The impact of wind on air temperature distribution in Athens and in Santorini

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1 Interntionl Conference Pssive nd Low Energy Cooling 133 The impct of wind on ir temperture distribution in Athens nd in Sntorini I. Livd, K. Nichou nd M. Sntmouris Lbortory of Meteorology, Section of Applied Physics, Physics Deprtment, University of Athens, Greece ABSTRACT Reserch studies of therml comfort in buildings im mostly t mintining ir temperture, especilly during summer period, within the cceptble comfort conditions nd with the minimum energy consumption. The contribution of wind is known to be n importnt prmeter for nturl ventiltion nd pssive cooling of buildings. In the present work the correltion between the vrition of wind nd ir temperture is studied for two different meteorologicl sttions in mjor Athens re, where the het islnd effect exists. The sme study is lso performed for nother sttion in Sntorini islnd nd comprison nlysis between the two regions is mde, since Sntorini is not exposed t the urbn environment nd furthermore it is directly ffected by the etesin winds nd the vicinity of the se. 1. INTRODUCTION The increse of living stndrds in buildings nd the persistence of high mbient temperture vlues during summer especilly in big cities hve incresed the instlltion of irconditioning systems nd consequently the energy requirements for cooling. Anlysis of summer ir tempertures nd wind velocities is thus necessry in order to provide pproprite informtion to building designers. Clcultion of cooling lod, pproprite sizing of AC systems nd interction of pssive nd hybrid cooling techniques nd components within the building re directly relted with the vilbility of locl climtic conditions, of which mbient temperture nd wind velocity re the most importnt prmeters. Tselepidki nd Sntmouris (1991) reported tht the frequency of high summer ir temperture vlues exceeding o C in Athens during June to September rnges from 0.32 to Besides the persistence of existing consecutive dys with high ir tempertures ws found sttisticlly significnt during this period. Especilly for July nd August it ws estimted tht cooling is necessry for more thn five hours per dy. Blrs et l. (1992, 1993) estimted the cooling power index for Athens, which ws proposed by Vinje. This index is bsed on ir temperture nd wind speed vlues nd corresponds to humn senstion. It ws found tht the occurrence of hot conditions over dy is sttisticlly significnt during summer period. The previling high ir tempertures together with low wind speeds ssist the persistence of hot conditions of high number of consecutive hours. Tzikopoulos et l. (04) ttempted to model the energy efficiency of bioclimtic buildings bsed on the estimted degree dys of Greece nd other Mediterrnen res nd the rest of Europe. The purpose of the present study is twofold. The cooling degree hours, DH(26), re estimted for two sttions in the extended Athens re nd for n insulr sttion in Sntorini islnd. Besides the impct of wind on the observed ir temperture distribution is nlyzed during June to September for five yer period (00-04). 2. METEOROLOGICAL DATA The study of hourly ir temperture nd wind speed vlues is of concern not only on tempo-

134 Interntionl Conference Pssive nd Low Energy Cooling rl but lso on sptil bsis. Tselepidki et l. (1992, 1994) observed importnt differentitions in neighbor regions in mjor Athens re.

2 134 Interntionl Conference Pssive nd Low Energy Cooling rl but lso on sptil bsis. Tselepidki et l. (1992, 1994) observed importnt differentitions in neighbor regions in mjor Athens re. However, comprison nlysis between summer conditions in Athens nd in islnds is lso very interesting. Livd et l. (1998) found tht the men return period of dily mximum ir tempertures, exceeding 26 o C is bout two dys in Cycldes for the period My to September. In the present work the hourly ir temperture nd wind speed dt were used for the period from June to September of the Ntionl Observtoty of Athens (NOA), s well s, of Ellinikon sttion tht it is locted in rurl re ner Sronikos gulf. Also nother islnd sttion is studied which is plced in Sntorini, in the southern prt of Cycldes (Fig. 1), where strong winds nd the bsence of het islnd effect re the most predominnt chrcteristics. The selection of the exmined period, 00-04, is bsed on the fct tht in ll studied sttions the men monthly ir tempertures were 0.5 o C to 3.5 o C greter thn the corresponding verge climtologicl vlues (Tselepidki et l., 1993). 3. ANALYSIS OF DEGREE HOURS The number of cooling degree hours with reference 26 o C, DH(26), is given in Tble 1 where it Figure1: Position of the three meteorologicl sttions in Athens nd in Sntorini islnd. is shown tht NOA is chrcterized by greter vlues from % up to 62% in comprison with Ellinikon sttion tht is plced in n open re djcent to the se t distnce of Km from NOA. In spite of the fct tht NOA is plced in n re with reltive vegettion, on the top of 417 meter high hill, its position ner the centre of Athens, mkes it exposed to the het islnd effect (Livd et l., 02). The estimted DH(26) vlues for NOA sttion nd for the period re 1.9 to 2.7 greter thn the corresponding vlues for the period (Tselepidki et l., 1993). This cn be explined by the higher ir tempertures nd the increse of the het islnd effect. In ddition, compring the DH(26) in Ellinikon sttion with the corresponding vlues in Izmir, in Turkey (Stmn nd Ylcinky, 00), which is plced in by of Aegen se t lmost the sme ltitude with Ellinikon, it ws found tht the DH (26) re of the sme order for the two sttions. The observed smll number of DH(26) vlues in Sntorini shows tht the bsence of the het islnd effect fvors the predominnce of lower ir tempertures during summer. Moreover the vicinity to the se, where the surfce tempertures rnge between 22 o C nd o C (Theohrtos nd Tsepidki-Livd, 1990) results in lower mbient temperture vlues in the whole region. Seeing tht during summer the locl Etesin winds (dily strong winds) re predominnt in Cycldes, it is rised question whether these winds fvor the mintennce of lower ir tempertures. Thus, the reltion between the mbient tempertures nd wind velocities is nlyzed in the following prgrphs. 4. IMPACT OF WIND ON AIR TEMPERA- TURE DISTRIBUTION The reltion between wind speed nd ir temperture ws studied for the totl dt set nd thereinfter for ech wind direction. Nmely, in Athens existing dt were clssified in three different dt sets bsed on wind direction reltive to the rel North (293 o - o, 46 o -1 o nd 136 o o ), while clms (wind speed lower thn 0.3m/s) were nlyzed seprtely. The purpose ws to study the impct of ) northern winds (293 o - o ) which correspond to locl Etesin winds during summer, b) SSE-WNW (136 o o ) winds which blow from the se, c) ENE-

