Radar Trends

Back in early August, when interior Alaska was seeing an unusually late spell of thunderstorm activity, I commented that it would be interesting to work with the Fairbanks radar data to investigate the variability of strong storms:

https://ak-wx.blogspot.com/2023/08/severe-thunderstorm-warnings.html

I ended up digging into this, and I've succeeded in producing an archive of integrated storm activity back to 2014.  The Fairbanks (Pedro Dome) radar has been around for a lot longer than that of course, and there is some historical data available back to 1997, but unfortunately there is just too much missing data in the early years to use it in this analysis.  There's also a complete gap from 2004 through 2013, and regrettably NOAA/NCEI says that data does not exist any more.

But we'll work with what we have.  My approach was to calculate the spatial area for which the radar reflectivity exceeded various thresholds at each radar scan, across the entire available scan domain, as archived in NOAA's Big Data project.  I used the base (lowest scan elevation) reflectivity and extracted the data on a 1km grid.

There is a myriad of ways that the results could be analyzed, but for starters I took the May-September average of each day's maximum area at or above the reflectivity threshold - such as 40 dBZ, see below.  A 40 dBZ reflectivity is typical of moderate convective rainfall in summer; a typical robust thunderstorm core would be more like 50 dBZ.

Of course many days have no thunderstorms and little or no rainfall across the domain, and most of each season's total activity is concentrated in a rather small number of days, but the season-long average of daily peak activity seems to provide a suitable integrated measure.  Note that I excluded days with less than 100 reflectivity scans in 24 hours; about two-thirds of days have more than 200 scans in May-September.

The overall year-to-year variability in 40+ dBZ radar echoes does not seem particularly great, but the monthly breakdown shows that both July and August of the last two summers were considerably more active than the decadal average.

Raising the bar to a 50 dBZ threshold emphasizes the high level of storm activity in the past two summers, especially in July (both years) and August (this year).  August 2023 actually stands out quite dramatically compared to the previous 9 years:

Taking it up still another notch to 60 dBZ, which would be typical of a strong storm producing at least some hail (possibly large), summer 2023 stands out even more compared to prior years, and there's a strong suggestion of an upward trend.  Despite the very short period of record, the linear trend in the first chart below is highly statistically significant.

This data obviously begs for further analysis and investigation, and I'll aim to do that as time permits.  For now, it's just interesting to note that perhaps the unusual spate of severe thunderstorm warnings issued by NWS Fairbanks this summer wasn't as out-of-line as I thought at the time.

Radar-Estimated Rainfall

In view of the extraordinary rainfall in the Fairbanks area this summer, I thought it would be interesting to look at radar data to get a better idea of the spatial distribution of the rainfall amounts in June and July.  Using a standard relationship between radar reflectivity and rain rate, I calculated daily rainfall estimates based on data from the Pedro Dome radar site just north of Fairbanks; the figure below shows the June-July estimated totals (click to enlarge).  Observed amounts at surface measuring sites are marked in black.

Here's a comparison of estimated and observed total precipitation by location:

Big Delta AP: 5.97" observed vs 6.22" estimated
Delta 6N COOP: 6.79" vs 6.78"
Nenana AP: 7.39" vs 9.53"
Fairbanks AP: 8.26" vs 6.95"
Eagle Summit SNOTEL: 8.50" vs 6.38"
Clear Sky COOP: 9.30" vs 9.28"
North Pole COOP: 9.77" vs  7.71"
Teuchet Creek SNOTEL: 10.10" vs 11.26"
Ft Knox Mine COOP: 10.94" vs 8.22"
Keystone Ridge COOP: 12.24" vs 8.08"
Mt Ryan SNOTEL: 12.50" vs 10.35"
Monument Creek SNOTEL: 13.30" vs 9.99"
Little Chena Ridge SNOTEL: 14.30" vs 11.92"
Upper Nome Creek SNOTEL: 16.60" vs 11.70"
Munson Ridge SNOTEL: 19.00" vs 18.56"

On average for the 15 stations, the radar algorithm underestimated the total precipitation by 11%, which is not too bad.  In a few spots the radar estimates were excellent, and in a few spots they were notably bad; for example, Keystone Ridge apparently observed 52% more rainfall than the radar indicated.  Localized differences like this might be related to local topographic enhancement or diminution of rainfall compared to what is estimated from radar reflectivity at the height of the radar beam (which increases with distance from the radar site).  It's also possible that the surface measurements are incorrect at some of the observing sites.  The charts below show comparisons of the daily rainfall amounts at Fairbanks airport and Munson Ridge SNOTEL; both of these sites report precipitation for the midnight-to-midnight period, which matches the period that I used for the radar calculations.

Regardless of local discrepancies and potential errors, the main point of the exercise is to get a rough look at the spatial distribution of rainfall during June and July, and the radar estimate serves this purpose quite well.  Based on the SNOTEL data, we already knew there was a broad area of enhanced rainfall in the hills east and northeast of Fairbanks, but the increased rainfall between Fairbanks and Nenana is interesting.  The radar estimate suggests that it was also very wet on the south side of the Tanana valley but north of the higher terrain of the Alaska Range.