The Structure and Evolution of Sea Breezes During the Qingdao Olympics Sailing Test Event in 2006

Transcription

1 ADVANCES IN ATMOSPHERIC SCIENCES, VOL. 26, NO. 1, 2009, Th Structur and Evolution of Sa Brzs During th Qingdao Olympics Sailing Tst Evnt in 2006 SHENG Chunyan 1,2 ( Sñ), Ming XUE 3,4, and GAO Shouting 5 (påë) 1 Shandong Provincial Mtorological Obsrvatory, Jinan, Shangdong Shandong Provincial Mtorological Institut, Jinan, Shangdong School of Mtorology, Univrsity of Oklahoma, Norman, Oklahoma 73072, USA 4 Cntr for Analysis and Prdiction of Storms Univrsity of Oklahoma, Norman, Oklahoma 73072, USA 5 Laboratory of Cloud-Prcipitation Physics and Svr Storms, Institut of Atmosphric Physics, Chins Acadmy of Scincs, Bijing (Rcivd 10 Dcmbr 2007; rvisd 10 May 2008) ABSTRACT Using data from automatic surfac wathr stations, buoys, lidar and Dopplr, th diurnal variation and th thr-dimnsional structur of th sa brzs nar th sailing sits of th Good Luck Bijing 2006 Qingdao Intrnational Rgatta from 18 to 31 August 2006 ar analyzd. Rsults show that xcluding rainy days and days affctd by typhoon, th sa brzs occur narly vry day during this priod. Whn Qingdao is locatd at th dg of th subtropical high at 500 hpa, th sa brz is usually strongr, around 3 4 m s 1. It starts at around 1100 to 1300 LST and lasts about 6 hours. Th dirction of th sa brz tnds to b southastrly. Whn Qingdao is undr th control of th subtropical high, th sa brz is usually wakr, lss than 2.5 m s 1 throughout th day, and bgins latr, btwn 1300 and 1500 LST. In this cas, th dirction of th sa brz is variabl from astrly to southastrly. Most sa brzs in Qingdao ar vry shallow, up to 300 mtrs dp. Strong sa brzs can rach 1.5 km in dpth and can push as far as 100 km inland. If th Huanghai sa brz movs inland and mts th sa brz of th Jiaozhou Bay in th wstrn part of Qingdao, th sa brz will strngthn and form thr boundaris du to th intraction of th two sa brzs. Ky words: Olympic sailing, sa brz, local coastal winds Citation: Shng, C. Y., M. Xu, and S. T. Gao, 2009: Th structur and volution of sa brzs during th Qingdao Olympics sailing tst vnt in Adv. Atmos. Sci., 26(1), , doi: /s y. 1. Introduction Th sa and land brzs rfr to nar surfac winds that blow from th sa to land during daytim and from land to sa at night causd by unvn surfac hating. Within th wathr forcasting community in China, th sa and land brzs hav not rcivd much attntion from forcastrs du to limitd availability of ral-tim dtaild obsrvations. For th Olympic sailing vnts to occur nar Qingdao, a coastal city in astrn China, in 2008, sa brz forcast is vry important bcaus th wind spd rror nds to b within 1 m s 1 and th wind dirction rror within 50 dgrs. In August, th wind is oftn undr 3 m s 1 in Qingdao, so th influnc of sa brz is vry important. In ordr to provid bttr wathr forcast srvics for th 2008 Olympic sailing vnts, svral tns of automatic wathr stations hav bn st up ovr th past two yars nar th sailing spots and in th coastal rgion of Qingdao, and thr buoys wr stablishd at sailing spots A, B and C (s Fig. 1). With ths data, th sa and land brzs nar th Corrsponding author: SHENG Chunyan, scy@mail.iap.ac.cn

2 NO. 1 SHENG ET AL. 133 Fig. 1. Gographical map of Qingdao and its nighboring rgions. Station sits (buoys and AWS) nar Qingdao 2008 Olympic sailing spots ( is th location of Huangdao Dopplr radar, + is th location of th lidar at Xiaomai Island) sailing spots can b studid in dtail for th first tim. Studis on th sa and land brzs can b tracd back to th 1940s. Simpl numrical modls wr usd by Haurwitz (1947) and Dfant (1951) to study th fin structur and dirction chang of sa brzs. Ths studis found clockwis rotation of sa brz wind dirction with tim. In ths modls, th sa brz starts at right angls to th coast and vrs (movs anticyclonically), until sunst whn it blows narly paralll to th coastlin. Numrical modls with