A cloud-scale model study of lightning-generated NOx in an individual thunderstorm during STERAO-A

Understanding lightning NOx (NO + NO2) production on the cloud scale is key for developing better parameterizations of lightning NOx for use in region al and global chemical transport models. This paper attempts to further the understanding of lightning NOx production on the cloud scale using a cloud model simulation of an observed thunderstorm. Objectives are (1) to infer from the model simulations and in situ measurements the relative production rates of NOx by cloud-to-ground (CG) and intracloud (IC) lightning for the storm; (2) to assess the relative contributions in the storm anvil of conv ective transport of NOx from the boundary layer and NOx production by light ning; and (3) to simulate the effects of the lightning-generated NOx n subs equent photochemical ozone production. We use a two-dimensional cloud model that includes a parameterized source of lightning-generated NOx to study t he production and advection of NOx associated with a developing northeast C olorado thunderstorm observed on July 12, 1996, during the Stratosphere-Tro posphere Experiment-Radiation, Aerosols, Ozone (STERAO-A) field campaign. M odel results are compared with the sum of NO measurements taken by aircraft and photostationary state estimates of NO2 in and around the anvil of the thunderstorm. The results show that IC lightning was the dominant source of NOx in this thunderstorm. We estimate from our simulations that the NOx pr oduction per CG flash (P-CG) was of the order of 200 to 500 mol flash(-1). NOx production per IC flash (P-IC) appeared to be half or more of that for a CG flash, a higher ratio of P-IC/P-CG than is commonly assumed. The resul ts also indicate that the majority of NOx (greater than 80%) in the anvil r egion of this storm resulted from lightning as opposed to transport from th e boundary layer. The effect of the lightning NOx on subsequent photochemic al ozone production was assessed using a column chemical model initialized with values of NOx, O-3, and hydrocarbons taken from a horizontally average d vertical profile through the anvil of the simulated storm. The lightning NOx increased simulated ozone production rates by a maximum of over 7 ppbv d(-1) in the upper troposphere downwind of this storm.