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GEORGE P. CRESSMAN

Abstract

An equation for vertical velocity is applied to data from 850, 500, and 200 mb. for calculations of vertical velocity and divergence in special cases characterized by pronounced failures of barotropic forecasts. The results of the calculations show that a non-development situation was adequately described by the equivalent-barotropic picture of a single quasi-horizontal surface of non-divergence. The onset of mid-tropospheric development, as shown by the appearance of large errors of barotropic forecast, was characterized by the appearance of a double surface of non-divergence, with a deep mid-tropospheric convergence layer in the vicinity of the trough line. This picture of development is confirmed by a second case study.

The appearance of the mid-tropospheric convergence layer is related to the low-level cold push, the tilt of the flow patterns with height, and the high-level jet stream, confirming synoptic studies by J. J. George, H. Riehl, and others.

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George P. Cressman

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George P. Cressman

Abstract

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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George P. Cressman

Abstract

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

Abstract

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

Abstract

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

Abstract

Kinetic energy budgets were prepared for the East Asia-West Pacific region to obtain a quantitative description of the sources and sinks of kinetic energy for the jet stream of that region. Budgets were prepared for locations of jet stream acceleration and deceleration for the period 12–16 January 1979, during the Global Weather Experiment. The region of generation of kinetic energy in East Asia was characterized by a large-scale direct solenoidal circulation, with a five-day average generation rate of 95 × 1010 kW or 34 W m−2. Orographic forcing over the east edge of the Himalayan plateau is suggested as a process partly responsible for the geographic reliability of the generation region over China. In the west Pacific region kinetic energy was destroyed by pressure forces at a rate of 61 × 1010 kW or 57 W m−2. This region was characterized by a vigorous indirect solenoidal circulation. The kinetic energy generation and destruction took place mainly at jet stream levels and are seen as successive phases of a modified inertial oscillation of the jet. Kinetic energy conversion at these rates greatly exceeds that in a typical vigorous extratropical cyclone, which could be in the range of 10–20 × 1010 kW.

The subgrid-scale motions were a sink of kinetic energy during jet stream acceleration and a source during jet stream deceleration. This observation is confirmed on a more general basis by a literature review demonsrating a fundamental consistency among the many studies of the energeties of large-scale systems. The kinetic energy changes of the small (subgrid) scale flow paralleled those of the large-scale flow and were about 11 W m−2. The magnitude of consistent subgrid- to grid-wale energy transfer over the Pacific (and elsewhere) indicates a need for considering the apparent phenomenon of negative viscosity in numerical atmospheric modeling.

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George P. Cressman
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GEORGE P. CRESSMAN

Abstract

The persistent occurrence of systematic errors of barotropic forecasts over mountainous areas is strongly suggestive of significant effects of mountains and friction which have not been included in the previously used forecast models. This study reports on experiments with a new barotropic forecast model which contains an improved mountain effect and a surface friction effect. For computation of the surface stress depending on the wind and on the terrain, a hemispheric map of the drag coefficient is obtained.

The results of tests of the forecast model on an initially zonal flow, and on ten observed meteorological situations, indicate that the effects of terrain on the evolution of atmospheric flow patterns can be of large magnitude, and can account for significant errors in numerical prediction. Some success at accounting for these effects is attained with the barotropic representation of the atmosphere.

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