complx forcing hav shown cyclonic rotation, and Numann (1977) gav thr rasons why th chang of th sa-brz dirction was non-uniform with tim. This was du to thr principal trms: (1) Coriolis (unknown from prvious work); (2) prssur gradints du to diurnal hating; and (3) larg-scal prssur gradints not affctd by diurnal changs. But thr ar many occurrncs whr th sa brz dirction dosn t chang or vn rotats cyclonically, which wr xplaind as th ffct of mountains (.g., Staly, 1957). Th dtaild structur of sa brz in th daytim has bn studid by,.g., Sha t al. (1991) and Clark (1984). Estoqu (1961) found that th formation and strngth of th sa brz was affctd by th largscal flow in addition to th hating condition. Many studis hav shown that th prvailing synoptic-scal flow has a dirct ffct on th strngth and landward pntration of th sa-brz front (.g., Wxlr, 1946; Haurwitz, 1947; Dfant, 1951; Estoqu, 1962; Frizzola and Fishr, 1963; Schrodr t al., 1967; Arritt, 1993; Ribl t al., 1993). Th dtails of th rsultant structur of th sa-brz front ar still not complt, but svral gnral faturs ar vidnt. Bchtold t al. (1991) and Arritt (1993) xamind sa-brz frontognsis and stability for a larg rang of synoptic wind spd and dirction during th day. Zhong and Takl (1993) modld th influnc of th larg-scal background flow on msoscal and microscal aspcts of th sa brz by dividing th sa brz into two typs: thos with sa brz fronts normal to th coastal lin and thos with fronts paralll to th coastlin. Th two gnral classs of sa brzs diffrd in many rspcts, including th associatd maximum vrtical vlocitis. Ribl t al. (1993) found that whn th flow is offshor, th sa brz front is associatd with a pronouncd discontinuity owing to incrasd convrgnc and subsqunt frontognsis. In addition, th offshor flow dos not allow th sa brz front to pntrat far inland. Whn th synoptic flow is onshor th front is wak but it pntrats much farthr inland. Houghton and Campbll (2005) rviwd many aspcts of th sa and land brzs and th forcasting tchniqus, pointing out that th larg-scal wind nar th surfac can affct th sa brz significantly. An analysis of sa brzs has bn prformd in a numbr of aspcts in various works (Frizzola and Fishr, 1963; Barbato, 1975; Wakimoto and Atkins, 1993; Buckly and Kurzja, 1997; Zhang t al., 2005). In Qingdao, th wathr in August is diffrnt from many of othr rgions whr th land and sa brzs ar studid. Th dominant wathr systm in this sason is th subtropical high prssur systm ovr th wstrn Pacific Ocan. Onc Qingdao is undr th control of this high at 500 hpa, th wathr is usually humid, stuffy and calm. At th dg of th high prssur systm, thr tnd to b thundrshowrs. In China, som studis on sa brzs xist. For xampl, Wu and Long (1993) analyzd th local sa brz faturs in Qingdao with data from coastal wathr stations. Xu t al. (1995) studid th fatur of sa brz in Longkou city on th northrn Shandong Pninsula. Zhuang t al. (2005) analyzd th climatic fatur of sa and land brzs in Rizhao city with Rizhao coastal data. Jin and Wang (1991) studid th ffct of landsa air tmpratur contrast on th strngth of sa brz with a thr-dimnsional numrical modl and found th dominant ffct of th tmpratur contrast in thir cass. Chang t al. (2002) found through 3D numrical simulation that th sa brz nar Qingdao is affctd significantly by th hating of Laoshan Mountain. Without marin data, ths studis did not provid th fin-scal analysis of sa brz faturs nar Qingdao. Furthrmor, ths studis did not look at th ffct of larg scal flows associatd

