Thursday, January 27, 2011
We managed to coordinate 5 radiosondes this morning, two within the valley (one of which was the NWS Operational sounding) and three running up Parley's Cayon, a major drainage to the East of Salt Lake. We've been speculating for a long time that the nocturnal drainage flows down this particular 'airshed' are of particular importance to the mass budget and cold air accumulation within the Salt Lake Valley. Now we have some data to help elucidate some of these processes. We also have the first hand experience of just how cold and windy it was. After launching a balloon half way up the canyon this morning (which took about 1.5 hrs) it took me a full 3 hours under a thick blanket to warm back up.
We'll do the same thing on Friday, and with clear skies overhead the drainage flow could be even more pronounced. In addition we may have some clouds and fog in the valley which may help to visually demonstrate the degree to which the drier canyon flows penetrate into the valley cold air pool.
Side wall IOP and convective boundary layers
We also launched balloons this morning and afternoon to look at difference along the east and west side walls of the salt lake valley. The idea here is that differential heating caused by the changing sun angle over the course of the day may create local thermal flows favoring east-west asymetries in the cold pool structure. We also got to observe the growth of the convective boundary layer within the bottom portion of the cold air pool.
Regional Flows and Lake Breezes.
Another topic for thermally driven flows is the regional scale diurnal wind reversal that occurs during daytime heating. In the morning hours the salt lake valley exhibits organized Southeasterly drainage flow towards the lake, which is the low point. In the afternoon, if sufficient solar heating occurs, the flow reverses to an up valley flow from the NW. When this reversal occurs there is sometimes also an embedded more local lake breeze that develops due to the heating differences of the water and land surfaces. We saw both of these occur today with some portions of the valley experiencing spikes in the dewpoint temperature as lake air was advected into town. Just after sunset there was also a fog front that moved in from the NW obscuring portions of the downtown area. Fog was much more extensive in valleys to our west, and some of this 'fog front' may have moved in from the Toole valley. 'Interbasin' exchanges of air such as this are a big complicating factor on our cold air pools.
We're looking forward to the next few days of intense observations and hope to share some more of the data with you soon.
Tuesday, January 25, 2011
Monday, January 17, 2011
Friday, January 14, 2011
The previous 24 hours have been fascinating with some changes in the cold air pool which were explicable and others that were not. To set the stage, we started the day on Thursday with a very sharp low level temperature inversion in place.
Then in the late afternoon and overnight hours a short wave trough moved into the region spreading light to moderate snowfall across the valley. As snow fall intensified around 10 PM the lower troposphere became almost completely saturated.
The surface temperatures did not change much, however in the layer of air near 800 hPa, which had been above freezing, a combination of evaporative (or sublimation) cooling and the melting of snow falling from aloft caused the temperature to decrease to just below the freezing point. This effectively weakened the total inversion strength but not nearly to a degree that would allow vertical mixing of the air. Nonetheless, pollutant levels dropped significantly, a change which was presumable do the the scouring of aerosol by the snow. This is also known as wet deposition. Some scientists in the Biology department were out collecting data from the snow pack today to see if this was in fact the case.
All of the above changes were quite easy to make sense of, but some of the changes this morning were a bit stranger. The layer of air which had been cooled seemingly spontaneously warmed again, climbing well above freezing:
In fact for a very brief period the airport weather observations indicated some freezing rain, which suggests that precipitation falling from mid level clouds was melting in the warm layer and then becoming supercooled in the shallow layer of air below freezing at the surface. Freezing rain is actually a very rare event in Utah, and it tends only to occur in situations like these.
The big question is why and how did this layer warm so quickly following the cessation of the strong precipitation. Some form of advection is likely responsible, but the source of the warm air remains a mystery.
In addition to the warming, we can also see from the above soundings that the inversion, while still very sharp, is shallower this morning than it was yesterday. In fact as of 11 AM this morning it was only about 500 meters deep. This means that the mixing volume for pollutants is quite small, and correspondingly the decrease in PM2.5 seen last night has quickly reversed with levels now spiking to 58 ug/m^3 (the NAAQS is 35).
