Observations of low-level baroclinity generated by anvil shadows

Abstract

Low-level cooling beneath the cirrus anvil canopies of supercell thunderstorms is documented in two Verification of the Origins of Rotation in Tornadoes Experiment cases and in the 17 May 1981 Arcadia, Oklahoma, supercell. Surface temperature decreases of 3°C or more occurred beneath the anvils within 45 min of the onset of overcast conditions. Cooling was confined to the lowest few hundred meters of the boundary layer, and believed to be due mainly to a deficit in the energy budget following a reduction of incoming shortwave radiation. In the three cases studied, the vertical wind shear was strong; thus, mixing prevented the formation of an inversion layer. Strong insolation at the ground outside of the anvil shadows coupled with the cooling beneath the cirrus canopies led to corridors of baroclinity along the shadow edges. It is shown that residence times in these baroclinic zones may be long enough for parcels to acquire considerable horizontal vorticity (e.g., ∼10-2 s-1) en route to a storm updraft. Enhancement of the horizontal vorticity of parcels ingested by an updraft may have implications for the dynamics of storm rotation.