The sensitivity of a simulated supercell to emulated radiative cooling beneath the anvil

Despite significant advances in computing power, radiative effects generally have been ignored in past three-dimensional numerical studies of convective storms. The exclusion usually has been justified on the assumption that radiative effects are unimportant on the time scales that convection typically persists, and using the argument that convection is "dynamically driven" rather than "radiatively driven." Even though the above arguments are true for some storms, past observations indicate that significant amounts of surface cooling (e.g., temperature deficits exceeding 5 K) can occur within the anvil shadows of long-lived convective storms, which tend to produce expansive anvils.

We have completed a pair of idealized simulations that illustrate the potentially important effects of radiative transfer processes on convective storm dynamics. One of the demonstration simulations (the control) was run without surface physics and radiation. In the other simulation, radiative cooling due to anvil shading was emulated by prescribing a cooling rate to the skin temperature at any grid point at which cloud water was present overhead. The imposed skin cooling rate is consistent with past observations. Low-level air temperatures were coupled to the skin cooling in this second simulation by the inclusion of surface sensible heat fluxes using simple bulk aerodynamic drag laws (latent and soil heat fluxes were not included).

Significant differences were observed between the two simulated storms, particularly in the evolution of the vorticity field and gust front. Though this emulation of radiative cooling is admittedly simple, it illustrates that a potentially important forcing is being missed in numerical simulations of long-lived convective storms.