3 134 SEA BREEZES EVOLUTION DURING 2006 OLYMPICS SAILING TEST EVENT VOL. 26 with th subtropical high. Fortunatly, svral Automatic Wathr Stations (AWS) nar th coastal rgion and th islands wr stablishd for th 2008 Olympic sailing vnts. Buoys wr st up at sailing spots A, B, and C, rspctivly. With ths fin-rsolution obsrvation data in and nar Qingdao, nw faturs and th fin structur of th sa brz in this rgion can b obsrvd. Is th sa brz frqunt nar th sailing spots? Is th sa brz nar th sailing spots strong? What tim dos th sa brz usually bgin? In this study, in ordr to answr ths qustions, improving th forcasting skill of th sa brz and offring bttr mtorological srvic for th sailing vnts, th sa brz during th Olympic sailing tst priod is studid. Th rst of this papr is organizd as follows. Sction 2 provids a brif introduction to th obsrving data. Sction 3 is an ovrviw of th wathr during th priod, and sction 4 discusss th gnral faturs of sa brzs. Sction 5 xamins th rlationship btwn th sa brzs with th tmpratur contrasts, sction 6 discusss th diffrnt faturs of th two typs of sa brzs for th days undr th control and at th dg of th subtropical high, and sction 7 furthr xamins th 3D structurs of th brzs. A summary is givn in th final sction. 2. Brif introduction to th obsrving data Th Good Luck Bijing 2006 Qingdao Intrnational Rgatta, also known as th tst vnt of th 2008 Olympics, was hld in th Fushan Bay of Qingdao (s Fig. 1). It was an Intrnational Sailing Fdration (ISAF) Grad 2 Intrnational Rgatta, which includd 11 vnts in nin classs. Mor than 503 world-class athlts from 40 countris participatd. Th tst vnt was hld from 18 to 31 August 2006 and th provision of wathr forcasting for th vnt bgan on 10 August. Thr ar thr sailing spots in th Bay ara, as markd by A, B, and C in Fig. 1. Thr is a buoy at ach spot. Ovr thirty automatic wathr stations wr stablishd in th coastal rgion. Ths buoys and AWSs can offr hourly wind, tmpratur, rlativ humidity and sa lvl prssur. On Huangdao Island in th southrn part of Qingdao, thr is a CINRAD- SA Dopplr radar (Fig. 1), which is a part of th Chins nw gnration wathr radar nt and can offr ral tim 6-minut Dopplr radar products. Th lidar in th Ocan Univrsity of China was also in-situ during th tst vnt. It is a kind of Dopplr windfinding radar and can giv th vrtical wind profil nar th sailing spot. Bcaus th lidar is a nw facility, it was tntativ during th tst vnt and its data is not continuous. Using availabl in-situ and rmotsnsing obsrvations, th wind faturs from 9 through 31 August ar analyzd in this study. 3. Ovrviw of gnral wathr pattrn In gnral, whn th ridg of th subtropical high at th 500 hpa lvl migrats from th Changjiang- Huaih rgion to th North China plains, th wathr in Qingdao is usually calm and stuffy, and tnds to hav thundrshowrs. According to th data from 1971 to 2000, wind spd is th lightst in August in Qingdao. During th 2006 Olympics sailing tst vnt, th dominant wathr systm was th typical subtropical high. Spcifically, th wathr pattrn of Qingdao can b dividd into thr typs: days whn Qingdao is undr th control of th subtropical high (to b rfrrd to as CNTL cas), at th dg of th subtropical high (to b rfrrd to as EDGE cas), and at th dg of a typhoon, rspctivly. Data show that sa brz did not occur on rainy days (14 to 16, 25 to 26, 29 and 31 August) and on th days affctd by a typhoon (17 and 18 August). Th sa brz occurrd on all othr days a. In ordr to study th faturs of th sa brz, th sa brz days ar dividd into two priods: th first priod from 9 to 13 August, during which th ara was undr th control of th 500 hpa subtropical high (th CNTL cas, s Fig. 2a), and th scond priod from 19 to 31 (xcluding rainy days), during which th ara was at th dg of th 500 hpa subtropical high (th EDGE cas, s Fig. 2c). It is found that undr th control of th 500 hpa subtropical high, th SLP is vry low and th surfac wind is usually southrly (Fig. 2b). At th dg of th 500 hpa high, th SLP is much highr and th surfac wind is usually northrly (Fig. 2d). 4. Obsrvd sa brz charactristics Figurs 3a and 3b show th diurnal volution of man wind spd at th thr buoys for th two wathr pattrns. It can b sn that thr ar two wind spd paks within th diurnal cycl, corrsponding to th sa and land brzs, rspctivly. Th land brz occurs in th arly morning hours btwn 0300 to 0900 LST in both situations b. Th timing of th a Hr, th sa brz is loosly dfind as an vnt with wind dirction in th coastal rgion nar th sailing spots changing from synoptic-scal wind dirction to astrly, southastrly or southwstrly at around noon and thn turning back to its original dirction in th vning. b 0800 LST=0000 UTC

