Evaluation of Extreme Severe Weather Environments in CCSM3

The ability of climate models to predict extremes is determined by its ability to adequately simulate certain environmental conditions both spatially and temporally. Some of the conditions conducive to the occurrence of extreme weather can be quantified from the convective available potential energy (hereafter referred to as CAPE) and vertical shear of the horizontal wind from 0-6 kilometer (hereafter referred to as shear) [Holton, 2004]. CAPE is a measure of the buoyancy of an air parcel, which indicates atmospheric instability. It has an annual cycle, which increases up to a maximum in summer (JJA) [Brooks et al., 2003]. Shear is at its maximum during the winter (DJF) and at a minimum during the late summer (JAS) [Aiyyer and Thorncroft, 2006]. A combination of CAPE and shear are has been shown to be an effective predictor of severe thunderstorms [ e.g. Brooks et al., 1994, Rasmussen and Blanchard, 1998, Brooks et al., 2003, Rasmussen, 2003, Trapp et al., 2007] In particular, high CAPE and high shear values generally indicate the occurence of significant severe thunderstorms, producing hail, strong wind gusts, and those producing significant tornados [Brooks et al., 2003]. However, high CAPE and high shear rarely occur simultaneously and just one of CAPE and shear being extreme can still cause severe weather. Since any projections of the future probability of extreme severe thunderstorms depend on the ability to model high values of CAPE and shear, there is a need to evaluate climate models in terms of these parameters. This study makes use of the statistical approach of extreme value theory to determine the ability of the Community Climate System Model Version 3 (CCSM3, Collins et al. [2006]) model output to simulate extremes in CAPE, shear, and CAPE*shear in comparison to the NCEP/NCAR reanalysis data