Study of Low-Level Jet and Extremely Heavy Rainfall over Northern Taiwan in the Mei-Yu Season

Abstract

Extremely heavy rainfall (at least 100 mm d?1) is one of the most disastrous weather phenomena in the early-summer rainy season of Fast Asia. This season is called the Mei-Yu season in Taiwan. Thirty-five cases of this rainfall event over northern Taiwan in May-June 1965?84 were selected for studying its relationship between the low-level jet (LLJ) and extremely heavy rainfall. As in Japan and China, extremely heavy rainfall is found to be closely associated with a LLJ. For example, there was an 84% likelihood that a LLJ of at least 12.5 m s?1 would be present at 700 mb 12 h before the start of the rainfall event. Conversely, when a LLJ was present over Taiwan in 1980?84, there was a 91% likelihood that such a rainfall event was in progress or would begin on the next day. Results provide evidence that the LIJ plays an important role in the development of this rainfall event. It appears that cumulus convection in the area of rainfall homogenizes the vertical wind profile so as to dissipate the LLJ or displace it to the immediate south, where its frequency is found to increase during and after the event This sequence of events provides observational support to the theoretical and numerical studies which show that the LLJ may form to the south of heavy rainfall as a part of the secondary circulation driven by convective latent heating.