Boundary-Layer Forcing as a Possible Trigger to a Squall-Line Formation

Abstract

The study of the life history of the 8 June 1966 squall line by Ogura and Chen (1977) indicates that a well-defined narrow hand of horizontal convergence was present at low levels prior to the appearance of first radar echoes. A two-dimensional model for the dry planetary boundary layer is developed and applied to this case in order to test the hypothesis that the prestorm convergence is produced by boundary-layer processes in association with a strong horizontal temperature gradient. The level 3 turbulence closure approximation by Mellor and Yamada (1974) is incorporated into the model as well as the similarity hypothesis of Businger et al. (1971) for the lowest constant flux layer. The basic driving mechanism is the diurnal variation of the temperature contrast across the observed dry line. Air on the northwest side was warm, while on the southeast side it was cool. The temperature contrast was introduced into the model as a lower boundary condition for the potential temperature. The model is integrated, starting from the early morning conditions through the late afternoon. The results indicate the development of ascending-descending motions as soon as a horizontal temperature gradient is established. In time, the mixed layer also develops and its depth increases. AS expected, it increases faster on the warm side than on the cool side. The intensity of the upward motion increases at a rate larger than that of the downward motion and the center of the ascending motion remains at a certain level (?800 mb). The center of the downward motion moves up in time. Thus, the upward motion is concentrated in the mixed layer at the location of the sharp gradient in the inversion. The major observed features in the velocity and temperature fields in the prestorm situation are well simulated by the model. Further, the result of a sensitivity test for a different initial wind field indicates that the location and intensity of the. resulting ascending motion is rather sensitive to the initial wind field. It is concluded that, if the synoptic-scale low-level wind blows in the right direction (off shore in sea-breeze terminology), a low-level horizontal temperature gradient of the magnitude observed in the 8 June 1966 squall line case is capable of generating upward motion with sufficient intensity to release the potential instability.