Tuesday, March 4, 2014

Higher Resolution Howard Pass Modeling

One of the highlights of last month's weather in Alaska was the reported extreme wind chill from the Howard Pass RAWS in the western Brooks Range; the event was discussed at some length on this blog.  I presented a cursory look at two relatively high-resolution model forecasts of the event, which showed very low wind chill but did not reflect wind speeds as high as reported from the RAWS anemometer (up to 91 mph sustained).  We speculated that if the extreme wind speeds really occurred, then higher resolution modeling might be required to capture the local flow processes.  Prompted by this notion, I recently ran the WRF model over a limited domain at higher resolution

To obtain a high resolution simulation without requiring prohibitive computing resources, I used a traditional nesting technique to embed progressively higher resolution domains within larger coarse-resolution domains.  The figure below shows the three smallest simulation domains at 9 km (white), 3 km (blue), and 1 km (red) grid spacing; an outer domain at 27 km grid spacing was also used, with initial and boundary conditions from the GFS model.  The location of Howard Pass is indicated with a white dot.  I performed the simulation on an Amazon cloud virtual machine, which is available at a relatively low hourly cost.


The model topography over the inner domain, with 1 km (horizontal) grid spacing, is shown below, along with the locations of 4 RAWS installations.  The model was initialized at 18 UTC (9am AKST) on February 14, and run for 30 hours.


The charts below show the hourly evolution of temperature and wind speed at the location of the Howard Pass RAWS, for the 1km simulation (red line) as well as the 5km WRF forecasts that were examined earlier.  The black lines indicate the reported conditions from the RAWS.  Clearly, the high resolution simulation showed wind speeds only slightly higher than the lower resolution runs in the latter stages, and still far below the RAWS reports.  The 1km WRF forecast temperatures were also considerably higher than the reported temperatures.




Looking at the other three RAWS sites within the inner domain, the 1km wind forecasts generally showed less discrepancy from the observations, although the forecast wind speeds were also much too low at the Noatak RAWS for most of the time.  It is interesting, however, to see that the 1km model produced a jump in wind speeds to observed levels for a few hours at the Noatak RAWS; the 5km WRF runs were unable to reproduce the higher wind speeds.






The modeled spatial distribution of wind speeds in the vicinity of Howard Pass is shown in the maps below at intervals of six hours; the RAWS location is indicated with a white dot.  Note that the wind vectors are not shown for all the grid points, as the grid is much finer than the spacing of the arrows suggests.  For scale, the plot area covers roughly 50x50 km.

It's immediately apparent that the model produced considerable variability in wind speeds within 10 or 20 km of Howard Pass, as we would expect in an environment of complex terrain.  According to these results, the highest wind speeds were located just downwind of the highest terrain but did not extend to the Howard Pass RAWS location; and the highest speeds were still well short of the RAWS observations.





For comparison, the wind speed in the lower resolution simulations at 30 hours is shown below for a region of approximately 200x200 km.  It seems that the area-average wind speed near Howard Pass was only slightly higher in the 1km simulation, but the higher resolution allowed more spatial variability to develop.



In view of these new results, is it less likely now that the reported extremes from Howard Pass represented reality?  Well, perhaps; but I would suggest that the 1km simulation may still be inadequate to capture what actually happened.  The major reason for this is that a 1km simulation is still too coarse to explicitly simulate the "boundary layer", which is the turbulent layer of air next to the ground (airflow higher aloft is usually laminar, not turbulent).  Numerical models like WRF use so-called parameterization schemes to calculate and represent the effects of sub-grid-scale transfers of heat and momentum in the boundary layer.  It probably goes without saying that the details of the boundary layer scheme will make an enormous difference for the model's predictions of wind speed near the ground - and I think it's possible that the WRF boundary layer schemes are ill-suited to capturing the kind of flow that was occurring near Howard Pass.  [Note that I re-ran the simulation through 12 hours with an alternative boundary layer scheme and obtained very similar results.]

