A study of the breeze circulation during summer and fall 2008 in Calabria, Italy

A study of the breeze circulation during summer and fall 2008 in Calabria, Italy
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  A study of the breeze circulation during summer and fall 2008 inCalabria, Italy Stefano Federico a,b, ⁎ , Loredana Pasqualoni c , Leonardo De Leo b , Carlo Bellecci b,c a Institute of Atmospheric Sciences and Climate  —  Italian National Research Council (ISAC-CNR), Section of Lamezia Terme, Italy b CRATI Scrl, Rende, Italy c Engineering Department, University of Rome  “  Tor Vergata ”  , Italy a r t i c l e i n f o a b s t r a c t  Article history: Received 13 November 2009Received in revised form 16 February 2010Accepted 23 February 2010 We present a study of the characteristics and importance of breezes at a coastal site in theCentral Mediterranean Basin. The site is located on the west coast of the Peninsular CalabriaRegion at the southern tip of Italy. This study adds new data on breeze circulations over aunique experimental coastal site characterized by the sea – land contrast in a  󿬂 at area which isalso in 󿬂 uenced by the complex orography of the region.The  󿬁 rst part of the study compares data from a surface meteorological station andmeteorological analysis at 850 hPa to show the importance of breeze circulation at the site.Results show that breezes dominate the local circulation and play a major role in the localclimate. Moreover, the characteristics of the breeze exhibit signi 󿬁 cant differences betweensummer and fall.The second part of the paper examines the thermal and large-scale forcings that play animportant role in the breeze circulation. A sharp difference in thermal forcing was observedbetweenJuly – AugustandSeptember.Thediurnalbreezeactsinphasewiththelarge-scale 󿬂 owand is particularly intense in summer. The modulation of the nocturnal breeze by the large-scale  󿬂 ow is also apparent.The third part of the paper focuses on two selected case studies one for summer and the otherforfallandexplainsdifferencesinthebreezecirculation.Finally,thebehavioursofsummerandfall breezes are presented and discussed.© 2010 Elsevier B.V. All rights reserved. Keywords: Sea and land breezesUpslope and downslope  󿬂 owsLarge-scale forcingThermal forcing 1. Introduction The sea breeze blows inland across the coast on  󿬁 neweather days. It is caused by the horizontal temperaturecontrast between the sea (cool) and the land (warm) duringdaytime. The temperature difference produces a pressuregradient between the sea and the land that forces the seabreeze. The opposite circulation occurs at night, when thehorizontaltemperaturegradient betweenthe land (cool) andthe sea (warm) causes the land breeze to  󿬂 ow. The observedsea breeze intensity, which can reach 10 ms − 1 , variesdepending on several factors including thermal forcing,large-scale winds, atmospheric stability, cloud cover, landuse, etc. The land breeze is a shallower and weakerphenomenon because the planetary boundary layer (PBL)over the land is stably strati 󿬁 ed at night. Defant (1951) andPielke (1981) present an excellent qualitative description of sea and land breeze development, in the absence of large-scale wind.In a region with irregular terrain, local wind patterns candevelop because of the differential heating between thegroundsurfaceandthefreeatmosphereatthesameelevationsome distance away. A larger diurnal temperature variationusually occurs at the ground, so that, during the day, elevatedterrain becomes a heat source, while at night it is a heat sink.As a consequence of the temperature gradient generated, an Atmospheric Research 97 (2010) 1 – 13 ⁎  Corresponding author. ISAC-CNR, c/o CRATI scrl, zona Industriale areaex-SIR, comparto 15, 88046 Lamezia Terme (CZ), Italy. E-mail address: (S. Federico).0169-8095/$  –  see front matter © 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.atmosres.2010.02.009 Contents lists available at ScienceDirect Atmospheric Research  journal homepage:  upslope  󿬂 ow (anabatic wind) develops on fair weather days.At night, downslope (katabatic) winds develop.Because of the speci 󿬁 city of the breeze circulation at aparticular site, many papers and books can be found in theliterature on the sea breeze and local winds, some of themreported in Simpson (1994) and Pielke (2002).This study investigates local circulation at a site