Saturday, November 28, 2015

Snowiest Place - CFS Reanalysis

Continuing with the discussion of normal snowfall patterns across interior Alaska, I thought it would be worthwhile to look at the CFS (Climate Forecast System) Reanalysis to see what kind of spatial variability of winter precipitation is represented in the model.  The CFS Reanalysis (CFSR) is NOAA's modern gridded historical analysis of atmospheric and oceanic conditions back to 1979; here's a paper describing the project.

The advantage of this data set for our purpose here is that the model - being run at fairly high resolution (about 38km grid spacing) - should be able to capture some localized features of the winter precipitation patterns across Alaska; it can "fill in the gaps" in the sparse observing network.  However, it's critical to bear in mind that the CFSR precipitation analysis is basically just a very-short-range model forecast and is not tied to observed precipitation, so it will have all the biases and errors that models are normally subject to.

Moving immediately to the results, the figure below shows the 1981-2010 mean November-March precipitation according to the CFSR over most of the state.  I apologize for the lurid color scheme, but it's designed to make it easy to pick out differences and specific values.  At first glance the precipitation patterns look very reasonable; it's good to see the dry zones along the Tanana River valley and immediately northwest of the Denali area.  However, the precipitation amounts are much too high in western Alaska; Bethel and Nome only see about 5" of precipitation in November-March, whereas the model says there is 10" or more.


Zooming in on the interior, and using a different color scale, we see numerous areas of enhanced precipitation in regions of elevated terrain; the black contours indicate the 500m elevation contour.  This fully supports reader Andy's comment that topography produces dramatic local variations in precipitation.  Andy mentioned the snowfall gradient from Takotna to Nikolai, and this is very clearly depicted in the model results; the marker labeled "2" shows the location of McGrath, which lies in the middle of a strong east-west precipitation gradient.  I've also added markers for several other locations, including those mentioned in the previous post.


The CFSR clearly shows the precipitation enhancement in the Nulato Hills to the west of Kaltag, and the model believes precipitation is even higher farther to the south, but as noted in Gary's comment, rain occurs more often as you go farther south.

From the point of view of "snowiest location", perhaps the most intriguing aspect of the CFSR data is the high precipitation amounts in the Kuskokwim mountains, and especially a bullseye of high precipitation at the northeast end of the range, about half way between Tanana and Minchumina.  It's interesting that reader Gary noted a preponderance of poor weather in this area north of Lake Minchumina, and I wonder if the region is particularly favored for upslope generation of snowfall during both westerly and easterly flow regimes.  According to the CFSR data, this might well be one of the snowiest places in interior Alaska, but unfortunately I'm not aware of any ground-level data to verify the claim.

The figure below compares observed 1981-2010 normal November-March precipitation to the CFSR data for 8 different locations.  Disregarding the pronounced wet bias, it's clear that the CFSR does have an ability to at least broadly discern relative differences in precipitation, and this gives a modest degree of confidence that the spatial patterns shown above probably do bear some resemblance to reality.


10 comments:

  1. http://earth3dmap.com/#?l=63.8894,-152.2678,0,0,41,77,15591

    Start at Lake Minchumina and fly the cursor and 3-d view W/NW/NE to gain an appreciation for the terrain and rivers...the North Fork Kuskokwim draining to the south/McGrath area, the Kantishna draining north from Minchumina. In combination they support weather advection from a SW direction over rising terrain and into often colder air.

    To the west the Yukon/Innoko River complex supports NE transport towards Poorman and Ruby, traditionally nasty places for low conditions in winter. In combination the Kuskokwim hills get lots of low weather conditions and snow as confirmed by local trappers, including me (also see Ruby wx obs).

    Depending upon moisture content remaining after upslope drying over the Alaska Range to the SE and any adiabatic warming, the Tanana Jet's SE>NW>SW flow can bring snow from the SE, but in my 32 yrs of winter camping at ~64.4N-150.8W, the snow produced is lower than a that due to a southwesterly vector.

    It snows more in the Kuskokwim hills than in Fairbanks, but high NE winds in winter/spring from the Yukon River, Livengood, and the Tanana Jet can compact and redistribute the base in exposed locations. The area's sand dunes testify to long term Aeolian transport.

    Gary

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  2. Here's a link to NOAA's snow info center:
    http://www.nohrsc.noaa.gov

    Modeled snow output for Alaska on November 30, 2015. Other dates and data can be selected, as can the various models used to derive the inferred data. A geographic area of interest can be boxed with the cursor for amplification:

    http://www.nohrsc.noaa.gov/interactive/html/map.html?ql=station&zoom=&loc=Latitude%2CLongitude%3B+City%2CST%3B+or+Station+ID&var=ssm_depth_gc&dy=2015&dm=11&dd=30&dh=0&snap=1&o9=1&o12=1&o13=1&lbl=m&mode=pan&extents=ak&min_x=-169&min_y=51.749999999995&max_x=-129&max_y=74.249999999995&coord_x=-149&coord_y=62.999999999995&zbox_n=&zbox_s=&zbox_e=&zbox_w=&metric=0&bgvar=dem&shdvar=shading&width=800&height=450&nw=800&nh=450&h_o=0&font=0&js=1&uc=0

    Not sure if it's representative of reality, but most historic data collection programs I've come across for snow depth in Alaska seem to come and go (likely due to personnel and funding), river basin flooding predictions in Spring being a possible exception.

    Gary

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    1. Gary, I did a couple of referencing between current reported snow depth and what the color scale shows for certain stations and it appears fairly accurate. I am going to dig into the historical record, as well.

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    2. Hi Andy. Any further insight into this experimental NOAA program from Richard and you would be worth reading.

      There are a few SNOTEL measurement locations in Alaska (http://www.wcc.nrcs.usda.gov/snow/).

      Not many folks are interested in poking a stick in the snow for science.

      Gary

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    3. Thanks Gary, I had not seen that page before. It's quite a remarkable interface, including a query tool to examine available observed data. Very interesting.

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    4. Yup, that's a work of interface art...they have some talented and motivated people.

      I've just started to explore the page especially the modeled versus observed info accessible by enabling the query function and clicking on the station locator feature. Even has written descriptive popups for the menu.

      Gary

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    5. I've seen their aircraft in Fairbanks (http://www.nohrsc.noaa.gov/snowsurvey/) that are probably used in some form to calibrate other data and also estimate "aeral snow water equivalent value". Likely an aid in forecasting the potential for Spring melt and floods in rivers.

      Gary

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  3. Richard, is there a way for the public to access the graphical portion of the CFSR? I enjoy this kind of stuff immensely.

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    1. Andy, unfortunately I'm not aware of a site to pull up graphical displays of CFSR data. ESRL is good for the older NCEP/NCAR reanalysis, but I don't think they cover CFSR. Part of the reason may be that the resolution is high enough that the total amount of data is difficult to manage; at our company we archive about 2TB of new CFSR data per year. So I made the graphics myself for this post. If you're interested in working with some raw data files, I can give you some pointers.

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    2. I actually e-mailed the CFS folks a few weeks ago looking for archived data. Unfortunately they do not store very much on site and NOMADS does not appear to have much.

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