Use In General Circulation Models Research Articles

Adjustment of the Ocean under Buoyancy Forces. Part I: The Role of Kelvin Waves

The early stages of the adjustment of an ocean toward equilibrium is examined using an ocean general circulation model. The initial state is one with uniform meridional potential temperature gradients yielding density gradients representative of those observed in the North Atlantic. The main adjustment in the time examined here (the first few months) is by coastal Kelvin waves. The first baroclinic mode dominates, so the behavior proves to be very similar to that in a shallow-water model. Also, the dynamics is locally close to that for an f-plane, but account must be taken of the way f varies with latitude. The essential result of the coastal adjustment is to reduce the temperature gradients around the perimeter of the basin, especially at the level where the first mode has the maximum effect. This study also examines the way this adjustment process is distorted in models due to the constraints imposed by computer limitations, namely in a model with too coarse a horizontal resolution and an artificially high lateral friction needed for computational stability. In the present case, the distortions can be quantitatively assessed and understood, and it is felt there are lessons to be learned about the use of general circulation models.

Changes in the radiative balance of the atmosphere due to increases in CO 2 and trace gases

Increases in CO 2 and trace gases in the earth's atmosphere can lead to changes in the radiative balance of the earth's atmosphere in two ways. Changes in the amount of these constituents leads directly to changes in outgoing longwave radiation. This change in outgoing longwave radiation causes the thermal structure of the atmosphere to change. These changes in thermal structure can then alter the distribution of outgoing longwave radiation. Changes in the distribution and amount of outgoing radiation due to various increases in CO 2 and trace gases are presented from a model with a spectral resolution of 5 cm −1. The effects of stratospheric aerosols, and the use of general circulation models for detecting changes in the radiative balance are discussed.

The use of general circulation models in the analysis of the ecosystem impacts of climatic change

The use of general circulation models in the estimation of the impact of climatic change on the global ecosystem is seen to depend primarily on their ability to reliably depict the seasonal and geographical distribution of the changes in surface climate variables. While present GCMs generally simulate the observed distribution of surface air temperature reasonably well, they show significantly different changes in the equilibrium temperature as a result of doubled CO2, for example. These disagreements are attributed to differences in the model's resolution and parameterization of subgrid-scale processes. Such model-dependent errors notwithstanding, much more information of possible use in impact analysis can be extracted from general circulation model simulations than has generally been done so far. The completeness, consistency and experimental possibilities offered by simulated data sets permit the systematic extraction of a wide variety of statistics important to the surface ecosystem, such as the length of the growing season, the duration of rainless periods, and the surface moisture stress. Assuming further model improvements, the elements of a model-assisted methodology for climate impact analysis are seen to be: (1) the determination of the seasonal and geographical distribution of that portion of simulated climatic changes which are both statistically and physically significant; (2) the transformation of the (significant) large-scale climatic changes onto the local scale of impact (the climate ‘inversion’ problem); and (3) the design of specific statistical parameters or functions relevant to local ecosystem impacts.

The use of general circulation models in climate impact analysis ? A preliminary study of the impacts of a CO2- induced climatic change on West European agriculture

An investigation is made of the possible impacts of a climatic change (induced by a doubling of atmospheric carbon dioxide concentration) on the European agricultural sector. Two general circulation models have been used to develop climatic change scenarios for the European study area. From the scenarios, information was obtained concerning the possible behavior of temperature, precipitation, solar radiation, and relative humidity in the altered climatic state. This meteorological information was then employed in two separate crop-weather models - an empirical/statistical model (for winter wheat) and a simple simulation model (for biomass potential). This type of approach represents a considerable departure from that employed by previous large-scale climate impact studies. Both the seasonal and regional components of a possible climatic change are incorporated directly in the two crop-weather models. The results of this investigation demonstrate that a simple crop-weather simulation model may be more suitable for the purposes of agricultural impact analysis than the linear regression models frequently used in such studies. In order for such an impact analysis to be accepted as a valid scientific experiment, a full presentation of the underlying assumptions and uncertainties is essential.

The Rossby Adjustment Problem in a Rotating, Stratified Channel, with and without Topography

Some numerical experiments On geostrophic adjustment in a gratified channel were carried out, partly to test the use of an ocean general circulation model (GCM), and partly to elucidate the dynamical effects of sloping bottom topography on the adjustment at different latitudes. Interesting dynamical effects include the release of potential energy through shelf waves, the nonlinear generation of barotropic flow by a baroclinic Kelvin wave and the nonlinear generation of shelf waves with approximately half the inertial frequency by internal Poincaré waves of near-inertial frequency. Problems that inevitably occur in the use of GCMs were also highlighted.