ANALYSIS OF MOISTURE TRANSPORTATION DURING A LOCAL HEAVY RAINFALL ON AUGUST IN TOKYO

Transcription

1 The Seventh Asia-Pacific Cnference n Wind Engineering, Nvember 8-, 9, Taipei, Taiwan ANALYSIS OF MOISTURE TRANSPORTATION DURING A LOCAL HEAVY RAINFALL ON AUGUST IN TOKYO ABSTRACT Tru Yamanaka and Ryz Oka Senir Research Engineer, Technical research institute, KAJIMA Crpratin --, Tamagawa, Tyfu-chi, Tky, Japan, t-yamanaka@kajima.cm Assciate Prfessr, Institute f Industrial Science, University f Tky -- Kmaba Megur-ku, Tky, Japan, ka@iis.u-tky.ac.jp Numerical simulatins were perfrmed n lcal heavy rainfall bserved at Tky n August. In this event, three lw-level systems, dry cld nrthwesterly, warm sutherly and cld nrtheasterly, were cnverged near rainfall regin arund Tky. The messcale mdel (MM) is successful in reprducing lw-level systems and the distributin f rainfall. And it is fund ut that cld nrtheasterly is the mst humid. Then the effect f urban area n lcal heavy rainfall is estimated. And anthrpgenic heat affects the distributin f rainfall regin, but has little affectin t misture budget within rainfall regin. KEYWORDS: LOCAL HEAVY RAINFALL, MOISTURE TRANSPORT, MESO SCALE Intrductin Intense thunderstrms ften ccur in summer arund Tky and prduce a lcal heavy rainfall which smetimes causes serius damages t urban infrastructure. In recent years, sme studies pinted ut that the cnvergence f lw-level airflw and the misture in lwlevel air play significant rles t initiate and develp thunderstrms. It was presented by [Yshizaki et al. (998)] that thunderstrms were generated n the cnvergence area f lwlevel airflw arund Tky during a heavy rainfall event. [Fujibe et al. ()] examined the relatin between precipitatin distributin and surface patterns using statistical apprach, and gt the results supprting the abve cnclusin. [Yshizaki et al. (998)] suggested the cnvergence f lw-level airflw culd be the trigger fr the initiatin f thunderstrm. [Sek et al. (7)] examined the distributin f precipitable water vapr derived frm glbal psitining system, and shwed that the precipitable water vapr near heavy rainfall regin was larger than thse f the surrunding regin. [Sek et al. (7)] nted that lw-level humid air was cncentrated and thickened by the cnvergence f lw-level airflw, and suggested that the amunt f the water vapr in the cnvergence regin related t rainfall amunt. Thse studies mentined abve clarified the rle f lw-level airflw and misture t initiate and develp the thunderstrms. In rder t predict accurately the ccurrence f thunderstrms, it is very imprtant t understand the actin f lw-level humid air befre water vapr in lw-level air transfrm int clud water. The detail f the prcess f misture transprtatin, hwever, was nt clear yet. In this paper, misture transprtatin during lcal heavy rainfall n August is investigated using Messcale Mdel th generatin (MM). Generally, misture exist much in lw-level air than in upper level air, hence lw-level airflw wuld play dminant rle in misture transprtatin. Since lw-level airflw is strngly affected by surface bundary cnditin, the effect f urban area n lcal heavy rainfall is estimated als in this paper.

