[Updated August 19 with polar plots, per reader request.]
A few readers of my original post about the Barrow urban heat island effect suggested that wind speed and direction are important in determining the relative temperatures at the Barrow ASOS and CRN stations. This is undoubtedly true based on the Hinkel et al study, but I thought I could follow up with an analysis of the 12 years of overlapping data (Hinkel used only a single winter). To do this, I calculated a daily mean wind vector from the surface observations in the twice-daily balloon soundings, then categorized the wind by both speed and direction and finally obtained the mean temperature difference between the two stations for each wind category. (Note that a more accurate daily wind vector could be obtained from the hourly ASOS observations; the balloon soundings generally go up only at 3 am and 3 pm AKST.)
The results are shown below, for daily maximum and minimum temperatures in high summer (top 2 charts) and two months from deep winter (bottom 2 charts). The blue, red, and green lines show the mean temperature differences (ASOS minus CRN) for weak (0-10 kt), moderate (10-20 kt), and strong (20+ kt) winds respectively. I required at least 10 instances in a single category to compute a mean temperature difference, so there are large gaps in the results for higher (less common) wind speeds. The purple columns indicate the frequency distribution of the wind direction categories; note that the categories are overlapping.
Based on this analysis, the most pronounced temperature differences between the Barrow ASOS and CRN stations are found for maximum temperatures in summer when winds are out of the east or east-southeast; in this scenario the trajectory of air reaching Barrow airport passes over a lot more land than the trajectory of air reaching the CRN station, and so the airport enjoys the benefit of more solar heating. The difference is greatly lessened, however, when wind speeds increase, as expected. When winds come from the west, there is little difference in summer temperatures between the two locations, and high temperatures are even a little lower at the airport with a stiff breeze from the west.
The daily minimum temperature differences for winter show a pattern that is nearly opposite to the high temperatures in summer, with temperature differences minimized for east or east-southeast flow, but with the airport staying a few degrees warmer when there is a light breeze from the west. The maritime influence is greatly lessened in winter owing to sea ice cover, but it still plays a role in elevating the CRN temperatures when the trajectory is from the east or east-southeast. Another way of looking at this is that we would normally expect Barrow temperatures to be noticeably higher in winter owing to the heat island effect, but when the flow is from the east or southeast, the air reaching the airport passes over more land and has time for radiative cooling, which brings the airport temperature down relative to the CRN location.
I'm sure there are several more interesting features of the data that could be discussed, but I'll leave it to readers to point these out and suggest hypotheses about their origin.
Update: reader Eric suggested using polar graphs to show the data; this is a great idea and I was able to get close to a true polar graph using the radar chart function in Excel. See below, along with the station location map for ease of reference.
I don't want to burden you but could you put these graphs in polar form - like a rose graph or something? I think it would make interpretations easier. And perhaps you could also add the station map like the previous post so we (or new readers) don't have to go back to get context.ReplyDelete
I find it interesting that there is such temp diff during summer as compared to winter. Perhaps I'm just too used to interior weather. Isn't there any other villages on the arctic coast that would have a similar weather station scenario to compare with? Basically is there a way to control for the sea and for the land - to better isolate the heat island effect?
What causes the variable temp difference during winter when there is no direct solar heating? And why is the max and min temps so different? Is this variability within noise and so all we are seeing is randomness? Of coarse, the different wind speed's consistent offset do imply non-randomness. I think that we need to rule all of this out to make it robust.
Eric, Happy to oblige - see the updated post. The Excel polar graphs are not quite right as my wind direction categories are in 30 degree brackets that don't quite line up with the correct angles, but it's close.Delete
It would be great to examine another Arctic town away from the coast and with two or more nearby (and reliable) weather stations, but I'm not aware of a location that meets these criteria. The Barrow results are hugely affected by the sea vs land contrast, so the temperature differences reflect both the urban heat island and maritime vs continental influences, both in summer and winter. I don't think there's any way to separate the two factors, and Hinkel's results were similarly affected. In my opinion, the largest observed temperature differences between Barrow and nearly rural districts are at least as much caused by the sea/land difference as by a true urban heat island effect.
I'll have to think some more about your additional questions.
FWIW there are four WX stations in the vicinity of Prudhoe Bay/Deadhorse, east of Barrow. Three are inland some distance and terrestrial, one is on a man-made structure connected to land and surrounded by the Arctic Ocean. The temp differences are typically apparent to some degree at least during periods of open water.Delete
Edit to the above. Similar to Barrow, one is located at the Deadhorse Airport, a potential island of heat (?). The others are somewhat remote. There is a fifth station near the Kuparuk Airport to the west, separated by some distance from the others.Delete
Thanks Gary. No doubt these would make for an interesting study - thanks for the info. Land/sea influences would probably still be an important factor, however.Delete
Great post Richard. With climate, many of the differences between stations hundreds of miles apart can be explained by conditions only a few miles away.ReplyDelete
There appears to be a certain amount of symmetry with direction. For instance, for polar plot 4 (winter minimum) the greater temp diffs are found with both northerly and southerly in equal amounts - but little in east/west winds. This leads me to think that there is a barrier of some type around at least one of the temp probes. Or maybe solar heating is uneven and directional thus effecting the results. Food for thought.ReplyDelete
Barrow ASOS location and pics:ReplyDelete
Barrow wind study using the ASOS at the airport:
Barrow CRN site:
A third Barrow weather site:
Just for the curious that like pictures and puzzles.