To illustrate the flow environment over Howard Pass, the chart below shows the vertical profile of temperature and wind speed at 12 hours into the 1km simulation.  A strong inversion was present as very cold surface-level air flowed up and over the pass from the North Slope, and wind speeds were highest just above the surface.



Unfortunately a mistake in the model setup meant that I didn't obtain the fine-resolution vertical profile data for later times in the model run, but we know that the winds above the surface strengthened dramatically in the next 18 hours.  The map below shows the wind speed at 825 mb, which was about 750 m above the level of Howard Pass RAWS and near the level of maximum wind speed.  Note that the 825mb pressure surface intersects the ground in the lower right, hence the lack of data.  The predicted 825mb wind speed was 80-90 mph or higher over a wide area above and west of Howard Pass; this is supported by similar plots for 850mb from the 5km WRF runs (see maps below).




According to the model, then, very high wind speeds occurred not very far above the surface on the lee side of the high terrain, but the model does not show the high momentum reaching the surface.  Presumably this is because the strong low-level temperature inversion caused the boundary layer scheme to produce relatively little vertical mixing of momentum; but it's an open question as to whether this is realistic or not.

To summarize, 1 km modeling of the Howard Pass event fails to reproduce the extreme conditions reported by the RAWS.  However, the discrepancy between the model and the observations still doesn't necessarily invalidate the RAWS data; in the words of a famous British comedy film, "it's only a model".  The obvious way to test whether the boundary layer scheme is artificially damping the surface wind speeds would be to re-run the model with still higher resolution (close to 100m) so that the boundary layer scheme can be dispensed with; this type of simulation is called Large Eddy Simulation.  Unfortunately, however, this would probably require some dedicated research funding to obtain the necessary computing resources.

14 comments:

  1. This is certainly an important event, regardless of the actual values of reported temperatures and wind. Thank you Richard for the analyses.

    For those that may not have kept or can't readily access the reported values of potential interest:

    At 14/2/2014 3:39 pm AKST, Temp -42 f, Wind Chill -97 f, North 71G78 mph
    At 15/2/2014 10:39 am AKST, Temp -37 f, Wind Chill -93 f, North 91G103 mph
    At 16/2/2014 7:39 am AKST, Temp -40 f, Wind Chill -76 f, North 23G96 mph

    The anemometer failed to report after the last time interval.

    The reported winds are very strong, even for a pass surrounded by not particularly high rising terrain. Few main-continent Alaskan remote stations typically exceed 100 mph. The Aleutian Island chain would be an exception for example.

    In my experience flying in winds 50+ mph >2500 ft AGL , the closer to the ground one goes, the lower the wind speed and the greater the mechanical turbulence (the boundary layer versus laminar flow noted).

    The Howard Pass RAWS sits on a "hill" above surrounding terrain I believe from viewing that location via Google Earth 3-D. It's possible that terrain affects the velocity of air flowing over it, similar to the effect of an aircraft wing that increases the local relative velocity of air flowing over the wing (due to Bernoulli's principle), compared with air flowing below or along side of the structure's curvature (flatter ground in the case of Howard Pass?).

    I have no idea if nearby terrain or rock outcrops can affect the reported wind in certain directions at that location.

    Gary

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    1. Gary, Thanks for the remarks. I think there's no doubt that air flowing through terrain constrictions can speed up in the manner you describe. It would be fun to have the modeling capacity to create a more reliable depiction of the processes at work.

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    2. I've wondered since the event's report what differential conditions could have caused such extreme winds?

      No doubt Umiat to the NE was reporting the cold and some wind. There were likely pressure and temperature gradients through constrained terrain known for wind, and perhaps the winds aloft were brisk. The POES IR shot I offered earlier visibly portrayed winds in that location.

      Maybe (?) those factors were taken into account above. Perhaps describing what elements went into the analysis would be beneficial and offer an opportunity for learning.

      Gary

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  2. Wow, that is quite a comprehensive evaluation Richard. Thank you for going to the trouble of doing this. Based on your analysis, I am wondering if there might have been something similar to a "Taku" wind event that they get several times per winter in Juneau. In that instance, an inversion layer at, or just above, mountain top levels acts as a lid that prevents upward motion of air and if the large-scale synoptics are just right, a large pressure-gradient force squeezes the wind through the mountain gaps. Based on your model runs, that type of setup map have existed.

