Numerical Modeling of a Midlatitude Squall Line: Features of the Convection and Vertical Momentum Flux

Abstract

A 4-h simulation is carried out for the 22 May 1976 squall line that passed through the mesonetwork of the National Severe Storm Laboratory in central Oklahoma. This squall line was more than 100 km wide, oriented north-south and traveled eastward at approximately 14 m s?1. It produced rainfall of 2-h duration at surface stations. The simulation was obtained from a three-dimensional convective cloud model with open lateral boundary conditions on the east and west, and periodic conditions on the north and south boundaries. The model domain is 96 km long (east?west) and 32 km wide (north-south) with a horizontal grid resolution of 1.0 km and a vertical resolution of 0.5 km. A squall line develops and moves eastward at 13.7 m s?1 during the last two hours of the simulation. The present meso?-scale model, however, can only simulate the leading edge of the squall line, with rain at specific surface locations lasting only 30 min. Realistic features of the modeled flow include the surface westerlies moving faster than the line behind the gust front, the strong easterlies in the lower cloud levels, and the cold boundary layer behind the gust front. Two-hour time means of the vertical momentum flux are calculated in a 60-km-wide domain (east?west) following the squall line. The vertical disturbance momentum flux for momentum normal to the line agrees with observations and is primarily confined to this region adjacent to the squall line. Horizontal-averaged time-mean momentum budgets are also calculated in this domain. For the normal component of momentum, this budget is in a quasi-steady state. It cannot be in a fully steady state as the gust front moves 1.2 m s?1 faster than the area of rain behind the line for the 2-h time mean. The parameterization of Schneider and Lindzen for the vertical momentum flux associated with active clouds is compared with mean data from the simulation. Their parameterization accounts for the in-cloud vertical momentum flux reasonably well, but ignores the remaining flux associated with convective-scale downdrafts, which is significant in lower levels.