An Analysis Of The Characteristics Of Tropical Cyclone Inverted Trough Torrential Rainfalls During The Meiyu-period Of Yangtze-Huai River Basin

Tropical cyclone (TC) is one of the most significant weather systems affecting summer heavy rainfalls in China. Jiangsu Province is located in the mid-latitude coastal region, where the tropical cyclone inverted trough (TCIT) impacts often occur. Meiyu rainstorms in Yangtze-Huai River basin have high frequency of occurrence during June-July, TCIT rainstorms has being gradually becoming an important severe weather affected Jiangsu during Meiyu period. Taking comprehensive advantage of statistical analysis, diagnostic analysis, comparative analysis and numerical simulation,the paper concentrates on the characteristics of TCIT rainstorms affected Jiangsu during Meiyu period, mainly includes the basic circulation characteristics, dynamic&thermodynamic characteristics, meso-microscale characteristics as well as dry-cool airmass activities working in TCIT rainstorms.The key components of the paper are as follows:1.We classify the TCIT rainstorms during Meiyu-period of Yangtze-Huai River Basin into three types,and summarize the the common circulation characteristics of TCIT rainstorms for the first time.Based on a census of the TCIT that affected Jiangsu Province during the summers of1986-2011, TCIT rainstorms in Jiangsu during the Meiyu-period of Yangtze-Huai River basin are selected. According to the moving directionsof TCs, TCIT rainstorms are classified into three types:the first type would always land on South(or East) China then move toward north, the second type would always move toward west along the coast of South China, and the third type would always move northward from East China coastal waters.On the basis of past analysis of TCIT rainstorms, the common circulation characteristics of TCIT rainstorms are summarized as follows:TCIT of lower troposphere would extend to Jiangsu Province. Over East China coastal waters, there are always south or southeast low-level jets(LLJ) which transports a large amount of warm-wet flow to Jiangsu, and interacts with the dry-cold airmass brought by mid-latitude westerly trough near the top area of TCIT, as a result an unstable stratification with warm air mass just under cold formed.Meanwhile, the speed convergence of south wind ahead of LLJ and wind direction convergence in two sides of TCIT,couple with high-level divergence resulted from horizontal inhomogmeous distribution of south wind at the entrance region of high-level southeast jet. Therefore, the vertical ascending motion would strengthen, which is necessary for the genesis and development of meso-microscale systems. 2.We diagnose the dynamic&thermodynamic characteristics of two typical TCIT rainstorms respectively belonging to the first and second type.Both of them were influenced by TCIT and westerly trough,but the activity of dry-cold airmass during heavy rainfall period was completely different. Through the south wind momentum at upper-level gradually extending towards low-level, the couple of low and high-level jet would give birth to the convergence in low-level and the divergence in high-level. The evolution of the vorticity and vertical velocity could better reflect the difference of dynamic characteristics between the two rainstorms.The movement and enhancement of secondary circulation stimulated by convergence of ageostrophic wet Q-vector respectively provided a good dynamic lifting mechanism for the genesis and development of heavy rainfall which occurred in the updraft of the secondary circulation,and it was respectively the convergence of Qy and Qx that directly led to ageostrophic wet Q-vector convergence.Both of the warm-wet transporting and south wind convergence ahead of LLJ made an important role in the forming of deep moisture column and warm-wet tongue.Rainstorms occurred in the warm-wet tongue area at the southeast side of southwest-northeast trending E index energy front, and the instability energy released with ageostrophic vertical ascending motion.3.We analyse the meso-microscale characteristics as well as the large-scale advantages of continuous tropical depression inverted trough rainstorm.Heavy rainfall resulted from eastwards movement of westerly high-level jet core, eastwards motion of Mongolia cold trough time and again, stability of inverted trough(mesoscale low vortex activities) and slow movement of LLJ. Ten rain clusters generated in quick succession between Yangtze-Huai River Basin,among the three stages, the activities of rain clusters of every stage was totally different from another two. The surface mesoscale convergence system would trigger rain clusters that distributed along the convergence line.Convective activity over heavy rainfall areas was so strong during first stage,the developments of rain clusters were associated with cold cover of value below-62℃, even cold center lower than-72℃,strong convective echo band with radar reflectivity greater than45dBZ moved slowly towards south,moreover, the process of heavy rainfall appeared adverse wind area on the radial velocity field. While during the latter two stages, rain clusters were corresponding to the isoline concentration band back of convective clouds,with new convective cells continuously supplement, the strong echo band maintianed. The water vapor flux and the allocation of negative(positive) helicity value at high(low) level could well indicate the intensity and area of the coming6h rainfall. 4.We discuss the influence of dry-cold air activities on lower/middle/upper troposphere to tropical depression inverted trough rainstorm based on the WRF mesoscale numerical simulation.The dry-cold air activities at upper troposphere reduced the precipitation,while both the dry-cold air activities at middle and lower troposphere would enhance the rainstorm,the latter one changed heavy rainfall area also.The more dry-cold air strengthened(weakened), the more precipitation increased(decreased),the more the location of heavy rainfall center moved southeastward(northwesatward) along with heavy rainfall area moving towards south(north).Changing relative humidity and temperature at the same time, the impact of dry-cold air at low troposthere to heavy rainfall area was more significant.To some extent,dry-cold air activities affected the mesoscale low vortex(convergence line) formed at the top of inverted trough.The changing of heavy rainfall area and intensity was well consistent with the allocation of divergence(convergence) at high(low) level, energy front and MPV.