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| The air is still polluted, but these clouds are actually a help rather than a hindrance |
Below is a loop of upper-air soundings collected each morning at the Salt Lake City airport over the past week. These soundings are plotted as a "skew-t" so a line of constant temperature slopes upward to the right. Note the two-stage evolution of the inversion event. First, the upper levels warm dramatically while the morning surface temperatures remain relatively steady. This occurs in the first 5 frames of the loop (note how the red line shifts rightward from about 800 to 600 mb). Second, a mixed layer forms very near the surface in the last 2 frames, as indicated by temperatures cooling with height to an level just above 850 mb.
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| Morning upper-air soundings from the Salt Lake City airport from 5–12 Feb 2016 |
Here are a couple of still images to further highlight this point. On the morning of 8 February, the inversion was based right at the surface. In such a situation, there's very little vertical mixing of the airmass. Pollution is trapped right at or very near the valley floor.
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| Morning upper-air sounding from the Salt Lake City airport on 8 Feb 2016 |
This morning (11 Feb), however, the inversion base is elevated and sits at 836 mb (5680 ft), almost 1500 feet above the valley floor. Below the inversion, there is a shallow mixed layer that is allowing for some vertical mixing of pollutants.
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| Morning upper-air sounding from the Salt Lake City airport on 11 Feb 2016 |
The growth of such a mixed layer is very common following the formation of low clouds during inversions in the Salt Lake Valley. Radiative cooling at cloud top drives turbulence that mixes the layer in and below the clouds. The end result is what meteorologists call a cloud-topped mixed layer. If you are in the Salt Lake Valley this morning, you are not in the inversion, you are beneath it, in the cloud topped mixed layer.
What does this mean for pollution? Well, it means that at night there's more mixing through a deeper layer, which reduces PM2.5 concentrations. The presence of the mixed layer also slows the long-term rate of rise (NOTE: Concentrations are still high, so this is no reason to roast marshmallows over a fire tonight). For example, PM2.5 levels at Hawthorne last night were not as high as they were the previous night. In addition, there is a flattening of the long term trend.
As discussed in the previous post, PM2.5 concentrations at University of Utah sensors are running higher than those aw Hawrhorne, but they also suggest a flattening out over the past 24 hours or so.
None of this means the air quality is good. We are still in unhealthy territory. What it does mean is that the rate of increase along the valley floor will probably slow. Those of you on the upper benches may see more sustained high PM2.5 levels in this scenario as the base of the inversion lifts and you are more continuously enveloped in the gunk. Perhaps this isn't a silver lining for you.
Science wonks can see Pataki et al. (2005) for more info on these cloud-topped mixed layers and their influence on mixing and transport in the Salt Lake Valley.
This is interesting and something I hadn't thought about. During past events, it does seem commonplace to see PM2.5 rise and then have a slowing in the rise rate or even a leveling out, but fog and daytime lifting cloud decks are common in such events, which could be a key player as you note. During past major events with nighttime fog, I've also noticed deposition either through the fog itself or snowfall, which you would also think would lower the PM2.5, but perhaps this is negligible?
ReplyDeleteI don't see any snow reported at the airport, but the media was suggesting there was some this morning. I think it could play a fairly small role in scavenging, but suspect most of the slowing of the rise is the growth of the mixed layer.
DeleteSee the Pataki et al. article as well. In that article, we use CO2, which is inert and a bit easier to deal with.
From this camera map (http://www.wrh.noaa.gov/slc/webcam_map/webcam_map.html) it looks like this is pretty much the case area-wide around the Great Salt Lake basin, west desert etc. The stratus deck this morning looks higher than yesterday morning, now around 5500 - 6000 feet. It seems like this may be a somewhat predictable evolution of the stratus deck about 24 hours or so after its initial development, assuming it doesn't burn off during the daytime (which it didn't yesterday, except briefly over the Salt Lake Valley).
ReplyDeleteI expect that the depth of the mixed layer and height of both cloud base and top to increase with time. Tomorrow they should be deeper/higher than today. After that it's tough to say as the system coming in for the weekend will perturb things in ways that are hard to anticipate.
DeleteI agree, although it gets complicated due to the depth of the overall Salt Lake Basin and surrounding terrain. At some point you would start to get a lot of spillover into adjoining basins if the cold pool deepens much, or merging with other cool pools. It is also interesting that most surrounding basins (such as the Uinta Basin and those in western Colorado, northern Nevada, much of southern Idaho) have none or only very limited fog/stratus.
DeleteIt is also interesting to watch the Mesowest obs, especially the Suncrest site at 6100 feet (south end of the Salt Lake Valley). It is apparently right at the top of the status deck and has been back and forth between warm/cold due to small waves on the top of the cold pool.
ReplyDeleteThanks Jim for all the enlightening information. Do you have any explanation why is the DAQ Hawthorne PM2.5 monitor consistently showing concentrations ~ 50 ug/m3, while the TRAX and NAA on essentially the same elevation are roughly 2x higher? That's a big difference.
ReplyDeleteA mystery that remains unsolved. See prior post.
DeleteAre the instruments different? If I recall right the DAQ one is more beefy. My naive thinking is suspicious that at least some of the difference in the values is in the instruments. But if so, which one to trust more?
DeleteYes, they are different. They have tracked fairly well in the past, but this is the first high RH/fog event. I'm not sure which instrument is closer to the truth at this time.
DeleteAh, missed your previous post. Guess a good topic for future research with potentially big implications.
DeleteWhen does the inversion season "officially" end due to the higher solar angle? Does the solar mixing prevent an inversion from setting up, or can it actually end an existing inversion?
ReplyDeleteWhere have you gone, head Weeny? The Wasatch skier world is curious to know how and why the forecast for this past Sunday so significantly underestimated the storm, how the Sunday-Monday slow-mix cleared the valley air, and whether we are in the clear on inversion season from this point. (And whether March is going to deliver with 250" , though I know you are not one to forecast long-term).
ReplyDelete