The Doug Lilly Symposium


Convective Storm Morphology as a Function of Environmental Profile Structure: An Eight-dimensional Numerical Parameter Space Study

Eugene W. McCaul Jr., USRA, Huntsville, AL

Early parameter space studies of convective storms focused on the sensitivity of storms to limited numbers of basic parameters such as convective available potential energy (CAPE) and vertical shear. From these studies emerged new hybrid parameters such as the Bulk Richardson Number as tools for predicting basic characteristics of storm morphology such as the likelihood of supercell structure. More recently, it has become apparent that storm morphology is also sensitive to other parameters of the storm environment. To allow investigation of these other sensitivities, new techniques for specifying starting vertical profiles were developed. These techniques, based on specifying the various parameters that are needed to construct an idealized thermodynamic and kinematic environment, along with a small number of simplifying assumptions, has led to the development of an eight-dimensional parameter space framework for the study of storm morphology. The eight independently adjustable parameters are: CAPE, hodograph radius, shape of the buoyancy profile, shape of the shear profile, depth of the mixed layer, depth of the moist layer, free tropospheric relative humidity, and cloud-base temperature (a proxy for the environmental precipitable water). To exploit this framework, reasonable "high" and "low" values are chosen for each of the eight available parameters, and environments are designed having all possible combinations of these parameter values. Numerical simulations are then conducted using a reliable cloud model, with all these different environments used as initial conditions. It is assumed that the vertical structure of the environment is a crucial determinant of storm morphology, and therefore that standard warm bubbles launched in a horizontally homogeneous environment are sufficient to reveal the basic trends in storm morphology within the prescribed parameter space. The simulations do indeed display a wide range of convective storm size, intensity, and structure. An overview of the results indicates that storms achieve greatest updraft intensity when steep lapse rates are present in CAPE-starved environments, with this trend much less evident or absent in shear-starved environments; when CAPE and hodograph radius are both large; when moist layer depth is large; when free tropospheric relative humidity is not too small; and when environmental temperatures and precipitable water are not too large. Storm updraft diameters increase when moist layer depth increases, and when the vertical shear increases. Storm precipitation is heavier in warm environments, but environmentally-based precipitation efficiency is greater in cooler environments. Vertical vorticity in storms is larger when CAPE and shear are large, and when low-level lapse rates are large and moist layer depths are small. Storm motions, long known to be sensitive to the hodograph, also exhibit some sensitivity to many of the thermodynamic parameters. There appear to be many avenues for fruitful future extension of this kind of parameter space study.wrf recording  Recorded presentation

Session 2, Severe Storm Dynamics and Prediction
Thursday, 2 February 2006, 1:30 PM-3:00 PM, A302

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