Tropical Cyclone Mekkhala (2008) Formation over the South China Sea: Mesoscale, Synoptic-scale and Large-scale Contributio

Abstract

Tropical cyclone formation close to the coastline of the Asian continent presents a significant threat to heavily populated coastal countries. A case study of Tropical Storm Mekkhala (2008) that developed off the coast of Vietnam is presented using the high-resolution analyses of the European Center for Medium-range Weather Forecasts/Year of Tropical Convection and multiple satellite observations. We have analyzed contributions to the formation from large-scale intraseasonal variability, synoptic perturbations, and Mesoscale Convective Systems (MCSs). Within a large-scale westerly wind burst (WWB) associated with the Madden Julian Oscillation (MJO), synoptic perturbations generated by two preceding tropical cyclones initiated the pre-Mekkhala low-level vortex over the Philippine Sea. Typhoon Hagupit produced a synoptic-scale wave train that contributed to the development of Jangmi, but likely suppressed the Mekkhala formation. The low-level vortex of the pre-Mekkhala disturbance was then initiated in a confluent zone between northeasterlies in advance of Typhoon Jangmi and the WWB.

A key contribution to the development of Mekkhala was from diurnally-varying MCSs that were invigorated in the WWB. The oceanic MCSs, which typically develop off the west coast of the Philippines in the morning and dissipate in the afternoon, were prolonged beyond the regular diurnal cycle. A combination with the MCSs developing downstream of the Philippines led to the critical structure change of the oceanic convective cluster, which implies the critical role of mesoscale processes. Therefore, the diurnally-varying mesoscale convective processes over both the ocean and land are concluded to have an essential role in the formation of Mekkhala in conjunction with large-scale MJO and the synoptic-scale TC influences.