Saturday, February 18, 2012

The Post-Frontal Crapshoot

Some forecast situations are fairly predictable with the tools we have today, some aren't.  Today's computer forecast models do a fairly good job simulating cold-frontal passages over northern Utah.  Thus, while we have a beautiful day today in the Salt Lake Valley and Wasatch Mountains, we know that change is coming tonight and that the Cottonwoods will see a pretty good burst of snow tonight.  There are uncertainties with regards to the distribution, intensity, and amount of snow, but the basic idea that there will be a snowstorm tonight is fairly predictable.  That wasn't the case 10 years ago when forecast models were nowhere near as good as they are today.

In contrast, the post-frontal environment tomorrow is more of a crapshoot when it comes to snowfall.  There are a number of reasons for this:
  1. Post-frontal precipitation is produced by shallow convective clouds that are small in scale and typically initiated over the Wasatch Mountains or, in some cases, the Great Salt Lake.  
  2. Shallow convective clouds, whether initiated by flow over the Wasatch Mountains or the Great Salt Lake, are very sensitive to small changes in relative humidity and temperature.  Therefore,  small errors in the relative humidity and temperature of the incoming flow can greatly reduce forecast accuracy.  
  3. Current computer models, even those run at 4-km grid spacing (e.g., the WRF model run locally by the National Weather Service), do not adequately resolve the processes responsible for these clouds.  Lower resolution models, like the GFS (~25-km grid spacing) and the operational NAM (12-km grid spacing) not only fail to resolve these clouds, but also fail to adequately resolve the Wasatch Mountains and Great Salt Lake.  
  4. Often lake-surface temperatures (and salinity) are poorly initialized in computer forecast models.   Sometimes this is because the developers of those models don't make the effort to incorporate recent lake-surface temperatures, which can only be obtained via satellite.  Other times, it's simply because there hasn't been a recent cloud-free satellite overpass.   
  5. Small particles known as aerosols, may play an important role in post-frontal precipitation efficiency.  At this time, such aerosol effects are poorly understood, pretty much entirely unobserved, and not considered at all by our computer models.  
In my view, this is an area ripe for immediate research.  In particular, would a dramatic increase in forecast model resolution to what we call cloud-permitting scales (horizontal grid spacings of 250 m or less) lead to forecast improvements?  Would computer model ensembles at such grid spacings lead to significant improvements in forecast skill?  Or, are uncertainties in terms of the land surface (e.g., the Great Salt Lake temperature), cloud microphysics (including aerosols), and the incoming large-scale flow too much to overcome?  

These are questions in need of answers, especially for those of us trying to figure out where to ski tour tomorrow.

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