2 The Seventh Asia-Pacific Cnference n Wind Engineering, Nvember 8-, 9, Taipei, Taiwan The cntents f this paper are rganized in the fllwing manner. Firstly, an verview f the target heavy rainfall is given. Secndly, misture transprtatin by lw-level airflw is examined using numerical mdel after the features f the mdel are intrduced. Thirdly, sensitivity study is given in rder t estimate the effect f urban area n the lw-level airflw and heavy rainfall. Finally, summary is presented in the last f this paper. Synptic envirnment n the lcal heavy rainfall event On August, lcal heavy rainfall was bserved in central part f Tky. The maximum hurly rainfall amunt f 8mm was recrded n an Autmated Meterlgical Data Acquisitin System (AMeDAS). The rainfall arund Tky cncentrated n small area within few kilmeters. Many huses were flded during this heavy rainfall event. The surface weather map at 9 JST n August which prvided Japan Meterlgical Agency (JMA) is shwn in Figure. It was abut hurs befre Figure : Surface weather map bserving maximum hurly accumulated precipitatin in at 9 JST n August. Tky. A synptic scale lw pressure passed n Tky frm nrthwest t sutheast withut a cld frnt line. It was fine in the daytime at Tky, and air temperature rse ver C arund Tky. Accrding t JMA, inflw f cld air int Tky appeared n upper level, s the atmspheric cnditin became unstable at Tky. Numerical mdel design Numerical simulatins were perfrmed by MM. Tw-way nested mdel dmains with high reslutin hrizntal grid spacing are used. Grid spacing f uter and inner dmains is km and km respectively. The uter dmain cvers the area f km 9km, and the inner dmain cvers the area f km 8km (refer t Figure ). The inner dmain includes the whle f Kant Plain and its surrunding muntains. The tp level f mdel dmain is hpa, and the mdel has 7 vertical levels. The initial and lateral bundary values f uter dmain are prepared by interplating NCEP final analysis data (FNL). MM has three types f land-use datasets fr lwer bundary cnditin. These datasets are supplied by PSU/NCAR and USGS. Instead f these datasets, the digital natinal land infrmatin data prvided by the ministry f land, infrastructure, transprt and turism (MLIT) f Japan is used in this study. T input the digital natinal land infrmatin data fr lwer bundary cnditin, MM and TERRAIN prgrams mdified by Kawamt and Oka (8) are used. Furthermre, anthrpgenic heat data f km hrizntal reslutin in and arund Tky is given int mdel. These anthrpgenic heat data are prvided by natinal institute f Advanced Industrial Science and Technlgy km Altitude (m) (AIST), Japan. The list f physical schemes used in this numerical simulatin is shwn in table. Fur Dimensinal Data Assimilatin (FDDA) is nt used in rder t estimate the effect f lwer bundary Kant Plain cnditin n lcal rainfall. 9km Inner dmain Tky Outer dmain Figure : Mdel dmain. Table : Mdel design. Cumulus Parameterizatin Nne PBL Scheme MRF Explicit Misture Scheme Gddard Micrphysics Radiatin Scheme Clud-radiatin scheme Surface Scheme Five Layer Sil mdel

3 The Seventh Asia-Pacific Cnference n Wind Engineering, Nvember 8-, 9, Taipei, Taiwan Numerical analysis Brief verificatin f numerical reprduced fields Figure (a) shws surface fields and air temperature recrded n AMeDAS at 9 JST (=UTC+9h). It was tw hurs befre bserving maximum hurly accumulated precipitatin in Tky. There were three systems arund Tky at a height f m; ) nrtheasterly cld frm sea (under 8 C ), ) sutherly warm frm sea (ver C ) and ) nrthwesterly frm muntain. These three systems had a scale f Kant Plain size (km), and cnverged n Tky urban area indicated as pen-dtted-ellipse in Figure (a). [Kwan et al. (8)] Nrthwesterly analyzed D fields derived frm dual dppler radar data during this rainfall event, and investigated fields n strm-scale which is a scale f hrizntal size (km) and time span ( hur). It was fund by [Kwan et al. 8)] that precipitatin cells develped at the cnvergence regin which was assciated with the three systems mentined abve. Accrding