Centripetal Acceleration and the Gradient Wind

Abstract

When the pressure gradient force and Coriolis effect are imbalanced, the flow does not remain straight but instead curves. The centripetal acceleration influences winds in such situations and accounts for this curvature. Centripetal acceleration is related to the gradient wind, which works with the pressure gradient force and the Coriolis effect to keep wind moving around a closed cyclone or anticyclone, rather than flying out on a tangent, out of the circulation cell. A derivation is presented that shows that since air moves clockwise (negative r) around an anticyclone and also a ridge (elongated area of high pressure) in the Northern Hemisphere, flow associated with anticyclones and ridges is supergeostrophic in the Northern Hemisphere, if the pressure gradient would be equal to that in the geostrophic case. Likewise, flow is subgeostrophic around a cyclone and also a trough (elongated zone of low pressure), since air moves counterclockwise (positive r) in the Northern Hemisphere around cyclones and troughs if the pressure gradient would be equal to that in the geostrophic case. In reality, flow in the upper-troposphere of the mid-latitudes seldom takes the form of an enclosed and isolated circulation—anticyclones and cyclones. Instead, the flow moves along in a series of Rossby waves that undulate from north-to-south and south-to-north, amid general west-to-east propagation. Convergence, divergence, and vorticity associated with the Rossby waves have a great role in modulating broad-scale flow and local weather conditions.