The Surface-Wind Response to Transient Mesoscale Pressure Fields Associated with Squall Lines

Abstract

The two most prominent surface pressure features associated with squall lines are 1) a surface mesohigh, centered within the heavy-rain region, and 2) a wake low, located at the back edge of the trailing area of light rainfall. The surface flow near these features is often highly unbalanced due to propagation and transience of the pressure fields. Centers of surface divergence and convergence are typically displaced rearward of the mesohigh and wake-low axes, respectively. In an attempt to explain the basic mesoscale characteristics of the surface flow in the vicinity of squall lines, a simplified model of a propagating mesohigh-wake-low couplet is developed using a one-dimensional slab model of the boundary layer. The component of the flow normal to the squall line is predicted, with advective and frictional effects included but the Coriolis force neglected. Model results are compared to the observed airflow near the mesohigh and wake low associated with an intense squall line that moved through Oklahoma and Kansas on 10?11 June 1985. The primary mesoscale features of the surface flow associated with squall lines are explained by the model. Displacement of the divergence and convergence axes to the rear of the mesohigh and wake-low axes is a result of propagation of the system. Strong winds are found out ahead of the mesohigh when the phase speed of the pressure wave matches the air-parcel velocities. This effect, along with the vertical transport of momentum by the convective line, can cause surging of the winds along the gust front. Strong surface convergence can occur behind the wake low even for weak pressure disturbances, which may account for the generation of new convection to the rear of some squall lines. In addition, as a result of reduced friction, strong and damaging winds may develop in the vicinity of wake lows that pass over open-water areas. Some squall lines possess very intense pressure gradients between the mesohigh and wake low (on occasion equivalent to that in the eyewall of a moderate hurricane); however, hurricane-force winds normally do not develop because air parcels do not stay in this gradient long enough to achieve extreme velocities.