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 April 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.
The Hydrometeorological ARray for Isv-Monsoon AUtomonitoring (HARIMAU), a 5-year project under the Japan EOS Promotion Program (JEPP) contributing to the Global Earth Observation System of Systems (GEOSS), has begun in 2005 to set up a radar-profiler network for observing the world's most active convective activities over the Indonesian Maritime Continent (IMC). Rainfall and wind distributions are displayed in nearly real time on the internet. Both scientific understanding and practical concepts on intraseasonal variations (ISVs) interacting with larger (seasonal and interannual) and smaller (diurnal or local) scale phenomena will be established. These are expected to contribute greatly and directly to climatic disaster prevention over the IMC and to global climate change assessment through studies on the global effects of the IMC-induced variations such as El Nino, and through construction of the first climatic database over the IMC.
In this study, the processes responsible for an intense wind event that occurred in west Sumatera on November 19, 2006, during the first campaign of the Hydrometeorological ARay for Isv-Monsoon AUtomonitoring (HAR-IMAU) were investigated.Strong winds of 17 m s À1 and a sudden temperature drop of 5 K were observed at an X-band Doppler radar site associated with the passage of a convective system, and some houses were severely damaged. The convective system developed under an environment of strong low-level easterly vertical shear associated with the easterly region of an equatorial Rossby wave. The northern part of the convective system possessed qualitatively similar structures to midlatitude bow echoes, including the convex shape of the convective line, a descending rear-inflow jet positioned at its apex, and mesoscale vortices on both sides of the rear-inflow jet. The low-level wind behind the convective system formed a channel of strong easterly wind as it passed through an area of relatively low topography in the mountain range. The enhanced easterly wind was thought to contribute to the formation of the bow echo-like structure in the northern part of the convective system. This easterly rear-inflow jet was further accelerated in the convective system and descended near the leading edge, forming divergent strong winds at the surface.The sounding data that were taken after the passage of the convective system indicated that dry air appeared in the lower troposphere associated with an enhancement of the southerly component of the wind. An analysis of objective reanalysis data suggests that the southerly was probably associated with westward-propagating mixed Rossby-gravity waves with a period of approximately 5 days. It is suggested that the dry air intruded into the convective system across the back edge of the precipitation area and caused enhanced evaporative cooling, which resulted in the e¤ective downward transport of the enhanced easterly momentum.
Observations of tropical convection and cloud clusters with two X-band Doppler radars were carried out on Manus Island, Papua New Guinea for two and a half months from 12 November 1992 during the Intensive Observation Period (TOP) of the TOGA-COARE. Several different types of clouds were observed by Doppler radars during the period, such as squall lines, convective clouds and stratiform clouds associated with cloud clusters, isolated convective clouds over the island in the day time, etc. Outlines of the observations on Manus Island and brief reviews of the results of these observations are described. Preliminary summaries of the analyses on the observations are: 1) A warm rain process dominates at the beginning stage of radar echo development and the maximum radar reflectivity is recognized at this stage, 2) The maximum echo top height is observed within 3 hours from the first echo, 3) The maximum echo area within the radar range was analyzed a few to several hours later than the time of maximum echo top height corresponding to the size of cloud cluster, 4) In the long-lasting stratiform echo, updrafts independent from those below the melting layer were identified above the melting layer. It is concluded that further studies utilizing these observational data may reveal the structure and the evolution mechanism of tropical cloud clusters.
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