The frontal width problem

Observations of two low-level frontal passages by a hot-wire anemometer pla ced at 3 m above the ground provide evidence that frontal zones acid the po stfrontal regions exhibit enhanced kinetic energy dissipation. These observ ations support a postulated linear relationship between turbulent kinetic e nergy dissipation rate epsilon and a characteristic frontal width L. A rigo rous theoretical development is not available to verify the postulate that an equilibrated frontal width is determined by a balance between frontogene tical forcing and turbulent dissipation of kinetic energy. Instead, a simpl e construct, based on a model of inviscid frontogenesis, is introduced. Thi s model exhibits downscale spectral energy transfer that decreases with hor izontal scale during frontogenesis. This model is invalid in the dissipativ e range of fully turbulent small-scale motions. In those cases, however, wh en frontal equilibration occurs at scales close to the scales where dissipa tion just begins to become significant, the inviscid model provides some th eoretical support for the postulated relationship between epsilon and L. Th e analysis is then extended to show how the vertical scale L, in a surface- based front is related to epsilon and N (N is the Brunt-Vaisala frequency) when the earth's rotation is not a factor but L, depends on epsilon, N, and f (f is the Coriolis parameter) when rotation is retained. Questions that remain unanswered by the present analysis concern the appropriate definitio n of a frontal scale, the role of precipitation in frontal scaling, and the equilibration of broad-scale fronts L greater than or equal to 10(5) m, wh en turbulent dissipation of kinetic energy may not be the controlling facto r in the equilibration of a front.