SIMULATIONS OF SHALLOW SUPERCELL STORMS IN LANDFALLING HURRICANE ENVIRONMENTS

Numerical simulations of the convective storms that form in tornado-pr oducing landfalling hurricanes show that shallow supercells are possib le, even though buoyancy is limited because ambient lapse rates are cl ose to moist adiabatic. Updrafts generally reach peak intensity at low levels, often around 2 km above the surface. By comparison, a simulat ed midlatitude supercell typical of the Great Plains of the United Sta tes exhibits a pronounced increase in storm size, both horizontally an d vertically. At low levels, however, the hurricane-spawned storms may contain updrafts that rival or exceed in intensity those of Great Pla ins supercells at similar levels. Simulations made using a tornado-pro ximity 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 o ften small in hurricane environments, its concentration in the strongl y sheared lower troposphere promotes the development of perturbation p ressure minima comparable to those seen in simulated Great Plains supe rcells. In a typical simulated hurricane-spawned supercell, the upward dynamic pressure gradient force contributes at least three times as m uch to the maximum updraft speed as does explicit buoyancy. Tilting an d stretching of ambient horizontal vorticity by the strong low-level u pdrafts 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 tha t may help explain why most hurricane-spawned tornadoes are weaker tha n their Great Plains counterparts.