Initiation of afternoon thunderstorms in Taiwan during TiMREX

This paper examines the initiation of afternoon thunderstorms over the island of Taiwan under synoptic situations with weak forcing. The motivation is part of an effort to develop a heavy rain nowcasting system for Taiwan. Based on a four year climatology of radar, satellite and lightning data Fang et al. (2011) has shown that for these weakly forced synoptic situations that the heaviest rainfall occurs between 1500 and 1700 LST along the lower western slopes of the mountains. Daily maximum total rainfalls are generally between 50 and 125 mm. A steep mountain range with elevations to 4 km runs north-south the length of the island. On the west side there is a flat plain about 45 km wide between the mountains and ocean while on the east side the mountains extend to the coast.

This paper utilizes the TiMREX data set, which includes the S-Pol radar and numerous additional soundings to examine the thunderstorm initiation location, number of initiations, intensity of the rainfall and kinematic and thermodynamic environment. There were 17 days during TiMREX within 150 km of S-Pol that were classified as days with weak synoptic forcing. These days are characterized by light winds, no synoptic fronts in the vicinity of Taiwan, no organized areas of precipitation and convective cloud development during the afternoon.

The storm initiation locations for these 17 days were not influenced by the environmental wind flow. It is likely that local flows such as sea breezes and anabatic flows dominate the air flow since the environmental winds were less than 10 m/s for these weakly forced synoptic situations. Typically these storms move little during their lifetime, rather they tend to grow in size and intensity by initiating new towers adjacent to the existing storms. They eventually dissipate in place after about 1700-1800 LST.

CAPE, CIN and average relative humidity between 800 and 500 hPa were examined for correlation with the extent of area receiving radar estimated rainfall amounts > 50, >100 and >200 mm. While noisy there was a tendency for the size of the area receiving > 50 mm to be larger as CAPE increased, average layer relative humidity increased and CIN decreased. The best correlation was with layer average humidity. There was no correlation with these three parameters for the size of the area receiving > 100 mm. The results suggested, that providing there was a nearby sounding available in the late morning, the above three parameters combined in a fuzzy logic algorithm might provide some skill in nowcasting the extent of the area along the lower foothills that would receive afternoon rainfall in excess of 50 mm.