Statistical Analysis of Warm-Sector Rainstorm Characteristics over the Southern of Middle and Lower Reaches of the Yangtze River in Summer
The warm-sector rainstorms over the southern of middle and lower reaches of the Yangtze River are defined, and they can be divided into four types based on synoptic situation, including warm shear-line, cold-front, subtropical high and severe southwest jet. By using the conventional and non-conventional observation data such as the hourly precipitation data of automatic weather station (AWS) from May to September during 2010-2016, we statistically analyzed the temporal-spatial distribution of warm-sector rainstorms and the nature of precipitation as well as the causes for their formation. In addition, based on NCEP FNL opearational global analysis data and the technology of mesoscale analysis, the system configurations of four kinds of rainstorms are built. (1) All the four types are decentralized local precipitation, and the precipitation usually occurs near the uneven surface, which is transitional zones between mountains, plains and lakes, etc. The warm shear-line type precipitation mainly happens in the mid-west of this area with wide range, most intensive intensity and most obvious extremes. The precipitation of the cold-front type is concentrated, and it has a higher intensity and a certain degree of extremes, distributing mainly in the middle of the area. The precipitation with the subtropical high often occurs in the mid-east of this area with weak intensity. The severe southwest jet mainly appears in the western part. (2) The warm shear-line and the severe southwest jet are dominated by night precipitation, and the subtropical high precipitation is mainly concentrated in the afternoon. However, the diurnal variation of the cold-front type does not show a distinct difference throughout the day. (3) The warm-sector rainstorms consist of steady and convective precipitation. The heavier the daily rainfall is, the more obvious convection the precipitation has. (4) With high humidity, unstable energy accumulation and other favorable background, the warm shear-line, cold-front and subtropical high are mostly generated by boundary layer (ground) mesoscale convergence lines, coupled with the place of the coupling of upper and low-level jet. The severe southwest jet is generally formed near the mesoscale wind velocity fluctuation and ground mesoscale convergence lines on the low-level jet stream. The stronger the low-level jet stream is, the greater the intensity of the rainfall is. (5) The rainfall region of the warm shear-line and the cold-front are respectively located in the wet zone with the south of warm shear-line in the lower level and the pre-front. The rainfall region of the subtropical high and the severe southwest jet are respectively located in the water vapor and the unstable atmosphere in the subtropical high and the left front of the strong low-level jet streams. Four types usually manifest as long-life moving mesoscale rain clusters. When it goes through mountains, rivers and lakes and other uneven surface, its rainfall intensity usually increases and moving speed slows down, which is the main reason for the formation of the local heavy rainfall of warm-sector rainstorms.