Data Assimilation of Lightning in WRF 3/4-D VAR Using Observation Operators

Compared to other types of satellite-derived data, assimilating lightning data into operational numerical models has received relatively little attention. NASA will launch in 2015 the GOES-R Lightning Mapper (GLM) that will provide continuous, full disc, high resolution total lightning (IC + CG) data. Previous efforts of lightning assimilation mostly have employed nudging. How-ever, we propose to develop a more sophisticated approach involving 3-D VAR and 4-D VAR toward which both NCEP and NRL are moving. The early stages of our research will utilize existing ground-based lightning data that can be assimilated prior to the launch of GLM; later phases will utilize GLM proxy data that will mimic what GLM will detect. A major difficulty associated with this exercise is the complexity of the observation operator defining the model equivalent of the lightning. It is using Convective Available Potential Energy (CAPE) as a proxy between lightning data and model variables. This operator is highly nonlinear. Marecal and Mahfouf (2003) have shown that nonlinearities can prevent a direct assimilation of rainfall rates in the ECMWF 4D-VAR (using the incremental formulation proposed by Courtier et al. (1994)) from being successful. In our case we also proved that the direct assimilation o