An Investigation of the Parameterized Prediction of Lightning in Cloud-Resolved Convection and the Resulting Chemistry
Pickering, Kenneth E
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The Deep Convective Clouds and Chemistry (DC3) field campaign provided a unique set of observations to further investigate the role lightning and lightning-generated nitrogen oxides (LNOx) have on the composition of the upper troposphere. With the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem), a cloud-resolved simulation of the 29-30 May 2012 severe supercell system investigated 18 flash rate parameterization schemes (FRPSs), the mean LNOx production per flash, and the transport and distribution of other trace gases. Most of the observed storm’s meteorological and chemical conditions were well represented when the model included convective damping and lightning data assimilation techniques. Newly-developed FRPSs based on DC3 radar observations and Lightning Mapping Array (LMA) data are implemented in the model, along with previously developed schemes from the literature. The schemes are based on relationships between lightning and various kinematic, structural, and microphysical thunderstorm characteristics available in the model. The results suggest the simulated graupel and snow/ice hydrometeors require scaling factors to more closely represent proxy observations. The model flash rates generated over the simulation period are compared with LMA observations. Thirteen FRPSs overpredicted flashes by > 100%. Generally, FRPSs based on storm kinematics and structure (particularly updraft volume) perform slightly better than schemes based on hydrometeors. However, the upward cloud ice flux FRPS best represents the observed lightning. The 10 WRF-Chem simulations included one run with no LNOx and nine runs with different LNOx production scenarios. The simulated CO and O3 are within 1% and 3% of aircraft observations, respectively, when compared with one model layer lower, which suggests the model slightly underestimates the convective transport. A LNOx production scenario of 82 moles per flash best represents the observed NOx mixing ratios in anvil outflow when combined with the observed flash channel vertical profiles and intracloud to cloud-to-ground ratios. This estimate is smaller than the mean 250 moles NO per flash suggested for a typical thunderstorm, but within the lower end of the estimated range (33-660 moles per flash). Analysis of the convective outflow using observations made 12-24 hours downwind indicates the mean daytime photochemical O3 production rate is about 1.6 ppbv per hour.