a Three-Dimensional Trajectory Study Within and around a Supercell Thunderstorm Model.

Some aspects of storm-environment interaction in the strongly sheared and veered environment are studied based on the three-dimensional dynamic trajectory calculation of air parcels. The input data needed are the three-dimensional perturbation pressure gradient forces (PPGF) and three wind components obtained from the storm model of Lin and Chang (1977). The computational domain has a volume of 20(km) x 20(km) x 14(km) covering the main updraft, the transition zone and the quasi-potential flow region of a model storm. Nearly 110 parcels were released uniformly over the western half of the domain at 3, 6 and 9 km level, respectively, with a constant distance of 2 km on each side. The dynamic trajectories were then computed using the Hamming's modified predictor-corrector method. The energetic characteristics of dynamic trajectories calculated are also investigated by examining the contribution of PPGF and thermal buoyancy to the kinetic energy change of a given air parcel. Further, the kinematic trajectories are calculated for the purpose of checking the physical consistency of the corresponding dynamic trajectories using the same launching positions and numerical method. Results obtained show that air parcels released on the storm's upwind side will either be forced to flow around the main updraft, or be forced to move downward forming the sloping downdraft. A physical role played by the three-dimensional PPGF in affecting the structure and internal dynamics of a model storm is shown to be of vital importance in several regions. In the quasi-potential flow region, incoming airflow is partly diverted away from the main updraft core thereby minimizing the lateral entrainment. In the transition zone, the negative buoyancy and PPGF act together causing air parcels to move downward forming the sloping downdraft on the left rear flank. Inside the main updraft, the combination of positive buoyancy and vertical PPGF terms is responsible for lifting the warm, moist air from the subcloud layer to the middle and upper levels. Furthermore, three dimensional airflow patterns as determined from dynamic trajectories are quite similar to those obtained from kinematic trajectories. These flow patterns are in good qualitative agreement with that obtained by Browning (1964) and Fankhauser (1971) for a conceptual storm model and by Klemp et al. (1981) for a three-dimensional cloud model.