Continuing with the theme of precipitation trends, I wanted to look at the rate of change of various precipitation thresholds over the last number of years to see if they are more or less frequent. To evaluate the rate of change, I looked at 4 different precipitation thresholds over the course of the Fairbanks climate record (good precipitation records extends back to 1915). Figure 1 shows the annual number of days for four different precipitation classes in Fairbanks (>= 0.01", >= 0.05", >= 0.25", and >= 0.50"). The only one of the trend lines that was significant at the 95% level (p<=0.05) was the number of days per year with at least 0.01" of precipitation. All the other thresholds were close to the even line but none showed a statistically significant trend (positive or negative). End of story, right?
A dangerous practice in the field of climatology is to make sweeping generalizations by looking at a single station's data. So, let's look to see how things have changed across a wide swath of Alaska. Unfortunately there are not many stations with a record as long as Fairbanks. We can increase the sample size to 21 stations when we set the begin date at 1951 instead of 1915. Figure 2 shows the stations we will use to look at the statewide trend in precipitation events.
Figure 2. Stations (21) used to assess long-term precipitation frequencies.
Looking at the statewide data, I was originally looking at "heavy" events so the categories are a little different. Actually, I was looking for national trends so the criteria were set somewhat higher than would ordinarily be appropriate for Alaska. In any event, the categories are: 1) at least 0.05", 2) at least 0.50", 3) at least 1.00", and 4) at least 2.00". That makes for a little bit of an apples to oranges comparison with Figure 1 – but let's progress nonetheless. Figures 3 through 6 below show the annual percentage difference of all stations from the 1951-2013 station average. To eliminate the effect of stations with large values overwhelming stations with low values, percentage differences were used instead of raw numbers. For example, a station that averages 50 days with 0.05" from 1951-2013 has a 10% increase if a year recorded 5 extra days with 0.05". A station that average 20 days with 0.05" from 1951-2013 has a 25% increase if a year recorded 5 extra days with 0.05". If you combine the raw numbers from those two stations, they would sow a 15% increase in the number of days with 0.05". However, comparing the percentages yields an increase of 17.5%. Therefore, comparing the annual percentage deviation is more meaningful since stations with larger averages would ordinarily drown out stations with low averages.
The number of days with >= 0.05" and >= 0.50" are noticeably increasing at a statistically significant rate (95% level) in Alaska. The >= 1.00" and >= 2.00" give mixed signals and neither trend is statistically significant. This is due in large part to small numbers and a relatively few stations responsible for most of the deviations.
Therefore, I feel comfortable stating that the number of small precipitation events and the number of moderately-sized precipitation events have definitely increased over the last 60 years here in Alaska. This is not so surprising given the fact that warmer air can hold more moisture and warmer air at the surface with constant temperature aloft (not analyzed) would lead to lapse rate instability. As for the frequency of 1" and 2" precipitation events, they are too infrequent to make judgments on.
Figure 3. Statewide average of percentage change from 1951-2013 average annual number of days with at least 0.05" of precipitation.
Figure 4. Statewide average of percentage change from 1951-2013 average annual number of days with at least 0.50" of precipitation.
Figure 5. Statewide average of percentage change from 1951-2013 average annual number of days with at least 1.00" of precipitation.
Figure 6. Statewide average of percentage change from 1951-2013 average annual number of days with at least 2.00" of precipitation.