Squall Lines preceding Landfalling Tropical Cyclones

Squall lines are well-organized mesoscale convective systems, which usually contain a convective line with a width-length ratio smaller than 1/3 and associated stratiform rainfall around it. Squall lines can bring severe weather such as strong wind, intense rainfall, hail and even tornado. Squall lines could appear in front of a frontal system. It could also appear preceding a landfalling tropical cyclone. Pre-frontal squall lines around the world have been well examined. However, very few efforts have been made on pre-typhoon squall lines in literature.

Observation shows that sometimes squall line may appear around the coastal area of China with the approaching of a tropical cyclone. These squall lines are usually 400-600km in front of the tropical cyclone center and sometimes move in parallel to the tropical cyclone. Before direct impact of the approaching tropical cyclone, severe disaster could be caused first by the pre-typhoon squall lines. Then the direct landfalling of tropical cyclone will make the disaster much worse. This double hit may complicate the disaster preparedness and mitigation.

Many interesting questions remain to be answered concerning pre-typhoon squall. What does the general mode of pre-typhoon squall could be? How different are they from the pre-frontal squall? How could the squall line interact with the approaching tropical cyclones? How could their interaction affect the intensity or track of both of the tropical cyclone and the squall? What is the favorable situation for a tropical cyclone to trigger a squall line? This work will try to look into some of these questions by analyzing squall lines preceding tropical cyclones that made landfall in China during 2007-2009 using satellite and conventional radar, surface, radiosonde, and NCEP global reanalysis data. Numerical simulation result of a squall line associated with landfalling Western-North Pacific typhoon Kammuri in Aug. 2008 will be presented as well.