Believe it or Not! Wildfire Smoke Is Cooling the Northwest

Today was another warm day, with Sea-Tac hitting 91F and temperatures rising above 100F in the Columbia Basin of eastern WA (see map).

But do you know that our temperatures this week would have been much warmer this week if it weren't for the substantial smoke covering our region?  Smoke that predominantly came from British Columbia?  Believe it or not, the proof is at hand!

The plume of smoke above us has reduced the amount of solar radiation reaching the surface...thus producing less surface warming.

Why is that true?  Because the smoke particles both scatter some of the solar radiation back to space and absorbs some of the sun's rays, the latter causing the air higher up in the atmosphere to warm.  By either mechanism, less solar radiation reaches the surface.

  Here at the University of Washington, the amount of solar radiation reaching the sensors at the top of our building has been substantially reduced by the smoke:  the numbers are shown below, dropping from 25.37 on August 1 to around 19.15 MegaJoules per meter squared on August 5-6.  Roughly a 25% drop in solar radiation!

Radiation in MJoules per m2
7/31   26.80
8/1     25.37
8/2     23.22
8/3     22.55
8/4     23.33
8/5     19.19
8/6     19.14
8/7     21.09
9/8     22.62

OK, the amount of solar radiation is less, but how can we prove that it cooled us off? 

To explore this issue, I asked Jeff Baars, a research meteorologist in my department, to explore the smoke's impact by taking difference between the UW WRF model forecast for surface (2-meter) air temperature and observations.  The UW WRF model does not include the effects of smoke (we are working on it!), so the difference between our model forecast and reality should reveal the effects of smoke.   The expectation is that the model without smoke will be too warm...but by how much?

Here is the difference between the forecast (72h) and observed temperatures at Seattle-Tacoma Airport from 23 July through last night, for the forecasts valid at 5 PM each day.  The various colors are for different resolution UW WRF forecasts (36, 12, 4, and 1.3 km grid spacing).  Before the smoke reached our region the WRF model forecasts were quite good, with very small errors.  But after the smoke hit, the errors reached around 10F, with the model being too warm.   Implication:  the smoke cooled Sea-Tac by 10F on two days, and less amounts on others!

What about Bellingham, a location even closer to the smoke source in BC?    Wow... the cooling effect of the smoke reached 12-14F!

We did a similar analysis for 5 AM, when temperatures are near their daily low, and found little effect.  This is not unexpected:  much of the smoke is higher up, where the air is cooler and the smoke particles are relatively small compared to the wavelength of infrared radiation, which is the key player at night.

The smoke was substantial today and even worse this evening, something shown by the small particle concentration (PM2.5) at Seattle and Tacoma (see below from Puget Sound Clean Air Agency). Levels of 60-90 micrograms per cubic meter are unhealthy of vulnerable populations.

According to the Canadian smoke forecast modeling system, we have several more days of this.

So should we thanks the Canadians for keeping us cool, or curse them for smoking us out?  I will leave that for you to decide.

from Cliff Mass Weather and Climate Blog http://ift.tt/2ww2nQq