4 NO. 1 SHENG ET AL. 135 Fig. 2. Exampl wathr pattrns for a CNTL at 0800 LST 13 August (a and b) and an EDGE cas on 0800 LST 23 August (c and d). Lft: 500 hpa prssur pattrn (units: gpm), right: surfac lvl prssur (hpa) (Th shadd part in th right of th figurs is th ocan). sa brz diffrs a lot btwn th two situations. For th sa brz undr th control of subtropical high (CNTL cas), th onst tim is btwn 1300 and 1500 LST, latr than th 1100 to 1300 LST onst tim found whn th rgion is at th dg of th high (EDGE cas). Th pak sa brz wind spd is rachd btwn 1500 and 1600 LST in th formr cas, whil it is rachd at or shortly aftr 1200 LST in th lattr cas. Th sa brz winds ar sustaind for 5 to 6 hours in both cass. In th CNTL situation, th wind is light and th sa brz is wak, with th man wind spd at th buoy sits rmaining blow 2.5 m s 1 throughout th day. For th EDGE cas, th winds ar gnrally strongr, with th strongst winds of th day bing associatd with th sa brz, narly 4 m s 1 at or shortly aftr noon. Th tst vnt occurrd from 18 to 31 August, whn Qingdao was mainly at th dg of th subtropical high. Considring th rquirmnt of 3 to 18 m s 1 wind spd and 50 dgrs wind dirction rror of th forcast accuracy for th sailing vnt, th sa brz provids th ncssary wind spd and stabl wind dirction for sailing, and that is why th tst vnt can b carrid through vryday during th Olympic tst vnts vn though th winds wr vry wak ovr land in Qingdao. It can b sn furthr from Figs. 3a and 3b that th wind spd is strongst at buoy A, th wind spd at buoy B is strongr, and that at buoy C is th last. This slight diffrnc of th wind spd can b attributd mainly to th diffrnt distanc of th buoys from th coast. Th wind spd drops quickly whn sa brz cass. Land brz turns to sa brz from 1000 to 1300 LST, with th wind spd dcrasing and thn incrasing again. Th sa brz in th EDGE cas dvlops to a gratr strngth and wakns quickly aftr around 1700 LST and th minimum wind spd is rachd btwn 2000 and 2200 LST. Aftr 2200 LST, th wind spd starts to pick up again as th land brz starts to form in th vning and arly morning hours. Th changs in th wind dirction ar shown in Fig. 3c through Fig. 3f. It can b sn that th wind is wakr and mor changabl in th CNTL

5 - 136 SEA BREEZES EVOLUTION DURING 2006 OLYMPICS SAILING TEST EVENT VOL. 26 s (m p d S d in W 1 ) Fig. 3. Wind spd at thr buoy stations (a and b) (m s 1 ), frquncy of cardinal wind (c and d), and th dirction of cardinal wind ( and f) at thr buoy stations of th sa brz days during 9 to 31 August 2006 (Th wind dirction in () and (f) ar givn in 1 through 8, with 1 rprsnting du north, and ach incrmnt of 1 rprsnting a 45 dgr clockwis incrmnt). Th lft column is for CNTL cass and th right column is for EDGE cass. cas. Th dirction of th sa brz is gnrally astrly to southastrly. For th EDGE cas, th wind is strongr and th wind dirction is mor stabl. In this situation, th prvailing wind is northrly in th morning and th sa brz is southastrly. Th dirction of th sa brz is normal to th coastlin. 5. Rlation btwn sa brz and land-sa air tmpratur contrast To xamin th land-sa tmpratur contrast and its rlationship with sa brz strngth, th daily tmpraturs ovr th sa (buoys) and land is analyzd. Th Fushan station that is about 2 km inland from th coast and closst to th buoys (s Fig. 1) is slctd to rprsnt th tmpratur ovr land. Figurs 4a and 4b show th man diurnal tmpraturs at th buoys and Fushan for th CNTL and EDGE days. It is found that th tmpraturs at th thr buoys ar narly th sam and hav a small diurnal chang. Th tmpratur ovr th land incrass significantly in day tim du to th daytim hating. At night, th tmpraturs ovr th land and th sa ar narly th sam for CNTL days, which xplains th wak land brz in this typ of wathr. Whil in EDGE days, tmpraturs ovr land at night ar 1 C 2 Ccoldr,

