CAPE (convective available potential energy) and the vector difference between the mean wind in the lowest 500 m and the lowest 6 km (a measure of vertical shear through midlevels) have been routinely combined in the Bulk Richardson Number (BRN, Weisman and Klemp 1982) as a simple means of estimating an environment's potential for producing supercells. Apart from the BRN computation, which was used as a means of investigating numerical supercell simulations, these two components and their relationship to actual severe weather have not been specifically explored in terms of scatter diagrams and parameter space relationships. Recently, the shear component of BRN (here called BRN shear) has been used in combination with a measure of low-level shear, 0-3km storm- relative helicity, as one potential way to suggest differentiation between tornadic and nontornadic supercell environments (Stensrud et al., 1997).
In the work by Stensrud et al. (1997), BRN shear is used to infer midtropospheric winds of sign)ficant magnitude relevant to a conceptual model of low-level mesocyclogenesis from Davies-Jones and Brooks (1993). Another important contribution of this "deeper-layer" vertical shear is the generation of vertical pressure gradients through the interaction of updraft with sheared environment (Weisman and Klemp 1986). Modeling work by McCaul and Weisman (1996) suggests that these shear-induced pressure gradients can contribute greatly to updraft strength, adding sigmficantly to vertical velocity potential from buoyancy alone.
Work by Davies (1996), Thompson (1997), and Stensrud et al. (1997) have suggested that certain magnitudes or ranges of BRN shear are more optimal for producing supercells with tornado potential. This paper attempts to build on this work, and suggests that BRN shear magnitudes supporting tornadoes are largely dependent on the amount of available CAPE in an environment. Rather than assessing BRN shear as a separate parameter characterized with ranges and values that are "more optimal", this paper suggests it may be more useful operationally to assess BRN shear magnitudes in tandem with buoyancy (CAPE), and in a format providing information additional to BRN. Further, as in Stensrud et al. (1997), results suggest that this information can be combined with measures of low- level shear (such as stormrelative helicity) to suggest environments of increasing likelihood for supercell tornadoes.
In this paper, a couple of large data sets will be used to show how BRN shear and
CAPE associated with strong and violent tornadoes tend to fall into a consistent
general area on scatter diagrams of both parameters. A brief discussion will speculate
about what this association may imply regarding processes relevant to forecasts of
supercells and tornadoes. Finally, examples will be offered regarding how this
information might be combined with other relevant parameters to contribute useful
information in suggesting supercell tornado likelihood.
Davies, J. M., 1996: Deep layer shear as a refinement to CAPE and low-level shear in tornado forecasting. Preprints, 18th Conf. on Severe Local Storms, San Francisco, CA, Amer. Meteor. Soc., 698702.
Davies-Jones, R., and H. E. Brooks, 1993: Mesocyclogenesis from a theoretical perspective. In The Tornado: lts Structure, Dynamics, Prediction, and Hazards, Geophys. Monogr., No. 79, Amer. Geophys. Union, 105-114.
Mc Caul, E. W., Jr., and M. L. Weisman, 1996: The dependence of simulated storm structure on variations in the shapes of environmental buoyancy and shear profiles. Preprints, 18th Conf. on Severe Local Storms, Sao Francisco, CA, Amer. Meteor. Soc., 718-722.
Stensrud, D. J., J. V. Cortinas, Jr., and H. E. Brooks, 1997: Discriminating between tornadic and nontornadic thunderstorms using mesoscale model output. Wea Forecasting, 12, 613-632.
Thompson, R. L., 1997: Eta model storm-relative winds associated with tornadic and nontornadic supercells. Accepted by Wea Forecasting.
Weisman, M. L., and J. B. Klemp, 1982: The dependence of numerically simulated convective storms on vertical wind shear and buoyancy. Mon. Weal Rev., 110, 504-520.
Weisman, M. L., and J. B. Klemp, 1986: Characteristics of convective storms. Mesoscale Meteorology and Forecasting, Amer. Meteor. Soc., 331-358.