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    1. Yes the inversion at Howard Pass may have had an effect, but Juneau is in a very different location, topography wise.

      The nearby glacial ice fields, temperature and pressure contrasts, and steep gradient outflows contribute to the Taku events. Adjacent terrain focuses air flow through the Gastineau Channel next to town.

      I lived there for almost a year and walked from Douglas to downtown Juneau to work. I was a bitter environment in winter.

      Gary

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    2. Brian, it seems the same ingredients were present, so the dynamics may have been the same. There was certainly a strong inversion near ridgeline and strong cross-barrier flow. It also appears that the flow was parallel to the ridge at about 500-600 mb, which is the third ingredient required for a Taku-type event.

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    3. Where haven't you lived, visited, or flown through Gary? As always, it is a great to have your in-person perspective that is crucial to bridging the gap between data-only analysis and place-analysis. There are other types of downslope wind mechanisms that might also be in play othar than a Taku-type event. Thank you for giving this so much thought guys.

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    4. Somehow I missed Bethel and points southwest, and the Alaska Peninsula south of Becharof Lake. But that's not for a lack of trying in bad weather at the time. They're no longer on my list thanks to Google Earth.

      I think Brian you, Richard, and Eric (plus the NPS folks that work the Noatak and placed the monitors, and Rick and others at the NWS who support the project) have seen something special going on here. I'm suggesting there were unique conditions that might have created the extreme short lived weather phenomenon in Howard Pass.

      Surely the UAF Supercomputer or some other entity could be tasked to explore the factors. We have winds in Alaska that affect life. Learning why and where they develop would be worthy of investigation.

      It's a project in need of funding as a predictor of future events.

      Gary

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    5. I wonder if there's a way to present a visual cross section of the valley terrain through Howard Pass? As I recall it does climb in elevation from the Colville River then descends into the Noatak River.

      That feature, the terrestrial arch, has to have had a contributing effect along with the channeling effect of surrounding terrain. The Google Sat images of the lakes depict good wave action from NE winds.

      From an earlier post fly this through the Pass and visualize the terrain: http://gmap3d.com/?d=1403570&s=AK&f=gap

      Gary

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    6. Gary, I like the idea of working with UAF to obtain the necessary computing resources. I'll see if I can make a connection there.

      Concerning your earlier question about the factors causing the extreme winds: the computer model solves the equations of motion and thermodynamics for the atmosphere, so it will be able to tell us in as much detail as required what the physical causes were. But first we have to obtain a simulation that shows conditions comparable to those that were reported.

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  3. What kind of resources would you need to do the high res sim? There are many cheap, if not free, distributed computing options available (e.g. boinc). The only thing you would need is time, a way to distribute the proper code and any needed license. If the model can be parallelized on a supercomputer it might be "parallelized" on a distributed system.

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    1. Eric, this is a good idea in principle - but to run the model in a distributed fashion requires intensive communication between the nodes, so it wouldn't be practical for nodes in different locations. The problem is that each tile needs to communicate its boundary conditions with the neighboring tiles at every time step...

      A super-high-resolution simulation could probably be done in a few days on the amazon cloud using one of the higher performance options, e.g. 16 CPU cores, 30 GB of memory. Not a supercomputer, but not trivially inexpensive to use.

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  4. Gary referenced satellite imagery and I am wondering if the imagery might be useful in identifying turbulent waves at or above mountain top level. It might strengthen the argument regarding downsloping, channeling, or gravity type waves.

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    1. The IR shot of that Brooks Range area I referenced in an earlier topic has gone cold. Find that or similar and it may show what you're discussing.

      "http://pafg.arh.noaa.gov/arhdata/sat/hrpt/14046014547/4f1f.jpg

      0145 2/15 POES IR pass shows some wind and temp action NE of Feniak Lake in the vicinity of Howard Pass. Cold in the Colville River Basin, and relatively warmer in the Noatak from my understanding."

      Gary

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