Development of A Lightning Nox Algorithm For WRF-CHEM

Abstract

NOx produced by lightning (LNOx) is an important factor in the formation of tropospheric ozone. Atmospheric chemistry models such as WRF-Chem can simulate the photochemistry of ozone (O¬3), but to produce accurate O3 concentrations, it is important to accurately specify the LNOx. Lightning is particularly important in contributing to the production of O3 in the middle and upper troposphere. This paper will describe our recently developed procedure to add a module to WRF-Chem that parameterizes lightning occurrence and the formation of NOx by lightning. Previous studies have shown that radar-derived storm height is strongly correlated with lightning flash rate. Using data from Lightning Mapping Arrays (LMAs) at several locations, together with data from WSR-88 Doppler radars, we further develop this relationship between lightning flash rate and convective storm height. LMAs provide information about in-cloud and cloud-to-ground flashes, both of which produce NOx. A lightning flash rate module for use in a regional scale version of the WRF-Chem model is designed based on this relation. The module contains an approximately second order power law fit between flash rate and radar reflectivity-derived echo top above the freezing level. This relationship then is used to predict lightning flash rate based on derived reflectivity. Finally, a module to calculate NOx production from the lightning flash rate is inserted into WRF-Chem. Previous estimates of LNOx production per flash are used in this module. The forecast NOx then is vertically distributed based on profiles of lightning sources from the LMA data. These distributions are a function of convective storm height above the freezing level. Results are evaluated using data from previous NASA airborne field studies of atmospheric chemistry.