Sherlock Holmes made use of a 7% solution of a certain drug to prepare for such cases. I will make use of more appropriate drug to heighten my mental prowess, one used by many scientists in our region: a tall Starbucks coffee.
Science is great fun, particularly for a difficult case like this. Using limited observations and knowledge of basic physical principles, we attempt to explain natural phenomenon. The enjoyment of an intellectual puzzle and a detective story.
And when the pieces come together, and when we gain an understanding of something that no one has understood before, the feeling is one of satisfaction and even elation. A feeling that once experienced, becomes addictive. A reason why many of us love being scientists.
So as Sherlock would say: "the game is on". And I will describe my chain of logic, starting with known facts and then examining each possibility until we determine the most probable cause.
Let's review the facts
1. A large tree fall occurred on the north side of Lake Quinault around 1:30 AM on January 27th. Many of the trees were old-growth, or at least, very large.
2. The tree fall area was quite limited in size: perhaps a half-mile on a side and extending from the lake toward the crest of about 2500 ft.
3. Several of the trees snapped off and this can only be explained by very strong winds (certainly at least 60-70 mph).
4. The trees fell to the south and thus the winds must have been from the north.
5. None of the limited surface observation locations in the area reported any winds even close to those needed to topple the trees. For example, a site just across the Lake only reported light winds during the tree fall.
5. The strong winds could NOT have been the result of microburst associated with a thunderstorm or strong convection. Weather radar showed no such feature and the lightning detection network had no strikes in the region.
6. An occluded front was approaching the coast at the time of big winds and tree fall.
The first question you should ask was whether the approaching weather system had strong northerly (from the north) winds associated with it. Or even northerly winds at all.
We know that the surface winds with system did not have strong northerly winds from the surface weather stations of the region. But if there were northerly winds aloft, there would be the possibility of a downslope windstorm, as northerly winds accelerated down the slope north of the Lake.
Fortunately, there are sufficient observations to answer this question. NOAA Earth Systems Research Lab (ESRL) maintains a device called a radar-wind profiler at Forks, Washington (up the coast a bit) that is capable of determining the wind and temperatures aloft in real-time. Here are the wind observations for 0000 UTC 27 January through 0000 UTC 29 January from the surface to 9 km above the surface. Winds are shown by the typical wind barbs and are color coded . The incident in question occurred at approximately 0930 UTC 27 January. A blow up at the critical time is shown below as well. Note that the front came in from the west and hit Forks before Lake Quinault.
The Bottom Line: No hint of northerly flow during the period in question. There were southeasterly winds at low levels, with increasing southerly and southwesterly winds aloft.
Here is the Langley Hill radial velocities from the lowest scanning angle at 0927 UTC. Green and blue indicate flow towards the radar, yellow/red/orange the opposite. Not easy to read without experience. But to my practiced eye, the radar suggests southeasterly winds of up to around 30 knots at low levels, turning to southerly and then southwesterly aloft. Consistent with the profiler. No northerlies
So if the air coming in off the Pacific was from the southeast east or south, where did the powerful northerlies come from? Perhaps the comments pushing a secret government project, aliens, or a meteorite strike were on to something.
Or perhaps not. There is no evidence of any space object reaching the earth in this region (I checked). And there IS a possible meteorological explanation: a rotor circulation associated with a strong mountain lee wave.
But first some atmospheric rotor 101. If fairly strong winds are approaching a mountain crest, they can undergo wavelike undulations in the lee of the barrier. A situation in which air surges down the mountain and then suddenly rises up, followed by potentially more down and up motions. If the wave has sufficient amplitude, a rotor can form underneath the wave, with flow moving in the opposite direction from the flow approaching the mountains. You see why this is interesting...here is a way to get northerly flow when the general flow is southerly.
Mountain lee waves can increase in amplitude as the winds approaching the mountain strengthen. But they can also amplify if there is a stable layer near the mountain crest, or if there is what is called a critical level above the crest level. A stable layer is one where air temperature does not cool rapidly with height, and a critical level occurs when the wind component perpendicular to the mountain reverses direction. These features help trap and amplify the low-level wave energy, producing stronger waves and stronger rotors.
But it is even better than this. A large rotor can in turn break down into highly intense subrotors that can have strong winds associated with them. Two colleagues of mine, James Doyle of the Navy Research Lab of Monterey and Dale Durran, a fellow faculty member at the UW, did a very nice paper showing the results of an ultra high-resolution simulation of these critters. Here is a vertical cross section across the lee slopes mountain that shows the rotor and subrotors (indicated by the red colors).
Could these conditions have occurred during the early morning hours of January 27th? I think the answer could be yes.
As the offshore front approached, the wind approaching the crest to the south of the lake increased (see topographic map, which indicates the key terrain features and the direction of the flow).
During the period in question, cooler air near the surface (in the southeasterly flow) was surmounted by warmer air above. This results in increasing stability above crest level. And with southeasterlies at the surface and southwesterlies developing aloft, this led to the development of a critical level, where the flow reversed. So all the factors supporting a strong mountain lee wave and potentially a rotor were in place.
But do any observations suggest such a development?
The development of a strong wave would result in substantial sinking along the lee slopes of the terrain feature. Sinking causes warming and pressure falls. We happen to have a weather observation just to the north of the terrain slopes (located on the south side of Lake Quinault, see map). Wow...there was a sharp pressure fall around 1:30 AM, just as the big blowdown occurred (see below). Suggestive.
But we have a tool that Sherlock would be envious of: high resolution numerical simulations. Considering the small scale of the blow down, I suspect we would need to run our model (called WRF) with uber-fine resolution (grid spacing of around 100 meters). The best the National Weather Service models do is around 4-km. Our UW WRF is 1.3 km. But for this case, UW graduate student Robert Conrick took WRF down to 444 meters and fellow student Nick Weber has produced some nice graphics.
So let us see whether we can simulate this event...or at least determine whether we are on the right track. I am going to show you a series of vertical cross sections, oriented SSE-NNW, that pass over the blowdown site. Each cross section will have potential temperature (solid lines), wind vectors in the cross section, wind speed (color shading) and vertical motion (blue for descent and red for ascent).
At 0400 UTC (8PM), you can see wave-like undulations in the temperature, modest downslope on the terrain and some weak northerlies over the blowdown area at low levels.
As 1240 AM (0840 UTC), the flow had strengthened greatly aloft and a rotor was obvious in the lower atmosphere over the valley.
But we have seen enough, I believe. The strong winds were not from UFOs, an angry Sasquatch, a microburst from convection, or some errant meteor.
An approaching front produced just the right conditions to produce a high amplitude mountain wave on the upstream ridge, which resulted in a strong rotor that produced powerful reverse flow (northerlies). As in the research work cited above, a very energetic subrotor was probably produced, and that resulted in a localized area of intense winds as it rotated down to the ground.
Perhaps we will try going down to higher resolution, but I have substantial confidence that the puzzle is solved. If I were Sherlock Holmes, I would take out my violin. But my reward, other than the satisfaction of completing a large puzzle, will be to catch up on the Olympics...or to watch one of my favorite TV shows---Air Disasters--but don't tell anyone.
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Announcement: A very interesting free lecture open to the public
The history of cloud seeding to enhance precipitation, and prospects for the future. Professor Bart Geerts, University of Wyoming
February 15th, Kane Hall, University of Washington Campus, 7:30 PM
For information and to register go here:
from Cliff Mass Weather and Climate Blog http://ift.tt/2o1eYrE
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