Simulations of Shallow Supercell Storms in Landfalling Hurricane Environments

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  • 1 Universities Space Research Association, Huntsville, Alabama
  • | 2 National Center for Atmospheric Research, Boulder, Colorado
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Abstract

Numerical simulations of the convective storms that form in tornado-producing landfalling hurricanes show that shallow supercells are possible, even though buoyancy is limited because ambient lapse rates are close to moist adiabatic. Updrafts generally reach peak intensity at low levels, often around 2 km above the surface. By comparison, a simulated midlatitude supercell typical of the Great Plains of the United States exhibits a pronounced increase in storm size, both horizontally and vertically. At low levels, however, the hurricane-spawned storms may contain updrafts that rival or exceed in intensity those of Great Plains supercells at similar levels. Simulations made using a tornado-proximity sounding from the remnants of Hurricane Danny in 1985 produce a small but intense supercell, a finding consistent with the available observational evidence.

Although the amplitude of parcel buoyancy is often small in hurricane environments, its concentration in the strongly sheared lower troposphere promotes the development of perturbation pressure minima comparable to those seen in simulated Great Plains supercells. In a typical simulated hurricane-spawned supercell, the upward dynamic pressure gradient force contributes at least three times as much to the maximum updraft speed as does explicit buoyancy. Tilting and stretching of ambient horizontal vorticity by the strong low-level updrafts promotes production of substantial vertical vorticity aloft in the hurricane-spawned storms. However, the weakness of their surface cold pools tends to restrict surface vorticity development, a fact that may help explain why most hurricane-spawned tornadoes are weaker than their Great Plains counterparts.

Abstract

Numerical simulations of the convective storms that form in tornado-producing landfalling hurricanes show that shallow supercells are possible, even though buoyancy is limited because ambient lapse rates are close to moist adiabatic. Updrafts generally reach peak intensity at low levels, often around 2 km above the surface. By comparison, a simulated midlatitude supercell typical of the Great Plains of the United States exhibits a pronounced increase in storm size, both horizontally and vertically. At low levels, however, the hurricane-spawned storms may contain updrafts that rival or exceed in intensity those of Great Plains supercells at similar levels. Simulations made using a tornado-proximity sounding from the remnants of Hurricane Danny in 1985 produce a small but intense supercell, a finding consistent with the available observational evidence.

Although the amplitude of parcel buoyancy is often small in hurricane environments, its concentration in the strongly sheared lower troposphere promotes the development of perturbation pressure minima comparable to those seen in simulated Great Plains supercells. In a typical simulated hurricane-spawned supercell, the upward dynamic pressure gradient force contributes at least three times as much to the maximum updraft speed as does explicit buoyancy. Tilting and stretching of ambient horizontal vorticity by the strong low-level updrafts promotes production of substantial vertical vorticity aloft in the hurricane-spawned storms. However, the weakness of their surface cold pools tends to restrict surface vorticity development, a fact that may help explain why most hurricane-spawned tornadoes are weaker than their Great Plains counterparts.

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