Revisiting an Old Concept: The Gradient Wind

The gradient wind is defined as a horizontal wind having the same direction as the geostrophic wind but with a magnitude consistent with a balance of three forces: the pressure gradient force, the Coriolis force, and the centrifugal force arising from the curvature of a parcel trajectory. This definition is not sufficient to establish a single way of computing the gradient wind. Different results arise depending upon what is taken to be the parcel trajectory and its curvature. To clarify these distinctions, contour and natural gradient winds are defined and subdivided into steady and nonsteady cases. Contour gradient winds are based only on the geostrophic streamfunction. Natural gradient winds are obtained using the actual wind. Even in cases for which the wind field is available along with the geostrophic streamfunction, it may be useful to obtain the gradient wind for comparison to the existing analyzed or forecast wind or as a force-balanced reference state. It is shown that the nonanomalous (normal) solution in the case of nonsteady natural gradient wind serves as an upper bound for the actual wind speed. Otherwise, supergradient wind speeds are possible, meaning that a contour gradient wind or the steady natural gradient wind used as an approximation for an actual wind may not be capable of representing the full range of actual wind magnitude.