Atmospheric Pressure Changes: The Importance of Deviations from the Balanced (Gradient) Wind

Abstract

The paper comprises a discussion of the effect of deviations from the instantaneously balanced, or gradient, wind on the translation and development of the surface pressure field. The deviation is shown to consist primarily of two components, which are associated respectively with vertical motion and horizontal acceleration in the air trajectory.

With certain types of pressure system, the first of the above components makes a contribution towards translation of the system. But its major role is in maintenance of the system against the dissipating effect of surface friction. The part played by this component in the intensification of anticyclones, end the formation of secondary depressions and cold pools is discussed : in particular, these common types of development are linked with vertical motion at the tropopause and in the stratosphere. The level just below the tropopause is shown to be crucial in the study of anticyclogenesis. The collapse of an anticyclone under excessive surface heating, the formation of "heat lows", and the pressure rise associated with convectional squalls are also attributed to this deviation.

By means of the deviation associated with horizontal acceleration, the changes in pressure force experienced by a moving element give rise to compensating flows which restrict any rapid local changes in surface pressure. In conjunction with the control exercised by the balanced wind(l), this accounts for the remarkable conservatism of surface pressure in face of the large convergence or divergence which atmospheriic motion appears to imply.

Apart from its controlling influence, the contribution from the horizontal acceleration increases with increasing asymmetry of the system. The eastward thrust of sharp ridges of high pressure is attributable to this deviation.