One of the major forecast problems over the tropics of southeast Asia is prediction of heavy rain. Key characteristics of the tropical atmosphere that make these events so devastating are the potential moisture availability and the inherent conditional instability. These allow (i) very large rainfall totals to occur, (ii) rapid communication between upper and lower levels, and (iii) the response to any large scale forcing to usually develop on the mesoscale. Thus extreme rain events often develop rapidly, occur over relatively small scales, and are difficult to forecast.

One long-term aim of this study is determine the requirements for improved prediction. There are potential benefits from (i) enhanced observations of the large scale flow, (ii) better analysis at the mesoscale, (iii) improved physical parametrisatons, (iv) increased resolutions, and, (v) improved understanding. It is somewhat unclear where research resources should be focussed. The study presented here is a first attempt to assemble local expertise from various countries in the region to make a unified attempt at investigating extreme rain events. By sharing our skills, experiences, knowledge and numerical systems, and by increasing the data base for significant, but relatively infrequent events, we think there will be broad scientific and operational benefits.

In this presentation we will describe observational and prediction aspects of four heavy rain events that occurred near Manila, Kuala Lumpur, Hanoi and central Java.

Event 1 occurred just north of Manila and coincided with the arrival near the Philippines of both a monsoon westerly wind burst, and Typhoon Utor, which was propagating westward while undergoing intensification and asymmetric structure change.

Event 2 occurred as a flash flood near Kuala Lumpur and was associated with the development of a mesoscale circulation and sustained conditional instability. On larger scales, it coincided with the arrival of an eastward propagating MJO and a cold surge event over the South China Sea.

Event 3 occurred near Hanoi with over 300 mm recorded over 2 days. The event was characterised by deep convergence, and was associated with the overlaying of a rapidly-sharpening upper trough of midlatitude origin, with a weak disturbance in the low level monsoon trough.

Event 4 occurred over central Java and was associated with the development of a mesoscale circulation over the Java Sea. On larger scales, it coincided with simultaneous surges in the trades of both hemispheres, the development of the near-equatorial troughs (in both hemispheres), and the strengthening of equatorial monsoon westerlies.

Examination of a continuous sequence of daily, operational 48-hour forecasts over a 60-day period including each rain event, reveals that peaks in forecast 24 hour accumulated rainfall (relative to other forecast times) clearly coincided with the actual events. Although this is rather encouraging, and suggests that the large scale forcing was accurately depicted, high resolution forecasts indicate that improvements in mesoscale initialisation and representation of physical processes are clearly necessary before prediction of the details of similar events is achieved.

Further information on the circulation changes, forecast climatology, mesoscale analysis and prediction, and dynamical aspects of these events will be presented at the Conference.