Numerical Simulations of the Microphysics and Electrification of the Weakly Electrified 9 February 1993 TOGA COARE Squall Line: Comparisons with Observations

A cloud scale model with a 12-class bulk microphysics scheme, including 10 ice phases and a 3D lightning parameterization, was used to investigate the electrical properties of a well-documented tropical squall line from the Tropical Ocean Global Atmosphere Coupled Ocean–Atmosphere Response Experiment (TOGA COARE). Consistent with observations, the simulated maximum updraft speeds across the squall line seldom exceeded 10 m s−1, which was expected given the relatively shallow 30-dBZ echo tops that rarely extended above the top of the mixed-phase layer (−20°C isotherm). Enhanced warm rain processes caused most of the liquid water to precipitate near the gust front at lower levels (below 4 km AGL), which accounted for the small amounts of graupel and cloud water content present in the mixed-phase region and, consequently, for generally weak charging and electrification. Most of the charge present in the squall line was generated within a few storm cells just behind the leading edge of the gust front that had sufficiently strong updraft speeds near the melting level to produce moderate values of graupel mixing ratio (>0.5 g kg−1). In contrast, the trailing stratiform region at the back of the line, which was mainly composed of ice crystals and snow particles, contained only weak net charge densities (<0.03 nC m−3). The spatial collocation of regions characterized by charge densities exceeding 0.01 nC m−3 and noninductive (NI) charging rates greater than 0.1 pC m−3 s−1 in this stratiform region suggests that NI charging is a plausible source for the majority of this charge, which was confined to discrete regions having small amounts of graupel (approximately 0.1–0.3 g kg−1) and cloud water content (CWC; ∼0.1 g m−3). The simulated weak updraft speeds and shallow echo tops resulted in a system exhibiting little overall total lightning activity. Although the 5-min average intracloud (IC) flash rate rarely exceeded 10 flashes per minute and only 3 negative cloud-to-ground (−CG) lightning flashes were produced during the entire 4 h and 30 min of simulation, this still was more electrical activity than observed. This tendency for the model to generate more lightning flashes than observed remained when the inductive charging mechanism was turned off, which reduced the total amount of simulated flashes by about 43%. The three CG flashes and the great majority of the IC flashes occurred within the strongest cells located in the mature zone, which exhibited a normal tripole charge structure.