Analysis On Synoptic Scale Characteristics Of Summer Mesoscale Convective System In Heilongjiang Province

The characteristics of satellite cloud pictures and synoptic elements distribution for Mesoscale Convective System (MCS) in summer over Heilongjiang Province were analyzed with the method of statistical analysis by using infrared cloud pictures and conventional meteorological observation data. The mainly results as following:(1) MCSs mainly produced in the western region of Heilongjiang Province, which is closely associated with the distribution of special mountain terrain in big and small Xinganling. The primary period of MCSs’is mainly at11-17clock in summer, and development period is on17-21clock and then MCSs dissipate at the period from21to02clock the next day. The process of MCSs’evolution has closely relationship with the variation of daily temperature, which suggests the variation of daily temperature have important effects on the evolution processes of the MCSs.(2) Analysis on the characteristics of MCSs’development process with TBB data show that the distribution of the minimum values of TBB is in the range from208to216K and the contour of gradients of TBB about0.6-0.7K/Km should identify the boundary of MCSs well. In general the MCSs are elliptical and eccentricity ratio is in0.5-0.7, the distribution of convection cloud top areas is centralized between10000Km2and40000Km2.(3) The synoptic pattern of MCSs was analyzed by the routine meteorological data indicate that the mainly system at850hPa and925hPa is the cold vorticity, as well as high-pressure ridge appears on500hPa when the MCSs come into being, the situation at700hPa is between them. The distribution in high and low level favored the formation of vertical ascending motion. And more, the warm advection appear from925hPa to500hPa has important effects on the formation of MCSs. The appearance of cold air on700hPa and500hPa increased the difference of temperature difference between higher with lower air, which enhanced the instability of atmosphere.(4) Analysis on the moisture condition during the formation and evolution of MCSs show that the average specific humidity increased in the development of MCSs, especially on850hPa and925hPa in the low level of troposphere, which favors to the enhance of vertical distribution of moisture with the feature of’drier on high level and wetter in the lower’.(5) Analysis on the instability condition during the evolution of MCSs indicate that MCSs moved toward higher temperature area at ground layer and cold area on700hPa and850hPa during the evolution of MCSs, which suggests that cold at higher level associated with warmer in the lower level favors to the development of convection. The warming effect is obvious at middle-lower level when the MCSs coming up, and with the development of MCSs the warming level moves to the higher level and cooling effect appeared at lower level, this is the results of water vapor condensation associated with release latent heat at higher levels and evaporation effects of water-drop during its’ dropping.(6) The analyses of triggering condition show that:The proportion of significantly streamline is the largest, followed by the jet stream before and after the MCSs occurred; The average wind speed increased in the development process of MCSs. Southwest jet is the dominant wind direction at850hPa and925hPa before and after the MCSs occurred, which benefits the development of strong convective. Wind direction convergence around the jet stream, speed convergence at the front of jet, cyclonic convergence on the left front of low-level jet, anticyclonic shear on the right of middle-level jet play an important role in the development for the formation of MCSs.(7) Physical indexes mentioned above have definite guiding significance on the forecasting on the development of the MCS.