Mapping the Impact of Surface Roughness on the Kinematics of the 3D Wind Field

It is well-documented that the wind profile (primarily the wind speeds and speed shear at low levels) are greatly affected by the underlying surface roughness, represented by z0 (e.g., Stull 2003). Large horizontal gradients in z0 produce vertical vorticity when the wind blows normal to the gradient in z0 (e.g., Coleman and Knupp 2009, 2013), and it may also produce convergence when the wind blows parallel to the gradient in z0. In addition, storm-relative inflow and helicity may be increased when the inflow is moving across an area of low z0.

It was shown by Coleman and Knupp (2013), using dual-Doppler measurements at one vertical level (250 m AGL), that positive (negative) vorticity was generated to the left (right) of the wind vector blowing down a lake in a neutral boundary layer. Additionally, using a method where radial velocities were converted to wind vectors using VAD analysis, then averaged over 60-90 min, similar results were shown over Mobile Bay, AL and over Guntersville Lake, AL. However, all these measurements were only at one vertical level, and most were approximated using VAD. Additionally, no measurements of convergence or divergence were made.

Preliminary modeling and measurements, using dual-Doppler radar and synthetic dual-Doppler radar and lidar, of the two- and three-dimensional wind field near large gradients in z0 have begun over Alabama.

Early modeling results show that while the vertical vorticity produced by a horizontal gradient in z0 is largest near the surface in a neutral boundary layer, there is some vertical vorticity produced as high as 500 m AGL. In addition, given a mass field the storm-relative helicity is increased over areas with low z0. And, when the wind field is parallel to the gradient in z0, significant convergence occurs at low-levels. This convergence could lead to an increase in the depth of the moist boundary layer and in low-level precipitable water along the gradient.

In addition to radar observations shown by Coleman and Knupp (2013), new single-Doppler data suggest maxima and minima in vertical vorticity due to changes in z0. Experiments are being conducted in several locations, using synthetic dual-Doppler radar and lidar in PPI sector scans at many elevations to determine the depth of the vorticity and convergence generated by horizontal gradients in z0. One ideal location being examined is on the western edge of the Huntsville metropolitan area, where there is a sharp gradient between very built up suburban areas and farmland. Another is just south of Tuscaloosa, where southerly winds produce convergence due to the high friction urban area, and the wetlands around the Black Warrior River stretch in a SSW-NNE orientation from south of TCL about 25 km further. Preliminary results from experiments in areas like these will be presented.