SEMINAR SERIES and COLLOQUIA

Ongoing research at The University of Alabama in Huntsville indicates that several mesoscale processes, not associated with a given rotating convective storm, may alter the intensity of that storm’s mesocyclone. The changes in mesocyclone vorticity may be significant, and are sometimes followed by tornadogenesis. In addition to the previously-documented drylines and mesoscale thermal boundaries, these processes include gravity waves, horizontal gradients in surface roughness, and topography.

As shown by Coleman and Knupp (2008), ducted gravity waves are associated with perturbations in convergence and vertical wind shear. Convergence ahead of the ridge of a ducted wave interacting with a mesocyclone increases the mesocyclone’s vorticity through stretching, while divergence ahead of the trough decreases the vorticity. Since the wind perturbations in ducted waves decrease with height, the vertical shear and streamwise vorticity in the storm inflow increase in the wave trough (for a wave approaching a storm from its right flank, the typical angle), and decrease in the ridge. A simple one-dimensional model shows that ducted waves approaching a rotating storm from its right flank cause periodic changes in the vorticity of the storm’s mesocyclone, but the overall change in vorticity over one full wavelength is positive. These results correlate well with several case studies of gravity wave interactions with mesocyclones. The more complex two-dimensional model, GRavity wave Interaction with TornadoeS (GRITS), also shows mesocyclone vorticity changes upon interaction with gravity waves.

Often, the low-level wind blows with a component normal to a horizontal gradient in surface friction/roughness length, associated with boundaries between water and land, or even between different types of land use. When this occurs, horizontal shear, and therefore vertical vorticity, may be produced. The vorticity is positive (negative) when the roughness length gradient is directed toward the left (right) of the wind vector. Numerical simulations show that vorticity on the order of 10-2 s-1 may be produced when a strong wind blows nearly perpendicular to the gradient in roughness length, and analysis of Doppler radar data indicates that this vorticity sometimes extends to a significant height in the boundary layer. This positive or negative vorticity may then be ingested by a rotating storm, changing the mesocyclone vorticity. Case studies confirm the effects of land-generated vorticity on a mesocyclone.

Topography also appears to affect the vorticity of a mesocyclone or existing tornado, and may also alter the environmental inflow of a storm. Cases have been documented in which tornadogenesis occurred as a supercell storm moved from higher terrain to lower terrain. Also, the intensity of damage being produced by an existing tornado sometimes increases as it passes over valleys, and decreases over hilltops. Such increases in vorticity as the depth of the ground-to-cloud base layer increases are consistent with the stretching of vorticity and with PV dynamics; however additional processes, including alteration of the storm-relative inflow on the lee side of a mountain or even lee rotors may be involved. Finally, channeling of the low-level flow by terrain features, primarily valleys, sometimes alters the low-level inflow near a convective storm, increasing the storm’s intensity and vorticity. Various changes in the low-level flow, including instability, horizontal shear, and increased storm-relative helicity, may affect a storm. A case near Huntsville, AL on 2 April 2009, as well as cases discussed by LaPenta et al. (2005) and Bosart et al. (2006), will be examined.

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