Convective Transport of Trace Species Observed During the Stratosphere-Troposphere Analyses of Regional Transport 2008 Experiment (START08)

During the Stratosphere-Troposphere Analyses of Regional Transport 2008 Experiment (START08) the NCAR/NSF Gulfstream V aircraft observed high concentrations of NO and NOy in the upper troposphere downwind of a weakening squall line in northern Texas, suggesting either convective transport of polluted boundary layer air to the upper troposphere or lightning production of nitrogen oxides in the convection. These hypotheses are tested by computing three-dimensional back trajectories using winds from a high-resolution simulation of the event with the Weather Research and Forecasting (WRF) Model. The WRF model simulation reproduces the storm structure and evolution with good fidelity. The back trajectories reveal two distinct layers of outflow air from different mesoscale convective systems (MCSs). Most air in the upper layer is transported northward from an MCS in southern Texas, while the lower layer is from both the northern squall line and the southern MCS. In both layers inconsistencies between observed concentrations of CO, NO, and O3, and predictions from a simple mixing model suggest that there is significant production of NO by lightning in the convective systems. This is consistent with lightning observations from the National Lightning Detection Network (NLDN). Additionally, the model simulation appears to slightly underestimate the depth of vertical transport by the MCS.