on theCalabria Peninsula (Fig. 1). Calabria extends between 38° and40° N latitude and between 15°30 ′  and 17°15 ′  E longitude.The region is bounded by the Tyrrhenian Sea (west) and bythe Ionian Sea (east and south). The Apennines run north – south along the peninsula and are characterized by  󿬁 ve maintopographical features reaching elevations of 1500 to2000 m:Pollino,CatenaCostiera,Sila, Serre,andAspromonte.The average width of the region is about 50 km in the east – west direction and 300 km in the north – south direction.There are three main plains by the sea (Sibari, Gioa Tauro,Lamezia).The physical characteristics of the peninsula, i.e., thepresence of elevated peaks in a symmetric position betweenthe Tyrrhenian and the Ionian Sea, present a rather uniquecondition for breeze development. Sea breezes and landbreezes act in phase with upslope and downslope winds togeneratestrongerandmorepersistentbreeze 󿬂 ows.Federicoetal. (2000, 2003), using a theoretical and modelling approach,studied the relative role of the sea – land and mountain – valleycontrasts in the local thermal convergence pattern over anidealized mountainous peninsula. They showed that thepresence of mountains strongly enhances the wind intensityand, for anidealized peninsula withthe aspect of Calabria (i.e.,east – west width and mountain elevation), the energeticcontribution of the mountain – valley contrast is larger thanthat of the sea – land contrast. Therefore, the local circulation iscaused by the presence of the sea but also by the presence of elevated peaks. Because we cannot distinguish the differentbreeze components (sea/land/mountain/valley), hereafter wewill call them diurnal (onshore) and nocturnal (offshore)components of the breeze.The Mediterranean climate is another important factorthat favours breeze development in Calabria. Calabria'sgeographical position is at the centre of the Mediterranean,where calm synoptic conditions often prevail (Trigo et al.,1999, 2002; Bolle, 2003), especially in summer and fall(Colacino, 1990; Piervitali et al., 1999). The favourableconditions for breeze development in the Mediterraneanhavebeendiscussedandreportedinseveralpapers,includingNeumann (1951) and Doran (1979) for Israel; Lalas et al.(1982)andKallosetal.(1993)forGreece;Cacciaetal.(2004)for France; and Ferretti et al. (2003) and Mastrantonio et al.(2008) for Italy.While previous studies over Calabria focused on theenergetic and mesoscale simulation of the breezes, this paperfocuses on experimental data collected at Lamezia Terme(Fig. 1). Fig.1. Calabria RegionintheCentralMediterranean.Left:topography(m,greyshading)withtopographicalfeaturesdescribed inthetext.Theblack dotshowsthelocation of the experimental  󿬁 eld.2  S. Federico et al. / Atmospheric Research 97 (2010) 1 – 13  The study does not follow a planned measurement campaignbut uses measurements from different sources that eitheroperated for a while or are permanently installed at the site.The experimental site is located 8 m above sea level and500 mfromthecoastline.Themountainsaroundtheplainareabout 10 km from the experimental site. Table 1 shows theoperational time for each instrument and Appendix  A  introduces the instruments and data.The aim of the study is to show the change in thecharacteristicsofthebreezecirculationbetweenSummerandFall 2008. For this purpose we show the characteristics andthe vertical structure of the breeze circulation for selectedcase studies and we show statistics for July – August andSeptember 2008, limiting the analysis to this period becauseof the availability of SODAR (Sonic Detection and Ranging)data. 2. Results  2.1. Breeze circulation for July –  August and September 2008 This section shows the importance of breezes to the localclimate.Fig.2(a)showsthatthesurfacestationwindrosefor July – August 2008. Winds come mainly from the W – SW direction(from 220° to 280°), which account for 65% of occurrences.A relative maximum (14%) is found in the E – NE direction(from40° to 100°). Windspeed is low,with morethan 90%of occurrences having speeds less than 5 ms − 1 .Fig.2(b) showsthatthewindroseat 850 hPafor thesameperiod as Fig. 2(a) derived from the ECMWF (EuropeanCentre for Medium-Range Weather Forecasts) operationalanalysisat850 hPa(Uppalaetal.,2005).Thesedata,availableat 6-h intervals, are representative of the large-scale circula-tions above the PBL. About 60% of the winds