Radar Observations of the Early Evolution of Bow Echoes

The evolution of 273 bow echoes that occurred over the United States from 1996 to 2002 was examined, especially with regard to the radar reflectivity characteristics during the prebowing stage. It was found that bow echoes develop from the following three primary initial modes: (i) weakly organized (initially noninteracting) cells, (ii) squall lines, and (iii) supercells. Forty-five percent of the observed bow echoes evolved from weakly organized cells, 40% from squall lines, while 15% of the bow echoes were observed to evolve from supercells. Thunderstorm mergers were associated with the formation of bow echoes 50%-55% of the time, with the development of the bow echo proceeding quite rapidly after the merger in these cases. Similarly, it was found that bow echoes formed near, and moved generally along, synoptic-scale or mesoscale boundaries in about half of the cases (where data were available). The observed bow-echo evolutions demonstrated considerable regional variability, with squall line-to-bow- echo transitions most frequent over the eastern United States. Conversely, bow echoes typically developed from a group of weakly organized storms over the central United States. Bow-echo life spans were also longest, on average, over the southern plains; however, the modal life span was longest over the eastern United States. Finally, the supercell-to-bow-echo evolution was most common across the northern plains, but the data sample is too small for this result to be considered significant.