From 24 to 26 November 2004, an extreme heavy rainfall event occurred in the mountainous
provinces of central Vietnam, resulting in severe flooding along local rivers. The Regional Atmospheric
Modeling System, version 4.4, is used to simulate this event. In the present study, the convective
parameterization scheme includes the original Kain-Fritsch scheme and a modified one in which a new
diagnostic equation to compute updraft velocity, closure assumption, and trigger function are developed.
These modifications take the vertical gradient of the Exner function perturbation into account, with an on-off
coefficient to account for the role of the advective terms. According to the event simulations, the simulated
precipitation shows that the modified scheme with the new trigger function gives much better results than
the original one. Moreover, the interaction between convection and the larger-scale environment is much
stronger near the midtroposphere where the return flow associated with lower-level winter monsoon
originates. As a result, the modified scheme produces larger and deeper stratiform clouds and leads to a
significant amount of resolvable precipitation. On the contrary, the resolvable precipitation is small when the
original scheme is used. The improvement in the simulated precipitation is caused by a more explicit
physical mechanism of the new trigger function and suggests that the trigger function needs to be developed
along with other components of the scheme, such as closure assumption and cloud model, as a whole. The
formalistic inclusion of the advective terms in the new equation gives almost no additional improvement of
the simulated precipitation
