Abstract:

In this study, using the data from the conventional soundings, the daily precipitation amount from 18 national weather stations in Hainan island, and the ERA-interim reanalysis in 2000-2017, the circulation types corresponding to heavy rain and weak rain events in Hainan island affected by cold low level easterly jet (LLEJ) were classified by the means of REOF (Rotated Empirical Orthogonal Function) and k-means clustering.The results show that two circulation types that lead to the heavy rain and weak rain events in Hainan island are identified.In general, the heavy-rain events have better water vapor and updraft conditions, while the weak-rain ones are relatively poor in the water vapor supply.For the type Ⅰ of heavy-rain events, the water vapor is transported by a deep eastward airflow from the surface to high level and is uplifted by the strong fronts resulting in strong precipitation.For type Ⅱ of heavy-rain events, an easterly airflow below 850 hPa conveys water vapor to Hainan island, and the updraft lifting by the South Branch Trough and surface fronts results in heavy rainfall.For the type Ⅰ of weak-rain events, Hainan island is affected by a strong dry cold air, with westerly airflow at 500 hPa and anticyclone circulation at 700 hPa, and the water vapor conditions at low level are poor.For the type Ⅱ of weak-rain events, Hainan island is mainly affected by the western Pacific Subtropical High circulation, and the anticyclonic circulation from 700 hPa to 500 hPa results in a subsidence motion.

Key words: Low-level jet, Cold air precipitation, REOF analysis, K-means clustering analysis