Tuesday, 22 January 2008
Relationship between lightning location and polarimetric radar signatures in an MCS
Exhibit Hall B (Ernest N. Morial Convention Center)
The relationship of lightning initiation and structure to the storm microphysics and structure depicted by polarimetric radar has been analyzed for a small mesoscale convective system (MCS) that occurred on 19 June 2004 during the Thunderstorm Electrification and Lightning Experiment (TELEX). Horizontal reflectivity (Z), differential reflectivity (Zdr), specific differential phase (Kdp) and correlation coefficient (RhoHV) data were gathered by a 10-cm, polarimetric radar located in Norman, Oklahoma. Three-dimensional lightning structure was mapped by the Oklahoma Lightning Mapping Array (OK-LMA), and ground strike points were mapped by the United States National Lightning Detection Network. OK-LMA data were processed to group mapped points into flashes and to determine the initiation location of each flash that contained more than 10 mapped points. The initiation location was calculated by sequentially eliminating outliers among the first 10 points that occurred in a flash, with no fewer than 5 points being used in the final initiation location. The initiation location and mapped points for each flash were superimposed on polarimetric radar data in order to investigate lightning relationships with storm structure. Two cells embedded within the MCS were the focus of this study. Both cells had lightning propagating through them at least 15 minutes before they began initiating lightning, but the two cells produced very different quantities of lightning. One cell was relatively shallow with little vertical growth, while the other cell showed good vertical growth with much stronger extremum in the polarimetric data. The stronger cell produced an order of magnitude more flashes than the weaker cell, and the altitude of lightning initiation increased with time during the first half of the cell's lifetime unlike the descending trend of the weaker cell. The lightning initiation points in the entire MCS tended to cluster together in one of two altitude ranges. The two-layer nature of the initiation points is consistent with a grossly tripolar structure of the charge distribution in the convective line. Initial results show that the lower level of lightning initiation occurred near the top of the melting level, where we speculate that melting graupel and cloud ice may be located. The upper level of lightning initiation occurred between the 25 and 35 dBZ reflectivity contours above local reflectivity maxima at lower levels. The polarimetric radar data suggest that this region is characterized by a boundary between graupel and cloud ice.