Saturday, May 10, 2014

Fairbanks vs Keystone Ridge Precipitation - Part II

The previous post on this topic illustrated some differences in precipitation climatology between the sheltered Fairbanks airport and the Keystone Ridge weather station; we saw that precipitation is both more frequent and more abundant in total on the ridge throughout the year.  This is to be expected given the elevation difference.  However, I also posed the question of whether the precipitation excess on the ridge is mainly caused by more frequent precipitation events or by heavier precipitation when it falls.  To look at this, we first need to calculate the mean precipitation per event, i.e. the mean daily precipitation excluding zero precipitation days.  This is shown in the chart below; we see that precipitation events are heavier on average at Keystone Ridge throughout the year, but the difference is very small in late winter and early spring, and interestingly also in mid- to late-July.

Using the mean precipitation per event, we can then examine some hypothetical situations to uncover the relative importance of frequency versus intensity.  For example, we can look at what would happen if the airport received precipitation as heavy as on the ridge, but with unchanged frequency.  Alternatively, we can look at the hypothetical outcome if the frequency at the airport were as high as the frequency on the hill, but with unchanged amounts per event.  The results of these two thought experiments are shown in the following chart.  The thin blue and red lines show the actual climatology for the two locations (as shown in the previous post).  The green line shows the result when we boost only the frequency at the airport, while the purple line shows what happens when we boost only the amount per event.

In general, we see that both the frequency difference and the intensity difference make a significant contribution to the overall difference in precipitation totals between the two locations.  However, there are some interesting variations in summer.  In mid- to late-July, the excess wetness at Keystone Ridge is dominated by increased frequency rather than increased intensity; however, on both sides of this period, and especially in August, the increased frequency is less important than the higher amounts falling on the ridge.  It is quite possible that some of these differences are due to sampling variability in the relatively short period of record; further investigation would be needed to examine the role of random sampling variations.

Finally, a ratio of the hypothetical precipitation excess from the two separate influences is shown in the chart below; higher values indicate times in the year when Keystone Ridge is wetter mostly because of heavier precipitation events, whereas low values show times in the year when the frequency difference dominates the total precipitation difference.

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