Simulated Convectively Influenced vs. Lightning Influenced Air: Methods for Calculation and Associated Uncertainties

Abstract:

Some atmospheric chemical processes are influenced by the presence of convection and/or lightning. Therefore, it is often important to determine whether a sampled air parcel previously has encountered these influences. This paper describes procedures for making these determinations and provides uncertainties associated with the procedures. Prior locations of the sampled air are obtained by calculating backward trajectories based on 1 h WRF output with 3 domains (horizontal grid spacing of 36, 12, and 4 km). Back trajectories are released along every DC-8 flight track from the recent NASA SEAC4RS campaign (~22,000 trajectories total). If lightning influence is sought, locations of the flashes usually are obtained from ground-based networks such as the National Lightning Detection Network (NLDN). Results show that the user-defined spatial and temporal separation between a trajectory and the flash location greatly influences whether the trajectory is considered lightning influenced. For example, a spatial threshold of 0.5 (0.25) deg lat/lon leads to 69% (60%) of the trajectories being lightning influenced. If model-derived convective influence is sought, a vertical displacement threshold corresponding to w≥1 m s^-1 is used to identify convectively influenced parcels. Remarkably, the model-derived method using the 4 km WRF output leads to only 5% (compared to ~60% from lightning) of the trajectories being convectively influenced. Additionally, the horizontal grid spacing (36 km vs. 4 km) exerts a major influence on the paths of the trajectories. The overriding conclusion is that very different results are obtained depending on whether simulated convectively influenced or lightning influenced air is being sought. Also, once this choice is made, the resolution of the model and the time/space thresholds that are chosen will have a major impact on the number of trajectories that are determined to be influenced. This paper will provide guidance on these choices.