Numerical Simulation and Analysis of a Prefrontal Squall Line. Part II: Propagation of the Squall Line as an Internal Gravity Wave

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Abstract
A numerical study of the squall line that occurred on 17–18 June 1978 was described in Part I of this paper. The squall line was collocated with a surface front during its initial development (at 0000 UTC 18 June 1978), but then propagated faster than the front, resulting in a separation of approximately 200 km by 0300 UTC and 300–400 km by 0600 UTC. In this paper (Part II), the movement of the squall line in the model is shown to be due to the propagation of a deep tropospheric internal gravity wave in a wave–CISK-like (Conditional Instability of the Second Kind) process. The thermal and dynamic perturbations associated with the hypothesized wave are shown to be consistent with internal gravity wave theory, and the characteristics of the wave are compared to similar results from other wave-CISK studies. The current literature favors the mechanism of gust front convergence to explain squall-line propagation, although there are other modeling studies that show specific instances of squall-line propagation as being due to internal gravity waves. It is suggested that a spectrum of scales of forcing may exist and be responsible for squall-line propagation, but many models and observations may be able to detect only the gust-front-type processes. The 17–18 June 1978 squall line probably did not propagate solely as the result of any one mechanism, but instead as the product of several active mechanisms. The dominant mechanism in these modeling simulations was an internal gravity wave, and it seems reasonable that the gravity wave was at least one of the mechanisms responsible for the actual propagation of the 17–18 June 1978 squall line. An unsuccessful attempt to model the squall line with a 5-km grid spacing and without a cumulus parameterization is also discussed. Briefly, the squall line did not develop properly on that scale and did not separate from the front.

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