Showing papers by "Geophysical Fluid Dynamics Laboratory published in 1976"

An Apparent Relationship between Eurasian Snow Cover and Indian Monsoon Rainfall

Abstract: A short record of year-to-year variations of summer monsoon rainfall over India is compared with that of winter snow cover over Eurasia as derived from satellite data. An inverse relationship between these two quantities is indicated, i.e., winters with extensive (little) snow cover over Eurasia tend to be followed by summers with less (more) rainfall over India.

On the Observed Annual Cycle in the Ocean-Atmosphere Heat Balance Over the Northern Hemisphere

Abstract: Based on the best presently available satellite radiation, atmospheric and oceanic data sets, the long-term mean heat balance of the earth and its normal seasonal variation are investigated over the Northern Hemisphere. Quantitative estimates for the various flux and storage terms in the atmospheric and terrestrial branches of the heat balance are given for 10-deg-wide latitude belts and for each calendar month. The results are presented in both graphical and tabular form. As was known before, the storage of heat in the oceans is found to dominate the energy storage in the combined atmosphere-ocean-land-cryosphere system. In the tropics, large changes in oceanic heat storage are found in the 10 N-20 N belt with a maximum in spring and a minimum in late summer. The main new finding of this study is that the inferred oceanic heat transports appear to undergo very large seasonal variations especially in the tropics.

On the Development of Spiral Bands in a Tropical Cyclone

Abstract: Development of the band structure in a tropical cyclone is investigated by solving an eigenvalue problem for perturbations of spiral shape. The perturbations are superposed on a baroclinic circular vortex accompanied with a radial and vertical basic flow. It is shown that the spiral bands in three different modes may be intensified in an inner area of a tropical cyclone. The baroclinicity of a basic field is not required for the development of bands in any mode. A spiral band which propagates outward can grow in the presence of the horizontal shear of the basic azimuthal flow. Without the basic circular vortex, this band is reduced to a neutral gravity-inertia wave with a particular vertical structure. The unstable spiral in this mode takes a pattern which extends clockwise from the center of a storm in the Northern Hemisphere. An azimuthal wavenumber 2 and a radial scale (twice the band width) of 200 km are preferred by this band. Another band with the characteristics of an inward propagating gr...

Numerical simulation of three-dimensional Benard convection in air

Abstract: A numerical model is developed to simulate three-dimensional Benard convection. This model is used to investigate thermal convection in air for Rayleigh numbers between 4000 and 25000. According to experiments, this range of Rayleigh numbers in air covers three regimes of thermal convection: (i) steady two-dimensional convection, (ii) time-periodic convection and (iii) aperiodic convection. Numerical solutions are obtained for each of these regimes and the results are compared with the available experimental data and theoretical predictions.At the Rayleigh number Ra = 4000 the present model is able to produce experimentally realistic wavelengths for the two-dimensional convection. The small amplitude wave disturbances at Ra = 6500 have period τ = 0·24. When they become finite amplitude travelling waves, the period is τ = 0·27. These values are in good agreement with theoretical and experimental results. A detailed study of the form of these waves and of their energetics is given in appendix A. As the Rayleigh number is increased to Ra = 9000 and 25 000, the convection manifests progressively more complex spatial and temporal variations.The vertical heat transport and other mean properties of the convection are calculated for the range of Ra considered and compared with experimental and theoretical data. A detailed comparison is also made between the mean properties of two- and three-dimensional convection at the larger values of Ra. It is found that the heat flux Nu is nearly independent of the dimensionality of the convection.

Simulation of Seasonal and Interhemispheric Variations in the Stratospheric Circulation

Abstract: This paper describes the stratosphere as simulated by the time integration of a global model of the atmosphere as developed at the Geophysical Fluid Dynamics Laboratory of NOAA. It is shown that the model is capable of simulating a number of the features of the seasonal variation in the stratosphere. For example, it qualitatively reproduces the seasonal reversals of zonal wind direction in the mid-stratosphere between westerlies in winter and the zonal easterlies prevailing during the summer season. In the mid-latitude region of the lower model stratosphere, zonal mean temperature is highest in the winter when solar radiation is weak. At the cold equatorial tropopause of the model, the seasonal variation of temperature is also quite different from that which would be expected from the seasonal variation of solar radiation. These results are in qualitative agreement with the observed variation. Attempts are made to identify the factors which are responsible for the various aspects of the seasonal ...

