The Effects of Anvil Shading From a Nearby Squall Line on the Structure and Evolution of a Discrete Supercell Thunderstorm

One way that a convective storm can modify its local environment is through diminishing the amount of solar radiation that reaches the surface beneath the storm’s cirrus anvil.  This is often referred to as “anvil shading”.  Past observations of both squall line and supercell thunderstorms have documented near-surface cooling of 3-6K resulting from anvil shading in the vicinity of thunderstorms.  This cooling leads to local changes in atmospheric stability, mixing and baroclinicity.  Numerical simulations of squall lines and supercell thunderstorms that include radiative transfer and surface fluxes have shown that the cooling induced by anvil shading can cause dynamical differences in the structure and evolution of these convective systems. In particular, anvil shading cools the pre-gust front environment (PGFE) of the storms, reducing the amount of vertical mixing and leading to a change in the strength of the near-surface vertical wind shear. This has been shown for both storm types to have implications for gust front evolution, with reduced shear leading to more rapid gust front motion in some shading scenarios.  For supercell storms in particular, this can affect low-level mesocyclone periodicity and intensity.

Recently, researchers have begun investigating how nearby convective storms may affect each other, and particularly how a nearby squall line may impact the evolution of a discrete supercell. Initial modeling results have revealed that the presence of a nearby squall line can lead to subtle changes in supercell intensity and potential severe weather production. These early simulations, however, neglected processes such as solar radiation.  The present study seeks to address this aspect of the storm interactions in light of the effects that anvil shading can have on the local environment as described above.  

To accomplish this task, numerical simulations including radiative transfer and surface fluxes have been run using the CM1 cloud model to investigate how squall line-induced anvil shading of the PGFE affects the structure, intensity, and development of supercells in that environment. The impact of squall line anvil shading on a supercell depends on several factors: the speed of the squall line, the speed and direction of the supercell’s motion, the maturity of the supercell before it enters the shaded environment, and the ambient environmental wind profile. In this presentation, we will compare the characteristics and evolution of an isolated supercell simulated with anvil shading to a supercell storm that also encounters shading ahead of a simulated squall line.  These results will also be compared with companion “clear sky” simulations where effects from radiation and surface fluxes are included, but cloud shading effects are turned off in order to isolate which changes in supercell evolution can be explained by the shading.  Particular attention will be paid to how the shading affects gust front and low-level mesocyclone evolution, as well as its effect on simulated severe weather proxies.