Analysis of the Effects Of Wind Channeling and Gradients in Roughness Length on Environmental Vorticity and Helicity

Authors have shown that channeling of the wind in valleys may change the wind direction and speed in those valleys. In addition, wind blowing between two mountains may be accelerated. Such wind channeling or tunneling sometimes produces quasistationary regions of positive and negative vorticity, that are often mixed to levels well above the surface in the boundary layer. Also, horizontal gradients in roughness length, with some component normal to the low level wind, produce similar areas of horizontal shear, and associated vertical vorticity. The vorticity is positive (negative) when the roughness gradient is directed toward the left (right) looking down wind. The changes in wind speed and direction caused by topographic effects may also change the storm-relative helicity. Bosart et al. (2004) and LaPenta et al. (2005) have examined the effects of wind channeling in tornado cases in the northeastern United States.

In this paper, Doppler radar data will be used to examine quasistationary regions of positive or negative vertical vorticity or storm-relative helicity associated with wind channeling and horizontal gradients in friction. Both dual-Doppler analysis and VAD-adjusted single-Doppler wind fields will be examined. Areas of background vorticity may then be located, and the effect upon storms interacting with these areas can be examined.

Several case studies will be presented. Some will show areas of shear and vorticity that are not associated with any storms. Others will show possible impacts on mesocyclones, including rapid changes in tropical cyclone-associated mesocyclones as they approached land, and the potential effects of wind channeling and tunneling during three tornado outbreaks.