Extended Abstract (188K)15.5
Numerical Investigation of the Role of Mid-Level Dryness on Tropical Mini-supercell Behavior
Glen Romine, University of Illinois, Urbana, IL; and R. B. Wilhelmson
Tropical cyclones that approach land are known to bring with them many hazards such as high winds, torrential rainfall and a destructive storm surge. Additionally, the convective environment that accompanies the cyclone may support the development of outer hurricane rainbands containing mini-supercells. This occurred in our numerical study of Hurricane Opal. Tropical cyclone mini-supercells can produce damaging tornadoes (McCaul 1991) and hence pose a significant public threat. The convective environment of these tropical cyclone mini-supercells differs from that usually found on the Great Plains. The tropical environment is characterized by the Miller "Type-II" thermodynamic profile, which is distinctly different from the Miller "Type-I" profile common of the Great Plains. McCaul (1991) developed a composite close-proximity thermodynamic profile for hurricane tornadic events, which was moist throughout the troposphere, similar to the Miller "Type-II" profile. Surface based instability in this profile is largely restricted to the lower troposphere, with peak buoyancy near 700 mb.
Storms forming within a moist environment have cold pools that are typically less intense than those that form in the presence of mid-tropospheric dry air. Entrainment of dry air into convective cells leads to evaporational cooling, which hastens the formation and strength of negatively buoyant air feeding the low-level cold pool. This in turn could lead to increased baroclinic generation of low-level vorticity that could influence tornado formation. Gilmore and Wicker (1998) examined the significant influence which mid-tropospheric dryness plays in supercell evolution. Notably, they found a direct relationship between midtropospheric dryness and cold pool intensity. Idealized simulations of supercell structure performed by McCaul and Weisman (1996, 2001) used a thermodynamic environment with fixed high relative humidity (90%) above the LCL. McCaul and Weisman (2001) recognized the need for a deeper examination of the potential role of midtropospheric dry air, noting that the magnitude of low-level vorticity was related to the cold pool intensity in their mini-supercell simulations.
Simulations of the life-cycle of Hurricane Opal (1995) have been carried out by the authors using the MM5 (5 grids) with 1.1 km horizontal resolution in the innermost grid (Romine and Wilhelmson 2002). Assessment of this hurricane simulation revealed a mid-level dry air boundary collocated with the development of deep convection analogous to mini-supercells. Additionally, soundings and water vapor imagery from the observed Hurricane Opal showed mid-level dry air drawn into the hurricane circulation. The role this dry air played in shaping the simulated convection properties will be further examined using the COMMAS cloud model at higher horizontal resolution (250 m). Representative soundings from the MM5 simulation on either side of an outer hurricane rainband will each be used to initialize the model. Comparisons will be made to a simulation with high humidity through the cloud bearing layer. Results from these simulations and how they relate to the hurricane simulated convection will be presented.
Supplementary URL: http://pampa.ncsa.uiuc.edu/~romine/opal.html
Session 15, Tropical Cyclone Tornadoes and Low-Latitude Severe Storms
Thursday, 15 August 2002, 4:30 PM-5:30 PM
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