The Effect of Vertical Shear on Tropical Cyclone Intensity Change

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  • 1 Hurricane Research Division, NOAA/AOML, Miami, Florida
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Abstract

The effect of vertical shear on tropical cyclone intensity change is usually explained in terms of “ventilation” where heat and moisture at upper levels are advected away from the low-level circulation, which inhibits development. A simple two-layer diagnostic balance model is used to provide an alternate explanation of the effect of shear. When the upper-layer wind in the vortex environment differs from that in the lower layer, the potential vorticity (PV) pattern associated with the vortex circulation becomes tilted in the vertical. The balanced mass field associated with the tilted PV pattern requires an increased midlevel temperature perturbation near the vortex center. It is hypothesized that this midlevel warming reduces the convective activity and inhibits the storm development.

Previous studies have shown that diabatic heating near the storm center acts to reduce the vertical tilt of the vortex circulation. These studies have also shown that there is an adiabatic process that acts to reduce the vertical tilt of a vortex. The effectiveness of the adiabatic process depends on the Rossby penetration depth, which increases with latitude, horizontal scale, and vortex amplitude. Large-scale analyses from the 1989–1994 Atlantic hurricane seasons are used to show that high-latitude, large. and intense tropical cyclones tend to be less sensitive to the effect of vertical shear than low-latitude, small, and weak storms.

Abstract

The effect of vertical shear on tropical cyclone intensity change is usually explained in terms of “ventilation” where heat and moisture at upper levels are advected away from the low-level circulation, which inhibits development. A simple two-layer diagnostic balance model is used to provide an alternate explanation of the effect of shear. When the upper-layer wind in the vortex environment differs from that in the lower layer, the potential vorticity (PV) pattern associated with the vortex circulation becomes tilted in the vertical. The balanced mass field associated with the tilted PV pattern requires an increased midlevel temperature perturbation near the vortex center. It is hypothesized that this midlevel warming reduces the convective activity and inhibits the storm development.

Previous studies have shown that diabatic heating near the storm center acts to reduce the vertical tilt of the vortex circulation. These studies have also shown that there is an adiabatic process that acts to reduce the vertical tilt of a vortex. The effectiveness of the adiabatic process depends on the Rossby penetration depth, which increases with latitude, horizontal scale, and vortex amplitude. Large-scale analyses from the 1989–1994 Atlantic hurricane seasons are used to show that high-latitude, large. and intense tropical cyclones tend to be less sensitive to the effect of vertical shear than low-latitude, small, and weak storms.

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