Land-Based Convection Effects on Formation of Tropical Cyclone Mekkhala (2008)

The effects of land-based convection on the formation of Tropical Storm Mekkhala (2008) off the west coast of the Philippines are investigated using the Weather and Research Forecasting model with 4 km horizontal grid spacing. Five simulations with Thompson microphysics are utilized to select the control-land experiment that reasonably replicates the observed sea-level pressure evolution. To demonstrate the contribution of the land-based convection, sensitivity experiments are performed by changing the land of the northern Philippines to be water, and all five of these no-land experiments fail to develop Mekkhala. The Mekkhala tropical depression develops when an intense, well-organized land-based mesoscale convective system moves offshore from Luzon island and interacts with an oceanic mesoscale system embedded in a strong monsoon westerly flow. Due to this interaction, a mid-tropospheric mesoscale convective vortex (MCV) organizes offshore from Luzon where monsoon convection continues to contribute to low-level vorticity enhancement below the mid-level vortex center. In the no-land experiments, wide-spread oceanic convection induces a weaker mid-level vortex farther south in a strong vertical wind shear zone, and subsequently farther east in a weaker monsoon vortex region. Thus, the monsoon convection-induced low-level vorticity remained separate from the mid-tropospheric MCV, which finally resulted in a failure of the low-level spin up. This study suggests that land-based convection can play an advantageous role in a TC formation by influencing the intensity and the placement of the incipient mid-tropospheric MCV to be more favorable for TC low-level circulation development.clos