3 Interntionl Conference Pssive nd Low Energy Cooling 1 Tble 1: Number of Dh(26) for NOA, Ellinikon nd Sntorini sttions during the period NOA ELLINIKON SANTORINI J J A S Sum J J A S Sum J J A S Sum Men Tble 2: Slopes of the liner correltion between wind nd ir temperture vlues for NOA, Ellinikon nd Sntorini sttions during dy nd night (vlues in prenthesis) period. Sttions Wind Direction June July August September o 0.26(-0.08) 0.05(-0.001) 0.62(0.) 0.60(0.41) ΝΟΑ 293- o 0.42(0.17) 0.42(0.17) 0.53(0.26) 0.29(-0.13) 46-1 o 0.71(0.49) 0.94(0.21) 0.57(0.59) 0.33(-0.07) o -0.37(-1.19) -0.49(-0.32) -0.09(0.94) 0.06(0.) Ellinikon 293- o 0.12(0.37) 0.36(0.26) 0.42(0.) 0.09(0.32) 46-1 o 0.56(0.09) 1.03(0.14) 0.74(0.04) -0.08(-0.) Sntorini ο -0.22(-0.03) -0.14(-0.28) -0.16(-0.) -0.26(-0.27) ο -0.19(0.001) -0.(0.07) -0.12(-0.02) -0.22(0.02) SE (46 o -1 o ) winds tht re ffected by Ymittos, 1027m high mountin plced est of Athens nd d) clms on the mbient tempertures. In Sntorini the clssifiction of wind directions ws mde in two sectors (0 o -179 o nd 180 o - 9 o ) depending on whether or not the sttion is ffected directly by the se. Figure 2 shows tht in Sntorini, ir temperture is reduced when wind speed is incresed, independently of wind direction nd especilly for wind speeds greter thn 5m/s, either during dy or night period for the whole summer period. Not to mention tht the reduction of mbient temperture ws greter when the wind ws blowing from 0 o to 179 o, from the seside, rther thn when it ws blowing from 180 o to 9 o, where the ir strem followed continentl route (Tble 2). In Ellinikon sttion (Fig. 3), it is observed tht in ll cses (except for the dy period during September) ir temperture is incresed with wind speed, independently of wind direction. On first sight this seems prdox, however, when the sme nlysis ws performed for winds blowing from the se (SSE-WNW directions) then the mbient tempertures were reduced for incresing wind speeds during the 24 hour period (except for September). On the contrry, when the wind ws from ENE to SE directions where the effect of topogrphy ws existing then higher mbient temperture corresponded to higher wind speeds (Tble 2). Finlly, the existing northern wind speeds cused higher mbient tempertures, lthough this ws not expected. This my be ttributed to the fct the northern irstrem is mixed with the hot ir from the urbn cnopy lyer nd thus high ir tempertures re observed independently of wind speed. In cse of NOA sttion (Fig. 4), it ws found tht for ll wind directions nd during the 24- hour period, the mbient temperture is proportionl to the logrithm of wind speed (except for the night period during September). The increse of ir temperture is greter when the wind speed is from ENE to SE directions when the ctbtic winds from Ymittos mountin become wrmer. During the night period the increse is smller wheres, for winds from SSE to WNW positive correltion is found for August-September (Tble 2). During September when the se is chrcterized by higher surfce tempertures then the ir mss is heted underneth nd this is strengthen by the het islnd effect nd thus higher ir tempertures re observed t NOA sttion for high wind speeds either during the dy or night period. On the contrry during the night period for northern or estern wind directions, lower ir tempertures correspond to higher wind speeds. In generl, it hs been found tht for lower wind speeds there is n brupt increse of ir temperture, while when the wind speed is in-