come from theNW sector. Apart from the obvious higher wind speed at850 hPa, the comparison between Fig. 2(a) and (b) revealstheimportanceof localcirculations because:a)the directionsin Fig. 2(a) are well focused (W – SW and E – NE) while at850 hPa they are more diffuse; b) the E – NE winds are morefrequent at the surface than at 850 hPa (5% and 14%,respectively);andc)theW – SWwindsaremuchmorefrequentat the surface than at 850 hPa (10% and 62%, respectively).Fig. 2(c) and (d) shows the same distributions of  Fig. 2(a) and (b) for September 2008. Local circulations are apparent.More precisely:a) winds come mainlyfrom the W – SW (33%)and E – NE (48%) directions at the surface while at 850 hPathey are more diffuse; and b) the E – NE winds are much morefrequent at the surface than at 850 hPa (1% and 48%,respectively).The comparison between Fig. 2(a) and (c) shows animportantchangein thecirculationatthesurface:in summerwinds come mainly onshore (65%) while in September theirmost frequent direction is offshore (48%).Thebreezeplaysa majorrolein thelocalcirculation. Fig.3(a) shows the distributions of wind direction and intensity asafunctionofthetimeofdayinJuly – August.WindsblowfromW – SWinthemid-morningandafternoon.Atthenightwindsblow from two main directions: E – NE and W – SW. Theirfrequency of occurrence is similar. The nocturnal breezecirculation accounts for the E – NE component. The large-scale 󿬂 ow accounts for the W – SW component. This componentwill be discussed in Sections 2.2 and 2.4. The diurnal breezedevelops between 8 and 23 h local standard time (LST).Fig. 3(a) shows the higher wind speeds associated withthe onset of the diurnal breeze regime in the mid-morningand afternoon hours, a typical condition of the breeze regime(Pearson, 1973, 1975; Pielke, 1974, 2002; Simpson, 1994).Fig. 3(b) shows the distributions of wind direction andintensity as a function of the time of day in September. Therole of the breeze is apparent: winds blow from W – SW in themid-morning and afternoon and from E – NE at night. Windspeed is higher in the afternoon. The diurnal breeze developsbetween 09 and 21 LST.  2.2. The role of SST and large-scale  󿬂 ow from 1 July to 30 September 2008 Breezes depend on several physical factors, includingthermal forcing, synoptic-scale  󿬂 ow, atmospheric stability,soil and vegetation characteristics, etc. Among these, thethermal forcing and synoptic-scale  󿬂 ow are particularlyimportant and are studied in this section.Fig. 4 shows the daily difference between the sea surfacetemperature (SST) of the St. Eufemia Gulf (Fig. 1; Appendix  A  explains how it is computed) and surface temperaturerecorded at the experimental site between 11 and 16 LST(diurnal difference) and between 00 and 06 LST (nocturnaldifference).The diurnal difference is negative (i.e., the land is warmerthan the sea) for July and August but it is positive for severaldays in September. The temperature difference averages are − 3,  − 4, and  − 1 °C for July, August, and September,respectively.The nocturnal difference between the SST and surfacetemperature is always positive with the exception of a fewdaysinsummer.Theaveragetemperaturedifferenceis2 °Cin July and August and 6 °C in September.From Fig. 4 it is apparent that the thermal forcing of thediurnal breeze is larger in July and August while the thermalforcing of the nocturnal breeze is larger in September. Whilethis result is expected, we highlight the substantial change inthe thermal forcing between July – August and September.  Table 1 Instruments, parameters measured, and measurement period.Instrument Data Measurement periodSurface MeteorologicalstationWind speed anddirection, temperature,global radiation,precipitation andrelative humidity,pressure, soiltemperature(10 cm depth).From 1 January 2007to 31 December2008. Permanentlyinstalled.SODAR Vertical pro 󿬁 les of wind componentsand turbulence.From 1 July to 30September 2008.RADAR WP Vertical pro 󿬁 les of the three windcomponents.From 1 June 2008.Permanentlyinstalled.3 S. Federico et al. / Atmospheric Research 97 (2010) 1 – 13  This feature is consistent with the change in the breezeregime between summer and fall recorded at the site.To quantify the impact of large-scale  󿬂 ow and channellingthrough the Marcellinara gap on local circulation, we use the850 hPawinddistributionderivedfromtheECMWFanalysis.Toavoid thein 󿬂 uenceof topography, we analyzetwo