An analysis of convective transport, Lightning NO.sub.x production, and chemistry in midlatitude and subtropical thunderstorms

Abstract

The impact of lightning NO.sub.x production and convective transport on tropospheric chemistry was studied in four thunderstorms observed during field projects using a 3-dimensional (3-D) cloud-scale chemical transport model (CSCTM). The dynamical evolution of each storm was simulated using a cloud-resolving model, and the output used to drive the off-line CSCTM which includes a parameterized source of lightning NO.sub.x based on observed cloud-to-ground (CG) and intracloud (IC) flash rates. Simulated mixing ratios of tracer species were compared to anvil aircraft observations to evaluate convective transport in the model. The production of NO per CG flash (P.sub.CG) was estimated based on mean observed peak current, and production per IC flash (P.sub.IC) was scaled to P.sub.CG. Different values of P.sub.IC/P.sub.CG were assumed and the results compared with in-cloud aircraft measurements to estimate the ratio most appropriate for each storm. The impact of lightning NO.sub.x on ozone and other species was examined during the storm in the CSCTM and following each storm in the convective plume using a chemistry-only version of the model which includes diffusion but without advection, and assumes clear-sky photolysis rates.

New lightning parameterizations were implemented in the CSCTM. One parameterization uses flash length data, rather than flash rates, as input, and production per meter of flash channel length is estimated. A second parameterization simulates indivdual lightning flashes rather than distributing lightning NO_x uniformly among a large number of gridcells to better reproduce the variability of observations.

The results suggest that P_IC is likely on the order of P_CG and not significantly less as has been assumed in many global modeling studies. Mean values of P_CG=500 moles NO and P_IC=425 moles NO have been estimated from these simulations of midlatitude and subtropical continental thunderstorms. Based on the estimates of production per flash, and an assumed ratio of the number of IC to CG flashes and global flash rate, a global annual lightning NO source of 8.6 Tg N yr^-1 is estimated. Based on these simulations, vertical profiles of lightning NO_x mass for subtropical and midlatitude continental regimes have been computed for use in global and regional chemical transport models.