P6.5
The sensitivity of simulated storm structure and intensity to the temperature at the lifted condensation level
Eugene W. McCaul Jr., USRA, Huntsville, AL; and C. Cohen
A parameter space numerical simulation study reveals considerable sensitivity of simulated storm updraft intensity to the prescribed initial temperature at the lifted condensation level, one of the eight key parameters used in the construction of the idealized soundings in our experimental framework. For example, when boundary layer conditions are warm and moist, such that the LCL is located at 0.5 km above the surface and the sub-LCL equivalent potential temperature is 354 K, peak updraft speeds reach 32 m/s in a moderately-sheared environment featuring convective available potential energy of 2000 J/kg. By contrast, reduction of the LCL temperature by 8.0 C (and similar amounts at all levels of the troposphere), such that the boundary layer equivalent potential temperature decreases to 325 K, permits peak updraft speeds in environments having otherwise similar key environmental parameters to reach 50 m/s.
Prior studies have shown that updraft overturning efficiency increases as the depth of the moist layer increases. In environments having subcloud equivalent potential temperature and other key parameters similar to the first environment above, but having a moist layer depth and level of free convection equal to 1.6 km, storm peak updrafts were found to reach 54 m/s. Reducing the temperatures of these environments by 8.0 C allows the peak updraft speeds to grow to 70 m/s, a value that, because of the release of the latent heat of fusion, actually exceeds the maximum expected from basic parcel theory.
The increases in peak updraft speeds occurring in association with decreasing environmental temperatures are the result of reductions in condensate loading in the cooler environments. This is shown also by the reduced precipitation mixing ratios in the storms there. Condensate loading is related to precipitable water content, which is about 60 mm for our warm, moist environments, and about half that value in our cooler environments. Such values roughly bracket those commonly encountered in many actual severe weather environments. The trends seen in the simulations suggest that, for otherwise similar mixed and moist layer depths and similar amounts and vertical distributions of instability and shear, convective storms in the relatively cool environments of the U. S. High Plains should have more intense updrafts than those occurring in warmer environments. These conclusions remain unchanged even if reasonable allowances are made for the effects of the reduced surface pressures found on the High Plains.
Poster Session 6, Numerical Modeling of Severe Local Storms
Wednesday, 14 August 2002, 3:00 PM-4:30 PM
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