A High Plains Squall Line Associated with Severe Surface Winds

Abstract

The characteristics of a severe squall line are examined using data acquired from the 1981 Cooperative Convective Precipitation Experiment (CCOPE). The case is unusual in that the squall line was decoupled from an immediate, surface-based inflow source due to a meso?-scale (20?200 km) outflow pool produced by a separate mesoscale convective system. Both systems participated in the development of a mesoscale convective complex which subsequently produced sustained, severe surface winds throughout its entire life cycle. Despite the absolutely stable, presquall atmospheric boundary layer, the squall line produced radar reflectivity values of 70 dBZ and storm-induced outflow of 30 m s?1. Aircraft soundings in the presquall environment suggest the storm was sustained by an elevated layer of high-valued ?c air overriding the cold dome produced by the developing MCC. The strongest surface winds were located upshear from the convective line and consisted of a northerly (alongline) component. Because the middle-level environmental flow was from the southwest, a simple vertical transport of middle-level momentum cannot account for the observed surface flow. The strong surface winds were primarily a result of the local surface pressure gradient associated with a mesohigh?mesolow couplet that accompanied the squall line. The squall line also maintained a strong, quasi-steady, supercell-like cell that could be tracked by radar for several hours. The kinematic structure, derived from a multiple Doppler radar analysis, shows that significant alongline flow was also generated by the rotational characteristics of the supercell. This feature distinguishes this system from tropical squall lines and many midlatitude squall lines which are composed of transient ordinary cells.