Analysis On The Severe Squall Line Process Occurred Over Yellow And Huai River Basins On 3^rd To 4^th June 2009

A severe squall line event occurred over Yellow and Huai River Basins on 3rd to 4th June 2009, with the characteristics of long life cycle, high moving speed, wide influence area, and leading to serious disaster. Using multiple surface observational data at high spatial and temporal resolutions, in conjunction with high resolution data produced using the Doppler weather radar three-dimensional digital mosaic system and the four-dimensional variational assimilation retrieval technique, the severe squall line process is analyzed focusing on its formation background, life cycle evolution, and system structures. The formation of the squall line is discussed mainly on the formation, accumulation, and triggering condition of convective instability energy, as well as the organization from convective cells to the squall line system. The life cycle evolution of the squall line is described from the evolutions of radar reflectivity structure, features of surface meteorological elements, as well as features of severe convective weather phenomena. The middle- and low-levels structures of strongest convection stage and system mature stage are analyzed using the three-dimensional retrieved wind field. The conclusions are as follows:(1) The formation of the squall line was closely associated with the cold air brought by the strong northwest flow behind the upper trough over southwest part of northeast cold vortex and the local warming of surface layer due to clear sky solar radiation. The dry ambient atmosphere at low level with a large temperature lapse rate was favorable for the occurrence of disastrous gale at surface during the squall line event.(2) During the evolution of the squall line, the convections developed to their strongest at the intensifying stage, when the strong convection region reached its widest and highest. The strong convection region weakened at its mature stage, while the horizontal scale of the squall line system reached its maximum. The differences of convection intensity and horizontal scale between intensifying and mature stage maybe closely associated with the changes of surface divergence and convergence, as the surface divergence and convergence were of stronger intensity but smaller extension at intensifying stage compared with that at mature stage.(3) Evolution of the squall line and the associated severe convective weather phenomena during its life cycle is described. The surface thunderstorm high and the gust front developed to the most intense at the mature stage, when the severe convective weather phenomena occurred over the largest area. At its mature stage, horizontal size of the squall line system reached the upper limit of meso-βscale, with double bands of strong radar echo inside. Features of cloud and weather phenomena associated with the two bands differ significantly.(4) Based on the retrieved wind field, the vertical distributions of horizontal divergence and vertical velocity at four subregions of the squall line system, namely the weak echo region before the squall line (P), the strong convective echo band in the squall line main body (C), the secondary strong echo region in the stratiform region (S), and the transition zone (T) are analyzed. There is descending motion within the T and S subregions at middle- and low-levels, with the descending in T being stronger. The stronger downdrafts and thinner layer of divergence at low attitudes within the T and S subregions are probably related to the disastrous gale at surface during the squall line event.