Explicit electrification and lightning in a 350 m WRF-ARW simulation of Hurricane Isaac (2012): Comparisons with observations and relationships with microphysics and kinematics

Lightning threats in present-day numerical weather prediction models are currently diagnosed from model variables such as graupel mixing ratio and ice water content that are known to be well correlated with the occurrence of lightning. To provide a more physically sound assessment of lightning threat, an explicit discharge model (with explicit solution of the 3D electric field and 3D discharge scheme) has been successfully implemented into the WRF-ARW model using hydrometeor electrification within the NSSL two-moment microphysics scheme. This talk will present the results from a high resolution 350 m simulation of the electrification processes within a hurricane in conjunction with available total lightning observations to augment our general understanding of some of the key cloud-scale electrification processes within these systems. The general environment and trends of Hurricane Isaac (2012), whose lightning activity was observed by the Earth Networks Total Lightning Network, were utilized to produce a reasonable tropical cyclone simulation. Overall, simulated storm-total flash origin density rates remain comparable to the observations. Because simulated reflectivities were larger and echo tops higher in the eyewall than observed, however, the model consistently overestimated lightning rates there. The gross vertical charge structure in the eyewall resembled a normal tripole or a positive dipole, depending on the location. The negative charge at middle levels and positive at upper levels arose primarily from noninductive charging between graupel and ice crystals/snow. As some graupel melted into rain, the main midlevel negative charge region extended down to the surface in some places. The large volume of positively charged snow aloft caused a radially extensive negative screening layer to form on the lighter ice crystals above it. Akin to continental storms and tropical convection, lightning activity in the eyewall was well correlated with ice water path (r > 0.7) followed by the graupel+hail path (r ≈ 0.7) and composite reflectivity at temperatures smaller than -10°C and the snow+ice path (r ≈ 0.5). Relative maxima in updraft volume, graupel volume and total lightning rates in the eyewall all were coincident with the end of an intensification phase.