NOx produced by lightning (LNOx) is an important factor in the formation of tropospheric ozone. Chemical models such as WRFChem can simulate some aspects of ozone's (O3) chemistry, but to produce accurate O3 concentrations, it is important to accurately specify the LNOx. Although lightning is a major source of NOx, WRFChem does not include that aspect of lightning production. The goal of this study is to improve the forecast of tropospheric ozone in WRFChem by adding a module 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 stations, together with data from WSR-88 Doppler radars, the relationship between lightning flash rate and convective storm height will be studied. LMAs provide information about both in-cloud and cloud-to-ground flashes. A lightning module for use in the WRFChem model will be designed based on this study. The module probably will contain an approximately second order power law fit between the flash rate and the radar reflectivity echo top above the freezing level. Once established, this relationship will be used to predict lightning flash rate in a particular area based on WRF-derived reflectivity. Finally, a module to calculate NOx production from lightning will be inserted into WRFChem and evaluated using data from previous NASA airborne field studies of atmospheric pollution. The relationship between lightning and NOx from previous studies will be used in this module.
a) What is the relationship between radar reflectivity and lightning flash rate? The exact mechanism of storm electrification is unknown. One of the latest theories (Deierling et al.,2005) describes a relationship between lightning frequency and the product of the downflux of solid precipitation and the upward mass flux of ice crystals. This method is hard to use in a regional model, and the current study will use a theory based on previous research by Futyan and Del Genio, 2007 involving the relationship between radar storm height and lightning flash rate.
b) Can this relationship above be used in a model such as WRFChem to simulate lightning flashes?
c) What is the relationship between lightning flash rate and NOx production?
Based on results from Objective 1, will including a lightning production module option in WRFChem improve the simulations of ozone? And can the lightning module be used in an operational sense?