A Survey of Squall Lines Preceding Landfalling Tropical Cyclones in China

Based on a three-year (2007-2009) mosaic of composite radar reflectivities, conventional surface and synoptic radiosonde observations, the general features of squall lines preceding landfalling tropical cyclones (pre-TC squall lines) in China are examined and compared with those squall lines of mid-latitude and tropical origins. Result shows that about 40% of landfalling TCs in China are associated with pre-TC squall lines. These squall lines most frequently occur in August from late afternoon to midnight. On average, they form in the front-right quadrant of their associated TCs about 600 km from the TC center. Most pre-TC squall lines form in the broken-line mode and have trailing-stratiform organization. These squall lines have a maximum length of 220 km, a maximum radar reflectivity of 57-62 dBZ, a life span of 4 h and move at 12.5 m/s.

The characteristics and environments of pre-TC squall lines are compared to their mid-latitude and tropical counterparts. Relative to the mid-latitude squall lines, pre-TC squall lines are generally shorter in both lifespan and length, though they do move at a comparable speed. Surface analyses show that pre-TC squall lines are associated with weaker pressure rises and smaller potential temperature drops. They are also found to form in environments with higher precipitable water and lower lifting condensation levels.

The approaching TC is found to have played an important role in the formation of its preceding squall lines. Composite analysis shows that pre-TC squall line generally forms in the transition area between the tropical cyclone and subtropical high with a modest 0-3km vertical wind shear (around 10 m/s) in the direction perpendicular to the long axis of the squall line. The formation of squall line is mainly due to low-level temperature advection, weak frontogenes together with rich water vapor supply by the outer flow of the TC, the latter has been found to be a key component for the predecessor rain events (PRE) ahead of TCs.