Numerical Simulations of Radiative Cooling beneath the Anvils of Supercell Thunderstorms

Abstract

Numerical simulations of supercell thunderstorms that include parameterized radiative transfer and surface fluxes are performed using the Advanced Regional Prediction System (ARPS) to investigate the effects of anvil shadows on the near-storm environment. If the simulated storm is nearly stationary, the maximum low-level air temperature deficits within the shadows are about 2 K, which is roughly half the cooling found in some previous observations. It is shown that the extinction of downwelling shortwave radiation by the anvil cloud creates a differential in the flux of downwelling shortwave radiation between the sun and the shade that is at least an order of magnitude greater than the differential of any other term in either the surface radiation or the surface energy budgets. The loss of strong solar heating of the model surface within the shaded regions leads to a reduction of surface temperatures and stabilization of the model surface layer beneath the anvil. The reduction in vertical mixing results in a shallow, strongly vertically sheared layer near the surface and calmer near-surface winds, which are limited to regions in the anvil shadow. This difference in radiative heating is shown not to affect the vertical thermodynamic or wind profiles above the near-surface layer (approximately the lowest 500 m). It is also found that these results are highly sensitive to the magnitude of the near-surface winds. If the initial hodograph is shifted such that the simulated storm acquires a substantial eastward propagation speed, the temperature deficit within the shadow is greatly diminished. This is due to both a weaker surface sensible heat flux and less time during which surface cooling and boundary layer stabilization can occur beneath the anvil.