Mechanisms accounting for the repeated occurrence of torrential rainfall over South Thailand in early January 2017

ABSTRACT Based on CMORPH precipitation estimates and ERA5 reanalysis data, this study investigates the mechanisms accounting for the repeated occurrence of torrential rainfall over South Thailand in early January 2017, which induced the strongest floods over Ko Samui and Ko Phangan in the last almost 30 years. It is found that the maintenance of a northeastward-moving mesoscale vortex that formed southwest of the Indochina Peninsula was the direct reason for the series of torrential rainfall events. Analysis of the vorticity budget illustrates that convergence-related horizontal shrinking was the most favorable factor for the maintenance of the vortex. Tilting was the second most favorable factor, whereas horizontal and vertical transport mainly caused a net export of cyclonic vorticity from the vortex’s three-dimensional range, which was detrimental for its maintenance. Further analysis indicates that tilting and vertical vorticity transport were sensitive to the vortex’s displacement and the enhancement of cyclonic vorticity at lower levels around the vortex, respectively, as the two factors showed completely different effects on the persistence of the vortex during two different stages. Graphical abstrac