Wind patterns and divergence field

4. Wind patterns and divergence fiel d on each category

19a). In addition, the amount of di vergence became large. These results showed that valley br eeze cir culati on devel oped in t he northern Kanto region in the mor ning f or this cat egor y as well. The inland di vergence area corresponding to t he down -flow area of valley br eeze circul ation was noted until 13: 00, whi ch was cl ose to the aver age for sunny li ght -wi nd days. On t he other hand, from Chiba to Ibaraki Prefectures, t he east wind from Kashi ma -nada did not appear and the south wind was predominant.

From 13: 00 (Fi g. 19c) to 15:00 ( Fi g. 19d) , t he sout herl y wind became even stronger, and the center of t he convergence area correspondi ng t o t he sea breeze front shifted to southern Saitama Prefecture. Moreover, t he di vergence area along t he pref ect ural border of Gunma and Sait ama accompanying valley breeze circul ation and the weak wind area around Saitama Prefect ure became indistinct , i ndicating a transiti on to ESB. On the other hand, after the peak of 13:00, wi nd speed decreased on t he coast of Sagami Bay, and t he di vergence area corr espondi ng t o the down -flow ar ea of the sea br eeze circulation gr aduall y shifted t o t he pref ectur al bor der of Kanagawa and Tokyo. This is consist ent with the obser vati ons by Fuj ibe and Asai ( 1984), which showed that i n the afternoon, the pressure gr adient which was concentr ated i n t h e coastal ar ea spr ead inland and the sea breeze stopped bl owing earlier i n the lower layer at t he coast. On the other hand, the di vergence area near Tokyo Bay was noti ceable throughout t he day, and the di vergence area spread f rom Tokyo Bay t o t he land area of Chi ba and Ibaraki Prefectures. In this cat egor y, south t o southwest wi nd was pr edomi nant i n Chi ba and Ibaraki Prefectures to

the east ern side of t hi s di vergence area , and the east wind from K ashi ma -nada was not obser ved thr oughout the day. On t he other hand, t he wi nd directi on was southeast under t he i nfluence of valley br eeze in t he wester n side of this di vergence area. It i s thought t hat these are the reason f or continuous di vergence area which was for med from Tok yo Bay t o the land area. At 17:00 (Fi g. 19e) , wi nd over the mount ains in t he nort hern Kanto regi on became weaker accompanyi ng the weakeni ng valley breeze and t he bor der with t he plain became a convergence area. The convergenc e ar ea accompanyi ng t he sea breeze front t hat penetrat ed the i nland began t o overlap this convergence area, and it became indistinct .

Categor y B (Fi g. 20) had a diur nal vari at ion quit e si mil ar t o the wi nd system i n Categor y A. In response t o Cat egor y B having a sli ghtl y smaller geostr ophi c wind speed compared with Categor y A, t he sout h t o southwest wind was weaker. In addition, the penetrat ion of t he sea breeze was sli ghtl y slower and the weak wind ar ound Saitama Prefect ure was seen over a wider area. Moreover, the wind directi on was more from southeast compar ed with Categor y A. In the Kashi ma -nada coast of Ibaraki Pref ecture, a sout heast wind was confir med i n the afternoon. A di vergence ar ea appear ed distinct l y around Tokyo Bay thr oughout the day si mi l ar to Categor y A, but its inl and expansion was small.

Categor y C (Fi g. 21) had a notabl y weak geostrophic wi nd, and it showed diurnal variati on closest to that of the average wi nd system o n sunny light -wind days ( Fi g. 7). On the coast of Sagami Bay, south erl y wind blew,

easterl y wind bl ew on the coast of Kashi ma -nada, and the wind orthogonal to the coastline appeared on the coast of Tokyo Bay, respecti vel y ; t ypical sea breeze systems were predominant. From 09: 00 (Fi g. 21a) to 11: 00 (Fi g. 21b) , wind was gen erall y weak compared with Categories A and B, but in additi on to the coast of Sagami Bay and Tokyo Bay, sea breezes ort hogonal to the coastline were noted along t he coast of Kashi ma -nada fr om Chiba to Ibaraki Prefect ures , and convergence areas were for med correspondi ng to the sea breeze front f rom Kanagawa P refecture t o the Tokyo Metropolit an area , and along the coast of K ashi ma -nada. In addi tion, around Chi ba Prefecture, a discontinuit y in t he wind dir ection was noted bet ween the east erl y wind from the Paci fic Ocean and south erl y wind from Tokyo Bay, creating a convergence ar ea. On the other hand, i n the nort hern Kant o region, a southeast erl y wind system towar d t he mountains appeared at t he same ti me as the for mation of a di vergence area from the pr efectur al bor der of Gunma and Saitama, and t he development of valley breeze was noted. Moreover, in southern Sait ama Pref ecture, after a weak north wi nd was first noted, a weak wind area was noti ced in a sli ghtl y larger ar ea.

