Analysis of the Deep Convective Activity Over the Western Pacific and Southeast Asia

Part I: Diurnal Variation

Abstract

Using the infrared (IR) irradiance data observed by GMS-1 geostationary satellite, it was tried to quantitize the deep convective activity in a manner which is physically more interpretable than the conventional areal average. This was partly accomplished by defining the intensity index of the deep convective clouds within the 1° longitude-latitude square mesh in the manner which takes the horizontal variation of the observed temperature and the vertical distribution of the atmospheric temperature into account.
Diurnal variation of the deep convective activity was investigated by using the abovementioned intensity index for the period during northern hemispheric winter (December 1978January 1979) and summer (July-August 1979). During northern winter, the amplitude of the diurnal variation is large over the Indonesian region and the northern Australia. Within this area it was found that there exist a distinct contrast in the phase of diurnal cycle between the land and the adjacent ocean. Composite analysis has revealed that the land shows the suppressed convective activity in the morning hours with the minimum around 9 o'clock local time. The convective activity is rapidly enhanced in the afternoon and reaches the maximum at around 18 o'clock local time. In contrast, the adjacent ocean clearly shows the enhancement of the convective clouds in the local morning and the suppressed convective activity in the afternoon and night.
During northern summer the intense diurnal variation was observed over the southern part of Tibetan Plateau. The amplitude is also large over the western Pacific to the east of Philippines. Composited diurnal cycle shows that there exists a contrast between the continent and the ocean, being similar to the one observed during northern winter. The continent and the large islands generally show the minimum convective activity around 9o'clock in the local morning and the maximum enhancement aound 18 o'clock in the evening. In contrast the oceanic area like western Pacific shows the maximum enhancement of the deep convective clouds around 6- to 9 o'clock local time in the morning and the suppressed activity in the afternoon. In addition to this land-ocean contrast, the evidences which suggest the regional effect on the diurnal variation are also discussed.