Using Single- and Dual-Doppler Analysis to Examine the Vorticity and Convergence Along Gradients in Roughness Length

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.

The circulation produced as air flows normal to a gradient in z₀ is easily predictable. However, the vorticity depends on the width of the zone where the difference in velocity takes place, which depends on horizontal diffusion. Similarly, the change in velocity that takes place when air flows from one type of land cover to another may be calculated, however the divergence depends on the length of fetch required to bring the wind to a new equilibrium speed.

Dual-Doppler analysis is an excellent method for determining wind fields at high resolution, and then profiles of vorticity and convergence. Examples of such analyses are shown from the north Alabama domain of the VORTEX-SE field campaign. These types of results are compared to high-resolution model estimates of vorticity and divergence near gradients in z₀.

In addition, single-Doppler analysis, in conjunction with VAD wind profiles, may be used to determine profiles of the magnitude in vorticity and divergence near gradients in z₀ in certain situations. This analysis requires a very uniform wind direction profile over the domain being examined, and it requires winds blowing along, or at a small angle to, the radar beam. Radial velocities may be converted to wind speeds using a simple trigonometric expression, assuming the wind direction is known, and wind vectors may be plotted. At least two examples of this technique will also be presented.

Such measurements of ambient vorticity and divergence due to gradients in z₀ may make convective initiation and/or tornadogenesis more likely in certain areas. Both of these points will be examined using case studies and/or climatology.