On the Squall Lines Preceding Landfalling Tropical Cyclones in China

Abstract

ased on a 3-yr (2007?09) mosaic of radar reflectivity and conventional surface and synoptic radiosonde observations, the general features of squall lines preceding landfalling tropical cyclones (TCs) (pre-TC) in China are examined and compared with their midlatitude and subtropical counterparts. The results show that about 40% of landfalling TCs are associated with pre-TC squall lines with high-occurring frequency in August and from late afternoon to midnight. Most pre-TC squall lines form in a broken-line mode with a trailing-stratiform organization. On average, they occur about 600 km from the TC center in the front-right quadrant with a maximum length of 220 km, a maximum radar reflectivity of 57?62 dBZ, a life span of 4 h, and a moving speed of 12.5 m s?1. Pre-TC squall lines are generally shorter in lifetime and length than typical midlatitude squall lines.Pre-TC squall lines tend to form in the transition area between the parent TC and subtropical high in a moist environment and with a weaker cold pool than their midlatitude counterparts. The environmental 0?3-km vertical shear is around 10 m s?1 and generally normal to the orientation of the squall lines. This weak shear makes pre-TC squall lines in a suboptimal condition according to the Rottuno?Klemp?Weisman (RKW) theory. Convection is likely initiated by low-level mesoscale frontogenesis, convergence, and/or confluence instead of synoptic-scale forcing. The parent TC may contribute to (i) the development of convection by enhancing conditional instability and low-level moisture supply, and (ii) the linear organization of discrete convection through the interaction between the TC and the neighboring environmental system.