Formation and Evolution of a Squall Line Preceding Landfalling Tropical Cyclone Kammuri (2008) in Western North Pacific

Squall line may form in various environments. Relative to the well-examined squall lines in front of mid-latitude trough, squall lines associated with landfalling tropical cyclones (TC) have not been paid much attention. A strong squall line occurred in Guangdong Province of China ahead of severe tropical storm Kammuri on 4 August 2008. This squall line produced a rain rate of 53 mm/h at Baiyun airport in Guangzhou and caused severe damage to electrical utility poles along a highway. This work is aimed to examine the formation and evolution mechanism of this pre-TC squall line.

The squall line formed via a broken-line mode near the coastal line of Guangdong province while Kammuri was developing from a tropical depression to a tropical storm. It had a dominant organization of trailing stratiform rainfall, a maximum length of 500 km and maximum intensity of 60-65 dBZ. It moved west-southwestward at a speed of about 11 m s-1 and lasted 3.7 h. The surface analysis shows an apparent cold pool, storm high, prelow and even wake low.

Synoptic analysis shows that synoptic-scale forcing may have played an essential role in convection initiation. The squall line forms in front of an upper-level trough near the entrance of an upper level jet. The vertical motion is contributed by quasigeostrophic forcing through an earlier dominant vorticity advection and a later dominant temperature advection in addition to weak low-level frontogenesis. Similar to the predecessor rain event of TC in the US, Kammuri is shown to have provided rich moisture towards where the squall line forms based on a backward trajectory analysis. The convective available potential energy is apparently increased likely as a result of the increase in water vapor.

High-resolution numerical experiments with WRFV3 confirm the synoptic analysis result. The TC does contribute to the formation and evolution of the squall line through continuous moisture supply and subsequent enhancement of instability, in addition to providing an environment beneficial to the linear organization of convections. When the TC is removed or the relative humidity in TC area is reduced in the initial field, scattered convections are still initiated but they fail to form a squall line and rapidly dissipate. Sensitivity experiments also show that the size and intensity of the approaching TC may affect the structure, length and intensity of the squall line. A larger and stronger TC tends to be associated with a longer and stronger preceding squall line.