Orographic Precipitation Studies
of the
Cascade Mountains of Washington

Modeling Studies, 2010

December 1-31, 2001
Mild wet winter month

This simulation was run for the entire month of December. The same set of climatological station as for October were used for comparison purposes with the exception of Startup 1 E for which the observational data were missing. This meant that there were six stations on the west slope and seven stations on the east slope. These results are summarized in Table 7.

Table 7. Comparison of Model with Observations for 2 km grid.
Location    Mean Obs Temperature ° C Obs Precip. cm.    Mean Model Temperature ° C Model Precip. cm.    Model Temperature Error Model Precip. Error
West Slope    2.7 33.7    2.1 32.7    -0.6 -3%
East Slope    -1.5 13.6    0.1 16.4    +1.6 +21%

The results of the 6 km grid were also compared with climatological data. In this case it was possible to use 70 climatological stations for the comparison. These were divided into two groups, 34 stations west of the divide and 36 stations east of the divide. From each of these two groups two more groups were created. From the western stations there were 13 that comprised the Puget Sound lowlands group and 17 that comprised the west slope group. From the eastern stations 14 were selected for the Central Basin group and 11 for the east slope group.

Table 8. Comparison of Model with Observations for 6 km grid.
Location    Mean Obs Temperature ° C Obs Precip. cm.    Mean Model Temperature ° C Model Precip. cm.    Model Temperature Error Model Precip. Error
West of the Divide    3.1 28.0    2.7 29.7    -0.4 +6.0%
East of the Divide    -0.1 7.2    0.7 10.5    +0.8 +46.0%
Puget Sound Lowlands    4.7 18.1    4.7 17.7    0.0 -2.1%
Central Basin    1.0 3.2    1.9 6.6    +0.8 +107.3%
West Slopes    2.2 36.9    1.5 40.8    -0.7 +10.6%
East Slopes    -1.8 15.1    -0.7 19.5    +1.1 +29.6%

For the month as a whole the model does an excellent job in estimating both temperature and precipitation west of the Cascade divide, but east of the divide the it is too warm and too wet. This is especially true in the Central Basin east of the Cascades.

Unlike the climatological data which shows a correlation between greater cross-mountain temperature difference and greater relative amounts of precipitation east of the divide, MM-5 here underestimates the temperature difference but overestimates the precipitation east of the divide. Clearly other factors are at work beyond the effects of cold-air pooling. Moisture depth and free air stability are likely very important in terms of how much precipitation carries across the divide. The smoothing of the topography in the case of the Cascades fills in the narrow valleys and reduces the ability of cold air pools to penetrate into the eastern valleys. It also lowers the ridge tops and so may uderestimate the blockage of air flow by the range as a whole.

One disconcerting feature of the model is its tendency to dramatically overestimate pecipitation in the Central Basin region. It does a much better job of estimating the precipitation at the eastern edge of the Cascades at locations such as Methow, Chelan, Wenatchee, Ellensburg and Yakima than it does for locations further east such as Waterville, Coulee Dam, Ephrata, Odessa, Hatton and Ice Harbor Dam which are overestimated by two to four times.

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