Saturday, September 8, 2018

Temperature Trends

In last week's post I mentioned that the abundant cloud cover over Fairbanks in August produced much lower than normal diurnal temperature variations; the average day/night temperature difference was almost the smallest on record.  Below is a chart (click to enlarge) showing the average diurnal temperature range in August, beginning in 1930 for the university's Experiment Farm and 1952 for the official Fairbanks climate record (when observations moved to the airport).  The downward trend is highly statistically significant at both locations; late summer now tends to see less difference between daily high and low temperatures than in the past.


It's of interest to see how the trends in daily high and low temperatures contribute to the diurnal range trend, and to see how this varies through the year.  Accordingly, the chart below shows smoothed daily values of the 1930-2017 linear trends in each quantity at the Experiment Station.  The relative positions of the blue and red lines show that daily low temperatures have warmed more than daily high temperatures at all times of the year, and therefore the diurnal range has been reduced throughout the year.  Note that both high and low temperatures have cooled slightly in mid-autumn, but the diurnal range has still been reduced.


The largest change in diurnal range has occurred in mid-summer.  Daily low temperatures have risen by more than +0.7 °F/decade from mid-May through the end of July, but high temperatures have not changed significantly in high summer, and so the diurnal range trend peaks at a rather remarkable -0.85 °F/decade (8.5 °F/century!) around July 1st.  It's clear therefore that the reduction in diurnal range in summer is attributable to nighttime warming rather than daytime cooling; as a simple example, the Experiment Farm has seen only four days with a freeze in June in the past 30 years, but a summer freeze was not uncommon in the earlier years (31 freezes in June and July of the 1930s and 1940s).

Using Fairbanks airport data since 1952, there are some differences as we would expect, but some of the same signals are evident: the winter peak in overall warming, the autumn lack of warming, and the summer trend (albeit less pronounced) towards smaller diurnal range.


If we re-do the calculations for Experiment Station over the shorter time period, there is a lot more similarity between the results for the two sites.


Finally, a chart of the June-July diurnal temperature range illustrates the changes at the two sites in the high summer months.  I'm not sure what was going on at the airport in the 1990s, and one does wonder if the transition to ASOS instruments in 1998 could have affected the numbers.  But the long history of consistent data from the Experiment Farm tells a clear story of remarkable change in this aspect of Fairbanks climate.


10 comments:

  1. Quite a trend, Richard. What comes to my mind when you showed how the minimum temperatures are mostly responsible is development/urbanization. I suspect that is a least part of what is going on. Notice how the trend in the first & last graphs seems to steepen sometime in the 70s or around 1980? That was a time of rapid growth in Fairbanks. And the change in instrumentation you mentioned bear looking at too.

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    1. Population growth could be part of the issue, but I'd expect that effect to be small in August given cloud cover. Changes in thermometer location at FAI seem more likely. Some of the change at the Experiment Station is almost surely due to changes in the nominal time of the daily coop obs.

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    2. Thanks for the comment, Jim. I agree that urbanization is involved, but without a pristine reference history it's hard to say how great that contribution might be. However, it would be worth looking again at some other sites with reasonably long histories. Back in 2015 I found some results that were a bit surprising...

      https://ak-wx.blogspot.com/2015/04/growing-degree-days.html

      Rick - it looks like the time of obs at Experiment Station changed from approximately 5pm to 8am in summer 1989, so this would produce an artificial cooling trend in the daily low temps, correct? We are now double-counting the cooler nights in the daily low temps. So the warming trend in daily low temps would actually be even greater if not for this effect. Of course the trend in daily high temps is also affected, so I'm not sure what the net effect would be on reported diurnal range.

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    3. In August it's moving into the time of year when lows regularly get double-counted, especially the last ten days of the month. 5pm obs time would be solidly double counting high temps, I'd think more so the the 8am obs does for lows in August.

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  2. Since seasonal thaw of the active layer in permafrost is responsive to air temperature, it would be interesting to pair some of these air temperature graphs with soil profile temperature data. I'm curious if these warmer low temperatures are having much of an impact on depth to permafrost.

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    1. Good suggestion, Tracy. I imagine it would be possible to get some idea of the long-term impacts by looking at interannual variability between warm and cool summers. If you have any good soil temperature data sources, let me know!

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  3. The Natural Resources Conservation Service (NRCS) manages the Soil Climate and Analysis Network (SCAN) within Alaska. I've never pursued downloading data myself, but here is an information flier about the program. https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcseprd775006.pdf.

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    1. Thanks! It seems there is data from a few recent years, but it looks like only one site (Kanuti Lake) is on permafrost and with only a couple of years of data. We may need a longer history to draw any robust conclusions. I'll look around!

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