The 500mb analysis from yesterday afternoon helps reveal the reason for the unusual conditions: see below (click to enlarge). The cold trough aloft is deeply entrenched over the western half of Alaska, and Fairbanks is under a zone of strong temperature gradient aloft with southerly flow; notice the tightly-packed dashed lines over the southern and eastern interior, denoting a tight gradient between very cold air over western Alaska and much warmer air to the east. (The dashed lines indicate 1000-500mb thickness, a good measure of overall temperatures in the lower troposphere.) Cold air is reaching Fairbanks from the base of the trough, but the frontal zone is proving a lifting mechanism to generate clouds and snow.
If and when the frontal zone shifts east and clouds clear out, temperatures will drop sharply in Fairbanks; most of the western interior is cold this morning.
On a related note, there were a few comments the other day about the variability of surface temperatures during cold spells like the current one. It's an interesting question as to whether surface temperatures are more or less variable when upper-air temperatures are low, so I looked at this by calculating the standardized anomaly of daily minimum temperatures for days in November through March since 1981; the chart below shows the relationship to 850mb temperature. (The standardized anomaly is the departure from normal, divided by the climatological standard deviation; both the normal temperature and the standard deviation vary greatly by date.)
The chart shows a hint that surface minimum temperatures tend to be less variable when the upper-air temperatures are low; for example, with very low 850mb temperatures below -30°C, the surface minimum temperatures mostly fall in the range of 0.5-2.5 standard deviations below normal. However, with very warm 850mb temperatures above freezing, daily minimum temperatures can be below normal or as high as 3 SD above normal.
The decreased variability during cold spells is seen in the chart below, which shows the standard deviation of the daily surface temperature anomalies in categories of 850mb temperature. The relationship is more noticeable for daily low temperatures than for high temperatures, which is why I used low temperatures in the scatter plot above. The results certainly confirm reader Gary's comment that "with 850 mb temps forecast to be -30 or below there's not a lot of room for variance in cold at the surface"; and I think this is because troughs of very cold air aloft are not generally conducive to very large temperature inversions near the surface.
Excellent analysis and review Richard. My only source of knowledge regarding cold surface temps under a similarly deep cold column of air was from flying. Unless we get some mixing wind or insolation (not much of that in January) it tends stay cold at the surface and not vary much. The troughs probably help as noted.
ReplyDeleteWe're finally getting some of that "Anthropogenic nucleation of air saturated to ice" stuff known as Ice Fog in Fairbanks. The next few days will be interesting and a first experience for many.
Gary
Richard could you offer an explanation for the current forecast surface High and an upper level Trough that's apparently providing cold temps in Interior Alaska? How do they function independently or combine together in their temperature regimes? It's confusing.
ReplyDeleteGary
Gary, pressure is actually quite low at the surface currently - are you referring to a forecast for high pressure to build in?
DeleteOne concept to bear in mind here is that the rate of decrease of pressure with height is related to average temperature in the layer - cold air is more dense, so pressure decreases more quickly with height; in other words, the 1000-500mb thickness is lower. If the air mass were warmer, then an upper-level trough as strong as the current one would produce much lower pressure at the surface. But nevertheless, there's only so much cold air can do to raise the pressure - there's no way to have high pressure at the surface with such a strong trough aloft.
Here's where I'm uncertain. These are current surface analysis charts for 01/19/16 that I looked at:
Deletehttps://www.weather.gov/afc/surface
http://www.wpc.ncep.noaa.gov/html/sfc2.shtml
I assume for today (maybe better for 1/18/16) that there is/was a trough aloft. It's the combination of the two (forecast surface pressure vs aloft) that's visually confusing.
Gary
Thanks, I see exactly what you mean: there is a weak surface high analyzed over the northern interior even while there is a trough aloft. This appears to be a good example of what cold air can do: the very cold air near the surface is raising the pressure slightly over the land area.
DeleteConsider a back-of-the-envelope calculation: a temperature drop from -20°C to -40°C gives a ~10% density increase, and if this is the average difference in the lowest 2% (20mb, ~150m) of the atmosphere, then the mass of the column would increase by 0.2%, which equates to 2mb. This is about the right order of magnitude for the pressure increase.
Thank you for the explanation...it does make sense. After I wrote above I read your latest posting and saw the 500 mb trough depicted on #4. So in reality the surface High is weak and an artifact (likely) of the cold air above. I've seen this before and now I grasp the connection.
DeleteWe only saw -46F at home but as usual there was and still is a slight NE breeze along College Road in town.
Gary
In case you don't have it, here's the link where I get the upper-air charts:
Deletehttps://www.weather.gc.ca/analysis/index_e.html