Effects of Heterogeneous Surface Roughness on Boundary Layer Kinematics and Wind Shear

Abstract

Different types of land cover are associated with different surface roughness, which produce variations in the frictional force on the wind.  Therefore, mean wind profiles at low levels often differ markedly over short distances where there is a gradient in surface roughness.  Horizontal gradients in surface roughness may produce vertical vorticity, circulation, and horizontal divergence.  The effect of roughness on vertical wind shear and storm-relative helicity is also qualitatively important and may lead to large gradients in helicity over short distances.  Recent studies also suggest an important role of friction in tornadogenesis. We show conceptually and theoretically how gradients in surface roughness produce quasi-ambient convergence and vertical vorticity, and gradients in vertical shear and storm-relative helicity.  We then present observational data and numerical simulations that demonstrate the effects of surface roughness on the kinematics and shear of boundary-layer flow, for future work examining the importance of these processes for tornadogenesis.