6 NO. 1 SHENG ET AL. 137 ) ( r tu r a p m T ) ( r tu r a p m T Fig. 4. Man diurnal tmpratur ovr th sa (at thr buoy sits) and ovr land (at Fushan station) for (a) CNTL and (b) EDGE cass, (c) man daily sa-land tmpratur diffrnc (land tmpratur minus sa tmpratur), and (d) maximum daily tmpratur diffrncs btwn ach buoy sit and Fushan station from 9 to 27 August Not that data ar missing from 28 to 31 August, and th tims in a, b and c ar in LST. producing strongr land brzs (s Fig. 4c). As a rsult, th maximum tmpratur contrast is gnrally highr for th CNTL days but is lowr for th EDGE days. Figur 4d shows th curvs of th daily maximum tmpratur diffrncs btwn Fushan and buoys A, B, and C from 9 to 27 August c. It can b sn that for th sa brz days, th diffrnc of th tmpratur is all ovr 4 K. But th sa-land tmpratur diffrnc for CNTL days is gnrally highr than that for th EDGE days. Th largst daily maximum tmpratur contrast appars on 12, 13 and 23 August, which ar ovr 6 C at all buoy sits. In fact, th sa brz is th strongst on 23 August ovr th ntir tst priod, whil th sa brz is not vry strong on 12 and 13 August. Th maximum tmpratur contrast on 13 August is th largst but th maximum wind spd of th day is lss than 2.5 m s 1. This indicats that th strngth of th sa brz is not only dtrmind by th land-sa tmpratur diffrnc. It is found in Fig. 2 that th surfac wathr systms ar quit diffrnt on 13 and 23 August. On 13 August th Qingdao ara is undr th control of a surfac low prssur systm and th prvailing wind is southrly. Whil on 23 August, th Qingdao ara is undr th control of a surfac high prssur systm and th prvailing wind is northrly, which is th dominant wind dirction for th EDGE cas. Th surfac wathr systm on 12 August is similar to that on 13 August and th prvailing wind is southrly too. Bcaus th sa brz is a shallow thrmal circulation, th rturn circulation of th sa brz in Qingdao is narly northwstrly. Thrfor, th northrly prvailing wind for th EDGE cas can hlp th stablishmnt of th sa brz rturn circulation and th sa brz is strongr. 6. Thr-dimnsional structur of sa brz 6.1 Vrtical structurs of sa brzs from Dopplr radar Vlocity Azimuth Display (VAD) wind profils and lidar wind profils Figur 5a shows th tim-hight cross sction of wind profils drivd from lidar masurmnts locatd at Xiaomai Island btwn th coastlin and th sailing sits for th aftrnoon of 21 August (s Fig. 1). c Unfortunatly, data ar missing from 28 to 31 August 2006

7 138 SEA BREEZES EVOLUTION DURING 2006 OLYMPICS SAILING TEST EVENT VOL. 26 Fig. 5. Tim hight cross-sction of low-lvl horizontal winds from lidar on 21 August (in LST) and (b) VAD wind profils from Huangdao Dopplr radar on 23 August (UTC). It can b sn that thr wr southastrly winds in th shallow boundary at 1451 LST, which is th sa brz. Th northwstrly winds in th uppr lvls ar associatd with th larg-scal wathr systm. In th morning, whn th sa brz bgins, th southastrly winds wr vry shallow. Evn by 1451 LST, or about 3 pm, th sa brz was only 70 to 80 mtrs dp. Th sa brz dvlopd gradually in th aftrnoon and rachd a dpth of about 150 m by 1622 LST on this day (Fig. 5a), indicating that th sa brz bcam strongr in th middl of th aftrnoon. Dopplr radar Vlocity Azimuth Display (VAD) wind profils can show th man horizontal wind profils nar th radar station. It is dployd from th 30 pixl valu of th lvl-ii Dopplr radar radial vlocity within a 30 km radius of th radar sit (Yu t al., 2006). Thrfor th VAD wind profils can indicat th 6-minut vrtical wind chang nar th radar sit. So far, this kind of data has not bn applid in th study of sa brzs in China. Th Olympic sailing spots ar