The Generation and Propagation of a Nocturnal Squall Line. Part II: Numerical Simulations

Abstract

The mechanisms behind the convective regeneration that occurred at midnight on 26 May 1985 in the Kansas PRE-STORM network (described in Part I) are examined with a numerical cloud model. Three mechanisms that could have caused the regeneration are identified: 1) an increase in low-level moisture, 2) an increase in low-level shear and 3) a mesoscale oscillation forced by previous convection. The first numerical experiments begin with a horizontally uniform environment. A primary squall line is generated in this environment and is then allowed to decay. Four hours after the decay, a second convective system develops at the leading edge of the gust front from the primary squall line. It is shown that this secondary generation is due to a mesoscale oscillation centered at 70 kPa that is forced by the previous convective system. Horizontal variations in the upstream environment are then considered. An increase in low-level moisture is examined first, and it is shown that the consequent lowering of the level of free convection (LFC) allows the gust front to trigger new convection. It is also found that the convective regeneration is stronger when the gust front collides with the moisture gradient during the upward phase of the mesoscale oscillation. Horizontal increases in low-level shear are then considered. In the region of enhanced shear, the lifting at the gust front is increased and consequently new convection can be triggered. In a comparison of the three regeneration mechanisms it is found that the strongest convection occurs for increases in low-level shear.