Search Results

You are looking at 11 - 20 of 20 items for

  • Author or Editor: George P. Cressman x
  • Refine by Access: All Content x
Clear All Modify Search
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.

Full access
George P. Cressman
Full access

president's page

Participation in the Society's Affairs

George P. Cressman
Full access
George P. Cressman
Full access
George P. Cressman

The development and motion of typhoon “Doris,” which was observed during the first two weeks of May, 1950, are studied. The development of the storm is examined with respect to previously published theories of storm formation. The original deepening occurred in the low latitude portion of an extended trough, after the fracture of the trough. This is in agreement with a model proposed by Riehl. The motion of the deepening storm relative to the high-level flow patterns differed from previously studied examples in that the deepening occurred as the low-level cyclone moved from under the west side of an upper anticyclone toward a position under an upper cyclone. The storm developed as two cyclonic vortices, which gradually merged into one, in agreement with a principle of Fujiwhara.

The motion of the storm northward, as it broke through the subtropical ridge line, is shown. After examination of several possibilities, this motion is attributed to the resultant of all the Coriolis forces acting on the storm, as discussed by Rossby. The suggestion is made that this resultant force becomes prominent in determining the motion of the storm due to changes in the radial velocity profile and the increasing geographical extent of the storm.

Full access
George P. Cressman

Abstract

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.

Full access
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.

Full access
George P. Cressman

Abstract

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.

Full access
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.

Full access
Karl R. Johannessen
and
George P. Cressman

A formula for the propagation of trough and ridge lines was tested on synoptic maps. It was found that it works best on the 500-mb chart. At this level the systematic error was found to be small for troughs (< 1 long.° per day) but considerable for ridges (+ 3 to +5 long. ° per day). However, the error scatter was about the same for troughs and ridges. It was found that computations of ridges over elevated terrain such as the Rocky Mountains region have a larger positive systematic error than ridges over flat country. This was not found to be the case for troughs. It is concluded that better results can be expected the closer the actual streamline pattern resembles the model for which the formula was derived.

Full access