Previsão do tempo, weather forecast: Is Wildfire Smoke Making Our Fog More Persistent?

Satellite picture for western Washington around 1 PM Saturday

The National Weather Service's forecast for Saturday had a problem. For central Puget Sound it was predicting clouds in the morning and then becoming mostly sunny (see below)

But instead fog held in ALL day, something shown by the Space Needle PanoCam at 3:10 PM Saturday. You don't see conditions like that holding in all day very much.

And the UW high resolution forecasts of fog/log clouds for 5 PM Saturday started with fog in the morning and rapidly burned it back (see forecast at that time).

An error that the NOAA/NWS HRRR model also made (forecast for 5 PM Saturday also shown):

In fact, this error--the unrealistic burn off of low-level cloud--has happened several times during the past few week, with fog and low stratus showing unusual persistence.

Local meteorologists and some of the NOAA modelers have been musing that perhaps, just perhaps, there is a reason for this uber-persistent low clouds and our model' inability to get it right: wildfire smoke.

And there are good physical reasons why smoke might be the explanation.

For example, smoke scatters some of the solar radiation back to space, lessening the solar heating available to burn off the fog.

In earlier blogs I showed the effect clearly by presenting the solar radiation on the roof of my building before and after the smoke moved in on Sept 11th (see below).

Smoke at any level, reducing solar radiation reaching the surface, could help the fog persist. Many operational weather forecast models do not have have this effect. However, the NOAA HRRRsmoke model AND the UW model do try to simulate the solar dimming from smoke. Thus, we need to find another explanation.

And there is one.

Cloud droplets generally form on small particles in the atmosphere called Cloud Condensation Nuclei (CCN). Smoke particles can act cloud condensation nuclei. So adding smoke can lead to MORE cloud condensation nuclei and MORE cloud droplets (see left side below). If the amount of water available doesn't change (which is normally the case) then having more nuclei leads to more, but smaller cloud droplets . And it turns out the having more particles leads to the cloud being more reflective, which REDUCES the solar radiation available to burn off the cloud.

More particles from smoke or pollution can make a cloud more reflective, something we see in the Pacific as marine traffic adds particles to low clouds, producing a feature called ship tracks (see below).

And there is more! Having a lot of small particles works against producing drizzle, which takes moisture out of the fog/stratus. Bigger droplets become heavier and fall out more quickly, collecting other droplet as they fall. Thus. having the moisture in smaller droplets helps maintain the fog!

Now here is the key issue. Most models, including the UW WRF and the NOAA HRRR model do not properly simulate these cloud effects and thus fail to forecast the ability of smoke to keep the clouds around.

The above is a physically plausible explanation, and certainly there were enhanced values of small particles in the lower atmosphere over Puget Sound on Saturday (see the small particles concentration at Seattle's Beacon Hill below).

Some of my colleagues at the NOAA Lab in Boulder, CO believe the above is a real possibility, and during the next month UW graduate student Robert Conrick will be exploring this mechanism with sophisticated modeling experiments. Will let you know what we find out.

Such situations are why science is so much fun, with interesting theories to test and real practical value when we solve the mystery.

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