A Statistical Analysis of the Relationship between Upper-Tropospheric Cold Lows and Tropical CycloneTrack and Intensity Changes over Western North Pacific

The geographical and temporal characteristics of upper tropospheric cold lows (UTCLs) and their relationship to tropical cyclone (TC) track and intensity changes over western North Pacific(WNP) during 2000-2012 are examined using the best track and global meteorological re-analysis data. An analysis of the two datasets shows that 73% of 346 TCs coexist with 345 UTCLs, and 21% of the latter coexist with TCs within an initial cut-off distance of 15º. By selecting those co-existed systems within this distance, we find possible influences of UTCLs on TC track and intensity changes, depending on their relative distance and on the sectors of UTCLs where TCs are located. Results show that the impact of UTCLs on TC directional changes are statistically insignificant when averaged within the 15º radius. However, left-turning TCs within 5º distance from the UTCL center exhibit large deviated directional changes from the WNP climatology, due to the presence of high-frequency abrupt left turnings in the UTCLs' eastern semicircle. The abrupt turnings of TCs are often accompanied with their slower movements. Results also show that TCs seem more (less) prone to intensify at early (late) development stages when interacting with UTCLs compared to the WNP climatology. Intensifying (weakening) TCs are more distributed in the southern (northern) sectors of UTCLs, with more favorable conditions for intensification within the 9-13º radial range. In addition, rapid intensifying TCs take place in the south-southwestern and east-southeast quadrants of UTCLs, whereas rapid weakening cases appear in the western semicircle of UTCLs due to their frequent proximity to mainland coastal regions.