Investigation of mesoscale pressure and temperature transients associated with bow echoes

Squall lines and their accompanying surface pressure, temperature, and moisture signatures have been well studied and documented over the past twenty years. Less studied is the smaller relative of the squall line, the bow echo. A search of the literature will not even reveal a consensus on a comprehensive definition of bow echoes; shape seems to be the only delimiting factor. Yet, bow echoes are often accompanied by extremely strong surface winds as well as heavy rain, hail, and possibly tornadoes, making them an important forecasting problem.

The Oklahoma Mesonet, a network of unprecedented longevity for such detailed spatial and temporal resolution – approximately 30 km at 5 minute periods, over ten years – provides a tool to examine the small-scale features in surface fields associated with bow echoes. The applicability of the squall-line prototypes of surface pressure and attendant temperature fields (e.g. the mesohigh and cold pool) are reviewed, particularly in relation to the onset, end, or complete lack of severe wind reports; stratiform precipitation; and the onset of any bowing. Preliminary results indicate the placement of the mesohigh in relation to the bow echo is typically very slightly behind or even on the convective line throughout its lifecycle, except just prior to new bowing development. The mesohigh also remains to the right of the bow apex throughout its existence. The associated cold pool is located farther behind the convective line and mesohigh, although its placement in relation to the bow apex varies. This differs from the positioning of these features within a typical squall line; there both the mesohigh and the cold pool are even farther behind the convective line, closer to the area of stratiform precipitation.

Development of new bowing is found to be preceded by a surge of the mesohigh ahead of the convective line. This surge is not accompanied by a similar surge in the cold pool, indicating it is not a response from the low-level gravity current resulting from the storm. Further research by the authors will focus on the exact cause of this surge. Upper-level gravity waves generated by mid-level heating in the stratiform precipitation region, or downward momentum transport and vorticity tilting, are the probable origins.