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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

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

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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George P. Cressman

Abstract

The jet stream of the west Pacific is a very steady feature of the winter circulation, with almost continuousintensification over East China and Japan and weakening in the central Pacific. In this study the National Meteorological Center spectral forecast model with normal mode initialization is used as a tool to provide diagnostic elements in a region of nonstandard data sources. Cross sections transverse to the jet stream in its intensification zone show a deep single-celled direct solenoidal circulation around a single frontal zone to be producing large increases of kinetic energy. Farther east, in the central Pacific, a strong indirect solenoidal circulation produces a strong decrease of kinetic energy.

The irrotational wind components at jet stream levels are suggestive of an inertial cycle as part of the jet stream dynamics. A simple model of a modified inertial cycle is integrated in which the geostrophicwind is allowed to vary as a function of the ageostrophic flow transverse to the jet axis. This produces dimensions of the cycle in agreement with observations. It leads to a conclusion that the indirect circulationcell and its climatological aspects are an inertial consequence of the constant baroclinic activity near Japan.

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GEORGE P. CRESSMAN

Abstract

The problem of spurious retrogression of very long waves by the non-divergent barotropic forecasts is shown to be the same problem discussed extensively by Rossby, Yeh, and Bolin. This difficulty is due to the failure of the non-divergent model to allow properly for the mutual adjustment of wind and pressure fields. The equation of continuity for a homogeneous incompressible fluid with an upper free surface, proposed as a remedy by Rossby nearly 20 years ago, removes much of the difficulty. Further improvements are obtained by inclusion of a tropopause in the manner adopted by Bolin. The results of a series of 10 test forecasts are shown in verification of the function of the divergence in a barotropic model.

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GEORGE P. CRESSMAN

Abstract

The system of objective weather map analysis used at the Joint Numerical Weather Prediction Unit is described. It is an integral part of the automatic data processing system, and is designed to operate with a minimum of manual supervision. The analysis method, based mainly on the method of Bergthórssen and Dööos, is essentially a method of applying corrections to a first guess field. The corrections are determined from a comparison of the data with the interpolated value of the guess field at the observation point. For the analysis of the heights of a pressure surface the reported wind is taken into account in determining the lateral gradient of the correction to be applied. A series of scans of the field is made, each scan consisting of application of corrections on a smaller lateral scale than during the previous scan.

The analysis system is very flexible, and has been used to analyze many different types of variables. An example of horizontal divergence computed from a direct wind analysis is shown.

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