The Effect of Vertical Shear on Tropical Cyclone Intensity Change: An Historical Perspective
Mark DeMaria, , OAR, Miami Florida
Numerous observational studies have demonstrated that the vertical shear of the horizontal wind tends to have a negative influence on tropical cyclone genesis and intensification. However, the physical explanations for this effect are somewhat variable. Perhaps the most commonly cited reason for this effect is "ventilation" where the heat released at upper-levels is advected in a different direction than the heat at low-levels. After an examination of previous modeling studies, and a case study from the Geophysical Fluid Dynamics Laboratory (GFDL) nested hurricane model, it appears that the "ventilation" hypothesis is not valid in many cases. Diagnostics of numerical model simulations have shown that the primary balance in the thermodynamic equation near the center of a tropical cyclone is between the diabatic term and the vertical motion term, and that horizontal advection is a secondary effect. In addition, idealized studies of mass-wind adjustment have shown that when heat is released on a horizontal scale less than the Rossby radius of deformation, most of the energy propagates away as gravity waves. Since the gravity wave speed is usually faster than the environmental wind speed, this process removes heat more rapidly than differential advection.
A more consistent framework in which to view the effect of shear is in terms of the advection of the potential vorticity (PV) associated with the storm circulation. If the vertical shear is not too strong, the storm PV is maintained as a vertically oriented column that moves with the vertically averaged horizontal wind. This motion results in asymmetric storm-relative inflow (or outflow) that varies with height, which can feed back on the convective patterns near the storm center. Depending on how the location of the convection is is altered by these relative motions, the storm intensity can be reduced. When the vertical shear becomes large, the storm PV becomes tilted in the vertical. Fairly large mid-level temperature increases occur in order to maintain hydrostatic and dynamical balance as the PV becomes tilted. These temperature changes can also affect the distribution of convection and the storm intensity. It is hypothesized that these two processes are the primary causes of the effect of shear on tropical cyclone intensity change, and not the simple differential advection of heat, as assumed in the "ventilation" theory.