A Numerical Study of a Squall Line over the Taiwan Strait during TAMEX IOP 2

Abstract

A two-dimensional numerical cloud model was used to investigate a squall line that occurred over the Taiwan Strait on 16 May 1987 during TAMEX (Taiwan Area Mesoscale Experiment). This squall line illustrated multicellular behavior as revealed by many Doppler analyses. The simulated squall line produced 13 new cells successively in a 6-h simulation, thus maintaining itself in a long-lasting state. The simulation result recaptured many of the interesting features also observed. The air in the front-to-rear inflow was ingested into the squall-line system primarily from the low levels in front of the system and accelerated upward and backward over a surface pool of cold air in the system-relative frame. Beneath this front-to-rear flow, a rear-to-front current developed between 2 and 4 km in altitude. It descended into the cold pool near the surface. Part of this descending air moved forward toward the gust-front region and part of it moved backward. The relative pressure maximum coincided with the cold pool near the surface and was overlaid by the relative pressure minimum in the midlevels where the convective region corresponded to the decrease of pressure deviation. When the aged cells moved back relative to the system, the decreasing pressure deviation moved back as well. The vertical pressure-gradient force associated with the dynamic part contributed to the formation of a new cell below the level of free convection, while the sum of the vertical pressure-gradient force associated with buoyancy and the buoyancy itself dominated the development of a cell once this cell had ascended above the level of free convection. The vertical transport of horizontal momentum normal to the squall line was found to he countergradient in a layer between 2.5 and 6.5 km, which could not be explained in terms of diffusion. In addition, this vertical flux of horizontal momentum normal to the squall line was independent of the mean vertical shear below 8.25 km, except at the 1.75-km height where the vertical shear changed signs very rapidly. An experiment was made to investigate the role of the ice-phase microphysics on the formation and structure of the simulated squall line. The comparison between our simulation results and other model studies will be presented here.