Topographic and land cover effects on mesocyclones and tornadoes

Evidence is increasing that local topography and land cover distributions may have an effect on mesocyclones and tornadoes. The recent outbreak of violent tornadoes in the Southeast U. S. on 27 April 2011 illustrates some of these effects. It may be shown, using theory and numerical simulations, that horizontal gradients in roughness length, with some component normal to the low level wind, produce quasistationary areas of horizontal shear, and associated vertical vorticity and circulation. The vorticity is positive (negative) when the roughness gradient is directed toward the left (right) looking down wind. In a neutral boundary layer, this vorticity may be detected at significant heights AGL. The intensity of mesocyclones passing across these gradients in friction may be altered, in some cases leading to tornadogenesis. Area of rough terrain or increased friction to the south of a pre-existing tornado may also alter its inflow, causing tornado decay in some cases. The channeling or tunneling of flow in river valleys, gorges, and other such topographic features may also locally change the wind direction and speed, affecting the storm-relative helicity and/or producing quasistationary regions of vertical vorticity that may be tapped by mesocyclones and tornadoes.

Mesocyclone and tornado intensification often occurs in areas where the height MSL of the land underneath it is decreasing, making for an increase in the height of the sub-cloud layer, and allowing for the possibility of increased vorticity stretching. The opposite may be said for mesocyclones and tornadoes going into areas with ascending terrain. Dynamic equations and numerical models will be used to demonstrate these effects. Individual case studies, in addition to long-term tornado climatology, show that certain geographic regions (on the meso-ƒ× scale) are more or less conducive to tornadogenesis and tornado intensity, depending on the background wind flow and the storm motion.