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 socalled "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, 1996. 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.