t [Kwan et al. 8)], the three systems influenced thse precipitatin cells t mature and t stagnate at Tky fr a while. Sutherly Nrtheasterly (a) 8km (a) 8km C 8 mm (b) 8km Figure : Hurly accumulated rainfall distributin at : JST; (a) bserved and (b) reprduced Figure : Surface fields and air temperature distributin at 9 JST; (a) bserved and (b) reprduced Since the three systems played dminant rles t frm thunderstrms in this heavy rainfall event, it is necessary t reprduce thse three systems accurately in rder t estimate misture transprtatin during this heavy rainfall event. The mdel is successful in reprducing three systems in Figure (b). Mrever, the three systems reprduced by mdel cnverge at the same lcatin as the bserved systems did (in Figure ). Figure (a) shws hurly accumulated rainfall estimated frm Radar-AMeDAS data at :JST. Radar-AMeDAS data is the bjectively analyzed weather radar rainfall data that are calibrated with AMeDAS. Peak rainfall amunt and distributin f the rainfall regin which are reprduced by mdel agree with thse f Radar-AMeDAS data, except that the reprduced rainfall regin is lcated n km easterly cmparing t Radar-AMeDAS data. Misture transprtatin Misture transprtatin is examined frm the distributin f water vapr maxing rati (q vp ) and lw-level. The distributin f reprduced q vp and vectr at altitude f m is shwn in Figure. The lcatins f surface bservatry statins in Kant plain are shwn in Figure as pen circle. At JST, in Figure (a), it was eight hurs befre bserving heavy rainfall. At that time, mst f Kant Plain is cvered with lwer q vp, frm g/kg t 7g/kg, than q vp near seaside. The average value f reprduced q vp n statin pints at JST is 7.8g/kg, and bserved ne was 8.7g/kg. The reprduced q vp is % less than the bserved ne. At 7 JST, in Figure (b), it was fur hurs befre bserving heavy rainfall. At that time, there is lw q vp air at Tky (S in Figure ) yet, hwever, high q vp air was transprted t Tky by nrtheasterly with very clear frnt line. Figure shws bserved q vp at 8 (b) C 8 mm

4 The Seventh Asia-Pacific Cnference n Wind Engineering, Nvember 8-, 9, Taipei, Taiwan (a) 9 JST g/kg NW NE NW S S NE S (b) 7 JST Figure : Reprduced q vp field at altitude f m (shade) and terrain (cntur), lcatin f bservatry statins (pen circle). 8 g/kg 8 Nrhthwesterly NW NW S Sutherly S S Observatin Reprductin Nrhtheasterly NE NE SITE Figure : Observed q vp at 7JST. Site names are same in Figure (b). sme statins (in Figure ). This Figure included reprduced q vp. Observed q vp at NE where nrtheasterly cvered was g/kg greater than nes at S, S and S where sutherly cvered. The mdel generally underestimates q vp cmparing t the bserved q vp, hwever, the difference f amunt f q vp between tw systems, nrtheasterly and sutherly systems, is almst same t bserved ne. In Figure 7, vertical transprtatin f q vp is investigated alng the SW-NE bld line shwed in Figure. The SW-NE line is almst perpendicular t the cnvergence line (in Figure (a) dented as pen-dtted-ellipse) and passes thrugh lcal heavy rainfall regin (in Figure (b)). At : JST, High q vp air which is.km in thickness cmes frm nrtheasterly side, and anther high q vp air which is.km in thickness cmes frm suthwesterly side. These tw high q vp air cnverge at 8: JST. The high q vp air frm suthwesterly side tends t g upward. Anther high q vp air frm nrtheasterly side runs upn the suthwesterly air, and prevents the suthwesterly air t g upward. A part f high q vp air frm nrtheasterly side, hwever, tends t g under the sutherly air, because the nrtheasterly air is cler and heavier than the sutherly air. Then, these tw high q vp air stagnate at the cnvergence pint. At 9: JST, the high q vp air frm nrtheasterly side is pushed by nrtheasterly cld utflw (pen arrw in Figure 7(c) (d)) assciated with previus rainfall. Then high q vp air is frced t g up at : JST (in Figure 7(d)). The high q vp air which is frced t g up is cndensed at upper-level and discharged the latent heat. This latent heat may cntribute