Derivation of physically motivated wind speed scales - the E-scale concept

The German BMBF-funded project RegioExAKT (www.regioexakt.de) has the following main objective: Determination of the trends in occurrence of, and threat by, severe convective storms in Germany until 2030, as well as the development of adaptation concepts for targeted main users. Among the threats which are investigated in RegioExAKT are straight-line wind events and tornadoes. Their intensity is usually estimated based on wind speed or damage scales, of which Fujita's F-scale is the most widely used.

Common to the existing scales is the drawback that their underlying relation between wind speed and scale classes is based rather on heuristic or empirical reasoning than on physical observables. To solve this problem, a class of new wind speed scales is proposed which rely on physically relevant quantities like mass flux density, energy density (pressure), or energy flux density. These so-called Energy- or E-scales can be applied to wind speeds of any intensity. Note that the well-known Mach scale is a special case of an E-scale.

Aside from its foundation in physical quantities which allow for a calibration of the scales, the E-scale concept can help to overcome the present plethora of scales for winds in the range from gale to hurricane intensity. A procedure to convert existing data based on the Fujita-scale or other scales (Saffir-Simpson, TORRO, Beaufort) to their corresponding E-scales is outlined. Even for the large US tornado record, the workload for E-scale conversion remains manageable, as mainly the F5 events would have to be re-rated.

Compared to damage scales like the “Enhanced Fujita” or EF-scale concept recently implemented in the USA, the E-scales are based on first principles and can consistently be applied all over the world for climatological purposes.