A Numerical Study of a Mesoscale Convective System Associated with the Heavy Rain Event over Guangdong Province in June 2005

The Mesoscale Convective System(MCS) that produced the heavy rainfall during 20-21 Jun 2005 in Guangdong Province is examined.The observations reveal that the MCS initiated and developed in front of a cold front.The convergence of vapor flux in the ahead of the jet stream core at 925 hPa level appears to be a crucial factor for its development.A 24-hour numerical simulation of the developing and mature as well as decline stages of the MCS is performed using the fifth generation Penn State/NCAR mesoscale model(MM5).The synoptic scale circulation evolutions and the mesoscale precipitation distributions associated with the MCS are reproduced successfully by the model.Using the higher-resolution output of the model,the meso-β-scale structure and evolution of the MCS at the mature stage are examined.The results show that:(1) with the development of the strong convective core,the associated positive relative vorticity core in the lower troposphere extends to the mid-upper troposphere,so as to form a deep and intense cyclonic air-column penetrating to the upper troposphere in the severe convective region of mature MCS.In the meantime,bands of positive and negative vertical vorticity,paralleled to the convective zone,are found in the middle troposphere.The maximum positive vorticity is near the severe convective zone,the negative vorticity near the rear of the convective zone and submaximum positive vorticity farther back.(2) On the surface map,the MCS is composed of a mesolow and a mesohigh which are located in the front of severe convective zone and behind the zone,respectively,i.e.,the severe convective zone occurred at the transition zone between the mesolow and the mesohigh.(3) The mesoscale circulation characteristics viewed in the coordinate system moving with the MCS include: a strong,nearly perpendicular convective scale updraft penetrates the troposphere of the severe convective zone;a convective scale downdraft within the lower troposphere is behind severe convective zone;two major branches of inflows which occurred respectively near 300 hPa and 900 hPa are ahead of the severe convective zone;the mesoscale updraft and downdraft are divided near 0℃ level in the stratiform precipitation region;the convergent inflows in the lower troposphere appear at 900 hPa and the divergent outflows in the upper troposphere appear at 200 hPa.