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A. WIIN-NIELSEN

Abstract

The kinetic energy of the horizontal, hydrostatic flow is divided into the kinetic energies of the vertically integrated flow and the deviation from this flow, the so-called shear flow. The energy transformation between the two types of flow is found in the general case of the primitive equations and also for the most simple quasi-non-divergent model. The two transformations are discussed, and the energy transformation in the quasi-non-divergent model in the two-parameter case is discussed as a function of wave number using linear theory. The energy conversion has been computed on a daily basis for the month of January 1959, and compared with earlier results of computations of transformations between available potential energy and shear flow kinetic energy. It is shown that the latter conversion changes the kinetic energy of the shear flow and not that of the mean flow. The residence time is estimated for the shear flow as well as the mean flow.

The energy transformation between the vertical shear flow and mean flow due to the non-divergent and divergent flow has been computed in the wave-number regime for the first 10 zonal wave numbers for each day in January 1959. It is found that the energy conversion between shear flow and mean flow is about 30 percent of the conversion between the available potential energy and the shear flow kinetic energy.

A further result is that the energy conversion between the shear flow and the mean flow due to the divergent part of the flow is estimated to be negative and ahout 10 percent of the conversion due to the non-divergent part of the flow.

The energy conversion as a function of wave number shows a maximum for the most unstable baroclinic waves.

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A. WIIN-NIELSEN

Abstract

The quasi-geostrophic, baroclinic stability problem is solved for an arbitrary zonal wind profile U = U(p) and for an adiabatic lapse rate. It is shown that the phase speed of the waves in this case depends on the vertical integrals of U and U 2. Due to the assumption of an adiabatic stratification there is no short wave cutoff, but the effect of the variation of the Coriolis parameter will in all cases give stability for sufficiently long waves.

A number of numerical examples show that the region of instability, in a coordinate system with wavelength as abscissa and wind shear as ordinate, is the largest when the wind maximum is situated in the upper part of the atmosphere, and when the curvature of the zonal wind profile at the wind maximum has an intermediate value.

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A. WIIN-NIELSEN

Abstract

The problem of control of the ultra-long waves in numerical prediction is considered. Sections 2–4 contain a discussion of the one-level forecasts. It is shown that the vertical variation of the horizontal wind and the static stability are the main factors in determining the value of divergence a t 500 mb. An independent estimate of the size of the term governing the ultra-long waves in the atmosphere agrees well with the one determined by Cressman on an empirical basis.

Section 5 points out that any two-parameter model has to contain an effect similar to the one contained in the one-parameter model controlling the ultra-long waves. A modification of a two-parameter model is made in such a way that the ultra-long wares are controlled.

Section 6 describes a perturbation analysis of the model developed in Section 5 in order to investigate the effect of the modification also on the shorter waves. It is found that a certain stabilization of the shorter waves is produced. Baroclinic instability and growth rate are investigated.

Sections 7 and 8 contain a justification of certain approximations used in the earlier sections regarding the vertical variation of static stability and the profile of vertical velocity.

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A. WIIN-NIELSEN

Abstract

Energy conversion between potential and kinetic energy is considered. Section 2 contains the derivations which are necessary to compute the energy conversion for a large region on the basis of vertical velocities and relative topography as obtained from a two-parameter model presently used by the Joint Numerical Weather Prediction Unit. The energy conversion is divided into three parts: (1) energy conversion due to a mean vertical velocity over the region, (2) energy conversion in meridional planes, and (3) energy conversion in the zonal planes.

Section 3 contains a discussion of the results obtained for the months January and April 1959. The energy conversion is positive for each day in both months, but the conversion in the meridional planes has a different sign in the two months, being positive in January and negative in April. The pattern of the mean meridional circulation is discussed and the frictional dissipation estimated.

Section 4 describes a procedure for an evaluation of the energy conversion for the different wave numbers, and discusses the results for the same two months. The same section contains a comparison with results obtained from a linear, adiabatic theory.

Section 5 contains a discussion of the modifications to the results in section 4 caused by the diabatic heating of the atmosphere It is made plausible that the maximum conversion found for the small wave numbers by an adiabatic computation is greatly altered due to the effects of the heating.

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A. WIIN-NIELSEN

Abstract

A comparison is made between the stability properties of the non-divergent and divergent one-parameter models. It is show that the introduction of divergence in the one-parameter model reduces the rate of growth of unstable disturbances arid confines the instability to a more narrow band of wavelengths.

Changes in zonal momentum, momentum transports, and energy conversions between mean flow kinetic energy and eddy kinetic energy are investigated in the linear case, as well as by extended integrations of the spectral form of the prognostic equations allowing only a few wave numbers.

Long-term variations in barotropic flow, where the flow is initially stable, are investigated using the spectral formulation, but allowing only as many wave numbers as are needed to investigate variations in the profile of the zonal wind.

