Saturday, July 25, 2015

Fire Weather Climatology

A couple of weeks ago I wrote about Alaska's daily fire acreage numbers from the last couple of decades and noted that the median burn rate drops off precipitously after about the first week of July.  Since then I've been wondering if we can identify any sharp changes in the Fairbanks weather climatology that would help explain the sudden shift to less favorable fire conditions at that particular point in the summer.

The first thing to note is that the normal temperature and rainfall don't indicate any sudden changes; the normal daily maximum temperature passes through its smooth seasonal peak at the beginning of July, and rainfall frequency and amounts generally increase into early August.  There's actually a downward blip in rainfall frequency in early July, although this is presumably an artifact of the sample size.

The cloud cover and humidity also increase as the summer advances.  The chart below shows the daily mean cloud cover and the normal daily minimum relative humidity as reported from hourly observations.  The upward trend in daily minimum humidity is quite striking, and since we know (see e.g. here) that low humidity is a key component of fire danger, this seems like part of the puzzle.  However, the seasonal change in humidity is gradual with no sudden change in early July.  The cloud cover does tick up a bit in early July, but this is a small shift in the climatology.

There are other variables that could be looked at (e.g. lightning frequency or wind), but as with the examples above, it seems likely that the normal curve for any other individual variable will also show rather gradual changes over time.  However, when we combine variables and look at the joint distribution of two or more variables, some different behavior can show up in the tails of the distribution.

For example, the chart below shows the frequency of warm and dry conditions that might be considered very favorable for fire growth.  In late June and very early July, 80F temperatures with sub-30% humidity occurs on more than 15% of all days, but the frequency drops rapidly to under 10% by July 15 and under 5% by July 21.  Comparing this to the normal fire growth rate curve (also below) and recalling that the second chart shows week ending dates on the horizontal axis, there is a rather good correspondence.  Within just a few days of the start of July, favorable fire growth conditions become much less common in Alaska's interior, and as a result the median fire growth rate comes almost to a halt.

A note to regular readers: I'm traveling at present, so posting and commenting activity on the blog will be light for the next several days.

No comments:

Post a Comment