists are beginning to supply meteorologists with a new understanding of large-scale hydrological phenomena and new large-scale river flow data; although issues remain with regard to large-scale evaporation and subsurface water data, which are central to the evaluation of GCMs. The increased collaboration between hydrologists and meteorologists has advanced research considerably; however, there are still issues to be resolved: (i) The quality of the GCM-derived water fluxes within the tropics is far from ideal. (ii) At the regional scale, there is a lack of agreement between rainfall databases. (iii) Land-surface schemes are oversimplified and, consequently, incorrectly generate large quantities of overland flow. (iv) No large-scale evaporation datasets. (v) Scaling issues need further research. Subsequent presentations addressed some of these remaining issues. David Grimes (University of Reading) gave the first specialist talk entitled Application of satellite-based rainfall estimates to riverflow forecasting in Africa. This presentation discussed the use of satellite rainfall data to drive a river-flow forecasting model for the Bakoye catchment, Mali, where the intense localised convectional rainfall is seasonally variable due to the intertropical convergence zone, and events are frequently missed by the raingauge network. Satellite-derived rainfall data may be used to drive the hydrological model, as an alternative approach to raingauges. David investigated driving the model with infrared (IR) satellite data and some extra meteorological information supplied by the European Centre for Medium-Range Weather Forecasts (ECMWF) re-analysis dataset. This method uses a typical IR algorithm, which relates cloud-top temperature to rainfall. A second method adds extra information on the storm type and the phase of the African easterly wave to the algorithm (vertical wind speed and near-surface relative humidity were found to have little relationship with rainfall). A phase number was assigned to each position on the easterly wave, with 286

Torrential events on the Spanish Mediterranean coast (Valencian Region). Spatial precipitation patterns and their relation to synoptic circulation

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