4 136 Interntionl Conference Pssive nd Low Energy Cooling Sntorini - June (v>4.9m/s) (v>4.9m/s) Sntorini - July (v>4.9m/s) (v>4.9m/s) b Sntorini - August (v>4.9m/s) (v>4.9) Sntorini - September (v>4.9m/s) (v>4.9m/s) c 18 d 18 Figure 2: Correltion of wind nd ir temperture vlues for Sntorini sttion (the bigger tringles nd squres correspond to V>4.9m/s). Ellinikon - June (v>4.9m/s) (v>4.9) Ellinikon - June (v>4.9m/s) (v>4.9) Ellinikon - August Ellinikon - September (v>4.9m/s) (v>4.9m/s) (v>4.9m/s) (v>4.9m/s) Tir( o C) c cresed bove threshold vlues then the ir temperture tends to stbilize presenting tendency to cncel the islnd effect. Figure 3: Correltion of wind nd ir temperture vlues for Ellinikon sttion (the bigger tringles nd squres correspond to V>4.9m/s). d 5. CONCLUSIONS From the study of Dh(26) in three meteorologicl sttions in Athens nd in Sntorini, the het islnd effect is evident in the centre of Athens with the totl number of Dh(26) vlues in NOA rnging from 65 to 8647 during the cooling

5 Interntionl Conference Pssive nd Low Energy Cooling 137 NOA - June dy night NOA - July dy night Tir ( o C) b NOA - August Dy NOA - September Dy Tir ( o C) c d Figure 4: Correltion of wind nd ir temperture vlues for NOA sttion (the bigger tringles nd squres correspond to V>4.9m/s). period of the yer. In n open re djcent to the seside t distnce of Km from the city centre the totl number Dh(26) is reduced vrying from 4624 up to 6469, however these vlues re considered reltive high when compred with the corresponding vlues for Sntorini ( ). This revels tht the het islnd effect is predominnt in the extended Athens re, even with smller intensity t rurl plces. The impct of wind speed on ir temperture distribution in Sntorini showed tht higher wind speed vlues correspond to lower mbient tempertures, either during the dy or night period nd especilly when winds flow from the seside. In Ellinikon sttion negtive correltion is found when the wind blows from the se nd especilly during June, July nd August. For ll other wind directions the ir msses re directly ffected by the het islnd effect, presenting its greter intensity in the centre of Athens nd thus resulting in higher ir tempertures independently of wind speed. Finlly, for NOA sttion ner the centre of Athens, the het islnd effect overrides the influence of wind speed during the 24-hour period nd this is mostly observed for lower wind speeds, while in no cse negtive correltion ws observed. REFERENCES Blrs, C., I. Tselepidki Livd, M. Sntmouris nd D. Asimkopoulos, Clcultions nd sttisticl nlysis of the environmentl cooling power index for Athens,Greece. J. Energy Conv. nd Mngement, 34, 2, pp Blrs, C., I. Tselepidki-Livd, M. Sntmouris nd D. Asimkopoulos, Anlysis of therml comfort conditions in Athens, Greece. Energy Conv. nd Mngement, 34, 4, pp Livd, I., S. Lykoudis, M. Sntmouris nd C. Blrs, High mbient ir temperture frequency distribution t Hellenic islnds. Energy nd Buildings, 28, pp Livd, I., M. Sntmouris, K. Nichou, N. Ppnikolou nd G. Mihlkkou, 02. Determintion of plces in the gret Athens re where the het islnd effect is observed, Theoreticl nd. Applied Climtology, 71, pp Stmn, A. nd N. Ylcinky, Heting nd cooling degree-hours for Turkey, Energy, 24, 10, pp Theohrtos, G. nd I. Tsepidki-Livd, 1990.The temperture of the se-surfce in the region of the Aegen nd its reltion to ir-temperture, Theoreticl nd. Applied Climtology, 42, pp Tselepidki-Livd, I. nd M. Sntmouris, Sttis-

6 138 Interntionl Conference Pssive nd Low Energy Cooling ticl nd persistence nlysis of high summer mbient tempertures in Athens for cooling purposes, Energy nd Buildings, 17, pp Tselepidki-Livd, I., M. Sntmouris, C. Moustris nd G. Poulopoulou, Anlysis of the summer discomfort index in Athens, Greece for cooling purposes, Energy nd Buildings, 18, pp Tselepidki-Livd, I., M. Sntmouris nd D. Melitsiotis, Anlysis of the summer mbient tempertures for cooling purposes, Solr Energy, 50, 3, pp Tselepidki-Livd, I. nd M. Sntmouris, Anlysis of summer climtic conditions in Athens, Greece for cooling purposes, Int.Conf. Solr Energy in Architecture nd Urbn Plnning, Florence, Itly, 17-21/5/1993, Proc., pp Tselepidki-Livd, I., M. Sntmouris, D. Asimkopoulos nd S. Kontoyinnidis, On the vribility of cooling degree-dys in n urbn environment: Appliction to Athens, Greece, Energy nd Buildings, 21, pp Tzikopoulos, A.F., M.C. Krtz nd J.A. Prvntis, 05. Modeling energy efficiency of bioclimtic buildings, Energy nd Buildings, 37, 5, pp

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