Land Component of Models

CLIMATE MODELS simulate the interactions between the atmosphere, and the ocean and land surface beneath it. The land properties that matter most to the atmosphere include land elevation; the the presence of mountains, for example, can strongly affect the flow of air, causing it to rise and fall, thus creating zones of high and low rainfall.

The albedo of the land, which determines how much of the incident sunlight is reflected, depends critically on the nature of the ground cover, whether it is snow, ice, sand, or vegetation. The moisture content of the soil has a strong effect on the hydrologi-cal cycle, for example, the amount of evaporation into the atmosphere, and the amount of run-off into rivers and lakes. The roughness of the land surface affects the flow of air, which can become very turbulent near the surface.

Since climate models have reached a new generation of sophistication, the type and scope of variables employed in models have also changed. Perhaps the most startling change in model specifications has been the replacement of single variables to represent the three principal components of land, ice, and atmosphere, with numerous variables to account for these parts.

In terms of the land component, the single variable used in early models, which did not distinguish between land and sea, has been replaced by a suite of variables that model not just land and sea, but as many as three layers of land, including overlays of snow or ice, and a vegetation layer. The elevation of the land has also been incorporated into models, together with its impact on the albedo of the land—that is, the degree to which solar energy is reflected back away from the surface. Recent work has focused on the interaction between component parts. For example, the pressure and temperature of the land and the air or sea adjoining it will have significant impacts on the horizontal and vertical fluxes of masses of air, water vapor, or water. The interaction between land and sea ice also has implications for the flow of salt to and from different components, and this is important because salinity leads to different forms of behavior, with respect to interactions, Seasonality has also been modeled, as its impact on temperature has important effects over many parts of the land, particularly for Eurasia,

In technical terms, improvements to the land component derive from more sophisticated handling of fractional processes and turbulence. The actions and behavior of non-solids in turbulence remains one of the most complex areas of mundane physics. [Page 594]Nevertheless, advanced computational power has gone a long way to help understand this behavior and predict how it is going to take place in a variety of environments. A second area of improvement concerns the handling of run-off of various types of precipitation. These issues have been tackled by a joint project known as the Common Land Model (CLM), which aims to pool the knowledge and understanding of research communities in various locations.

The land component of climate models most readily shows the impact of humans. Rapid urbanization, deforestation, and changes in land cover all have significant effects on the other components of the model, and also directly impact the atmosphere and its circulation. Cities, for example, are noticeably hotter than the surrounding areas, and this introduces new forms of circulation into the system. These changes also occur much more rapidly than changes in the sea or atmosphere. The increasing heterogeneity of the land, which the changes generally bring about, increase the complexity of models and the difficulties involved in attempting to simulate climate systems.

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