Simulated squall lines with and without cloud shading effects

Frame and Markowski (2010) have demonstrated the feasibility of using the tilted independent pixel approximation (TIPA) to account for the three-dimensionality of cloud shading effects in idealized numerical simulations of supercells in which radiative transfer effects were included (in addition to surface fluxes, which couple the surface temperature forcing to an atmospheric response). In this preprint, we describe the results of simulations of a long-lived squall line in which cloud shading effects are included using the TIPA. The simulations are compared to control simulations that also include surface fluxes but have a radiatively transparent cloud. In the simulations with cloud shading, the stabilization of the lower boundary layer reduces vertical mixing and alters the low-level vertical wind shear relative to the control simulations, which ultimately affects the structure of the gust front updraft and larger-scale squall line characteristics. Not surprisingly, the modifications of the hodograph (e.g., whether it is lengthened or shortened) and squall line structure depend on the ground-relative wind profile, i.e., the position of the hodograph trace relative to the origin.