about 20 km northast of th Huangdao radar station (Fig. 1), just within th ffctiv radius of th VAD wind profil product. Although th vrtical rsolution of VAD data is rlativly low, th VAD wind profils from 9 to 31 August 2006 show clar signs of sa brz winds in th aftrnoons whn buoy data indicat thir prsnc. W not that vn though th radar is not locatd at th sailing sits, it is locatd on a larg pninsula that is subjct to a similar influnc of land and sa brzs. Wind changs in th dirction of VAD winds corrspond wll with thos sn in buoys and automatic wathr station data. Th dpth of th sa brz is gnrally lss than 300 mtrs in th VAD data during this priod, a fatur similar to that of th sa brz in Longkou in th northrn part of th Shandong Pninsula (Xu t al., 1995). Th sa brz can not b sn in th Huangdao Dopplr radar rflctivity products (not shown) bcaus th hight of th radar is 169 mtrs, gnrally highr than th hight of th sa brz. Th sa brz on 23 August 2006 is, howvr, as dp as 1 km (Fig. 5b). Th VAD wind profils show that th surfac winds turnd to southwstrly at around 0900 and 1000 LST, and to southastrly at 1240 LST, maning that th sa brz is fully stablishd by 1300 LST, which is coincidnt to th obsrvation from th buoys and th automatic wathr stations (AWS). At th bginning, th sa brz is confind to blow 300 mtrs in hight. With th dvlopmnt of th sa brz circulation, th hight of th sa brz incrass gradually to 600 mtrs by 1400 LST. At around 1700 LST, th southast sa brz is th strongst of th day and rachs a hight of about 1 km. 6.2 Horizontal structurs of sa brzs in Dopplr radar masurmnts As discussd abov, th strongst sa brz vnt during th Olympic tst priod occurrd on 23 August 2006, and only this sa brz can b clarly idntifid in low-lvl PPI (Plan Position Indicator or constant lvation) scans of th Huangdao Dopplr radar. According to th Dopplr radar rflctivity, at 1300 LST on 23 August, thr was a thin boundary in th 0.5 dgr lvation rflctivity fild of th Huangdao radar. At this tim, no boundary can b idntifid nar th Jiaozhou Bay in th wstrn part of Qingdao ara. Half an hour latr, this thin boundary bcoms strongr and mor clar. A clar boundary of grn chos and nhancd rflctivity is found running through th middl of Jiaozhou Bay in a southwst-northast orintation (Fig. 6a). This corrsponds to nhancd low-lvl convrgnc along th sa brz front. Th sa brz can also b idntifid in th radial vlocity fild (Fig. 6b). Diffrnt from th rflctivity, th ntir Jiaozhou Bay shows rd radial vlocitis (radial wind is away from th radar sit), indicating th prsnc of sa brz. That is to say, thr ar two sa brzs, on is nar th astrn part

8 NO. 1 SHENG ET AL. 139 Qingdao Qingda A A Qixia Qixia Laizhou Laizhou Laiyan Laiyang Pingdu Laixi Pingdu Laixi Gaomi Jimo Gaomi Jimo Fig. 6. Rflctivity and radial vlocity imags at 0.5 dgr lvation from Huangdao radar, indicating th prsnc of sa brz fronts. Rflctivity at (a) 1336 LST and (c) 1703 LST 23 August; radial vlocity (right panls) at (b) 1336 LST and (d) 1703 LST 23 August. of Huanghai Sa and its sa brz front can b sn from Dopplr radar, th othr is from th local Jiaozhou Bay. Th sa brz nar th Jiaozhou Bay is too wak to b sn in Dopplr radar rflctivity but can b confirmd by data from th AWS station nar Qingdao (s Fig. 7a). At around 1400 LST, th sa brz from th Huanghai Sa travls across th Jiaozhou Bay and gradually forms a circular boundary in th radar rflctivity fild. At 1500 LST, th thr-boundary structur of th sa brz can b sn clarly from th AWS data (s Fig. 7b). By 1700 LST, th sa brz has dvlopd to its maximum strngth and is about 100 km inland. Bcaus th sa brz front is far inland and th Jiaozhou Bay is out of Figs. 6c and 6d, it is found that th northrn portion of th sa brz front is at Qixia City (markd Qixia in Figs. 6c, d), and its southwstrn part is at Chnggzhuang, Zhuchng City (this city is out of Figs. 6c, d). Th dpth of th sa