Part 4:
Model Parameters
For meteorological models, there are dozens of parameters which influence the accuaracy and detail of the output information. We will address the two most important parameters -- surface factors and grid factors.
Surface Factors
The type of surface and the location of the surface can have an enormous effect on a numerical model. Different factors need to be accounted for depending on the soil types, vegetation types, terrain of the land, elevation, and useage of the land. A mountain region will have different properties than a grassland area. For each type, the model must account for different characteristics, which will influence the accuracy of the model. Therefore, it is extremely important for a model user to know the surface characteristics over which the model is placed.
The properties of the grid that is used also has a significant impact on the quality of the information produced by the model. First, the size, or domain, of the model is important. Does the model need to cover ten thousand square kilometers or one thousand square kilometers? Does the model need to cover the entire time zone or just the state of specific interest? The size of the domain determines the amount of input information and the amount of output information, and therefore determines the model run-time. The longer it takes for the model to run, the higher the cost to the user. A second factors is the grid resolution. We mentioned this subject before in the Grid section of Session 5. The resolution of the grid (the size of the individual cells) influences the detail of the information for the region and therefore influences run-time. The user must decide if a high resolution is necessary or if a lower one will give enough detail and accuracy. A third issue is whether or not to use a nested grid. A nested grid, as we covered in Session 5, is a higher resolution grid placed inside a lower resolution grid. This allows a higher level of detail where it is needed most. However, a nested grid also affects computer run-time. And if a nested grid is used, is it a one way or a two way grid? A one way grid has data from the outer grid feed into the higher resolution nested grid. A two way grid feeds data from the outer grid to the nested grid but also feeds data from the nested grid to the outer grid. A two way grid is more detailed and precise, but it also more complex. As a result, it will have a longer computer run-time.
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