Big and small
Global climate models attempt to depict the complex interplay of atmosphere, oceans and land worldwide, but often provide insufficient information at the regional level. That is why attempts are being made to couple them with regional models. "The physical conditions are taken into account much better in regional models than in global climate models," says Dr. Gregory S. Jenkins, Assistant Professor of Meteorology at Penn State University. "Regional climate models could provide more useful information for users."
For example, global climate models use about 30 data points for the US, while regional models use about 800 data points. Regional models take relief into account more than global models. Although mountains are important for climate and weather, in global models even influences from mountains like the Rocky Mountains become insignificant. It is also difficult to define coastlines with global models: however, regional models can resolve the coastal areas in great detail. "To the Northeast, violent storms form along the Atlantic coast; this is important for both weather forecasting and climate studies," says Jenkins.
Penn State University researchers use a global climate model called Genesis developed by the National Center for Atmospheric Research and a regional climate model they created. It was examined how the results differ depending on whether a global or regional model was used.
"We wanted to see what changes we get by assuming increasing greenhouse gases in variables like precipitation, soil moisture, and snow depth," Jenkins said on March 17. February 1998 at the annual meeting of the American Association for the Advancement of Science. "We also wanted to understand how the variables in both models differ within different regions of the US."
The regional model used by Jenkins was originally developed for short-term weather forecasting. Later, however, it was modified so that the climate can be studied over the long term. For the simulation of the climate over seasons and years, the vegetation and local variabilities are included in the calculation.
"For some variables, like temperature, we seem to get the same results with both models," says Jenkins. "But for example soil moisture increases in the global model but not in the regional model."
Scientists calculated global and regional models based on carbon dioxide concentrations twice today's levels. Overall, the global models consistently delivered high precipitation. However, the regional model is better suited for forecasting rainfall amounts that are similar to actual observations. The regional climate simulation with twice the carbon dioxide concentration for the US Northeast delivered warmer temperatures and resulted in more rain and less snowfall in winter. Little snow means little snow cover for the spring melt. "This type of result is important for water balance," says Jenkins.
The regional models rely on the data from the global models. Problems arise when interpolating, for example to increase the number of data points by a factor of thirty.
One way the researchers tested the models was to try to simulate extreme events of our time, such as El Niño or the 1988 drought. The models showed difficulties in predicting both precipitation and droughts. For example, both calculations did not correctly simulate the 1988 summer drought in the central US.
"Coupled global and regional models may not yet provide the best results because coupled atmosphere-ocean models that provide the boundary conditions for the regional climate model are still in the early stages of development," says Jenkins. "Ultimately, coupled atmosphere-ocean models will show us how extreme events like El Niño can moderate or exacerbate the effects of global warming. This will change our understanding and the results obtained with regional climate models."
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