An Intense, Quasi-Steady Thunderstorm over Mountainous Terrain. Part IV: Three-Dimensional Numerical Simulation

Abstract

A three-dimensional numerical simulation of an intense, quasi-steady left-moving thunderstorm observed over mountainous terrain is presented. The observational analysis of the evolution of convection leading to this storm is presented in Part I, and a detailed analysis of the Doppler radar-observed storm structure is presented in Parts II and III. This storm was particularly interesting because it initially grew in an environment characterized by terrain-induced boundary layer convergence before a massive mesoscale cold front passed underneath. The front cooled and moistened low levels while veering the surface winds to the north, creating a hodograph of winds strongly backing with height. After frontal passage the initial storm cell grew explosively and turned to the left. The observed storm evolution after the frontal passage was reproduced well by the numerical simulation. An observed secondary updraft which was not simulated, was attributed to residual effects of the prefrontal environment, which was not considered. The overall success of this simulation led to the conclusion that the storm structure was largely governed by the environmental wind shear and was only weakly influenced by its triggering mechanism. The microphysical structure was reproduced only moderately well. The model had the greatest difficulty in simulating the echo intensity. This is attributed to the characteristics of the assumed Marshall?Palmer graupel distribution. However, no apparent degrading effects on the dynamical structure were found as a result. The dynamical structure compared well with that of right-moving cells described observationally and simulated numerically by a number of authors. In particular, it was found that the leftward movement was induced by pressure forces projected to low levels within an anticyclonically rotating updraft in approximate cyclostrophic balance. The rotation was produced by the tilting of horizontal voracity (associated with the wind shear) into the vertical and subsequent stretching. Trajectory analysis of updraft and downdraft parcels revealed the existence of both entrainment and pressure-forced downdrafts. It is demonstrated that much of the vertical pressure gradient acceleration of parcels may be accounted for by pressure in approximate hydrostatic equilibrium with the mean density anomaly of the local environment surrounding the parcel.