Wednesday, 6 October 2004
The original formulations of the supercell composite parameter (SCP) and significant tornado parameter (STP), as documented in Thompson et al. 2003, have been modified to include the effective shear terms developed by Thompson et al. 2004. Specifically, the Bulk Richardson number shear term in the SCP has been replaced by the effective bulk shear, while the 0-3 km storm-relative helicity (SRH) has been replaced by the effective SRH. The effective shear terms make little difference in the SCP calculation for typical supercell cases, but they result in a more realistic assessment of the storm environment in cases of very shallow or deep buoyancy, and with elevated storms. Another motivation for these changes is to reduce the numerical values of SCP, namely cases where SCP becomes exceptionally large for certain wind profiles (e.g., large SRH and large Bulk Richardson shear term). The normalized values of both SRH and Bulk Richardson shear can range as large as 5 to 6 in supercell environments, and SRH tends to be large when the BRN shear is also large. Therefore, the product of the normalized values can exceed 25-35 and result in cluttered contour analyses of the SCP field. By replacing the Bulk Richardson shear term with the effective shear, we eliminate the excessively large SCP values and discriminate between supercells and non-supercell storms as well as the original formulation.
The STP has also been modified to include effective shear and convective inhibition. The original 0-6 km bulk shear term has been replaced by the effective bulk shear with corresponding lower and upper thresholds, and the 0-1 km SRH has been replaced by the effective SRH. The 100 mb mean parcel convective inhibition has also been added to the STP formulation to reduce specific value areal coverage of the STP in hourly objective analyses (see Bothwell et al. 2002). These changes to the STP have reduced both parameter values and lowered the false alarm, while maintaining the ability to discriminate between significantly tornadic and nontornadic supercells.
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