资源环境科技发展态势分析平台

A suite of six idealized supercell simulations is performed in which the surface drag coefficient C-d is varied over a range of values from 0 to 0.05 to represent a variety of water and land surfaces. The experiments employ a new technique for enforcing a three-force balance among the pressure gradient, Coriolis, and frictional forces so that the environmental wind profile can remain unchanged throughout the simulation. The initial low-level mesocyclone lowers toward the ground, intensifies, and produces a tornado in all experiments with C-d >= 0.002, with the intensification occurring earlier for larger C-d. In the experiment with C-d = 0, the low-level mesocyclone remains comparatively weak throughout the simulation and does not produce a tornado. Vertical cross sections through the simulated tornadoes reveal an axial downdraft that reaches the ground only in experiments with smaller C-d, as well as stronger corner flow in experiments with larger C-d. Material circuits are initialized enclosing the low-level mesocyclone in each experiment and traced backward in time. Circulation budgets for these circuits implicate surface drag acting in the inflow sector of the supercell as having generated important positive circulation, and its relative contribution increases with C-d. However, the circulation generation is similar in magnitude for the experiments with C-d = 0.02 and 0.05, and the tornado in the latter experiment is weaker. This suggests the possible existence of an optimal range of C-d values for promoting intense tornadoes within our experimental configuration.