Effects of convectively-generated gravity waves on bow echo evolution

An idealized simulation of the 13 March 2003 bow echo over Oklahoma was run using the Cloud Model 1 (CM1). Numerous gravity waves were generated by the simulated convection throughout its lifetime, but a specific slow-moving wave generated shortly before bowing has been examined. This slower gravity wave, moving at approximately 11 m s^-1, was generated by an increase in low-level latent cooling associated with an increase in rear-to-front flow and low-level downdrafts shortly before bowing. The wave moved ahead of the convective line and was manifested at the surface by a positive pressure surge. The low-level vertical motion associated with this wave, in conjunction with higher-frequency gravity waves generated by the multi-cellularity of the convective line, increased the immediate pre-system CAPE by approximately 250 J kg^-1 just ahead of the bowing segment of the convective line, aiding convective updraft strength in that segment.

The latent heating profile, at the time of the large increase in low-level latent cooling shortly before generation of the wave, was next placed as a constant heat source within a dry simulation. A slow-moving gravity wave and surface pressure surge was successfully simulated and appeared similar to the full-physics simulation. To further isolate the cause of the pressure surge, the latent heating profile due solely to evaporation and melting was placed in a second dry simulation, which also produced the slow-moving gravity wave and surface pressure surge. Because of the frequent association of the pressure surge with new bowing development, this result suggests a strong connection between evaporation and melting rates and new bowing. To further evaluate this connection, a second full-physics idealized bow echo simulation was performed. Modifications were made to graupel density and fallspeed within the microphysical parameterization that altered the low-level latent cooling rates. The resulting effects on the slow-moving gravity wave will be discussed.