grid points:15.5° E, 39° N and 17° E, 38.5° N. The  󿬁 rst point is over theTyrrhenian Sea west of Lamezia Terme; the second is over theIonian Sea, near the eastern entrance to the Marcellinara Gap.The  󿬁 rst grid point is used to evaluate the frequency of windswith directions between 210° and 330° (NNW – SSW, hereafterwesterlies) and the second is used for winds with directionsbetween 30° and 160° (NNE – SSE, hereafter easterlies).Table 2 shows the occurrence of westerlies and easterliesfor wind speeds greater and less than 8 ms − 1 . This thresholdwas chosen because it has a major effect on the breezecirculation pattern for the largest thermal forcing involved inLamezia Terme breezes (Lyons, 1972). Large-scale  󿬂 owusually acts in phase with the diurnal breeze in all threemonths because westerlies are far more frequent thaneasterlies. At the same time, it acts against the nocturnalbreeze. Weak westerlies ( b 8 ms − 1 ) are more frequent inSeptember compared to summer while intense westerlies( N 8 ms − 1 ) are more frequent in July. Weak easterlies aremore frequent in summer and negligible in September, nointense easterlies were found for the period considered.Considering the above results for thermal and large-scaleforcing it follows that:a) the large-scale  󿬂 ow cannot account for the high percent-age of easterlies recorded in September 2008, which aredue to the nocturnal breeze; andb) for July – August 2008, because the thermal forcing of thenocturnal breeze is weak, westerlies often suppress thenocturnal breeze development (about 50% of the time).These issues are further investigated in Section 2.4.  2.3. Two selected case studies for summer and fall The “ circulationoftheday ” wasclassi 󿬁 edfrom1Julyto30September by a semi-objective method. The method isobjective because measurements must satisfy preciserequirements. Nevertheless, these requirements are de 󿬁 nedafterasubjectiveanalysisofallthedays.Daysweredividedintwo classes: complete and incomplete breeze circulation(Table 3). Complete breeze circulation days must satisfy thefollowingrequirements:a)surfacewindmeasurementsshowboth the diurnal (onshore) and nocturnal (offshore) breezecomponents; b) winds at the  󿬁 rst (35 m) and second (45 m)SODAR level show both the diurnal and nocturnal breezecomponents; c) the nocturnal breeze component blows for atleast two consecutive hours at the surface and at  󿬁 rst and Fig.2. (a)WindroseatLameziaTermesurfacestationforJuly – August2008.Colourshadingshowsthewindintensity(ms − 1 );(b)asin(a)butforECMWFanalysisat 850 hPa; (c) as in (a) but for September 2008; and (d) as in (b) but for September 2008.4  S. Federico et al. / Atmospheric Research 97 (2010) 1 – 13  second SODAR levels; and d) no precipitation is recorded.Incomplete breeze circulation days are those not satisfyingthe requirements a – d.We show measurements for two selected days withcomplete breeze circulation, one for summer and the otherfor fall.Fig.5showstheevolutionofthemeteorologicalparametersmeasuredbythesurface station on11July. Themeasurementsshow a bell-shaped global radiation. No precipitation wasrecorded. Surface temperature followed the radiative forcingwith values between 21 and 29 °C, while relative humidityranged from about 65% (evening and night) to 80% (mid-morning and afternoon).Wind speed and direction followed the typical diurnalevolution of breeze circulation days at the site. Wind speedwas less than 1 ms − 1 during the evening and night, while itincreasedto4.0 ms − 1 intheafternoon.Nocturnalwindswerefrom E – NE (i.e., offshore) but, between 08 and 09 LST, theyshifted to W – SW (i.e., onshore), causing the onset of thediurnal breeze (note also the sharp gradient in temperatureassociated with this shift).Fig. 6 shows the SODAR measurements of the zonalcomponent. This component is perpendicular to the along-coastline direction and it is the best component for observingthe breeze development. For this instrument, time is given inUTC (LST=UTC+2 h). Below 100 m, the nocturnal offshore Fig. 3.  (a) Frequency of wind direction (upper graph) and of wind speed (lower graph) as a function of time of day (LST) recorded at Lamezia Terme for July – August 2008. Grey shading shows the frequency of occurrence (%); and (b) as in (a) but for September 2008.5 S. Federico et al. / Atmospheric Research 97 (2010) 1 – 13
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