Synoptic-Dynamic Analysis of Early Dry-Season Rainfall Events in the Vietnamese Central Highlands

The Central Highlands are Vietnam's main coffee growing region. Unusual wet spells during the early dry season in November and December negatively affect two growing cycles in terms of yield and quality. The meteorological causes of wet spells in this region have not been thoroughly studied to date. Using daily rain gauge measurements at nine stations for the period 1981–2007 in the Central Highlands, four dynamically different early dry-season rainfall events were investigated in depth: I. Tail end of a cold front; II. Tropical Depression-type disturbance; III. Multiple tropical wave interaction; and IV. Cold surge with Borneo Vortex. Cases I and IV are mainly extratropically forced. In case I, moisture advection ahead of a dissipating cold front over the South China Sea led to high equivalent potential temperature in the southern highland where this air mass stalled and facilitated recurrent outbreaks of afternoon convection. In this case, the low-level northeasterly flow over the South China Sea was diverted around the southern highlands by relatively stable low layers. On the contrary, low-level flow was more orthogonal to the mountain barrier and high Froude numbers and concomitant low stability facilitated the westward extension of the rainfall zone across the mountain barrier in the other cases. In case III, an eastward travelling equatorial Kelvin wave might have been a factor in this westward extension too. The results show a variety of interactions of large-scale wave forcings, synoptic-convective dynamics and orographic effects on spatio-temporal details of the rainfall patterns.