Some measurements on undercutting cold air

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Abstract
From time series of closely‐spaced upper‐wind (pilot‐balloon) measurements in the rear of synoptic lines of wind shift and cooling, the configuration of the elevated wind‐shear layer and the strength of the flow beneath are deduced. The height of the shear layer does not rise uniformly with time, but at one stage remains constant or decreases, before resuming a sustained rise. It is demonstrated by the method of dynamic similarity that this phenomenon is broadly the counterpart of the friction head on density surges in stratified liquids.Cumulative inflow into the advancing wedge of cold air is compared with the total mass in the cold air wedge and found to be generally in excess of this, the exceptions occurring with sea‐breezes or analogous phenomena on a larger scale (coastal fronts). In one case where sufficient observations are available, vertical velocities are computed. The excess inflow is shown to be associated with a vertical circulation involving considerable mass transfer upward across the shear zone.

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