OBSERVATION AND MODEL ANALYSES OF POSITIVE CLOUD-TO-GROUND LIGHTNING IN MESOSCALE CONVECTIVE SYSTEMS

<正> The analyses of spatial and temporal characteristics of positive cloud-to-ground(CG)lightning for four mesoscaleconvective systems and two severe local convective systems in 1989 and 1990 show that positive CG flash rate usuallyhas two peak values.The major peak occurs during the developing stage of the storm and most of the positive CGflashes originate at the lower part of the storm.The minor occurs during the dissipative stage of the storm and most ofthe positive CG flashes originate at the upper part of the storm,especially in the region of the wind divergence in thestorm anvil.The positive CG flash rate is almost an order of magnitude larger in the developing stage than in thedissipative stage.The appearing time of the peak of negative CG flash rate is in accordance with that of the valley of pos-itive CG flash rate.The higher the intensity of the radar echo,the higher the positive CG flash rate.Most of the positive CG flashes oc-cur when the weak echo area is larger,and mostly originate in the region where the radar echo intensity is about 10dBzand in the back region of the moving storms.The spatial distribution of the positive CG flashes is much more dispersivethan that of the negative.The mesoscale analysis reveals a bipolar lightning pattern.The mean bipole--length reaches itsminimum during the mature stage of the storm and reaches the maximum during the developing stage of the storm.The vertical distribution of the charge density is calculated by a one-dimensional charging model.Then,we discussthe producing condition of the positive CG lightning and forming cause of charge structure mentioned above.