Numerical Simulations of the Effects of Coastlines on the Evolution of Strong, Long-Lived Squall Lines

Abstract

This study develops conceptual models of how a land?water interface affects the strength and structure of squall lines. Two-dimensional numerical simulations using the Advanced Regional Prediction System model are employed. Five sets of simulations are performed, each testing eight wind shear profiles of varying strength and depth. The first set of simulations contains a squall line but no surface or radiation physics. The second and third sets do not contain a squall line but include surface and radiation physics with a land surface on the right and a water surface on the left of the domain. The land is either warmer or cooler than the sea surface. These three simulations provide a control for later simulations. Finally, the remaining two simulation sets examine squall-line interaction with a relatively cool or warm land surface. The simulations document the thermodynamic and shear characteristics of squall lines interacting with the coastline. Results show that the inclusion of a land surface did not sufficiently affect the thermodynamic properties ahead of the squall line to change its overall structure. Investigation of ambient shear ahead of the squall line revealed that the addition of either warm or cool land reduced the strength of the net circulation in the inflow layer as measured by ambient shear. The amount of reduction in shear was found to be directly proportional to the depth and strength of the original shear layer. For stronger and deeper shears, the reduction in shear is sufficiently great that the buoyancy gradient circulation at the leading edge of the cold pool is no longer in balance with the shear circulation leading to changes in squall-line updraft structure. The authors hypothesize two ways by which a squall line might respond to passing from water to land. The weaker and more shallow the ambient shear, the greater likelihood that the squall-line structure remains unaffected by this transition. Conversely, the stronger and deeper the shear, the greater likelihood that the squall line changes updraft structure from upright/downshear to upshear tilted.