Squall Lines

Supercell Thunderstorms

Bow Echos

Dry Line

The bow schape is suggested to result from the massive outflow the cell produces. It is coupled with the so- called "rear-inflow jet" at mid levels which transports dry environmental air from above the boundary layer into the downdraft, which enhances evaporation of the precipitation which in turn increases the magnitude of negative buoyancy; in addition, high-momentum air from aloft is brought down to the surface, which both results in high wind speeds near the surface. In this way, an intense gustfront develops, which plays a crucial role in the system's development and sustenance. Apparently, the origin of the rear-inflow jet is associated with the (baroclinic) generation of horizontal vorticity within the system and with the formation of a counter-rotating midlevel mesoscale vortex couplet at the tips of the bow. These mesoscale convective vortices (MCVs) are thought to arise from tilting of horizontal vorticity which has been generated baroclinically (e.g. along the gustfront). Usually, owing to the Corilos force, the northern (cylonic) vortex is better pronounced than the southern vortex.

What is a bow echo? A bow echo is a term meteorologists use to describe a line of thunderstorms which has a distinct convex shape, like a backward "C", pointing into the direction of movement. The origin of how the bow echo forms is complex but basically pressure differences within the core of the storm and the environment cause a descending jet of high speed wind to reach into the lower portion of the storm. Since this air is descending and accelerating it acts as a wedge and rapidly lifts air on the leading edge of the line, near the center, to form new thunderstorms while the portion of the line to the north and south lags behind. As this air descends it also causes drying and evaporation of the precipitation resulting in a decrease in radar reflectivity, or notch, just behind the bowing portion of the line. The strongest winds are perpendicular to the bowing segment and are generally comprised of straight line winds oriented in the direction of the bow echo movement. To the north and south at the end of the bow echo there can be two separate vortices called bookend vortices - the one to the north rotating counter-clockwise and the one to the south rotating clockwise. The northern, cyclonic rotating bookend vortex can spawn mesocyclones but in this case it did not. The southern bookend vortex usually is not as strong as the northern one and is short lived. Here is one example of a bow echo that produced damaging straight-line winds in Salix, Iowa on July 17, This example shows 4 radar reflectivity pictures of the Sioux Falls Doppler Radar showing the evolution of the bow echo from upper left to lower right with a well defined bow echo in the lower right corner during the time of wind damage reports.