brz is up to 3 km according to th VAD Wind Profil in Fig. 5b. Th location of th sa brz front can b sn clarly from th radial vlocity imag (Fig. 6d). Th structur of th sa brz front can also b sn from AWS data although th data ovr land far from th coast ar spars (Fig. 7c). Aftr 1700 LST, th sa brz bcoms wakr from th northastrn part. Th sa brz front in rflctivity data disappard gradually. At 2000 LST, th sa brz front can still b idntifid in th surfac chart although th short front can t b sn in th surfac chart (Fig. 7d). It can b sn that th convrgnc lin btwn th sa and land brzs nar th sa brz front rmaind vn though th sa brz has did. At this tim, no sa brz front signatur can b found in th Huangdao radar data. 6.3 Th vrtical structur of th sa brz in Dopplr radar masurmnts Th vrtical structur of th sa brz front can also b xamind using th RHI (Rang-Hight Indicator) radar scan data. Figur 8 shows such vrtical cross sctions at 1700 LST 23 August 2006 along th whit lin in Fig. 6d that is normal to th sa brz front. It can b sn that th sa brz is vry shallow vn though this is th strongst stag of th sa brz (around 1700 LST). Thr xists a rgion of nhancd rflctivity btwn th 75 and 86 km rangs northwst of th radar and blow 1.5 km in hight,

9 140 SEA BREEZES EVOLUTION DURING 2006 OLYMPICS SAILING TEST EVENT VOL. 26 Jiaozhou Bay Fig. 7. Sa-brz fronts analyzd basd on AWS data at (a) 1300, (b) 1500, (c) 1700 and (d) 2000 LST 23 August which is th rgion of sa brz front dg (Fig. 8a). In th radial vlocity fild (Fig. 8b), a layr of outgoing radial vlocity (rd colors) is found blow th 2 km lvl, and this dpth sms to dcras to lss than 1 km nar th sa brz front. Abov this layr, th flow spd is narly zro, and a rgion of rvrsd flow (towards radar) is found at about th 4 km lvl nar th 65 km rang. Such rsults ar consistnt with th findings with VAD data. 7. Discussion and summary With data from automatic wathr stations, buoys, lidar and Dopplr radar that ar routinly availabl and spcially dployd for th Good Luck Bijing 2006 Qingdao Intrnational Rgatta that occurrd from 18 to 31 August 2006 off th coast of Qingdao, Shandong Provinc, China, th thr-dimnsional structur and charactristics and diurnal volution of sa brzs nar th sits of sailing tst wr analyzd. Th main conclusions ar summarizd as th following: (1) Excluding days impactd by rain or a narby typhoon, a sa brz is found on all days ovr approximatly a 20 day priod in lat August 2006 that covrs th Olympics sailing tst vnt. Whn th Qingdao coastal rgion is locatd at th dg of th subtropical high at th 500 hpa lvl, th sa brz usually starts at around 1100 to 1300 LST and lasts about 6 hours, and its wind dirction tnds to b southastrly. Whn th rgion is undr th control of th subtropical high, th sa brz usually bgins latr btwn 1300 and 1500 LST, and th sa brz wind dirction is astrly to southastrly. (2) Undr th control of th subtropical high, th winds at all thr spots ar wak and rmain blow 2.5 m s 1 on avrag and th wind dirction is mor variabl. Th wind dirction is gnrally astrly to southastrly. Th sa brz found at th dg of th subtropical high is strongr. Th man wind spd is 3 4 m s 1 in this cas and th sa brz dirction is usually southastrly and normal to th coastlin. If th sa brz wind spd is strongr, its dirction tnds to b mor clos to th normal dirction of th coastlin. Othrwis, th dirction is closr to th dirction of th systmatic or background wind. (3) In gnral, th diffrnc in th sa and land air tmpratur is ovr 4 C for th sa brz days. Th subtropical high can affct th sa brz du to th diffrnc in th prvailing wind. At th dg of th subtropical high at th 500 hpa lvl, th prvailing wind in Qingdao is usually northrly and it can act as th rturn circulation of th sa brz. Thrfor, th northrly prvailing wind can hlp th stablishmnt of th sa brz, thus th sa brz is strongr and