Some Results from a Simplified Three-Dimensional Numerical Model of Atmospheric Turbulence

Abstract: A simplified set of subgrid-scale transport equations is used to compute the stresses in a three-dimensional model of thermal convection in the atmosphere. Terms appearing in the full transport equations thought not to be essential to the large-scale dynamics are discarded, leaving prognostic equations to be solved for the subgrid-scale energy and the virtual potential temperature variance only. Equations for the Reynolds stresses and the subgrid-scale temperature-velocity correlations are considerably simplified and can be solved algebraically. A scale analysis of the full transport equations is offered as partial justification for the present approach in the case of nearly isotropic turbulence. The problem studied is that of a well-mixed layer bounded above by a region of strong stable stratification. The present model gives a significant improvement in the representation of the large-scale variables as compared with the more conventional eddy viscosity approach. In three experiments testing di...

Open Boundary Conditions for Dispersive Waves

Abstract: Approximate outgoing radiation conditions have been widely used at open boundaries in dispersive wave computations. Exact outgoing radiation conditions are constructed here for infinitesimal surface inertia-gravity waves, barotropic Rossby waves, and non-hydrostatic internal gravity waves incident at infinite plane open boundaries. They are also constructed for infinitesimal two-dimensional surface gravity waves incident at a square open boundary, a finite straight open boundary in a channel, and a circular open boundary, The constructions use Laplace transforms, various Fourier transforms, and a wave field splitting technique. The exact outgoing radiation conditions all involve weighted space and time averages. Their numerical implementation would be most complex and would require computer storage approaching that which one is trying to avoid by the introduction of open boundaries. Alternatives are discussed.

An Iterative Time Integration Scheme Designed to Preserve a Low-Frequency Wave

Abstract: A two-step iterative time integration scheme is formulated, by which the amplitude of a low-frequency wave in a primitive equations model is preserved fairly well for a period of short-range weather prediction while the high-frequency noises are damped. The desired computational characteristics are obtained by separating the terms of the equations at the corrector step. Numerical examples are presented which show the damping property of the proposed scheme. The new scheme does not require more data space in a computer than the amount used in the Euler-backward method.

Baroclinic instability in an eccentric annulus

Abstract: A study has been made of baroclinic instability in a differentially heated, rotating fluid annulus whose channel width varies azimuthally. Both laboratory experiments and an a.nalytica1 model employing a linear normal-mode analysis have been used. The experiments show three types of flow. For slow rotation the flow is 'symmetric’, whereas at high rotation speeds baroclinic waves occur at all azimuths. At intermediate rotation speeds it is possible to have a mixed flow which is ‘symmetric’ in the narrow part but has baroclinic waves in the wide part of the annulus. This result suggested the analytical investigation of the stability of a barocIinic flow whose meridional scale varies downstream. It was found that this model also permits three possible types of flow: everywhere stable, everywhere unstable, and also a mixed flow which is locally unstable where the meridional scale is largest but locally stable where the scale is smallest.

The Trapeze Instability in an Equatorial β-Plane

Abstract: From preliminary reports of the GATE experiment it was concluded that there is some strong evidence that mesoscale waves with a periodicity close to 2 days exist in the equatorial regions. The dynamics of unstable internal gravity waves due to trapeze instability was discussed by means of a two-dimensional, β-plane numerical model. It was concluded that the trapeze instability may be the means by which the observed 2-day waves are excited.

An Open Boundary Condition for Models of Thermals.

Abstract: Numerical models of thermals usually introduce a closed box within which convection occurs. A boundary condition which allows inflow and outflow through the edges of the computational domain is discussed. The results of numerical experiments using this boundary condition are then compared and contrasted with those obtained when a rigid boundary is assumed. It is found that the flow development is especially sensitive to the choice of the upper boundary condition even before warm fluid reaches the top of the computational domain.

Intrusions and double-diffusive convection

Abstract: A STRATIFIED fluid layer in which two components contribute to the vertical density distribution need not be stable even though the net density decreases upwards. If one of the components is unstably distributed, then molecular diffusion can release its potential energy—a phenomenon known as double-diffusion convection1. Considerable progress has been made over the past 10 years in the study of double-diffusive convection in situations where the properties vary in one direction (vertically) only. In the ocean, horizontal advection is nearly always important and its neglect in the previous laboratory experiments makes it difficult to apply the results in a wider context. A first step in the study of two-dimensional effects in double-diffusive convection was made by Turner and Chen2, and here I follow up one of their suggestions in detail in the hope that it may help to understand the layering which occurs when an outflow intrudes into an ambient fluid with the same density but different component concentrations3.