I was at a meeting a few weeks ago and one of the folks there, who works in community planning, was wondering if Fairbanks summers are becoming wetter. After all, the past five years (especially 2014 and 2016) have featured two of the wettest summers of record. If summers are getting wetter, that is something that planners need to take into account when considering things like downtown storm drainage capacity, rural road culvert sizing or generally higher river levels. Happily, here in Fairbanks we have enough historical data to take a stab at answering that question.
Since summer precipitation often comes in the form of showers and thunderstorms (especially the first half of summer) and these are much less frequent (climatologically speaking) over the flats as compared to areas closer to higher terrain, it's not a good idea to use the usual Fairbanks "threaded" climate record, since we know that the Fairbanks Airport, being out on the flats, receives significantly less rain than areas not far to the north and east. Luckily, we have precipitation observations taken in almost the exact same place, from the UAF Ag Farm, since July 1911. While there are some data quality problems with the Ag Farm observations over the decades, summer precipitation looks reasonable. Amazingly, there appears to be only one month that is actually missing, August 1969. For this one month I used the College Observatory data (taken on West Ridge near what is now called Jack Townshend Point, about 3/4 of a mile northeast of the Ag Farm). Here is scatter plot of the accumulated June through August precipitation at the Ag Farm for the past 106 years (1912 through 2017):
Of course, it's entirely possible (even likely) that changes in summer precipitation are not best described by a simple linear fit. Often in Alaska, a piecewise linear regression (i.e. "hockey stick") provides a better estimate of trend – but not in this case. There is no evidence of any significant changes in the linear trend.
Another change we can check for are any abrupt "step" increases. When we do this, the results are also mixed:
The last analysis I'll look at here is a simple smoothing of the observed precipitation:
So where does this leave us as to the original question? Are summers becoming wetter? It looks to me like the answer is an unequivocal "maybe".
On the yes side, the ordinary linear regression trend is significant at the 90% level, as is the 20-year minimum length step change.
On the no side, the trend of the median summer precipitation is not significant, and there is no significant step change when requiring a 25-year (or longer) length.
In the maybe camp, the cubic spline analysis is certainly suggestive of a significant change, just barely falling into the no trend camp (using a 95% confidence interval).
While this analysis is perhaps not satisfying from a community planning perspective, since "maybe" seems like it's not an "actionable" answer, from a climate perspective it is interesting that we are close to being able to detect an increase, which for precipitation in Alaska is not (yet) usually the case. However, increasing precipitation during the 21st century is exactly what the the climate model consensus have for nearly all of Alaska, and we may be starting to see that reflected in Fairbanks. The next several years will help to clarify the trend for the early 21st century.
Nice analysis. A better question for planners etc than "are summers getting wetter?" might be along the lines of "could there be a change big enough to make a difference in decisions?" That appears pretty unlikely.ReplyDelete
Rick, I appreciate your use of multiple statistical tests. It's something worthy of emulating.ReplyDelete
Rain in early summer is more thundershower while in the late summer it is less so. How has the mean amount of rain in May/June changed vs July/August? Do thunderstorms with their short outbursts produce about the same total as slow moving rain clouds? I would think the rate of rain would be just as important as amount. More thunderstorm rain would have different planning than more slow drizzling.
Just based on the multi-decade means, at the airport, convection makes up significantly less of the warm season total than the stratiform rains. Over the Yukon-Tanana Uplands it's closer.Delete
Other questions might be: Are soils wetter, vegetation and soils more moist, water tables higher, and rivers fuller and seasonal flow more? I tend to look at the buffered secondary effects rather than the raw source data.ReplyDelete
https://www.fs.fed.us/pnw/exforests/ and http://www.lter.uaf.edu/research may offer answers.
Great work, Rick - thanks. Is this data in GHCN?ReplyDelete
UNXA2 data should all be in GHCN-DDelete