Sunday, December 29, 2013

Solar Heating Minimum

** Updated on 12/30 **

As Richard noted in a recent post, there is little to no solar energy for much of Alaska this time of year. In fact, there is no functional solar heating anywhere north of Fairbanks for the next several weeks. In looking at hourly data for Bettles from NCDC, I looked at the average hourly temperature for every day of the year to see when solar energy stopped influencing daily temperatures (n=45 years). Bettles was chosen due to its location far away from any alternate heat source (e.g., an ocean).

Though trial and error, it appears that when the sun is 3° above the horizon or less, there is no perceptible diurnal temperature difference. At 3° above the horizon, the theoretical solar energy is 9 watts/square meter. At the latitude of Bettles, that translates to a 55-day period of no solar heating. The next two charts show the annual solar elevation for Bettles and for Barrow.



The chart immediately below shows the average hourly temperature for the 55-days at Bettles where the sun is less than 3° above the horizon. Interestingly, there is a very slight bump at the equivalent of solar noon – even on days when there is no sunrise at all!


The final graphic (map) shows the number of days with no effective solar heating; i.e., to maximum solar elevation is 3° or less. The 835 GHCN stations are shown as green dots.


** Updated Section **

Here are some charts showing the hourly temperatures and sea level pressure readings for the days with no sunrise and no sunset at Bettles (Dec. 12 to Dec. 31 & June 12 to July 2 respectively). Note: Due to Leap Years and the procession of the equinoxes, the dates will be slightly different from year to year.







The final (?) chart shows the hourly temperatures for Barrow between December 1st and January 27th. December 1st was chosen to account for the variability in sea ice pack closure around Barrow. January 27th was chosen due to that being the date that the sun is still below the horizon. The local effect at Bettles disappears. When only those days when the sun was within 2 degrees of the horizon were chosen, the line was still effectively flat. Interestingly, the point at which a daily insolation pattern was established in the Spring wasn't until the sun was 5° above the horizon (~Feb. 14th).

Also, the 3-hour pattern that Eric noted is highly exaggerated for Barrow. It turns out that this is due to some days recording temperatures in 3-hour increments instead of hourly. Those occurrences are much more frequent in the earlier part of the climate record when the temperatures at Barrow were significantly colder. It will take more work to tease out those obs.



6 comments:

  1. Very nice, Brian. It's great to visualize the length of this deep winter period. As far as Bettles is concerned, I wonder if there is a diurnal pressure cycle even in deep winter that is sufficient to disturb the inversion slightly around noon, on average. It would be interesting to see if the hourly wind and/or pressure climatology shows anything; but don't go to any trouble to work on it.

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  2. Are the 3 hour bumps on the avg daily temp filtering artifacts?

    I've often seen this non-apparently-solar midday heating. Maybe they are from planetary waves propagating north from solar heated regions?

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    1. Eric, thank you for the comment. I updated the post with additional charts at the bottom showing the temperate and SLP for the time periods of no sunrise and no sunset. As for the 3-hour bumps, I had noticed them too but as near as I can tell there it is just an interesting coincidence. Those bumps disappear when looking at the new charts I uploaded. Both you and Richard may be on to something as to an explanation regarding the slight temperature bump near solar noon. There does appear to be an air pressure minimum in late morning that might advect slightly warmer temperatures from southerly latitudes or the warmer west coast.

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    2. About 30 years ago we flew roundtrip for a couple winters from Fairbanks to the North Slope via Bettles (fuel stop). It was at least a monthly event for a day, and involved locating and tracking transmitter-implanted fish in the Colville and Sagavanirktok drainages.

      In December, even though there was minimal insolation at ground level, the surrounding elevations were exposed. The Brooks Range was aglow at mid-day. Temps at altitude were typically 15-30 deg F or more above the valley floors, which often reached -50F or colder.

      I can see a scenario where by there was heating and air circulation AGL that may have briefly influenced the daily temps in Bettles. It lies in a valley surrounded by several drainages, and any mixing or warming above could be reflected at the surface.

      I'd look for a trend in mid-day wind direction, and any available info on soundings if they are available.

      Gary

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    3. Eric, I added one more chart using Barrow data. The local effect seen in Bettles is not apparent at Barrow. Perhaps the more consistent wind in Barrow mutes any local effects like the one that Gary mentioned for Bettles. Also, the 3-hour temp thing you noticed for Bettles is more prominent at Barrow. It is a result of temperatures measured in 3-hour increments instead of 1-hour increments.

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