BOW ECHOES and BOOKEND VORTICES
The above figure is a prototype of the evolution (stages A through E) of a
bow echo (from Fujita 1978).
The black contour lines are meant to depict radar reflectivity. Dr. T. Theodore Fujita, a professor
at the University of Chicago, coined the term "bow echo" in the late 1970s. The terminology
was based on how bands of rain showers or thunderstorms "bow out" when strong outflow winds
associated with the storms reach the surface and spread out like pancake batter. The bowed rain
band is near the leading edge of the damaging winds, and frequently marks the location of where
the rear-inflow jet contacts the ground. The storm system on which Fujita based the "bow echo"
terminology produced a strong derecho over northern Wisconsin and adjacent states on
July 4, 1977.
Derechos typically are associated with a long lived bow echo or a series of bow echoes. These bow echoes may vary in size, but usually go through an evolution that displays at least some of the aspects of the prototype shown above. The time span involved in the schematic varies with the size of the bow and with the background thermodynamic and kinematic (wind/mass field) environment. Small bows tend to evolve more quickly than larger structures, and those that form in very thermodynamically unstable and/or strongly sheared environments usually evolve more rapidly than those forming in more settled regimes. A smaller-scale bow, for example, one whose length extends through perhaps three or four average-sized counties, might evolve from stages A through E in about an hour. In contrast, for a larger-scale bow (such as those associated with many of the more significant, longer-lasting derechos described in Noteworthy Events), the time span involved is on the order of several hours.
During the development of a bow, counter-rotating storms (or areas of storms) commonly appear at both ends of the larger-scale bowing segment, straddling the rear-inflow jet. These storms are known as bookend vortices. In the above schematic, such features are likely to be best-developed during stages "C" and "D" at the locations marked by the red dots. The presence of bookend vortices can enhance the rear-inflow jet and thereby initiate or accelerate the bowing process. If a bow echo persists for some time, i.e., for more than two or three hours, the influence of Coriolis force becomes significant. This causes the poleward (cyclonic) member of the two vortices to become dominant and, over time, the overall convective system to become increasingly comma-shaped. To emphasize that the poleward member of a bookend pair most often is dominant, the red dots on the left side of the dotted downburst ("DB") path in the schematic have been drawn larger than those on the right, with the hatched depiction of the southern vortex in stage "E" signifying its demise. A composite radar view of a comma-shaped derecho-producing convective system with a bookend vortex is shown below. The radar image most closely corresponds to stage "D" in the schematic. The location of the system's northern bookend vortex is highlighted by the open circle in northwest Ohio.
