Production of lightning NOx and its vertical distribution calculated from three‐dimensional cloud‐scale chemical transport model simulations

Abstract

A three‐dimensional (3‐D) cloud‐scale chemical transport model that includes a parameterized source of lightning NOx on the basis of observed flash rates has been used to simulate six midlatitude and subtropical thunderstorms observed during four field projects. Production per intracloud (PIC) and cloud‐to‐ground (PCG) flash is estimated by assuming various values of PIC and PCG for each storm and determining which production scenario yields NOx mixing ratios that compare most favorably with in‐cloud aircraft observations. We obtain a mean PCG value of 500 moles NO (7 kg N) per flash. The results of this analysis also suggest that on average, PIC may be nearly equal to PCG, which is contrary to the common assumption that intracloud flashes are significantly less productive of NO than are cloud‐to‐ground flashes. This study also presents vertical profiles of the mass of lightning NOx after convection based on 3‐D cloud‐scale model simulations. The results suggest that following convection, a large percentage of lightning NOx remains in the middle and upper troposphere where it originated, while only a small percentage is found near the surface. The results of this work differ from profiles calculated from 2‐D cloud‐scale model simulations with a simpler lightning parameterization that were peaked near the surface and in the upper troposphere (referred to as a “C‐shaped” profile). The new model results (a backward C‐shaped profile) suggest that chemical transport models that assume a C‐shaped vertical profile of lightning NOx mass may place too much mass near the surface and too little in the middle troposphere.