Extended Abstract (708K)11B.4
A climatology of environmental parameters that influence severe storm intensity and morphology
U. S. Nair, University of Alabama, Huntsville, AL; and E. W. McCaul and R. M. Welch
Recent numerical simulation work using idealized initial atmospheric profiles has provided quantitative evidence that perhaps at least nine basic, theoretically independent, environmental parameters strongly influence the intensity and morphology of deep convective storms. These environmental parameters are: convective available potential energy (CAPE), lifting condensation level (LCL), level of free convection (LFC), deep layer vertical shear, low level shear, altitude of maximum parcel buoyancy, precipitable water, mean dewpoint depression in the midtroposphere, and convective inhibition (CIN). In this study we document the warm season climatology of these parameters for the continental United states.
The National Climatic Data Center's Comprehensive Aerological Reference Data Set radiosonde database covering the period 1950-2000 is used for assessing the values of the environmental parameters. The mean values, peak values, standard deviations and thresholded frequencies of occurrence of the parameters are calculated for all stations having continuous time series of soundings for at least 10 years. Separate computations are performed for 00 UTC and 12 UTC soundings. The data values are interpolated to a regular grid, and their spatial distributions are examined.
The data maps reveal strong patterns in many of the key parameters. CAPE exhibits a strong maximum from Texas northward into the Plains, with much smaller characteristic values over the East and West. At the same time, shear parameters are also larger over the Plains because of the influence of lee troughs that typically form just east of the Rocky Mountains. LCL heights are generally low in the Gulf Coast and Southeast, but increase over the High Plains and the desert Southwest. Maps of parameter mean values show that the central Plains tend to have environments with a balance of CAPE and shear that favors intense supercell storms, whereas storms in the Southeast are usually shear-starved. Storms over the desert areas tend to have high LCLs and are vulnerable to outflow dominance, but are probably relatively efficient at overturning because of their high LFCs. Scatterplots of CAPE versus shear indicate a negative overall correlation between the two parameters, which holds true throughout the continental U. S. The negativity of this correlation is not inconsistent with the apparent spatial coincidence of favorable CAPE and shear over the Plains; in that region, mean shears tend to be stronger than elsewhere, so that the scatter of points is displaced toward larger shears. The small-CAPE portion of the CAPE-shear scatterplot tends to contain a higher proportion of events with low altitudes of maximum buoyancy, which simulations have shown enhances storm strength in small-CAPE regimes. These findings may help explain how severe storms can occur over the wide ranges of CAPEs and shears observed in previous studies.
Session 11B, Climatological Studies of Severe Storms
Wednesday, 14 August 2002, 4:30 PM-6:00 PM
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