The sensitivity of single air parcel trajectory calculations to starting elevation

Abstract

Trajectory models are frequently used to characterize the atmospheric transport pathways for airborne gases and aerosols. Users of these models must specify a starting elevation for their calculations. The variation of wind with altitude causes trajectory models to be sensitive to the starting elevation, particularly when single trajectories rather than Lagrangian particle dispersion simulations are used to characterize atmospheric transport.In this work we systematically investigate and quantify the sensitivity of single trajectory calculations to the starting elevation. The analysis was based on an eight-year database of daily, 48-h back-trajectories calculated for ten sites. Trajectories were calculated at four different starting elevations, and the horizontal difference between endpoints was determined for five upwind travel times. Trajectory model calculations were found to be strongly sensitive to starting elevation. A 500m difference in starting elevation leads to an average horizontal separation of 326km after 48h. Mean horizontal separations of 627km and 886km were found for starting elevation differences of 1000m and 1500m, respectively. A seasonal dependence of the sensitivity was found, with the smallest separations occurring during the summer, the largest during winter, and intermediate values during the fall and spring. A linear relationship was observed between trajectory model sensitivity and difference in starting elevation. Empirical equations were presented to approximate this relationship.