Synoptic-Dynamic Analysis, Predictability and Convection-Permitting Simulations of an Extreme Precipitation Event Affecting Ho Chi Minh City on 26 September 2016

An extreme precipitation event severely hit Ho Chi Minh City (HCMC, Vietnam) on 26 September 2016. This event was the most extreme event since the beginning of regular measurements in 1961. Rainfall amounts between 100 and 200 mm were measured at eight stations in the metropolitan area of HCMC. Due to the short duration of the event, namely less than two hours from approximately 1645 until 1830 local time, the event led to flooding of various (main) streets during rush-hour traffic and to various flooded buildings.

The event and its synoptic-dynamic causes are analyzed using satellite-derived rainfall estimates (NASA GPM IMERG), geostationary satellite imagery (JMA Himawari-8), cloud property retrievals from polar-orbiting satellite measurements (AVHRR PATMOS-x), radiosonde measurements, and the new ECMWF fifth generation reanalysis ERA5. The analysis suggests that the event was mainly caused by two factors:

1. After breakdown of the daytime sea-breeze circulation over the Malay Peninsula on 25 September 2016, an associated convergence line moved eastward with the monsoonal westerlies and passed the Gulf of Thailand during nighttime. The convergence line can be identified in 10-m wind convergence, convergence of vertically integrated moisture flux, and cloudiness. It reached the coast of southwestern Vietnam on the morning of 26 September and initiated convection over the Ca Mau peninsula in southern Vietnam as it continued to move northeastward.
2. In association with Typhoon Megi northeast of Luzon (Philippines), northeasterly low-level winds occurred over the northern and central South China Sea on 26 September 2016, which induced convection over the southern parts of the central Vietnamese Annamese Cordillera due to orographic lifting.

A cold pool that was associated with the orographically induced convection then collided with the convergence line in the vicinity of HCMC, leading to an intensification of convection and keeping convection stationary and very active over HCMC for about 2–3 hours.

In the presentation, the predictability of the event in multi-model ensembles from global Numerical Weather Prediction models will also be discussed. Convection-permitting simulations with the Weather Research and Forecasting (WRF) model will be used to further analyze the synoptic-dynamic development of the event in more detail.