What is a bow echo?

The term "bow echo" was introduced by Fujita in 1978 to describe a radar pattern that has a "distinct shape, like that of a bow or crescent." Fujita found that his bow echoes were often associated with severe surface wind swaths caused by downbursts. He also conjectured that bow echoes are associated with certain kinematic features such as elevated rear inflow and bookend vortices --- conclusions that have since been verified by Doppler radar observations as well as by numerical simulations.

Recently, it seems that anything that is arc-shaped on radar is called a bow echo, regardless of its size, the presence of downbursts, or the processes that led to the arc shape. Furthermore, there has been a very recent introduction of new, sometimes undefined terms such as "squall-line bow echo" and "bow echo MCS." These issues raise the questions: "what is a bow echo?" and "would a dynamical definition for a bow echo be useful?

To begin answering these questions, we analyze a series of idealized numerical simulations with two configurations: one set specifies a "quasi-infinite" squall line, wherein along-line boundary conditions are periodic to avoid simulating the ends of the line; the second set specifies a squall line with a limited extent, wherein along-line boundary conditions are open-radiative, and line ends are part of the simulated system. We find that any simulated convective system can become bow shaped if line ends are allowed. However, under certain environmental conditions we also find smaller bow-shaped convective features in either simulation configuration; these appear to be the "bow echoes" identified by Weisman (1993) as a "dynamically unique form" of convective organization, and may be truer to the county-scale bow echoes studied by Fujita. So far, the only dynamically distinct feature we find that separates these bow echoes from other bow-shaped systems is elevated rear inflow at the leading edge of the system. Other features such as bookend vortices and severe surface winds can occur with all types of convective systems having line ends (and, thus, bowed shapes).

Interestingly, all of our simulations with line ends produce stronger surface winds than simulations without line ends, suggesting that forecasters are wise to look for bow shaped echoes as an indicator of potentially severe surface winds. However, our simulations are consistent with recent studies of BAMEX cases, wherein small-scale bow shaped echoes (like those studied by Fujita and Weisman) produced the most damaging surface winds.