Saturday, February 15, 2014

Model Depiction of Howard Pass Event

In view of Brian's suggestion of "further investigation" of the Howard Pass wind chill report, I thought it might be worth looking at some of yesterday's numerical model forecasts of conditions in the western Brooks Range.  The idea here is that if a numerical model forecast showed wind chill values anywhere near the reported values, this might boost the credibility of the RAWS data for this event.

There is a multitude of global weather forecasting models that could be examined for this event, but the coarse resolution of global models wouldn't tell us much about the terrain-modified flow conditions near Howard Pass.  Fortunately, the National Centers for Environmental Prediction produce a daily 5-km resolution run of the WRF forecast model in two different configurations for an Alaska domain; more information can be found here:

http://www.emc.ncep.noaa.gov/mmb/mmbpll/nestpage/

The model is initialized at 9 am AKST every day, which is conveniently just a few hours before the purported record wind chill measurement at Howard Pass.  Below are images showing the 6-hour forecasts of 2m temperature and 10m wind from the two alternative configurations of the model (NMM and ARW); these are forecasts for approximately the time of the minimum reported wind chill (-97F at 3:39 pm yesterday).  Note that the black contours show terrain elevation at 600m (thick line), 800m (medium), and 1000m (thin).  The location of Howard Pass is indicated with a white dot.



The forecasts are considerably different, with the NMM version showing much lower temperatures almost everywhere, and especially over and north of the north side of the Brooks Range.  A cursory comparison to observations suggests the NMM was closer to reality at high elevations (Killik Pass, -30F) and on the North Slope (Umiat, -34F), but the ARW was better in the lower terrain to the south and west (Noatak RAWS, -15F).

Let's look at the NMM a bit closer: the charts below show the forecast wind speed and wind chill at 3 pm yesterday.  Note that the wind arrows "begin" at the grid point that they represent, i.e. they point away from the grid point in question.  Also, not all the grid points are plotted as the grid is much finer than the arrows suggest.  The forecast wind speed at Howard Pass was only 25 mph, compared to 60-70 mph sustained according to the RAWS.  Nevertheless, with a forecast temperature of -36F, the forecast wind chill was -73F.  As one would expect, the lowest wind chills are along the ridge line where extremely cold air from the North Slope was being lifted and forced over the ridge and through gaps in the high terrain.




In the NMM forecast, the temperatures proceeded to drop overnight, and wind speeds increased slightly, with the result that the predicted wind chill dropped even lower by 9 am today: the forecast wind chill was below -80F over a large area near and west of Howard Pass.  The lowest forecast wind chill value at the Howard Pass location was -83F at 11 am.





I hope to look at some of the station comparisons in more detail next week, to see if we can gain any more insight as to how well the NMM run performed.  The biggest difference from the Howard Pass RAWS data is obviously the wind speeds, which purportedly reached an almost incredible 91 mph sustained at 11 am today.  It may be that an even higher resolution model run would be required to capture the localized flow effect that would have caused such extreme speeds - if indeed they actually occurred.


16 comments:

  1. The curious aspect to me was the exceptional wind data generated by the HP site versus others in the vicinity. I wonder if that's been typical over a range of time. Howard Pass is windy, but that air has to go somewhere before it markedly slows, and I'd expect nearby sites to reflect that. As for temps, well there are numerous causes for downstream warming.

    The areas south of Delta Junction, north of Healy, NE of Palmer, and Turnagain Arm near Anchorage get localized winds, but the terrain is far more conducive to gap flow, versus the Howard Pass area as I recall. A 3-D Google Map might reflect that.

    Gary

    ReplyDelete
    Replies
    1. For the visually driven:

      Google 3-D of Howard Pass area:
      http://gmap3d.com/?d=1403570&s=AK&f=gap

      Google overhead of the RAWS station:
      http://amazon.nws.noaa.gov/cgi-bin/hads/interactiveDisplays/displayMetaData.pl?table=dcp&nesdis_id=39617266

      Note the station location relative to the "3-headed lake" to the NW of the station's location. Use that to find the 3-D view as required.

      Not a particularly narrow gap in terrain. But note the wind on the lake surfaces. Typical. We lost a friend in an aircraft hunting accident nearby. Turbulence was indicated.

