This work describes the recent implementation of explicit lightning physics within the Weather Research and Forecasting (WRF) Model. Charging of hydrometeors consists of five distinct noninductive parameterizations, polarization of cloud water, and the exchange of charge during collisional mass transfer. The three components of the ambient electric field are explicitly solved for via the computationally efficient multigrid elliptic solver. The discharge process employs concepts adapted from two well-documented bulk lightning models, whereby charge reduction is imposed within a prescribed volume centered at grid points characterized by electric field magnitudes exceeding a given breakdown threshold.This lightning model was evaluated through benchmark convection-allowing (3 km) model simulations of three contrasting convective systems: a continental squall line, a major hurricane (Rita 2005), and a winter storm. The areal coverage and magnitude of the simulated hourly flash origin density (FOD) for the continental squall line are qualitatively comparable to that of the total lightning data observations from Earth Networks Total Lightning Network (ENTLN). In agreement with the ENTLN observations, no FOD are simulated for the winter storm case. The simulated spatial FOD pattern of the hurricane and the eyewall gross charge structure were both in reasonable agreement with observations. The simulated FOD for all three cases were also evaluated against those obtained with the recently developed McCaul diagnostic lightning prediction schemes and exhibited overall good qualitative agreement with each other for Rita and the continental squall line.

Using the Advanced Weather Research and Forecasting numerical model, the impact of horizontal grid spacing on the microphysical and kinematic structure of a numerically simulated tropical cyclone (TC), and their relationship to storm intensity was investigated with a set of five numerical simulations using input data for the case of Hurricane Rita (2005). The horizontal grid spacing of the parent domain was systematically changed such that the horizontal grid spacing of the inner nest varied from 1 to 5 km by an increment of 1 km, this while keeping geographical dimensions of the domains identical.Within this small range of horizontal grid spacing, the morphology of the simulated storms and the evolution of the kinematic and microphysics field showed noteworthy differences. As grid spacing increased, the model produced a wider, more tilted eyewall, a larger radius of maximum winds, and higher-amplitude, low wavenumber eyewall asymmetries. The coarser-resolution simulations also produced larger volume, areal coverage, and mass flux of updraft speeds $5 m s 21 ; larger volumes of condensate and ice-phase particles aloft; larger boundary layer kinetic energy; and a stronger secondary circulation. While the contribution of updrafts $5 m s 21 to the total updraft mass flux varied little between the five cases, the contribution of downdrafts #22 m s 21 to the total downdraft mass flux was by far the largest in the finest-resolution simulation.Despite these structural differences, all of the simulations produced storms of similar intensity, as measured by peak 10-m wind speed and minimum surface pressure, suggesting that features in the higher-resolution simulations that tend to weaken TCs (i.e., smaller area of high surface fluxes and weaker total updraft mass flux) compensate for features that favor TC intensity (i.e., smaller-amplitude eyewall asymmetries and larger radial gradients). This raises the possibility that resolution increases in this range may not be as important as other model features (e.g., physical parameterization and initial condition improvements) for improving TC intensity forecasts.

This study presents the assimilation of total lightning data to help initiate convection at cloud-resolving scales within a numerical weather prediction model. The test case is the 24 May 2011 Oklahoma tornado outbreak, which was characterized by an exceptional synoptic/mesoscale setup for the development of long-lived supercells with large destructive tornadoes. In an attempt to reproduce the observed storms at a predetermined analysis time, total lightning data were assimilated into the Weather Research and Forecasting Model (WRF) and analyzed via a suite of simple numerical experiments. Lightning data assimilation forced deep, moist precipitating convection to occur in the model at roughly the locations and intensities of the observed storms as depicted by observations from the National Severe Storms Laboratory’s three-dimensional National Mosaic and Multisensor Quantitative Precipitation Estimation (QPE)—i.e., NMQ—radar reflectivity mosaic product. The nudging function for the total lightning data locally increases the water vapor mixing ratio (and hence relative humidity) via a simple smooth continuous function using gridded pseudo-Geostationary Lightning Mapper (GLM) resolution (9 km) flash rate and simulated graupel mixing ratio as input variables. The assimilation of the total lightning data for only a few hours prior to the analysis time significantly improved the representation of the convection at analysis time and at the 1-h forecast within the convective permitting and convective resolving grids (i.e., 3 and 1 km, respectively). The results also highlighted possible forecast errors resulting from errors in the initial mesoscale thermodynamic variable fields. Although this case was primarily an analysis rather than a forecast, this simple and computationally inexpensive assimilation technique showed promising results and could be useful when applied to events characterized by moderate to intense lightning activity.