Tuesday, May 20, 2014

Thunderstorm & Lightning Climatology

Thunderstorm season is quickly approaching in Alaska. So when can we expect the lightning to commence and how frequently is it observed at various locations? The Bureau of Land Management (BLM) has lighting detection antennas around the State to monitor potential wildland fire activity. They generously provide the historical data for easy download to the general public as a series of GIS data sets. The data begins in 1986 and runs through 2013 (27 years). However, they implemented a new system for the 2013 season so that data is not necessarily comparable to the previous years and is therefore not utilized in the analysis for this post.

Nearly all lightning strikes occur during the months of May through September and about 85% are due to surface heating. Figure 1 shows the cumulative distribution of strikes by month for the years 2000-2012. As you can see, the overwhelming vast majority of strikes occur in the months of June and July.

Figure 1. Cumulative distribution of lightning strikes recorded by BLM in Alaska from 2000 to 2012.

This provides an interesting perspective on the year-to-year variability but doesn't tell us a lot about the spatial variation. Since the BLM data is geocoded, we can place the data on a map. Figures 2 and 3 show all of the lightning strikes from 1986 to 2012 around Anchorage and Fairbanks respectively. Each map is exactly the same scale and represents an area of 1,120 square miles. Most residents of Alaska are not surprised to find out that more lightning occurs in the Interior than the Southern Coast. However, the disparity is fairly striking (pun intended). The Anchorage area receives 1 lightning strike for every 45.0 square mines annually. The Fairbanks area receives 1 lightning strike for every 2.35 square mines annually. That means lighting is approximately 14 time more common around Fairbanks than Anchorage.

Figure 2. All lightning strikes in the vicinity of Anchorage recorded by BLM in Alaska from 1986 to 2012.

Figure 3. All lightning strikes in the vicinity of Fairbanks recorded by BLM in Alaska from 1986 to 2012.

Looking at just he Fairbanks area, we see that 50,030 lightning strikes have been observed during the 1986-2012 time period for an area within 50 miles of Fairbanks. Half of those strikes occurred between June 16th and July 12th; a 27-day window! Figure 4 shows the distribution of strikes by 10-day time intervals around Fairbanks.



Figure 4. All lightning strikes within 50 miles of Fairbanks from 1986 to 2012 and a chart showing the frequency of strikes during 10-day intervals.

A look at the observations from the Anchorage and Fairbanks International Airports gives a good impression as the proximity of these storms to the respective airports. During that 27-year time period, there were 26 days with thunderstorms at the Anchorage International Airport and 210 days with thunderstorms at the Fairbanks International Airport. That is an average of 1.0 per year in Anchorage and 7.8 per year in Fairbanks. Since the number of lightning strikes around Fairbanks is 14 times more than around Anchorage, and the number of thunderstorm days per year in Fairbanks is 7.8 times more than around Anchorage, this tells us that any given thunderstorm day in Fairbanks is likely to experience twice as many strikes as a thunderstorm day in Anchorage. One factor not accounted for is cloud-to-cloud lightning. I am not sure if the BLM data captures those events or not. Figure 5 shows the annual number of thunderstorm days at the Anchorage and Fairbanks International Airports. 


Figure 5. Number of thunderstorm days per year in Anchorage and Fairbanks (observed at the respective airports) from 1986 to 2012. 


Postscript:

There are several papers that provide maps of thunderstorm frequency across the state. One is linked to in the first paragraph on this post. Others can easily be found using popular search engines.


11 comments:

  1. Very nice post, Brian. The report you linked showed that the peak in the Fairbanks climatology was very, very close to the solstice. I wonder if the new, much better data set shows the same thing? Also, I wonder if one could show that 50% of all lightning falls within just a 3-4 week period? It's a remarkably brief, but quite intense, lightning season for the interior.

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    1. For all lightning strikes within a 50-mile radius of Fairbanks (n=50,031), half of all strikes occur between June 16th and July 12th – a 27-day time window. I'll add a map/table shortly.

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    2. The post has been updated with a new figure and accompanying paragraph.

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    3. Thanks for the update, Brian. The ramp-up and drop-off in activity are extraordinarily rapid! It's really surprising to me that May doesn't see more lightning, with its long hours of sunshine and often plenty of cold air aloft. It would be interesting to see exactly what it is that changes in early June to allow lightning to occur.

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    4. Rick would be able to speak to this much better than I but my guess is that the thermal inertia from winter to summer still leans toward winter until the end of May. Frosts and freezes are still common well into May due to snow cover and frozen soils. The semi-permanent summertime thermal trough doesn't become established until all of the cold air is scoured out. One the trough forms, it becomes a lifting mechanism that facilitates vertical motion.

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    5. In Fairbanks May is often a dry month, with a comparative lack of moisture and lower humidity. The ground is cold, and the trees are sucking up anything that's damp to reform their growth structure. It's the time to paint outdoors if your into that activity.

      Low moisture plus the other factors Brian notes = lack of thunderstorms. That may be indicated in daily readings of humidity, not sure.

      While it gets wet in late July>September, the Sun's influence declines and the lifting mechanism Brian mentions probably decreases.

      Gary

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    6. Good points, Brian and Gary both. I'll look up the dewpoint measurements to see if there is an obvious correlation with thunderstorm activity.

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    7. If as I suggested May is comparatively a dry month both thunderstorm and rain-wise, then there must be reasons.

      Is it a lack of moisture influx from coastal areas? Is the influx or dew point activity the same as June? I'm not sure. Maybe it's just the perpetual daylight and heat of mid-June > mid-July that's the driver?

      All I know after 49 yrs is that if I don't paint in May it'll rain when I get ready to. Maybe it's the paint that's the culprit!

      Gary

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  2. Are the thunderstorm readings for just above the airport? The Anchorage airport gets almost no strikes compared to the Chugach mountains and so could under-represent. My limited experience with Anchorage lightning is that it's weak. Of course, Fairbanks is weak too compared to the midwest US.

    My observation with storms in Arizona is that they really don't form until the surface dewpoint was above 55. Perhaps the same applies here.

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    1. The ASOS equipment at the airports have optical and radio lighting detection systems. Since lightning can be heard at the equivalent of AM frequencies, it is pretty east to hear. As for the optical sensors, I am not sure how those work. Lightning more than about 5 miles from the airport are seldom recorded in the ASOS observations.

      The intensity of any particular lightning bolt is influenced by a number of factors. However, the vertical height of the thunderstorm is a good proxy measure to estimate lightning rate and intensity.

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    2. If it is clear over Anchorage airport but thundering over Flattop, then using the airport visible reading would fail as a proxy for a thunderstorm day. You would need to show that such a proxy is valid. Isn't the lightning geocoded with time? Most storms are isolated and so time can be clumped to one cloud.

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