The aim of this study is to examine the structure of convective cells in the rainfall systems with horizontal scale from 2 to 100 km in the western Pacific warm pool region.

Doppler radar observation (observation range is 150 km in radius) data was obtained in the Republic of Palau, in the western Pacific warm pool region, from December 2004 to January 2005. With the radar reflectivity data, the characteristics of number, area, and echo top of the convective area were analyzed objectively.

Analyses on the characteristics of the convective area were made for the period from December 15, 2004 to January 15, 2005. A rainfall system was detected by a threshold of 10 dBZ radar reflectivity on the horizontal plane at the altitude of 1km. A rainfall system was categorized as MCS (Mesoscale Convective System) with horizontal scale longer than 100 km or sub-MCS with that of less than 100 km. Analysis period was divided into MCS periods and sub-MCS periods. MCS period was defined as the period that MCSs existed in the observation range, and sub-MCS period was defined as the period without MCS for more than 6 hours in the observation range. Characteristics of convective areas in MCS period and sub-MCS period were examined. A convective area was detected by a threshold of 30 dBZ radar reflectivity on the horizontal plane at the altitude of 1 km. The convective area was a area including one or more convective cells. During the analysis period, no period without The rainfall systems for more than 3 hour was recognized.

As a result of analysis, there were 19 sub-MCS periods. The total time of them occupied about 60 \% of the whole analysis period. Except two or three cases when cloud clusters passed over observation range, most of the MCS periods lasted less than a half day. The analyzed echo top (5.4 km) of sub-MCS periods was 2 km lower than that of MCS periods (7.4 km).

For the sub-MCS period with the longest duration, number, area, and echo top of the convective area were analyzed. Most of the convective areas in the period were less than 25 km$^2$ and they were composed of single convective cell. Averaged echo top during the period (4.5 km) were 0.9 km lower than that of 19 sub-MCS periods (5.4 km). The number and total area of the convective area had a peak in the afternoon (around 15 LST), and the average echo top and average area of the convective area for each radar volume scan had a peak within 6 hours after that. The highest echo top of the convective area tended to appear around the time of the peak of the average area of the convective area. When the number of the convective area became largest, the isolated convective areas included in the same sub-MCS lined up on a line. Their echo tops of most of the isolated convective area were below the altitude of 5 km. However, one of adjacent convective areas on the line exceeded the average echo top (5.4 km) of 19 sub-MCS periods and reached around 7 km.

A period of extremely small number, area, and echo top of convective area was included in the longest sub-MCS period. During the period, dry air was from the tropopause to the altitude of 1 km〜2 km. Strong inversions were at 1〜2 km and 4〜5 km. Most of echo tops were at 3 km in height near the stable layer. After the duration of small number of the convective area for about 9 hours, the convective area composed of only one convective cell with the area of 20 km$^2$ had an echo top penetrating the inversions of 5.5 km in height. Afterward, the increase of the number, area, and echo top of the convective area were recognized again.

In this study, the charactersistcs of convective cells in the long lasting stable condition above 2 km were revealed quantitatively from the analysis of a variation of number, area, and echo top of the convective area on the western Pacific warm pool region.