Characteristics Of Lightning Flash Size And Parameters At Flash Initiation Stage In A Supercell Storm

Lightning flash initiation and size,are the new parameters to characterize lightning activity.The research on the relationship between these new parameters and storm structures is valuable for further understanding of flash initial break and subsequent development,deep exploration of characteristics and formation mechanisms of lightning activity,and promotion for lightning warning,forecast and data assimilation.This paper studies on the flash initiations and sizes of a supercell storm,in New Mexico on October 5,2004,using the observations from the New Mexico Lightning Mapping Array and the Albuquerque,New Mexico,Doppler radar(KABX).A new method to calculate parameters at flash initiation stage is designed and applied to this supercell storm and the relationship between flash size and initiation and storm structure is studied.Details are as follows:Basic characteristics of flash initiation and size: 1)The median values of the duration,three-dimensional displacement,vertical displacement and the average displacement velocity for the upward(downward)negative leaders at IS are 13.5 ms(7.5 ms),1.4 km(1.0 km),0.9 km(0.5 km),and 9.2×10~4 m/s(1.2×10~5 m/s),respectively.2)With time going on,the speed of upward negative leader at IS decreases,while that of the downward negative leader increases.3)At IS,negative leaders are dominantly tilted,considering that the median angles between the 3-D displacement direction and the vertical direction are 40° for upward leader and 54° for downward leader,respectively.4)the probability density distribution of flash size described by flash convex hull and total channel length can be well fitted by negative power function,while the flash size bias to the small value.The median of flash horizontal extent is 6.1 km while the vertical extent is 4.3 km,and there are 83% of flashes whose horizontal extent is greater than vertical extent.5)The flashes with horizontal extent greater than vertical extent are mainly initiated at 8.5 km,and those with vertical extent greater than horizontal extent are mainly initiated at 11 km.6)The more horizontal the leader displacement direction at IS is,the less the flash vertical extent is.Relationship between flash initiation and storm structure: 1)Flash initiation position inside the convection zone is higher than that of outside,while flash initiation displacement,velocity,and vertical angle is smaller than that of outsize.And these trends maintain with the evolution of storm.2)the closer the lightning is to the updraft,the higher the flash initiation position is.The flash initiation position in the rear flank downdraft is lower than that in the updraft while the flash initiation height in the right anvil is close to that in the updraft.3)flash initiations firstly firstly appeared in the updraft,and then in the rear flank downdraft,lastly in the right anvil.Relationship between flash size and storm structure: 1)during the temporal evolution of the supercell,the mean flash size is anti-correlated with the flash rate,following a unary power function,with a correlation coefficient of-0.87.In addition,the mean flash size is linearly correlated with the area of reflectivity greater than 30-dBZ at 5 km normalized by the flash rate,with a correlation coefficient of 0.88.2)In the horizontal,flash size increases from the region near the convection zone to the adjacent forward anvil.The region of minimum flash size usually corresponds to the region of maximum flash initiation and extent density.The horizontal correspondence between the mean flash size and the flash extent density can also be fitted by a unary power function,and the correlation coefficient is greater than 0.5 in 50% of the radar volume scans.Furthermore,the quality of fit is positively correlated to the storm intensity.3)In the vertical,the height of the maximum flash initiation density is close to the height of maximum flash extent density,but corresponds to the height where the mean flash size is relatively small.In the discussion,the distribution of the uplifted,small and dense charge regions when and where convection is vigorous in the storm,is deduced to be responsible for the relationship that flash size is anti-correlated with storm intensity or flash activity intensity,and the characteristics that high flash initiation,small flash initiation displacement and velocity appear in the convection zone.