Horizontal gradients in roughness length z0 (a good indicator of surface friction) produce local, sometimes large, gradients in wind speed in the BL. This is especially true near sharp interfaces between different types of land use, including urban/farmland and land/water interfaces. When a component of the BL wind vector is normal to the gradient in z0, vertical vorticity is produced. This vorticity is positive (negative) when the gradient in z0 is directed to the left (right) of the wind vector. When a component of the BL wind is parallel to the gradient in z0, divergence is produced. This divergence is positive (negative) when the gradient in z0 is anti-parallel (parallel) to the wind vector. Examples of the wind, vorticity, and divergence fields along gradients in z0 are shown using both single- and dual-Doppler analyses from data during the non-IOP period of VORTEX-SE and from other UAH studies.
The ambient vertical vorticity generated by gradients in z0 may be ingested by a mesocyclone or even a tornado, causing it to intensify or weaken, depending on the sign of the vorticity. In addition, it is possible that enhanced convergence (divergence) associated with along-wind gradients in z0 may enhance or weaken storm updrafts and associated tilting of environmental streamwise vorticity.
Climatological evidence that areas with ambient positive (negative) vertical vorticity have a relative maximum (minimum) in tornadogenesis has been presented in previous conference posters (e.g., Coleman 2015). However, in this paper, more comprehensive climatological evidence is presented, utilizing 171 tornadogenesis events within the domain of the UAH ARMOR radar over a period of 10 years. This evidence shows that tornadoes, especially weak ones (EF0-EF1), tend to form more frequently in areas where gradients in z0 would theoretically produce positive ambient vorticity, and possibly convergence.
Several case studies, some during VORTEX-SE, will be examined where horizontal gradients in friction appeared to play a role in the genesis, prevention, weakening, or intensification of tornadoes.