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Model Validations
The following series of images gives examples of what several
global climate models project for the US under an equilibrium doubled CO2. These results are
from very early versions of the respective models, but nevertheless demonstrate several issues in model
validation. Recent model results show improved accuracy.
In each case we also will examine the model validation as compared with the
present climate. Models used in this comparison are the Oregon State
University (OSU) 2-level model, the NASA GISS low-resolution model, and the
GFDL model. The comparison will consist of a series of 4-panel displays of
regional patterns of temperature and precipitation over North America. In
each 4-panel display, the panel in the upper left corner will be the
observed values that serve as the reference pattern for comparison with
model results.
Figure 14 gives the comparison of model validations on mean January temperatures. Maps for each of the three models give the difference in temperature between the model produced value and the observed temperature for that local part of the domain. If a model gave a perfect simulation of the present climate, its map would be a field consisting of all zeros. If the numbers are positive, the model overpredicts the temperature (gives values that are too warm), and negative numbers give a simulated climate that is colder than observed. The GISS model gives values that tend to be positive, suggesting that simulated January temperatures are warmer than observed. Most values are within 2 degrees C of observed, although a few are markedly larger. The largest errors tend to be positive and occur at high latitudes.
The GFDL model is a higher-resolution model that gives a more even balance of positive and negative values but also has large positive numbers in the northeastern part of North America similar to the GISS model. The OSU model also has large positive and negative values but agrees with the other models in overpredicting January temperatures in eastern Canada.
Figure 15 gives a similar plot of July temperatures. In this case, the GISS values are more negative and some of the largest negative values are in Northeastern Canada. The GFDL model is biased too warm but with lower magnitude than for January. The OSU results show large negative values off the east coast of the US and in the mountainous western US. It can be concluded that each model has its own bias that differs from region to region and month to month.
Global climate models also have been used to simulate paleoclimates (climates of the past). In one example where a climate model is used to simulate climate change as Earth emerged from the last ice age, the distribution of spruce pollen observed to have been emitted over North America during this transition agrees very well with the locations where spruce trees would have grown for the climates simulated by the model.
