A study of two propagating heavy-rainfall episodes near Taiwan during SoWMEX/TiMREX IOP-8 in June 2008. Part I: Synoptic evolution, episode propagation, and model control simulation

This paper is the first of a two-part study to investigate two rain-producing episodes in the longitude-time (Hovmöller) space upstream from Taiwan during the eighth intensive observing period (IOP-8, 12-17 June 2008) of the Southwest Monsoon Experiment/Terrain-influenced Monsoon Rainfall Experiment (SoWMEX/TiMREX), with a goal to better understand the mechanism and controlling factors for their organization and propagation. Both in a prefrontal environment, the first episode moved eastward and the second was a rare westward-moving event, and each caused heavy rainfall in Taiwan, on 14 and 16 June, respectively. In Part II, the roles played by synoptic conditions and terrain effects are further examined through sensitivity tests. With the aid from a successful simulation with a grid spacing of 2.5 km, the structure and organization of convection embedded in the two episodes are shown to be different. With stronger low-level vertical wind shear in its environment, the first episode consisted of well-organized squall-line-type convective systems and propagated eastward mainly through cold-pool dynamics. However, the convection of the second episode was scattered and less organized with weaker vertical shear, and individual cells traveled with background flow toward the north-northeast. Throughout the 6-day case period, the southwesterly low-level jet (LLJ) is found to have much control over the general region of convection, and thus dictates the overall rainfall pattern in the Hovmöller space at the regional scale. The rapid development of the mei-yu front and LLJ over southeastern China during 16-17 June, to the west of the previous location of the jet, is found to result in the westward movement of the second episode.