t enhance the cnvective activity. It des nt clarify which high q vp air is frced t g up yet. Therefre, t investigate ptential misture surces f the heavy rainfall, we pursue the ascent air parcels f the cnvective cluds using backward-in-time lagrangian trajectry mdel supplied by Read/Interplate/Plt versin (RIP). This trajectry mdel cmputes the three-dimensinal displacement f an air parcel using interplating technique fr time and space. As input fr the trajectry mdel, we use the MM-utput fields in the inner dmain which have high reslutin fr space (km) at every minutes. Ttal time perid f trajectry cmputatin is hurs with time increment f minutes. Initial lcatin f air parcels fr backward trajectry calculatin are set within a radius f.km frm the strng ascent regin (vertical velcity is Height (km) (a) : JST (b) 8: JST (c) 9: JST (d) : JST g/kg 8 SW (km) NE SW (km) NE SW (km) NE SW (km) NE Figure 7: Crss sectin f q vp n the SW-NE line (in Figure (b))

5 The Seventh Asia-Pacific Cnference n Wind Engineering, Nvember 8-, 9, Taipei, Taiwan Nrthwesterly airflw grup Sutherly airflw grup Nrtheasterly airflw grup Statinary grup q vp (g/kg) Altitude(km) Figure 8: Backward trajectries. The radius f circle dentes q vp value f parcel. Shade clr dentes terrain altitude. And circle filled clr dentes the altitude f parcel. mre than m/s) f grwing cnvective clud. The hrizntal distributin f the air parcel starting pints is unifrm hrizntal grid with m spatial grid at the cnstant height f km. Figure 8 shws backward trajectries which are calculated fr air parcel within the strng ascent regin preceding the heavy rainfall shwed in Figure (b). The radius and the filled clr f circle at terminal f each trajectry dente water vapr mixing rati (q vp ) and altitude f air parcel at that time, respectively. The air parcels can be classified int fur airflw grups by its riginate regin. These fur airflw grups cnsist f the three airflw grups frm a lng distance, nrtheasterly, nrthwesterly and sutherly airflw grups, and statinary grup which is frm a shrt distance. Figure 9 shws the time series f mean diagnstic values alng the trajectries within each grup. These mean diagnstic values are altitude, water vapr mixing rati (q vp ) and virtual ptential temperature. Three airflw grups frm a lng distance, nrtheasterly, nrthwesterly and sutherly airflw, slwly change the quantity f mean q vp during hurs until each airflw grups g upward rapidly at : JST. These three airflw grups, hwever, well cnserve the riginal quantity f mean q vp and mean virtual ptential temperature. In Figure 9(a), it is fund that the air parcels f nrtheasterly and sutherly airflw grup riginate in lw-level atmsphere. The air parcels f nrtheasterly airflw grup cntain the highest mean q vp amng fur grups. Mean q vp cntained by sutherly airflw grup is g/kg at least less than thse cntained by nrtheasterly airflw grup. The air parcels f nrthwesterly airflw grup riginate in middle-level atmsphere and keep its altitude until : JST. The air parcels f nrthwesterly airflw grup cntain the lwest mean q vp amng fur grups. The air parcels frm statinary grup riginate in lw-level atmsphere, as thse f nrtheasterly and sutherly airflw grups riginate in. In Figure 9(c), it is fund that virtual ptential temperature f statinary grup is higher than thse f nrtheasterly and sutherly airflw grups. It indicates that air parcels frm statinary grup are lighter than thse frm nrtheasterly and sutherly airflw grups. In Figure 9(a), the air parcels frm statinary km g/kg K (a) Altitude NE Grup NW Grup S Grup Statinary Hur (JST) (b) water vapr mixing rati NE Grup NW Grup S Grup Statinary Hur (JST) (c) virtul ptential temperature NE Grup NW Grup S Grup Statinary Hur (JST) Figure 9: Time series f diagnsis alng trajectries. (a) altitude, (b) water vapr mixing rati and (c) virtual ptential temperature. Table : Mean diagnstic value fr the airflw grups Grup Rati (%) Altitude (km) q vp (g/kg) Nrtheasterly. 8. Sutherly.. Nrthwesterly.9. Statinary 7..

