The concept of:
Excess Rotational Kinetic Energy (ERKE),
as developed by Donaldson and Desrochers, has proven to be effective in prediction of tornadoes from mesoscyclones. ERKE can be described as the amount of rotational kinetic energy in excess of that necessary for the maintenance of mesocyclonic shear. ERKE provides high accuracy and improved lead times for significant (F2 or greater) tornadic storms, and Brooks has found that the variation in the shear values within different levels of the mesocyclone is an important parameter in the development of severe weather. The most damaging tornadoes occur with a strong low-level mesocyclone tied to a strong mid-level mesoscyclone. However, increasing the value of mid-level (4 - 7 km) shear beyond a critical point results in the suppression of the low-level mesocyclone, despite intense rotation at mid-levels.
This project will test PC-compatible software that the operational forecaster can use in real time to evaluate the tornadic potential of a mesocyclone once it has been detected using the WSR-88D radar. This software will compute ERKE within the mesocyclone at one level or vertically integrate ERKE over low and mid levels, and relate the value of ERKE to the probability that the mesocycone will produce a tornado. It will further be able to predict the intensity based, on the Fujita scale of the tornado expected at the surface.
Burgess evaluated 37 mesocyclones, looking at variations in shear, size, and rotational velocity in an attempt to find distinguishing characteristics in tornado-producing mesocyclones. Donaldson and Desrochers have evaluated 23 mesocyclones, calculating the values of ERKE and documenting which mesocyclones produced tornadoes. Using this dataset in conjunction with data gathered by the principal investigators, the amount of ERKE associated with strong and weak tornadic development has been estimated and used as threshold values for the predictive code in the computer program. Calculation of the shear in mid-levels will give an indication of the probability of low-level mesocyclone development, and resultant tornado development. By calculating the value of ERKE in the low-level and mid-levels of the storm, and comparing these results with surface reports, the relationship between ERKE at these levels and tornado formation can be incorporated into the predictive scheme. With this approach, ERKE will become a better estimator of weak tornadoes.
