The chart below shows the number of days in each winter on which sustained wind speeds of at least 5 knots (blue) and 10 knots (red) were observed between November 1 and February 8. The discontinuity in 1996 is definitely related to the introduction of ASOS instrumentation, so the numbers are not directly comparable between the pre-1996 and post-1996 years; but the longer-term history is interesting nevertheless.
Remarkably, this winter since November 1 has seen only 3 days with a wind speed report of at least 10 knots, and only 17 days of 5-knot winds or greater; both of these are record lows. Put another way, the wind speed has remained below 5 knots for every hourly observation during 83 of 100 days since November 1. The peak sustained wind speed since November 1 is only 11 knots (on January 1), which is also a record low. Therefore based on the data in the hourly observations, it would seem that no winter since 1950 was so lacking in wind as this winter (through February 8). (We should bear in mind, though, that we can't be sure about the comparison to pre-1996 years. The late 1960's appear to have been relatively windless.)
The next question is - obviously - why has this happened? It's surprising, because we normally associate calm conditions with unusual cold in winter - but of course it was extremely warm until late January, and the inversion strength was generally a bit weaker than normal. The mean pressure has been a little higher than normal (see below) over much of the state but does not suggest a major reduction in the normal north-south pressure gradient. I quickly looked at the 850 mb wind speed and it has been slightly higher than normal since November 1, so we can't blame the overall flow pattern. Thus we have a mystery that is worth investigating more carefully in a future post.
Figure. Eielson AFB wind speed between November 1st and February 10th.
Obviously there are a few outlier years. The winder of 2010-2011 has no days with calm winds and a lot of Missing ("M") observations. Most likely, those are calm wind observations. The other years 1996-1997 to 2003-2004 are a little more uncertain. 1997 was when the ASOS was installed so that explains part of it. If we ignore the anomalously high years, 2014-2015 appears unremarkable.
Good to read the analysis Richard...winter fun speculating as to why the lack of wind in the greater Fairbanks-North Pole (?) area.
ReplyDeleteIt certainly has been blowing in the hills at some altitude above the airport's ~450' MSL elevation. Also if one cares to look the periodic Mesowest reports of nearby winds in the valley to the SE-S-SW-NW it's apparent the wind has been more active.
And when the E>W Tanana Jet kicks up the wind directions point all around the clock as if the circulation in the city basin is somewhat circular (CW?) or at least confused, a note I shared with Rich years ago.
Given the frost is still here accompanied by air pollution I suspect it'll take the arrival of March with its breeze and increased insolation to break the spell.
Gary
PS: I did see some weak heat wave refraction over darker terrain at the airport today so a warmup via the Sun is slowly coming.
If there is a height dependency on winds, we should be able to check it a couple of ways.
ReplyDeleteI don't have time to do it but one way to check how low those 850 winds go is to drive into the hills either off Chena Pump or Farmers Loop. At some point there should be a marked decrease in snow/ice accumulation in the trees.
Another more remote way is to look at the wind history of all of the co-op stations around the Tanana Valley and see if there is a noticeable elevation dependency. There are a few in the hills at different elevations.
Good ideas Eric...I see a road trip is in order. Take a GPS for elevation info. I have watched Murphy Dome from afar but it's hard to tell hoar frost from normal snow cover. Driving up there would tell the tale.
DeleteAnother aspect is that Fairbanks sits in a shrouded bowl on a northerly bend in the Tanana River with hills in an arc SE to SW. If air behaves like water, and flow from the SE may cause an eddy pattern of ground level air circulation typical of rivers. Downstream of any discontinuity in the river bank and an eddy forms like a whirlpool.
As far as northerly winds...if they don't penetrate the inversion then they must flow on over the top headed for Hawaii. If so, the ground level air just mills around but doesn't leave.
Gary
Eric - I plan to look at some other locations, but most coop stations only report temperature and precip.
DeleteGary - good suggestion re: dependence on wind direction. Perhaps the particular combination of upper-level wind direction and vertical stability has favored the unusual stagnancy of air in the valley.
I believe the key to defining stagnation, in addition to analyzing air flow patterns, would be analyzing the quality of the local air over time:
Deletehttp://co.fairbanks.ak.us/airquality/
I suspect the two (accumulation of visible frost and pollutants) are linked this winter. How they compare in previous winters???
As to why the stagnation...minimal wind at the surface as you noted ...why now? That's worth investigating per your suggestion above.
Gary
Archived air quality data and typical monthly trends:
Deletehttp://www.aqfairbanks.com
http://www.aqfairbanks.com/science/
Some local science from the University's Geophysical Institute:
http://www.gi.alaska.edu/node/740
http://www.gi.alaska.edu/research/atmo
State contracted research. Good analyses of local meteorology in the reports:
https://dec.alaska.gov/air/anpms/comm/fbks_pm2-5_science.html
Gary
Richard, I took the liberty of adding a chart at the end of your post with Eielson wind speeds.
DeleteVery good Brian. Eileson lies more in the swath of the Tanana Jet than Fairbanks. It may be similar when cold prevails, but if the wind blows SE>NW...all bets are off. Blair Lakes wx report is the key regarding the Jet.
DeleteGary
Great job Richard. Insistently, Anchorage has seen a low winter wind speed too but not as dramatically. It is the 3rd lowest on record (Nov. 1 to Feb. 8). The average speed of 4.44 mph is one of 9 winters (1953 to present) with an average speed under 5.0 mph. At 850 mb, the average wind speed on 17.9 mph is only 1 mph below normal and near the middle of the pack. The R-square value between 850 mb wind speed and 10 m wind speed is only 0.21. Also, the R-square between 10 m wind speed and 2 m temperature is only 0.02. Again, these are Anchorage numbers.
ReplyDelete