6 The Seventh Asia-Pacific Cnference n Wind Engineering, Nvember 8-, 9, Taipei, Taiwan grup are gradually lifted t an altitude f km during the nrtheasterly and sutherly airflw appraching t cnvergence regin, because the nrtheasterly and sutherly airflw g under the statinary grup. T get an insight in cntributin f fur airflw grups t the amunt f q vp in a cnvective clud, the cntent rati f each airflw grup within a cnvective clud is calculated and shws in table. The cntent rati is the rati f the number f each grup s air parcels t ttal number f air parcels. This table als shws the mean value f altitude and amunt f q vp fr the trajectries within each grup at : JST when trajectry starts. While nrtheasterly airflw grup cntains the highest mean q vp amng fur grups, the cntent rati f nrtheasterly airflw grup takes nly %. The air parcels frm sutherly airflw grup cntains less mean q vp than thse frm nrtheasterly airflw grup des, but the cntent rati f sutherly airflw grup takes twice as much as thse f nrtheasterly airflw grup. The air parcels f nrthwesterly airflw grup are very dry and the less cntent rati amng fur grups. Therefre, nrthwesterly airflw can nt cntribute misture t cnvective cluds. Sensitivity study Lw-level air plays dminant rles in misture transprtatin. Since the lw-level airflw is strngly affected by surface bundary cnditin, the effect f urban area n lwlevel airflw is estimated in this sectin. In rder t estimate the effect f urban area n lwlevel airflw, mre tw numerical experiments are perfrmed: WITHOUT_AH case and NON_URBAN case. The WITHOUT_AH case has the same mdel design as the cntrl experiment (mentined abve sectin, CONTROL case) except anthrpgenic heat is nt included. In the NON_URBAN case, the land-use index f urban area is replaced with the ne f farmland. Figure shws hurs accumulated rainfall reprduced in each case. During these hurs, mst f rainfall in Tky was bserved. In the WITHOUT_AH case, distributin f heavy rainfall regin is shifted easterly cmparing t the CONTROL case. In the NON_URBAN case, amunt f rainfall seems t be less than the ther tw cases. T investigate the relatinship between inflw f q vp int rainfall regin and rainfall amunt, the misture budget within rainfall regin is examined. The misture budget within a regin is given by Eq. (), FV = SV + R + SFC WithutRa inf all () where FV dentes flux cnvergence f q vp int the regin, SV dentes time increment f q vp in the regin, R dentes rainfall amunt during a secnd, SFC WithutRainfall dentes net time increment f cndensate withut rainfall (e.g. clud water, rain water, ice, snw and graupel). Figure shws the misture budget within the vlume which has the big bx shape in hrizntal (Figure ) f the height frm surface t the tp f mdel. And Figure als shws the hrizntal cnvergence f lwer-level airflw f the height frm surface t.km. mm 7 (a) CONTROL (b) WITHOUT_AH (c) NON_URBAN Figure : hurs accumulated rainfall amunt. A big square dentes cntrl vlume t calculate the misture budget. Small squares dente the regin int which air parcels are injected initially fr backward trajectry calculatin.

7 .: 8:. -. : 8: FV SV R SFC WithutRainfall - : : : : qvp budget (g/m/s) FV SV R SFC WithutRainfall.: 8:. -. : 8: FV SV R SFC WithutRainfall - : : Cnvergence (/s) - Cnvergence (/s) qvp budget (g/m/s) Cnvergence (/s) qvp budget (g/m/s) The Seventh Asia-Pacific Cnference n Wind Engineering, Nvember 8-, 9, Taipei, Taiwan : :.: 8: : : 8: : :. -. : (a) CONTROL (b) WITHOUT_AH (c) NON_URBAN Figure : misture budget (upper rw) and lw-level airflw cnvergence (lwer raw). The psitive value f cnvergence dentes inflw int the cntrl vlume. In all three cases, the lw-level airflw cnvergence reaches psitive peak value at abut 9 JST, and then the lw-level airflw cnvergence changes t negative after rainfall amunt (R in Figure ) reaches peak value at JST. The negative value in the lw-level airflw cnvergence after JST is caused by the cld-utflw assciated with rainfall. In the WITHOUT_AH case, the misture budget and lw-level airflw cnvergence are almst same as thse f the CONTROL case befre JST. This is suggested that anthrpgenic heat affects the distributin f rainfall regin (Figure ), but has little affectin t misture budget. In the NON_URBAN case, the value f lw-level airflw cnvergence is negative befre 7 JST. This means that upper level airflw cnvergences befre 7 JST in NON_URBAN case. In NON_URBAN case, the psitive peak value f the lw-level airflw cnvergence is less than thse in the ther tw cases. Since the lw-level airflw cnvergence as the transprter f misture is negative r weaker in NON_URBAN case, inflw f qvp (FV) in the NON_URBAN case is less than thse in the ther tw cases. This suggested that the existence f urban area enhances the lw-level airflw cnvergence which plays significant rle in misture transprtatin int rainfall regin. T investigate the difference f ptential misture surces between three cases, CONTROL, WITHOUT_AH, NON_URBAN, backward trajectry analysis are perfrmed. The way in which air parcels are injected int a strng ascent regin is the same as mentined in previus sectin. Figure shws backward trajectries which are calculated fr air parcels within sme strng ascent regins preceding rainfall shwed as small bxes in Figure. It fund that the air parcels within strng ascents can be classified int fur airflw grups by its riginate regin in all three cases. These fur airflw grups are mentined previus sectin; these airflw grups cnsist f nrtheasterly, nrthwesterly and sutherly airflw grups and statinary grup. The cntent rati is calculated and shws in Figure. While the sum f the cntent rati f the nrtheasterly and the sutherly airflw grups take % bth in Altitude(km) q vp (g/kg) Altitude(km) q vp (g/kg) Altitude(km) q vp (g/kg) (a) CONTROL (b) WITHOUT_AH (c) NON_URBAN Figure : Backward trajectries as in Figure 8 but fr air parcels within same strng ascent regins in each cases.

