The Effects of Differential Friction on PBL Kinematics and Possible Influences on Tornadoes and CI

This paper examines two topics: 1) the influences of horizontal gradients in friction on the kinematics of the boundary layer (BL), including the generation of ambient vertical vorticity and horizontal divergence; and 2) the potential influences of these frictional effects on convective initiation (CI), mesocyclones, and tornadoes.

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 case 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 convective updrafts, leading to or suppressing CI in some areas.

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 convergence. Climatological convective rainfall analysis also shows a minimum in the Black Belt region of Alabama, where low friction causes divergence, with maxima in convective rainfall to the north and south of the Black Belt.

Several case studies, some during VORTEX-SE, will be presented where horizontal gradients in friction, and associated vorticity, appeared to play a role in the genesis, intensification, and weakening of tornadoes. Convective initiation due to frictional convergence will also be demonstrated.