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George P. Cressman

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George P. Cressman

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The analysis and use of charts of absolute vorticity are described. Measurements from a winter and a summer series of maps gave 608 and 641 mb, respectively, as the pressure at the mean equivalent barotropic surface. It is shown that, on charts near the equivalent barotropic surface, the absolute-vorticity patterns give indications useful in short-range forecasting, since the lines of constant absolute vorticity are advected with nearly the speed of the wind. Examples are presented, showing typical contour and vorticity patterns for a rapidly moving pattern, a stationary pattern, and a situation of rapid trough development.

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George P. Cressman

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With the vorticity equation for horizontal motion as a beginning, and with the aid of several assumptions, the equation ∇ H ·ν = ν T ·∇ H In η is derived, giving the horizontal divergence as the product of (ν T ) the vector wind-difference between the level in question and 600 mb, and the logarithm of the absolute vorticity (ν) at the level where the divergence is desired. The divergence at 850 mb is computed for a variety of situations with the above equation and is compared with vertical velocities, with the divergence computed from the wind field, and with observed weather distributions, the comparisons yielding favorable results. Finally, suggestions are made for the use of the method for divergence determination in the prognostic routine.

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George P. Cressman

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This study indicates that the jet streams which are observed on meridional cross sections appear first at high latitudes and usually shift slowly southward to low latitudes, where they eventually disappear. It is common to observe the presence of more than one upper west-wind maximum at the same time but at different latitudes. Average variations in the speeds of these maxima are studied. Splitting of a single west-wind maximum into two distinct maxima is occasionally observed on a hemispheric scale.

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George P. Cressman

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In an attempt to use the equation developed by Rossby for the motion of waves in a single-layer barotropic atmosphere as a prognostic tool, the effect of the upstream variation of wave length is studied. With the aid of the concept of group velocity an expression is obtained for trough displacement which takes into account the change of wave length with time and the acceleration of the long waves. Tests of the results indicate that the inclusion of the upstream wave-length variation in the forecast of trough displacement gives a significant improvement in the forecast verification. The results can be expressed qualitatively as follows: If the wave length of the long waves increases upstream at the initial moment, the eastward speed of the wave under consideration decreases with time. If the wave length decreases upstream at the initial moment, the eastward speed of the wave increases with time.

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George P. Cressman

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This study of the interrelations and interactions in the long waves in the upper westerlies on daily charts has disclosed several results having prognostic significance. The relation of the movement of the long waves to the wave length and the speed of the zonal westerlies is discussed. The basic current of westerlies is studied and some indications for its variations are shown. Some interactions between the long waves, which have prognostic value, are those involving change of wave number, change of wave length, and change of amplitude. A Study of these interactions leads one to conclude that daily upper-air charts showing several of the long waves are a prerequisite for the forecasting of the long-wave pattern.

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George P. Cressman

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By means of a modification of the slice method of Bjerknes and Petterssen, the effects on the convective cloudiness of a net inflow to, or outflow from, the region under consideration are examined. The results give a physical description of these effects. They also show that the effect of a given mass transport through the slice under consideration is greatest when the actual lapse rate exceeds the moist-adiabatic value only slightly. Applications to tropical meteorology are discussed.

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