In the aft ernoon, t he sea bree ze was i ntensified , and the cent er of the convergence area near the Tokyo Metropol itan area shifted inl and with t he penetrati on of sea br eeze. However, compar ed wit h Categories A and B, its penetrati on was somewhat sl ow. An area of di vergence around Tokyo Bay was confir med until around 15:00, but it did not spread i nland si gnificantl y. In contrast , a convergence area appear ed surrounding Tokyo Bay from t he

prefect ural bor der of Tokyo and Saitama t o Chiba Prefecture, and it r emained there. In addition, t he d i vergence area at the pr efect ural border of Saitama and Gunma corresponding to the valley breeze circul ation gr aduall y decreased in area and became weaker. However, it was still obser ved for longer than in Categories A and B, conti nuing until about 15: 0 0. Based on this, the transition t o the ESB in Categor y C was late compared with Categories A and B.

Categor y D ( Fi g. 22) is also characteri zed by a weak geost rophic wi nd si milar to Categor y C. However, it showed a diur nal variati on i n wind system with a sli ghtl y larger east wind component in response to a larger east wind component of t he geostrophi c wind. At 09: 00 ( Fi g. 22a) , a di vergence area was noted from the prefect ural border of Gunma and Sait ama to Tochi gi Prefect ure, confir ming the appearance of a vall ey breeze. However, the southern Kanto regi on was generall y covered by nort heast wind, and there was al most no penetration of sea breeze. At 11:00 ( Fi g. 22b), t he devel opment of vall ey br eeze and expansi on of t he di vergence area were noted i n t he norther n Kanto Region, and the east erl y wind became str ong on the coast of Kashi ma -nada and Chi ba Pref ecture. The penetration of t ypical sea br eeze was li mit ed to the coast of Sagami Bay and Tokyo Bay, and a convergence area developed from K anagawa Prefect ure to the Tokyo Metropolit an area .

At 13:00 (Fi g. 22c), south wi nd was obser ved ar ound Kanagawa Prefect ure, and southeast wind was obser ved ar ound Tokyo Bay. Convergence areas were

also linked i n the ar ea surr ounding Tokyo Bay from Kanagawa Prefect ure, through the Tokyo Metropolitan area to Chi ba Pr efectur e. At 15:00 (Fi g. 22d) , south wind spread to the prefectur al border of Tokyo and Sait ama, and a strong convergence ar ea was obser ved her e . At the same ti me, southeast wi nd from the Pacific Ocean con verged ar ound Chiba Prefecture; thus, these convergence ar eas j oined. In contr ast, a di vergence area along t he pref ectur al border of Gunma and Sait am a was seen until this ti me, whi ch i s si milar to Categor y C; it became weaker ther eafter.

In Cat egor y E (Fi g . 23), east erl y wind was generall y the predominant wi nd.

From 09:00 ( Fi g. 23a) to 11:00 ( Fi g. 23b) , from the prefectur al bor der of Gunma and Saitama t o Tochi gi Pr efect ure, di vergence area s were noted , and valley wind was observed, which was si mil ar to Categor y D. However, most areas in the sout her n Kanto r egion and the coast of Kashi ma -nada were covered by northeast wind. The penetration of sea breeze was bar el y obser ved.

In r esponse t o strong northeast wind ar ound Tokyo Bay, a convergence area was for me d from K anagawa Prefecture to t he Tokyo Metropolitan area .

In the afternoon, east erl y wind was obser ved i n most of t he Kanto region, and wind speed increased furt her. The penet ration of sea breeze fr ont was not clear, but at 15:00 ( Fig. 23d), weak sout h wi nd i n Kanagawa Pr efecture and southeast wi nd ar ound Tokyo Bay were present, cr eating a convergence area along the coast of Tokyo Bay and the Tokyo Metropolitan area . On t he ot her hand, a di vergence area accompanying valley wi nd ci rculati on i n t he norther n Kanto regi on appeared until ar ound 15:00, si milar t o the other categories, and

gr aduall y became indi stinct t hereaft er.

Here , t o confir m that the convergence/ di vergence areas as thus far described correspond to the upward and downwar d fl ow areas i n the lower at mospheric layers, composite aver aging of the verti cal wind obt ained by the anal ysi s dat a of MSM was conducted . Fi gure 24 shows t he di stribution of vertical wi nd speeds at 975 hPa (approxi matel y 300 m above sea level) and surface wi nd at 15:00 for each categor y of sunny li ght -wi nd days during the li mited peri od 2006–2011 ( 82 days) when the MSM data are available . Although a detailed discussion coul d not be per for med due to the lack of cases, t he charact er istics of the surface wind were si mi lar to the obser vational data in all cat egori es . The up -flow areas around the pr efectur al border of Tokyo and Saitama and the down -flow area s in Tokyo Bay and the norther n Kant o region were obser ved. T he convergence and di vergence areas corresponded t o the upwar d and downward flow areas, respecti vely. Fi gur e 25 shows t he meri dional and verti cal component s of wi nd on the vertical north –sout h cr oss sect ion along t he 139.5 °E line at 15:00. In Categor y A, sea and valley breezes already bl ew continuousl y. On t he other hand, i n Categories B, C, and D, t he upward flow at t he sea breeze fr ont and the downward fl ow accompanyi ng the valley breeze system were obser ved. The existence of valley br eeze and sea breeze ci rculations t hat was expected from the convergence/di vergence fiel ds of the surface wind was observed in the same way.