Investigation of an extreme rainfall event during 8–12 December 2018 over central Vietnam – Part 1: Analysis and cloud-resolving simulation

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 in-
teraction 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. Furthermore, the low-level
convergence in this event was strong enough, and the air
was unstable enough to trigger most of the convection near
the shoreline (further inland). As a consequence, heavy rain-
fall occurred along the coastal zone and coastal sea. (2) The
strong easterly wind played an important role in transport-
ing moisture from the western North Pacific (WNP) across
the Philippines and the SCS into central Vietnam. ... mehr(3) The
Truong Son Range also contributed to this event due to its
barrier effect. (4) In addition to cumulonimbus, the low-level
precipitating clouds such as nimbostratus clouds were also
major contributors to rainfall accumulation for the whole
event. The analyses of local thermodynamics also indicate
that the southward movement of the low-level wind con-
vergence zone caused the southward movement of the main
heavy rain band during the event.
The cloud-resolving storm simulator (CReSS) was em-
ployed to simulate this record-breaking event at a grid size
of 2.5 km; evaluated results show that the model simulated
the surface wind well and captured the southward movement
of the low-level wind convergence. The overall rainfall can
be captured quite well, not only in quantity but also in its
spatial distribution (with a similarity skill score (SSS) ≈ 0.7
and threat score (TS) > 0 at 700 mm for 72 h rainfall). Thus,
the CReSS model is shown to be a useful tool for both re-
search and forecasts of heavy rainfall in Vietnam. The model
performed better for the rainfall during 9–10 December 2018
but not as good on 11 December 2018. In the sensitivity test
without the terrain, the model had poorly simulated the sur-
face wind, which resulted in the model generating insuffi-
cient rainfall for this event and also not capturing the spatial
distribution of the rainfall. Thus, the test confirms the impor-
tant role played by the local topography for the occurrence
of this event.