Interesting Weather Information

Monday, March 18, 2013

Thunderstorm Primer - Part 3 - Linear Mesoscale Convective Systems - Bow Echoes

A Bow Echo is pretty much what it sounds like it is - a line of thunderstorms shaped like a hunter's bow. Bow Echoes range from small, covering a few counties and lasting for a couple of hours to very large covering several states and lasting up to 24 hours.  The big ones are often considered to be a special classification called "derechos". The word derecho (pronounced DAY- ray - cho) is Spanish for "straight" referring to the damaging straight-line winds that often accompany derechos. This post will cover bow lines later after the Thunderstorm Primer is complete I will post  a detailed description of derechos.

Bow Echo over Eastern Oklahoma (index map lower right) may 24, 2003 10:48 GMT.

Ted Fujita, the F-Scale guy, was the first to describe in detail how this type of Linear MCS evolves. The schematic below is based on Fujita's original from 1978.

A large cumulonimbus develops and evolves  because of the strong rear inflow jet (RIJ). As the line moves to the right on the diagram then central region bows in response to the mid-tropospheric RIJ. The north end of the line often develops into a mesoscale low and there may be enough turning in the northern bookend vortex to spawn a tornado. The rotation of the southern bookend vortex is opposite the rotation of Earth (i.e. opposite the coriolis effect) and does not develop.

The northern end  and the southern end are subject to rotation due to their location north and south of the Rear Inflow Jet. At the southern end the rotation is opposite to the coriolis effect and does not develop. The rotation at the northern end is enhanced by the coriolis effect effect and the vortex strengthens often leading to a coma shaped radar echo with the wide part with the northern vortex. You can see this on the radar image above.

The vortices are called "Bookend Vortices" and there is enough spin that tornadoes frequently develop in association with the northern vortex.

Because the inflow jet is dry, evaporation at the rear of the storm cools it, makes it more dense and then it can descend towards the ground in a long line of straight line winds along the leading edge of the line. Wind damage is widespread when this happens.

Leading edge vortices (still called tornadoes almost everywhere) can develop as the inflow interacts with the straight line winds moving down out of and ahead of the line.

The video above is from a derecho, essentially a long-lived, large bow echo that moved through the Cincinnati area during the evening of June 29, 2012.  I will have much more on derechos in a post after the conclusion of Thunderstorm Primer.

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