      Gary

      Delete
    2. As usual Gary, your scientific insight and local knowledge of many places in remote Alaska is extremely beneficial to those of us who do not reside in the Interior.

      Delete
    3. It's all fun Brian, at least I hope so. Do you suppose they sited the instrument in that spot for a reason (besides big rocks to hold it down)...like exceptional local weather? That's my guess, but the NPS may know the rest of the story.

      Now when there's a visual image to upload we'll really learn something from these instruments.

      Gary

      Delete
    4. I'd like to add this sobering recollection of the potentially hazardous weather to be found in and around the Howard Pass area of Alaska:

      http://aviation-safety.net/wikibase/wiki.php?id=40608

      Gary

      Delete
    5. Like most of the remote RAWS sites NPS has put out, location is largely determined by the suitability of helicopter landing area. I believe it's swampy down lower.

      Delete
    6. Hi Rick. Yes it's swampy at lower elevations. Those knolls would have been favored by ancient hunters scanning for migrating game. Typically there's a surface scatter of lithics about indicating their prior occupation.

      Here's a better link to the accident I linked above. Note the witness hunter's observation of the weather that prevailed at that time. My experience was similar when flying in that area years ago.

      http://www.ntsb.gov/aviationquery/brief2.aspx?ev_id=20001208X06388&ntsbno=ANC96FA131&akey=1

      Gary

      Delete
    7. I read through the report Gary. Indeed, very sobering.

      Delete
    8. I was told that aircraft was used by the manufacturer in a pre-retail sale test program. The testing typically involves high speed maneuvers and load testing as part of FAA certification.

      As noted in the NTSB report a portion of the wing skin separated from the aircraft and was found some distance away from the crash site. The attaching rivets also pulled through the metal skin. All signs of an over stress condition at some point(s).

      In the report there was the observed high wind event at Howard Pass (and assumed turbulence), which may have led to component failure and loss of aircraft control. Like I said, nasty place at times.

      Nature is neither kind nor cares. Pay attention.

      Gary

      Delete
    9. A couple of documents I found in regard to the siting decisions for these new stations in NW Alaska:

      A report on site evaluation:
      science.nature.nps.gov/im/units/arcn/documents/documents/ARCN-VS24_TerrestrialVegSoilsSiteEval_2009.pdf

      A review of a siting proposal:
      www.wrcc.dri.edu/nps/reports/arcnsitecommentsfinal100312.pdf‎

      One point I noted is that rock outcroppings near the site help disguise the installation from the point of view of caribou.

      Delete
    10. The second link in my previous comment may have been garbled.

      http://www.wrcc.dri.edu/nps/reports/arcnsitecommentsfinal100312.pdf

      Delete
    11. Excellent sourcing Richard, and a good read as well. Their task is clear, and I support the efforts for the gathering of information. I'll leave the potential for noting climate change to the experts that analyze the resulting data.

      Early Man recognized the effectiveness of fences erected from stone as a means to provide cover for hunters plus guide the prey to a suitable site for the kill. They're visible throughout that country and elsewhere. A few large rocks placed on end is all it takes to divert or direct their migration. That probably would be counter to the NPS' goal to minimize the visual impact of any structure, similar to the shortening of the sensor's mast.

      I hope the wind data returns to Howard Pass.

      Gary

      Delete
  2. Thanks for that excellent numerical analysis Richard! Given that the difference between the forecasted minimum WC (-83°F) and the measured minimum WC (-97°F) was 'only' 14°F, it seems that someone would have a hard time arguing that it was superficially erroneous.

    ReplyDelete
  3. Thanks much Richard; this is really helpful.

    One thing we have found in modeling studies in the Alaska Range is that to get the winds basically correct on the north side of Isabel Pass (along the Richardson Hwy and Delta River) required going down to around 1km resolution, so even the 5km high res models by be too coarse.

    ReplyDelete
    Replies
    1. Rick, useful information - thanks. I am tentatively planning to run a WRF simulation at higher resolution to get a closer look. It would be very fun to get the model to reproduce the event with reasonable accuracy.

      Delete
  4. Thanks for Posting ! first time I have found a genuine post related to 3D Interior Modelling


    ReplyDelete