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A. WIIN-NIELSEN

Abstract

The equation for the vertical velocity in a quasi-non-divergent, three-parameter model has been solved for a certain simple flow pattern. The importance of vertical variation of static stability and of the horizontal wind is discussed from the solutions. Examples showing the distribution of divergence relative to the synoptic systems are computed and compared with those obtained by other investigators. A general discussion of the factors influencing the mid-tropospheric divergence follows in section 4, and section 5 contains finally some remarks on the divergence in very long waves.

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A. WIIN-NIELSEN

Abstract

Different factors influencing the changes in the zonally averaged wind are investigated. It is evident from recent investigations of errors in the zonally averaged winds in the non-divergent, one-parameter model that the convergence of the meridional transport of zonal momentum concentrates too much momentum in the middle latitudes and predicts too small amounts of momentum in the low and high latitudes.

The other factors influencing the changes of zonal momentum, i.e., mean meridional circulations, friction, and vertical transport of momentum, are investigated on an averaged basis in the, following sections: In section 3 it is shown that the divergent, one-parameter model will reduce the errors in zonal momentum predicted by the non-divergent model, due to an implied mean meridional circulation. The corrections to the predictions of changes in zonal momentum caused by mean meridional circulations in a two-parameter model are investigated in section 4 by the aid of operationally computed initial values of the vertical velocities. It, is shown that reductions in the errors of the non-divergent model with respect to zonal momentum can be expected with a careful arrangement of the information levels in a two-parameter model.

Section 5 contains a similar investigation of the averaged contribution of vertical advection t o changes in zonal momentum. It is found that this contribution is smaller than the one resulting from mean meridional circulations, and further that the contribution from the vertical advection of momentum is not likely to reduce the errors found in the non-divergent predictions.

The main conclusion from the study is that the contributions from mean meridional circulations and surface friction are the most important for the reductions of errors in the prediction of zonal momentum in the non-divergent model. Some reduction of the errors can he expected in the divergent, one-parameter model or in a two-parameter model with a proper arrangement of the information levels. In order t o incorporate surface friction in a realistic way, and further in order to avoid the artificial constraint of a non-divergent level appearing in a two-parameter model, it is most likely that more than two parameters ale needed for accurate forecasts of zonal momentum.

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A. WIIN-NIELSEN

Abstract

It is shown that calculations of the energy conversion from available potential energy to kinetic energy based on vertical velocities obtained by the so-called adiabatic method may lead to erroneous results. An analysis of the method shows that it measures the difference between the energy conversion from available potential energy to kinetic energy, and the generation of available potential by diabatic heating. This result holds for both zonal and eddy avai1able potential energy.

The main conclusion is tested using numerical results from several energy conversion and energy generation studies.

The adiabatic method is compared with other methods to estimate vertical velocities.

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A. WIIN-NIELSEN
and
A. D. VERNEKAR

Abstract

The mean meridional circulation calculated earlier from observed values of transports of momentum and sensible heat by solving the zonally averaged form of the quasi-geostrophic omega equation is used to investigate the influence of this secondary flow on the zonally averaged values of the wind and temperature in the atmosphere. The contributions from the horizontal transport processes and the mean meridional circulations are computed separately in order to estimate their relative importance. It is found that the mean meridional circulation counterbalances the horizontal transport of momentum in the upper troposphere, while the two effects work in the same direction in the lower part of the atmosphere. With respect to changes in the zonally averaged temperature field, it is found that the effect of the mean meridional circulation opposes the effect of the horizontal transport of sensible heat almost everywhere.

The recent results of calculations of the mean meridional circulation are also used to discuss the role of zonal heating and friction in quasi-geostrophic models.

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JACQUES DEROME
and
A. WIIN-NIELSEN

Abstract

The middle-latitude standing wave problem is investigated by means of a quasi-geostrophic, linear, steady-state model in which the zonal current is perturbed by the lower boundary topography and by a distribution of heat sources and sinks. All the perturbations are assumed to have a single meridional wavelength and the dissipation is considered to take place in the surface boundary layer using, as a first approach, a horizontally uniform drag coefficient.

After investigating some basic properties of the model atmosphere, some computations are made to determine its response to the combined forcing by topography and by diabatic heating for January 1962. The resulting perturbations are found to be in rather good agreement with the observed standing waves. The results also indicate that the standing waves forced by the topography are in about the same position as those forced by the diabatic heating and that the former have somewhat larger amplitudes than the latter.

The effect of allowing the drag coefficient to have one constant value over the continents and a smaller constant value over the oceans is examined and found to be quite important when the ratio of the two values is 6, but small (yet such as to bring the computed and observed eddies into closer agreement than in the case of a uniform drag coefficient) for a ratio of 2.

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