A Numerical Study of a Nontornadic Supercell over France

摘要

A case of a nontornadic supercell over France is simulated with a three-dimensional nonhydrostatic cloud model. The simulation starts from an operational data analysis without any superimposed perturbation. The initial convective cell is triggered by a mesoscale convergence line associated with a preexisting thermal boundary. The model succeeds in simulating right- and left-moving storms arising from a splitting process of the initial convective cell. The right-moving storm exhibits the characteristics of a supercell with a hooklike structure in the precipitation field, a midlevel rotating updraft, and a low-level cyclonic vortex. A vorticity budget analysis is performed along backward parcel trajectories for the initial storm and for the supercell phases, with emphasis on the impacts of the preexisting thermal boundary and the associated low-level variations of shear. Similar mechanisms as those found for the cases over homogeneous environment operate globally. Indeed, the simulation exhibits a couplet of cyclonic and anticyclonic vortex on the flanks of the updraft leading to the split of the initial storm. The supercell derives its low-level cyclonic vorticity from tilting of baroclinically produced horizontal vorticity within the storm's forward flank region. Nevertheless, some differences in the rotational characteristics and in the formation of the initial cell arise from the heterogeneous environment compared to those of homogeneous environment. The vorticity analysis shows that the veering with height of the cyclonic and anticyclonic vertical vorticity cores at midlevels is not symmetric due to the heterogeneous field. It is also found that an additional mechanism operates in the low-level cyclonic vertical vorticity generation.