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Warren L. Godson

Abstract

From a consideration of temperature changes in isobaric surfaces a new pressure-tendency equation is derived which is applicable to mass changes of a layer or pressure changes at any level including the earth's surface. Using this ‘isobaric tendency equation’ orographically and thermally induced pressure systems are studied with respect to their development and variations in their intensity. An analysis of the terms in the ‘isobaric tendency equation’ as applied to extratropical cyclones suggests that deepening occurs when one or more of the three following conditions are met: (1) there is strong warm-air advection in the lower strato-sphere, (2) there is a low level of nondivergence permitting subsidence at the level of the tropopause, and (3) there is potentially unstable warm-sector air.

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Warren L. Godson

Abstract

A basic problem in atmospheric radiation is the computation of flux transmission (hemispheric radiation) for a narrow interval of the water-vapor spectrum. Individual lines are spaced more or less at random and have widely divergent intensities. Even inside a narrow spectral interval, the line half-width and the variation of line intensity with temperature vary appreciably from line to line.

With the assumption of a completely random line spacing, the above problem can be formulated rather simply for the limiting cases of thick layers and thin layers, even for an arbitrary atmosphere and spectrum. With the additional assumption of a logarithmic ogive for line intensities, a flux-transmission function can be developed, dependent on only two parameters, which gives reasonable accuracy over the entire transmission range. This accuracy can be further improved by semi-empirical corrections to the two basic parameters.

If the requisite spectral parameters are considered known precisely, the method proposed should yield values of the flux divergence, as required for calculations of radiative heating and cooling, with relative errors of the order of 2 to 3 per cent. Because these errors are largely random, the techniques can be applied with confidence in computations involving integrations over the entire spectrum.

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Warren L. Godson

Abstract

The “parcel” method of treating dynamic instability is extended to the case of an arbitrary flow pattern. The criteria for instability become relatively complex and depend on the three-dimensional gradients of temperature and geostrophic wind, as well as on the latitude. Fundamentally different types of instability and stability may arise; these are illustrated by typical examples. Methods for the computation of the various terms in the fundamental stability-criterion parameters are presented, and these methods, as well as the generalized criteria themselves, are extended to the case of saturated air.

It is suggested that marked quasi-horizontal, or dynamic, instability will superimpose a strong field of “perturbation divergence” on the ambient field of mass divergence and hence should be a significant factor in the weakening of anticyclones and the formation and deepening of cyclones.

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Warren L. Godson

Abstract

The subsequent motion of a perturbed air-parcel is outlined, together with the criteria that determine the four fundamental types of dynamically stable and dynamically unstable motions. The specification of such motions for synoptic studies is presented, and the physical significance of the motions themselves discussed. The importance of lateral diffusion in the atmosphere is deduced, especially when dynamic instability is present. A region of dynamically unstable motions is identified as one showing lateral “perturbation divergence.” The sectors of compensating convergence can also be established, especially for the class of instability characterized by a unique “axis of outflow” for the perturbed parcels.

In theory, marked instability should be associated with a negative development tendency for the corresponding surface pressure system, and marked stability with a positive development tendency. Statistical studies of a large number of dynamic-instability computations adequately verified these concepts and revealed the fact that at 500 mb only one quarter of all cases showed actual dynamic stability. When the computations are grouped with respect to distance from the 500-mb polar front, maxima of stability are found in the quasibarotropic cold and warm air-masses and in the frontal transition zone; a significant maximum of instability is found in the baroclinic warm-air zone. This instability is especially well marked for strongly baroclinic cases. The physical significance and the consequences for forecasting of these synoptic results are discussed.

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Warren L. Godson

The form of the gradient-wind equation involving the motion of pressure systems is briefly derived in order to indicate the assumptions involved. The equation is applied to closed highs and lows aloft, verifying qualitatively their empirically-observed motion in the direction of their strongest winds. Criteria for the speed of translation of high pressure centers and ridges are deduced from the gradient-wind equation. It is shown that both roots of the binomial solution for anti-cyclonically curved trajectories or contour lines must be admissible and that the classically-discarded root offers a logical explanation for the so-called super-gradient wind.

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Roy Lee
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Warren L. Godson

Abstract

The existence of an Arctic stratospheric jet stream in winter, hitherto largely inferred from mean geostrophic wind sections, is considered on the basis of actual winds for the Canadian Arctic area during the winter of 1955–1956. Temperatures at the 100-millibar level at a number of stations over the Canadian Arctic were examined to throw light on the intensely baroclinic zone below the jet stream.

Meridional movements and intensity changes of the jet stream during this winter, as inferred from a statistical study of the 100-mb temperature field, are in accord with the conventional view that the jet stream is maintained by differential radiational heating and cooling of the ozone layer across the boundary of polar night.

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