When snow cover is absent or merely a trace in Fairbanks (blue line), the temperature tends to be increasingly above normal as October proceeds; this reflects the increasing departure from normal that the lack of snow represents. Of course, this is a chicken and egg problem: do the warm temperatures cause lack of snow, or vice versa? It's both, but as the season advances, a lack of snow becomes an increasingly unusual influence that prevents valley-level temperatures from dropping as they normally do. Also, by late October even well above normal temperatures are not normally a hindrance to snow accumulation (this year being an extreme outlier with such abnormal warmth).
It's also interesting to see that prior to about 18 October, a thin snow cover (1-3") is associated with below-normal temperatures, but after that date it is more often observed with unusual warmth. This is simply because thin snow cover is greater than is usually observed at the earlier dates, but becomes less than usual later. The same is true for 4-6 inch snow depth; but snow more than 6 inches deep is associated with colder than normal temperatures throughout the autumn and early winter, and especially in October.
I love the analysis on this blog. I wish I had found this so much sooner.
ReplyDeleteA few questions about the above post. 1. The zero snow line ends around today. I assume this is because there is always snow by now. What does the line look like without 2013?
2. What is the interannual varibility of the different lines (scatter plot)?
3. Because the rate of increase of temp in the beginning is the same for all lines - wouldn't this imply that temp changes are not influenced by snow as much as the sun and initial temp at the beginning of the season?
Thanks for the blog again. Where are you getting the climate data?
Eric,
DeleteThanks for the comments - glad you like the material. Here is a brief reply, I may have time to add more graphics later.
1. I required at least 10 years in a snow category to calculate a median temperature anomaly for each date. Zero snow is not unprecedented in early November, but is rare. The results are very similar without 2013 (the median is a fairly robust measure of midpoint).
2. Interannual variability is large and increases as you move deeper into winter. For example, for no snow on Oct 31, the 25th percentile of temperature anomaly is +5.6 F and the 75th percentile is +16.3 F; the median as shown is +10.9 F.
3. It's interesting that the slope of the lines is similar at first, but I'm not sure I understand the point you are making. I'll have to ponder this some more.
Climate data are from the GHCN dataset,
http://www1.ncdc.noaa.gov/pub/data/ghcn/daily/
http://www1.ncdc.noaa.gov/pub/data/ghcn/daily/readme.txt
Yes this is interesting. Again a wonderful product from a skilled analyst.
ReplyDeleteI suspect that without an insulating cover of snow, the ground, structures, various water bodies, and flora are still transferring heat to the air above. Insulation eventually ends that process.
This strange year in Fairbanks we've experienced external heating from an influx relatively warm air, and consequently a lack of freezing precip in the form of snow.
Until today. Maybe. Zombies hate snow on Halloween.
Gary
http://co.fairbanks.ak.us/airquality/CRCurrentPhoto.jpg
DeleteNot sure how well this will link, but as forecast snow arrives in the Fairbanks area at noon.
Gary
So you did get your snow after all, Gary. (The barren ground was getting disconcerting.)
ReplyDeleteRichard, about the lines being the same slope: If snow had a large cooling effect then we would see a noticable decrease in slope for snow days vs barren days - despite the interannual varibility. Instead we see the same rate of warmth. This tells me that snow cover is not as important as the influx of different air masses and their temperatures. The barren line is so much warmer because of the initial conditions that created the warm and rainy weather. When we get into winter the same weather patterns produce the instability/stability that we see every winter - week long inversions and chinooks. The colder the more instability. The colder the more snow on average. This is how I read the graph.
Thanks for the analysis. I've been thinking about creating my own database for weather analysis but haven't bothered to start. Perhaps I will anyway :)
Eric,
DeleteI think there might be a way to sort this out using the 850 mb temperatures - if the snow vs temp correlation is mainly caused by the weather regime, then a similar analysis for 850 mb temps should give similar results. But if the correlation is mainly caused by the (low-level) cooling effect of snow, then the results would look very different at 850 mb. I'll dig this up and see what happens.