Investigation of An Extreme Rainfall Event during 8–12 December 2018 over Central Vietnam. Part I: Analysis and Cloud-Resolving Simulation
Abstract. An extreme rainfall event occurred from 8 to 12 December 2018 along the coast of central Vietnam. The observed maximum rainfall amount in 72 h was over 900 mm and set a new record, and the associated heavy losses were also significant. The analysis of this event shows some key factors for its occurrence: (1) The interaction between the strong northeasterly winds, blowing from the Yellow Sea into the northern South China Sea (SCS), and easterly winds over the SCS in the lower troposphere (below 700 hPa). This interaction created strong low-level convergence, as the winds continued to blow into central Vietnam against the Truong Son Range, resulting in forced uplift over the coastal plains due to the terrain’s barrier effect. As a consequence, heavy rainfall occurred along the coast. (2) The strong easterly wind played an important role in transporting moisture from the western North Pacific across the Philippines and the SCS into central Vietnam. (3) The Truong Son Range also contributed to this event due to its barrier effect. (4) In addition to a cumulonimbus, the low-level precipitating clouds such as nimbostratus clouds were also major contributors to rainfall accumulation for the whole event.
The Cloud-Resolving Storm Simulator (CReSS) was employed to simulate this record-breaking event at high resolution, and the overall rainfall can be captured quite well not only in quantity but also in its spatial distribution (with a Fractions Skill Score ≈ 0.7 and Threat Score > 0 at 700 mm for 72 h rainfall). Thus, the CReSS model is shown to be a useful tool for both research and forecasts of heavy rainfall in Vietnam. The model performed better for the rainfall during 9–10 but not as good on 11 December. In the sensitivity test without the terrain, the model did not generate nearly as much rainfall for this event. Thus, the test confirms the important role played by the local topography for the occurrence of this event.
