CONTINENTAL TO SUBCONTINENTAL CLIMATE MODELING
COMPARISON OF MODEL SIMULATIONS WITH OBSERVED PRESENT-DAY CLIMATE
Figure 2 illustrates present-day mean January and July temperature and precipitation in the Western United States as simulated by GENESIS (figs. 2A-2D) and RegCM (figs. 2E-2H), compared with observed values that have been interpolated onto a 15-km grid (figs. 2I-2L). In the observed climate, January temperatures are above freezing only in southern portions of California, Arizona, New Mexico, Texas, and along the Pacific Coast. Extremely cold winter temperatures occur largely on the northern Great Plains. Due in large part to its coarse depiction of topography, GENESIS for January simulates relatively warm winter temperatures across much of the interior Western United States; extremely cold temperatures are restricted to the far-northern Great Plains in Canada. In contrast, the RegCM simulation provides a spatial temperature pattern that better matches the observed present-day pattern. The location of the freezing line in the RegCM simulation is very close to that of the observed data, and cold temperatures on the northern Great Plains are more accurately depicted. For July temperatures, the results are similar: GENESIS simulates the general, broad pattern of July temperatures, but the Western Cordillera is not evident. The RegCM simulation provides an acceptable pattern of warm temperatures in the Southwest and cooler temperatures along the Western Cordillera mountain chain from northern Mexico to British Columbia.
Precipitation in the Western United States is presently dominated by two distinctly seasonal features of atmospheric circulation. Winter precipitation, associated with low-pressure cells and westerly wind patterns (jet stream) off the Pacific Ocean, is the dominant feature along the Pacific Coast and into the northern Rocky Mountains (fig. 2K). In contrast, summer rainfall, associated with convective storms from monsoonal flow originating from the subtropical Pacific Ocean and Gulf of Mexico, occurs in the southern and eastern portions of the West (fig. 2L). The wet-west/dry-east pattern of January precipitation is present in the GENESIS simulation (fig. 2C), although the spatial distribution does not closely match observations (fig. 2K). The RegCM simulation of January precipitation is a better match with the observed pattern, although, as with GENESIS, the simulated precipitation is too high in the Great Basin and the Southwest. The results for July show that both models simulate too much precipitation throughout the Rocky Mountains and that the RegCM captures the gross features of observed precipitation.
| Figure 2 . Maps showing comparison of present-day climate (average or mean January and July temperature and precipitation) as depicted by A-D, GENESIS; E-H, RegCM; and I-L, observed conditions. Observed present-day climate data (I-L) is interpolated onto a 15-km grid from climate stations using a locally weighted trend-surface regression approach (e.g., Lipsitz, 1988). The GENESIS maps (A-D) show, in part, how the crude depiction of topography in the model (see fig. 1A) is translated into the simulation of temperature and precipitation. Because of the reduction in overall elevation in the smoothed representation of topography in the model, lower values of January temperatures are not simulated correctly and the overall representation of topography as a broad dome centered over Wyoming, Colorado, and Utah spreads out areas of higher precipitation relative to those of the observed present-day climate. The RegCM maps (E-H), on the other hand, show that this higher resolution model, even though forced by the coarse-resolution GENESIS, produces simulations that are closer in overall appearance to observed climatic patterns (I-L). (Click here for scalable 490K PDF version) |
SIMULATION OF A POTENTIAL 2 X CO2 CLIMATE
The GENESIS 2 X CO2 simulation used a 50-m-thick "slab ocean" and atmospheric CO2 concentrations prescribed at 680 ppm--twice the level used in the present-day simulation (Giorgi, Shields-Brodeur, and Bates, 1994). The model simulated 20 years before achieving an equilibrium climate. A 3 1/ 2-year RegCM simulation was conducted using the last 3 1/ 2 years of the GENESIS run as boundary conditions. The coarse-scale fields of GENESIS are interpolated to the finer scale boundary of the RegCM to provide forcing or inputs for the regional simulations. Giorgi, Shields-Brodeur, and Bates (1994) provide complete details and analyses of these simulations. Figure 3 shows a comparison of the simulated January and July temperatures and precipitation for the present-day climate (control simulation) (figs. 3A-3D) and 2 X CO2 climate (figs. 3E-3H). Figures 3I-3L show the climatic differences (anomalies) between these two simulations. The anomaly maps for January and July temperatures (figs. 3I and 3J) indicate general and substantial warming of the region for the 2 X CO2 simulation. January temperatures are as much as 5oC warmer on the northern Great Plains and 3oC or more warmer in parts of Oregon and Idaho, whereas the southwestern deserts warm by only 1oC to 2oC. The spatial pattern for July is quite different--British Columbia and the Sonoran Desert warm by 4o-5oC, whereas the northern Great Plains and the Oregon/Idaho region warm by 3oC or less.
The anomaly maps for precipitation (figs. 3K and 3L) indicate that, for the 2 X CO2 simulation, winter precipitation is greater than in the modeled present-day climate (control simulation) over the Pacific Northwest and along the Pacific Coast to southern California. Scattered areas of increased winter precipitation also are located over portions of the Great Basin, northern Mexico, and parts of the Great Plains. January precipitation values that are less than those modeled for the present-day climate are located over the Sonoran Desert, the Four Corners region, eastern Oregon, and most of Utah, Wyoming, Montana, Alberta, and the northern Great Plains in the United States. July precipitation for the 2 X CO2 climate is generally greater than that modeled for the present-day climate in the northern half of the region and is less than that simulated for the present-day climate in the southern half of the region. In the 2 X CO2 simulation, arid summer conditions are simulated for the Southwestern United States, California, and most of the Great Basin, as well as for portions of the northern Great Plains of the United States.
These simulations demonstrate the potential complexity of climate change in the Western United States. The amount of warming and the changes in precipitation in the 2 X CO2 simulation (figs. 3I-3L) vary greatly with geography and with season, with most regions of the Western United States receiving a mixture of winter and summer precipitation that is quite different from that of the present-day simulation.
| Figure 3. RegCM simulation of January and July temperature and precipitation for the present-day climate (A-D), for the 2 X CO2 climate (E-H), and the difference (anomalies) between the 2 X CO2 climate and the present-day climate (I-L). The RegCM present-day climate simulation (A-D) used the GENESIS simulation of present-day climate (see figs. 2A-2D) as lateral boundary conditions or inputs, whereas the RegCM 2 X CO2 climate simulation (E-H) used the GENESIS 2 X CO2 simulation for lateral boundary conditions. The climate anomalies (RegCM of 2 X CO2 climate minus RegCM of present-day climate) are shown in I-L. The temperature anomalies show greater warming in winter than in summer, with the maximum warming in winter in the interior of the continent (I). Precipitation anomalies reveal increases of precipitation in the 2 X CO2 simulation relative to the present-day climate along the West Coast in January (K), and in a broad region extending from the Pacific Northwest to the Great Plains in July (L), with generally drier conditions simulated elsewhere in the interior. A hint of a stronger summer monsoon in the 2 X CO2 climate is also apparent. (Click here for scalable 181K PDF version) |
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