Koji Okumura (1), Takehiko Satomura (1), Taikan Oki (3)
(1) Division of Earth and Planetary Science, Graduate School of Science, Kyoto University
(3) Institute of Industrial Science, University of Tokyo
The behavior of tropical rainfall activity was investigated using
meteorological radar data from the GEWEX Asia Monsoon Experiment in
Tropics (GAME-T).Radar data was obtained at Chiang Mai(98.9E,18.8N) from 1998(GAME IOP) to 2000, and at Om Koi (98.4E, 17.8N) from 1999 to
2000. In rainy season, Chiang Mai radar performs ordinary
observation with 240km radius, once per an hour. In addition, extra
observation was made during the GAME observation program with 120km
radius, once or twice per an hour. Om Koi radar data had better time
resolution. This radar performs low level scans whose elevation angle
ranged from 0.5 to 2.9 , once per about 5 minutes.
Diurnal variations of Chiang Mai radar echoes were as follows; 1)In the afternoon, many convective echoes appeared around the radar and began to expand and merge with other echoes.
2)Total echo area reached its maximum at about 15-16 LT everyday.
3)Echoes completely vanished by 5-6 LT of the next day. Daytime solar
heating is thought to be a main cause of the diurnal variation
described above. After the sunset, however, new intense small echoes that looked like convective echoes were observed at the edge of some expanding echo regions. There might be other factors that increased atmospheric instability. One echo area also showed diurnal
variation. It attained maximum value at 20 LT owing to a
kind of organization, although radar echo amount averaged over whole radar coverage reached its maximum at 15LT.
Each day was classified into two categories. The first one has a period of echo area variation that was shorter than 24 hours, and the second one was longer. Total rainfall amount of the shorter period category was larger than the longer one.
Being averaged in the same manner as Chiang Mai radar data, Om Koi
radar observation showed similar characteristic to Chiang Mai's.
To study effects of topography on diurnal variation of precipitation, averaging region was divided by the mountains (Danwa Range) which run
north to south, middle of Om Koi radar coverage, into three fragments; (1)east plain of mountains (2)mountainous area (3)west plain of mountains. In the west of mountains, echo amount did not have strong diurnal cycle. In the mountainous area,
maximum time of echo was 15 LT, while in the eastern area radar echo
reached maximum at 20 LT. From watching movie of radar images,
this diurnal cycle difference among three region seemed to be caused
by eastward moving rainfall clouds that initiated almost the same time in a day at western side of mountains.
Submittal Information
Name :
Date :
Koji Okumura
31-May-01-23:56:19
Organization :
Theme :
Division of Earth and Planetaty Science, Graduate School of science, Kyoto University
