Thursday, September 8, 2011

A wildfire in...western Washington

I have decided to stay local today (at least local for me) and talk about a wildfire that's ongoing in the Olympic Mountains of western Washington.  With the huge wildfires in Texas, it seemed like a good topic to briefly talk about. I know...there are three tropical cyclones in the Atlantic that would also be worthy subjects. However, I feel like I've somewhat neglected my many blog readers who live in the Seattle area.  Besides--I can see the smoke from this fire from my window, and that makes this a compelling subject for me.

As noted in the news article I linked above, this fire (known now as the 'Big Hump Fire') started last Thursday, but has really grown this week.  They also note in the article that we haven't had any lightning recently so...they assume the fire was manmade somehow.

We can see the smoke from the fire on Tuesday's high-resolution MODIS imagery:
MODIS image from the Aqua satellite on Tuesday, Sept. 6, 2011.
My crudely-drawn arrow points at the approximate location of the fire.  You can see smoke plumes trying to spread out from it, though they don't seem to be sure of where to go.

So, where is the smoke from this fire going to go?  We can once again use NOAA HYSPLIT trajectory model to look at smoke plume trajectories over the next 24 hours.  This model is freely available and you can run your own simulations using it online at
NOAA HYSPLIT model forecast trajectories for 24 hours starting at 12Z, Sept. 8, 2011.
In the image above, I started a new trajectory every three hours over a 24 hour period.  You can see in the map view that all of the plumes seem to head generally to the south or southwest.  This roughly agrees with the 850mb winds forecast by models for this evening over the region:
UW 4km WRF 12-hour forecast of 850mb winds (barbs), temperature (colors) and heights (contours) valid 00Z, Sept. 9, 2011.
 Overall we do have flow out of the north-northwest along the coast and into the mountains (though the mountains are interrupting the flow somewhat).  It's pretty weak flow, though, so the smoke won't get evacuated very quickly.

But let's return to the trajectory model above. The lower panel of the graphic above shows a time-height series for the plumes.  Time increases as you go to the right, and each star represents a time when the model started another plume trajectory calculation--every three hours.  You can see how high above the ground each of the plumes gets by following the lines that come out of those stars.

Notice that for plumes starting or already traveling during the mid to late afternoon (from about 18Z through 00Z), the plumes get pretty high--up to 1000 meters above the ground.  But then, once we get to the evening (after 00Z), the plumes stop getting that high--they get trapped very low near the ground.  This continues until the following day in the afternoon, when once again the plumes are able to rise higher.

This shows the effect of the nocturnal inversion extremely well (at least, in this model).  Remember from my discussions about diurnal cycles and about the nocturnal boundary layer wind maximum that at night the land surface cools off very rapidly.  This, in turn, causes the air close to the surface to cool more than the air above it.  The result is a layer at the surface where temperature increases with height--a stable "inversion".  Stable layers inhibit vertical motion.  Thus, at night, we can see that the stable layer that has formed near the surface is keeping the smoke plume low to the ground--it can't rise up.

In contrast, during the afternoon when the sun is beating down, the ground surface warms up very quickly.  Similarly, the air near the surface warms faster than the air above.  This leads to a layer near the surface where the temperature decreases with height--a more unstable layer.  The more unstable the layer, the more easily you can have vertical motion.  We see that in these smoke plume trajectories--the smoke rises much higher during the afternoon when the near-surface layer becomes less stable (and we get more convective mixing).

As I noted in my last blog post, the Pacific Northwest will be stuck under a ridge for the next several days, with warm temperatures and low humidities persisting.
UW 4km WRF 72-hour forecast of surface dewpoint temperature valid 12Z, Sun., Sept. 11, 2011.
The forecast graphic above shows dewpoint temperatures on Sunday morning throughout the Pacific Northwest.  Note the purple blob over the Olympic Mountains--those are dewpoints down in the 30s and 40s.  With high temperatures forecast to be in the upper 80s on that day (the joys of living under a ridge...), that represents very low relative humidity.  This fire could be difficult to put out.

One slightly positive effect of this wildfire is that its smoke can contribute to some absolutely amazing sunsets.  Below is a screen capture from Dr. Dale Ireland's high-definition webcam that was pointing in the direction of the fire as the sun went down on Tuesday evening.
Image from Dr. Dale Ireland's page.
Incredible.  On his webpage, Dr. Ireland actually has a time lapse movie of the entire day on Tuesday.  Outside of watching it to enjoy the beautiful weather and excellent scenery across Hood Canal and in the Olympics, at around 4 PM during the day you can see the fire really start to explode--the smoke plume shoots up into the sky almost like a volcanic eruption.  The smoke then spreads out as it hits the capping inversion aloft (yet another example of how a stable layer stops vertical motion).  Then, the sun setting through the smoke-filled sky fills the environment with brilliant and exotic hues of red.  This video is definitely worth a watch.

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