8 The Seventh Asia-Pacific Cnference n Wind Engineering, Nvember 8-, 9, Taipei, Taiwan CONTROL case and in WITHOUT_AH case, thse in NON_URBAN case take nly %. On the ther hand, the cntent rati f the nrthwesterly airflw grup in NON_URBAN case is % much than bth in CONTROL case and in WITHOUT_AH case. The cntent rati f the statinary grup is nt different amng the three cases. As the afrementined remark, the air parcels Rati CONTROL WITHOUT_AH NON_URBAN NE S NW Statinary Grup Figure : The rati f the number f each grup s air parcels t ttal number f air parcels. f the nrtheasterly and sutherly airflw grups are very humid lw-level air, and the air parcels f the nrthwesterly airflw grup are dry middle-level air. This suggests that the amunt f q vp transprted int strng ascent regin in NON_URBAN case is less than bth in CONTROL case and in WITHOUT_AH case. Cnclusins Misture transprtatin during lcal heavy rainfall event arund Tky n August is investigated using messcale mdel. And, the effect f urban area n lcal heavy rainfall is estimated als in this study. Feature f the misture transprtatin during heavy rainfall event n August are summarized as fllws;. The mdel successfully reprduces lw level systems preceding rainfall. And hurly rain fall amunt and distributin f the rainfall regin reprduced by mdel agree with thse bserved.. The air parcels within the cnvective cluds can be classified int fur airflw grups by its riginate regin; fur airflw grups cnsist f the three airflw grups frm a lng distance, nrtheasterly, nrthwesterly and sutherly airflw, and statinary grup which is frm a shrt distance. Amng these fur airflw grups, nrtheasterly airflw grup has the largest amunt f q vp, and sutherly airflw grup has the secnd. These tw airflw grups, nrtheasterly and sutherly airflw grups, wuld cntribute mainly t misture transprtatin int cnvective cluds.. Anthrpgenic heat affects the distributin f rainfall regin, but has little affectin t qvp budget.. Existence f urban area enhances the lw-level air cnvergence which plays significant rle in misture transprtatin int rainfall regin. References Fujibe, F., Sek, H., Shji, Y. (), Relatin between precipitatin distributin and surface patterns in the Kant Plain in the afternn f summer, Tenki, Vl., N., (In Japanese) Kawamt, Y. and Oka, R. (8), Develpment f urban climate analysis mdel mm: part, Imprvement f parameterizatin f grund surface and incrpratin f anthrpgenic heat release, Jurnal f Envirnmental Engineering (Transactins f AIJ), Vl.7, N., -. (In Japanese) Kawan, S., Ichikawa, H. and Hayashi, H. (8), Thunderstrm causing shrt time heavy rainfall at the center f Tky n August th, Tenki, Vl., N., 8-8. (In Japanese) Sek, H., Shji, Y., Fujibe, F. (7), Evlutin and airflw structure f a Kant thunderstrm n July 999 (Nerima heavy rainfall event), Jurnal f the Meterlgical Sciety f Japan, Vl.8, N., -77. Yshizaki, M., Uekiy, N., Sek, H., Takayama, H., Kusunki, K. and Tsukuba Area Precipitatin Studies Grup (998), Analysis f thunderstrms ccurred ver Kant Plain n August 99, Tenki, Vl., N., 9-. (In Japanese)