Assimilating Lightning Data Into WRF Using a Nudging Approach that Modifies the Vertical Profiles of Temperature and Humidity

Total lightning observations (intra-cloud plus cloud to ground flashes) are assimilated into the Weather Research and Forecasting (WRF) model using a Newtonian nudging method. Results are analyzed for two cases with copious amounts of lightning: the 27 April 2011 tornado outbreak across Mississippi and Alabama and the 15 June 2011 severe wind and hail event over the southeast United States. In each case, the WRF model configuration consists of three nested grids with resolutions of 9, 3 and 1 km. The lightning observations, which are similar to those that will be available from GOES-R, are assimilated at 9 km resolution. Two way interaction is allowed between all three grids, and convection is explicitly resolved at all three resolutions. The lightning data are assimilated over a 6 h period, and forecasts are made over the subsequent 6 h period. The presence of lightning allows certain assumptions to be made about the temperature and relative humidity profiles of the storm environment. These assumptions are used to assimilate the lightning observations through nudging of the temperature and relative humidity fields. General improvements in the results are obtained. However, by assimilating in only one direction, such as moistening of the atmospheric column or warming near the surface, biases sometimes occur that are inconsistent with NCEP Global Forecast System Final Analysis datasets and NCEP Stage-IV precipitation analyses. The detailed methodology, as well as the advantages and disadvantages of the assumptions and how they affect the assimilation and model results, will be described in the paper.