10 NO. 1 SHENG ET AL. 141 (a) (b) Fig. 8. (a) Vrtical cross-sction (RHI display) of Dopplr radar rflctivity and (b) radial vlocity normal to th sa brz front at 1700 LST 23 August Th radar is locatd at th lowr-lft cornr of th plots. bgins arlir. (4) Th dpth of th sa brz is usually vry shallow and is hld to blow 300 mtrs. For strongr sa brzs, it can b 1.5 km dp and can push inland as far as 100 km. If th Huanghai sa brz propagats inland and mts th sa brz of th Jiaozhou Bay on its wstrn part, th sa brz will strngthn and form a thr-boundary structur as a rsult of th intraction btwn th two sa brz systms. As discussd abov, xcluding th maximum daily tmpratur diffrnc, th larg scal wathr pattrn can affct th strngth of th sa brz too. Whn th Qingdao coastal rgion is at th dg of th subtropical high (EDGE priod), Qingdao tnds to b affctd by wak cold air and th prvailing wind is usually northrly in th morning (s Fig. 3f), which is sam as th dirction of th sa brz rturn circulation. Onc th land-sa tmpratur diffrnc is larg nough (usually ovr 4 K), th surfac wind will chang from sa to land and th prvailing wind can act as th sa brz rturn circulation and hlp th stablishmnt of th sa brz, thrfor, th sa brz is strongr and bgins arlir. Undr th control of th subtropical high, th tmpraturs ovr th land and sa ar both high and th associatd strong hating in th daytim rsults in mor mixing nar th surfac, thrfor, th wind is light and wind dirction mor variabl ovr th ntir day. In this situation, th astrly wind is not favorabl to th stablishmnt of th sa brz winds and th sa brz is wakr. Furthrmor, th dirctions of sa brz for th CNTL and EDGE days ar slightly diffrnt as wll. For th sa brz in EDGE days, th dirction is southastrly, which is normal to th coastlin. In CNTL days, th dirction of th sa brz is from astrly to southastrly, closr to th systmatic wind dirction (s Fig. 3). This may b causd by th diffrnc of th prvailing wind and th diffrnc of th sa brz strngth. If th sa brz is strongr, its dirction is closr to th normal dirction of th coastlin. If th sa brz wind spd is wakr, its dirction is closr to th dirction of systmatic or background winds. In Houghton and Campbll (2005), th sa brz starts at right angls to th coast and vrs (shifts anticyclonically) until sunst, to bcom narly paralll to th coastlin. As discussd in this study, th vring fatur of th sa brz documntd in Houghton and Campbll (2005) for th coastal rgion dos not happn in Qingdao rgion. Bcaus of th significanc of th Qingdao coastal rgion in th upcoming Olympic vnts in summr 2008, and th gnral lack of dtaild studis on th ssntially daily sa brz vnts, spcially thos using spcial obsrvational data, w bliv that this study rprsnts an important stp towards bttr undrstanding and vntual improvd prdiction of land and sa brzs in this rgion. Acknowldgmnts. Th authors thank Jianlin Wang of Qingdao Municipal Mtorological Burau for providing som of th data usd in this study. This work was mainly supportd by National Natural Scinc Foundation of China Grant (Grant No ), th Chins Acadmy of Scincs Grant (Grant No ) and th Natural Scinc Foundation of Shandong Provinc Grant (Grant No. Q2007E03). Spcial thanks go to Prof. Gang Fu, Ocan Univrsity of China and an anonymous rviwr, for thir constructiv suggstions that improvd this papr significantly. REFERENCES Arritt, R. W., 1993: Effcts of th larg-scal flow on charactristic faturs of th sa brz. J. Appl. Mtor., 32, Barbato, J. P., 1975: Th sa brz of th Boston ara and its ffct on th urban atmosphr. Ph. D. Dissrtation, Boston Univrsity, 223pp. Bchtold, P., J.-P. Pinty, and P. Mascart, 1991: A numrical invstigation of th influnc of larg-scal winds on sa-brz-and inland-brz-typ circulations. J. Appl. Mtor., 30, Buckly, R. L., and R. J. Kurzja, 1997: An obsrvational and numrical study of th Nocturnal sa brz. Part I: Structur and Circulation. J. Appl. Mtor., 36,

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