During the latter half of the first CPEA campaign (CPEA-I), X-band Doppler radar (XDR) observation was carried out from 10 April to 9 May 2004 in west Sumatera. In this paper, characteristics of precipitating systems and their relation to the Madden-Julian Oscillation (MJO) are investigated based on the analysis of the XDR data.
Significant diurnal variations of precipitation were observed both in the convectively inactive and active phases of MJO, in which the area of intense echoes with reflectivity greater than 40 dBZ attained a maximum around 16 LST, whereas the area of weaker echoes attained a maximum in the evening. However, while the area of weaker echoes showed significant drop in the evening (21 LST) in the inactive phase, such a drop occurred in the early morning in the active phase. During the convectively inactive phase from 10 to 22 April, the precipitation was caused by systems that formed within the observation area. Shallow convective cells appeared in the mountain range around midday, which subsequently developed into deep convective systems. These convective systems then migrated southwestward in many cases, but in some cases they were observed to split and then migrated both southwestward and northeastward. During the convectively active phase from 23 April to 6 May, much of the precipitation occurred associated with the development of pre-existing cloud systems within super cloud clusters (SCCs). While the precipitation area with weak to moderate reflectivity became considerably larger than that in the convectively inactive phase, the area and the top heights of intense echoes were generally suppressed. Environmental factors responsible for these modulations in precipitation are also discussed.
The structure and evolution of precipitation systems were further investigated for some selected cases. The precipitation system observed on 17 April showed typical evolution during the convectively inactive phase, whereas the precipitation system on 11 Apri1 was observed to split into two systems that migrated southwestward and northeastward. The northeastward migrations of the precipitation systems occurred as a result of successive generation of new convective elements on the northeastern side of old ones. The precipitation systems observed on 23-24 April and 5-6 May occurred during passages of SCCs, and moved eastward associated with intrusions of low-level westerlies. These systems were composed of convective regions around the leading portions of the low-level westerlies, and stratiform regions behind. Analyses of reflectivity and Doppler velocity fields for the latter case suggested that the elevated orography in the west Sumatera temporarily blocked the eastward propagation of the system and the intrusion of the low-level westerly.