The NAM-3km covers the continental US at a grid spacing of 3 km, four times the resolution of the 12-km NAM in which it is nested. With such high resolution, you would think the NAM-3km would be especially useful for precipitation forecasting over the complex terrain of the western US, but it isn't, because it has a major overprediction problem.
Tom Gowan, a graduate student in my research group, recently led a study examining the performance of several forecast models at mountain locations across the western U.S. during last winter. I have been holding off on publicly sharing these results broadly since the paper describing this work is still in review, but the results are too pertinent to forecast needs right now not to share at this juncture. In the case of the NAM-3km, we used pre-operational test runs from last winter that were kindly provided by NCEP.
The plot below shows the ratio of mean-daily precipitation produced by the NAM-3km to that observed at SNOTEL stations. Overprediction is evident at the majority of sites, with on average the NAM-3km producing 1.3 times as much precipitation as observed.
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| Source: Gowan et al., in review. |
A major reason for this is that the NAM-3km produces far more major precipitation events than observed, especially over the interior western US. In the plot below, the frequency bias is the ratio of the number of forecast events to the number of observed events in each event size bin. The NAM-3km has by far the largest overprediction problem. Note that the NCAR ensemble also produces too many large events, although the magnitude of the problem is not as acute.
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| Source: Gowan et al. in review. |
The loop below shows steady, drippy precipitation over the Wasatch and nearby ranges during the overnight period.
Now, it is always dangerous to say a model is wrong before the forecast verifies, but I'm going to say it anyway. This forecast is wrong. There's little evidence to support such huge precipitation totals. Even in the NCAR ensemble, 7 of the 10 members produce less than 0.2" of precipitation, and the wettest goes for about .57".
This issue plagued the NAM Conus Nest when it was run at 4-km grid spacing and it appears to carry over to the higher resolution upgrade.
The bottom line is this. If you want great deep powder skiing, consider using the NAM-3km for your holodeck experience. However, if you live in the real world, avoid using the NAM-3km precipitation forecasts unless you want to be severely disappointed on a regular basis.
The bottom line is this. If you want great deep powder skiing, consider using the NAM-3km for your holodeck experience. However, if you live in the real world, avoid using the NAM-3km precipitation forecasts unless you want to be severely disappointed on a regular basis.
I'm a total Fred here, but any ideas as to why higher resolution overestimates precipitation? Also, 3km still seems really coarse-grained for representing the features of things like mountains.
ReplyDeleteThere are a number of possibilities, but one that could be important is that scheme that is used to simulate precipitation processes is simply not up to the task of dealing with such fine-scale information. These issues are not evident, for example, in the HRRR, which is comparable resolution, but uses a different precipitation scheme.
DeleteJim
Is the overforecast of orographic precipitation in the 3-km NAM compensated by an underforecast in adjacent valley areas? Not knowing much detail about the parameterization of forecast models, I am wondering if the orographic wind fields are somehow unrealistic, or if the problem is mostly with the precip development process. In this case, my impression is that perhaps it is treating areas of shallow orographic clouds near crest height as efficient precip producers when in reality they are not. So, some random thoughts.
ReplyDelete