Evolution of Mesoscale Convective Systems during Tropical Cyclone Formations in the Western North Pacific

This study is an extension of Cheung and Elsberry (2004a, b) in identifying the existence (or not) of mesoscale convective systems (MCSs) and their potential contributions during tropical cyclone (TC) formations in the western North Pacific (WNP). The hypothesis is that mesoscale features such as MCSs determine the position and timing of TC formations within generally favorable environmental conditions. Ninty-nine TC formations in the WNP (0-22oN, 90-160oE) during the period September 1999-December 2003 are examined. MCSs two days before the TC formation alert was issued for each case are identified by the deep-convective areas [infrared channel-1 (IR1) temperature < 214 K with an area greater than 4„e104 km2 and eccentricity > 0.5] in the GMS-5/GOES-9 satellite imageries. For diagnosing different mechanisms of formation, the 99 cases are classified into six categories according to their large-scale formation patterns: monsoon shear (MS), southwesterly flow (SW), southwesterly and northeasterly flow (SW-NE), trade wind northeasterly flow (NE), monsoon confluence (MC), and easterly wave (EW). Whereas MCSs activities are much affected by the diurnal variation, one to several MCSs are identified in most of the 99 formations in this study. On average more MCSs are found 48 h before formation in the monsoon-related patterns (SW, SW-NE and NE), whereas in the EW pattern only one MCS is identified in every formation and the MCS only appeared within 24 h prior to formation and usually developed until the formation time. Because the MCSs in the MS pattern are longer-lived than those in the MC pattern, the percentage area of deep convection within 200 km of the system center is the smallest in the MC (and EW) pattern. However, the near-center average IR1 temperature in the EW is the lowest, which indicates that the single MCS in this type of formation has developed well into the upper troposphere. Examination of QuikSCAT oceanic wind data during 1999-2003 also indicates that the locations of strengthening winds are well correlated with those of the deep convections, which supports our hypothesis of MCS contribution to intensification of tropical disturbances. In addition, several characteristic evolution profiles of the average IR1 temperature during formations are also identified.