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ABSTRACT
Numerical simulations of supercell thunderstorms including parameterized radiative transfer and surface fluxes are performed using the Advanced Regional Prediction System (ARPS) model to investigate how low- level air temperature deficits within anvil shadows affect the simulated storms. The maximum temperature deficits within the modeled cloud shadows are 1.5-2.0 K, which is only about half that previously observed. Within the shadows, the loss of strong solar heating cools and stabilizes the near-surface layer, which sup- presses vertical mixing and modifies the near-surface vertical wind shear. In a case of a stationary storm, the enhanced easterly shear present beneath the anvil leads to a thinning of the outflow layer and corresponding acceleration of the rear-flank gust front far ahead of the overlying updraft, weakening the low-level mesocyclone. It is further shown that the direct absorption and emission of radiation by clouds does not significantly affect the simulated supercells. Varying the time of day of model initialization does not prevent the simulated storms from weakening. This behavior is mirrored for storms that slowly move along the major axis of the anvil shadow. If the rear-flank gust front moves into the anvil shadow and the updraft moves normal to the shadow (i.e., northward movement of the updraft), cyclic periods of intensification and decay can result, although this result is likely highly dependent on the storm-relative wind profile. If the gust front does not advance into the shaded region (i.e., southward movement), or if the storm moves rapidly, the storm is relatively unaffected by anvil shading because the rear-flank gust front speed and outflow depth remain relatively unchanged.
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1. Introduction
The importance of low-level baroclinic zones on the development of low-level rotation in supercell thun- derstorms has been well established. The importance of storm-scale boundaries as a source of baroclinically generated horizontal vorticity to be ingested by the up- draft, particularly the forward-flank gust front, has been demonstrated (e.g., Klemp and Rotunno 1983; Rotunno and Klemp 1985; Markowski et al. 2012) by several in- vestigators. Furthermore, Markowski et al. (1998a) and Rasmussen et al. (2000) observed that the low-level mesocyclone within a supercell typically intensified just after a storm passed from the warm to the cool side of a preexisting mesoscale boundary (i.e.,...