A STUDY OF NON-GEOSTROPHIC FLOW WITH APPLICATIONS TO THE MECHANISM OF PRESSURE CHANGES

Abstract

Surface pressure changes can occur only when an accelerational field exists. The regularity of occurrence, the distribution, and the magnitudes of the accelerational fields found in the atmosphere have been determined from the available data. The most direct method used was to plot maps of the deviation of the observed wind from the geostrophic wind. Charts of the horizontal divergence, as determined from the observed winds, were prepared for several levels. Charts were also drawn of the non-geostrophic temperature changes, which are defined as the difference between the actual 12-hour temperature changes and the temperature changes which would result from geostrophic advection of the temperature field. It is shown that the magnitudes of the divergence and the non-geostrophic temperature changes are consistent with the observed deviations from the geostrophic wind. The errors of each method are investigated and it is concluded that they are not sufficient to affect the order of magnitude of the results. All of the charts exhibit definite patterns which show a considerable degree of correspondence with the weather conditions. It is concluded that accelerational fields regularly occur in the atmosphere which are one order of magnitude greater than cyclostrophic accelerations and accelerations due to the variation of the Coriolis parameter. The equation for the pressure tendency is discussed with reference to the observational data. Since the total divergence in a vertical column is the relatively small difference between large divergences of opposite sign, the divergence integral in the tendency equation apparently cannot be evaluated from the data. Furthermore the sum of the divergence and advective integrals yield only the surface pressure tendency, which is already available. It does not appear that the divergence can be prognosticated as accurately as the pressure field. It is pointed out that the vertical velocities associated with a field of divergence may cause large pressure and temperature changes aloft with no surface pressure change. This shows that it is not possible to determine the regions responsible for surface pressure changes by considering the changes in the several layers. The influence of vertical stability on surface pressure changes was investigated statistically with indeterminate results. A model of a cyclonic development based on the latent heat of condensation is discussed. It appears that this mechanism is incapable of explaining pressure changes of the magnitude commonly observed. A mechanism by which additional accelerations and pressure changes might result from the deformation of the field of mass by an initial accelerational field is presented. Sufficient evidence has not been accumulated to determine whether this mechanism operates in the atmosphere.