Comparative Study Of Characteristics And Development Mechanism On Two Types Of Heavy Rainfall Of Warm Area Convergence Line In South China Coast

According to the circulations at 850hPa by the objective and automatic method, the warm area heavy rainfalls in South China coast during April to June in 2009 ～2014 are divided into two types:one is the southly pattern with the meridional convergence line around Pearl River Estuary, which is formed by the convergence of southeasterly, southerly and southwesterly; The other is the southwestly pattern with the zonal of convergence line at the east of Pearl River Estuary, which is formed by convergence of westerly and southwesterly. Statistics on 6-hr strong rainfall sample library shows that the frequency is 73.2/year for the southerly convergence line storms on average, while the frequency of southwesterly convergence line pattern is 50.3/year. After the onset of summer monsoon in South China Sea, the occurrence frequencies of both patterns increase remarkably. The synthetic diagnosis of circulations of two convergence line storms show that, there are a broad warm high ridge at 500 hPa and the temperature ridge is at upstream of the high ridge, so an obvious warm advection is at high ridge area. And it is a no front regions for the southerly pattern. For the southwesterly pattern, the circulation is a weak trough with a temperature trough falling behind the height trough with weak perturbations, the front region is at mid-latitudes, the Southe China coast is located in the warm area of the mid-latitude front region. The variable characteristics of two types of warm area storm convergence lines show that there are strong vertical motion center at the vertical cross-sections. The convergence layer in the strong rainfall region is quite deep and the several vertical convergence centers are overlapping. Both two types are all with abundant water vapor and with deep convective instability energy layers, and strong release of latent heat of water vapor condensation. The release of latent heat leads to the warming up and stretching of air column over the convergence lines, thus it maintains the strengthen deep convergence and vertical upward motion. There is stronger latent heat release in southwesterly pattern, but a stronger warm advection at middle and upper layers is larger than the latent heat release in southerly pattern.To investigate the thermodynamically development mechanism of warm area convergence line storm, WRF numerical simulations show that, condensation latent heat plays an most important role on warming air and stretching air column, enhancing character of low value systems of convergence line. Then latent heat release over the convergence lines strengthening convergent circulation at lower layers and weakening divergent circulation at middle-high layers. The influence can be strong to 30～50% of the wind circulation strength of the two types of the warm area convergence line storm over South China coast. The research here shows that the convergence line systems of the warm area of South China are the important synoptic systems of the warm area storm in South China coast.Terrain tests of southly pattern show two effects:one is the that the yunwu mountain block the southly convergence line, climbing up the southly convergence line. The other one is that a long narrow tube formed between the yunwu mountain and tianlu hill converges and rises the southly convergence line. Terrain tests of southwestly pattern show terrain effect of dynamic lifting is weak. Vertical movement comes from systemly convergence and ascending branch of secondary circulation. Southwestly convergence line almost parallel to lianhua mountain.Different from the blocking southly convergence line by yunwu mountain, Southwestly convergence line is cooperated with friction by parallel mountain ranges. Movement speed is slowed down, the time of the precipitation is extent,its accumulation precipitation in total always exceeds it of southly pattern.Combining the latent heat sensitivity test with terrain test, results show the differences between two types on latent heat and topographical dynamics mechanism. For southly pattern, low-level convergence line systemic convergence rising, high-level suction and dynamic lifting by topography have a great deal of positive contribution on vertical movement. Dynamic topography uplifting may not be comparing with other two,but it as a trigger affecting the formation of precipitation, then precipitation release condensation latent heat. So topographical effect influents latent heat indirectly. Southwestly pattern with strong convective instability, the strong rising of low-level convergence line system and ascending branch of secondary circulation work together on triggerring severe convection to heavy successional rainfall. The topographical effect is weak. Latent heat is very important for Southwestly pattern, topography only extend the time of precipitation, magnitude of precipitation and strong-updraft are most related to the release of condensation latent heat.