The forecast remains quite complex for the next few days with minor disturbances rippling across the region keeping periods of clouds and light precipitation in the mix. We'll have to see how each successive event impacts both the thermal structure and the concentration of pollutants.
Sunday, January 9, 2011
Inversion is a short name for temperature inversion. A temperature inversion is a high stability layer in the atmosphere where temperature, rather than showing its normal decrease with height, increases with height. Such layers are often surface-based, with coldest temperatures at the surface and temperatures increasing with height above the surface. Such surface-based inversions are usually formed at night and in winter when the ground cools faster than the air above. In fact, inversions are a common nighttime feature year around, since cold temperatures near the ground are produced commonly after sunset when the ground loses heat through outgoing long-wave radiation.
Temperature inversion layers can also form in the free atmosphere above the surface. This often happens when warm air is advected (i.e., blown in) above a layer of cold air.
The word 'inversion' is used in the local newspapers to refer to the mid-winter pollution events that affect the Salt Lake Valley. Inversions and pollution layers are, in fact, not synonymous. Inversions are a meteorological phenomenon; pollution is produced by a variety of human activities. Pollution can be emitted into and stored in a layer of high atmospheric stability, such as a temperature inversion, but the atmosphere, even when temperature does not increase with height, has sufficient stability to store pollutants and keep them from mixing vertically. In fact, the atmosphere in the Salt Lake Valley in winter, when polluted, is often isothermal, with temperature being more or less constant with height. Further, non-polluted inversions can form in areas where pollution sources are not present. The Salt Lake Basin is a topographic basin where stable layers (including inversions) often form, but the air pollution problem is not caused by inversions, it is caused by pollutants that are emitted into the stable layers. The solution to our pollution problem must come from reducing pollutant emissions. In the meantime, our PCAPS meteorological research program is gaining a better understanding to the meteorological processes that lead to the formation, maintenance and destruction of stable layers within the Salt lake Valley, and we hope this will, with effort, lead to improvements in forecasting stable layer and pollution events.
Friday, January 7, 2011
Living in Utah is quite a privilege, most of the time. But experiencing cold air pools like the present one is not the best of times. Everyone down in the Salt Lake Valley has been exposed to a mystery brew of fog, stratus, and smog for the past 24 hours. The composite image above was taken from the Suncrest development at the south end of the valley this evening. It's impossible to see anything down in the Valley, but the the Oquirrhs are in the distance to the left and Lone Peak is to the right.
We're now in day 7 of IOP-5 with ~30 hours to go. This event has had a number of unexpected features, but we're now locked into the most intense part of the event in terms of one measure of cold-air pools: the deficit in temperature air from a particular level would have if it was lifted upwards without exchanging heat to the level of the top of the Wasatch (~3000 m). The larger the deficit, the more negatively buoyant the air is, and the greater tendency for the air to sink back to its original level. The observed temperature deficits this morning of over 25 C are pretty impressive (the purple shades in the accompanying figure). These deficits and the cold air pool won't be wiped out until the next push of cold air begins to move in aloft Saturday night.
That'll usher in a welcome breath of fresh air.
Tuesday, January 4, 2011
The soundings show an evolution that is quite different from what we expected. While a shallow layer near 800 hPa does reach saturation overnight and persists through the evening, the valley never “socks in” with low level clouds as predicted by the NAM. We suspect that the discrepancy between the model forecast and the observed soundings is related to the model failing to predict the very shallow cloud layer near 700 hPa. This cloud layer altered the longwave radiation balance in the lower atmosphere, preventing the lowest layers from cooling and reaching saturation. Radiative cooling is evident at the top of this cloud layer which leads to a nearly dry adiabatic layer between 800 hPa and 700 hPa as seen in the 09Z sounding.
The image below shows the 21Z, 03Z and 09Z ISS soundings from top to bottom.
Light snow flurries have been observed overnight and this morning, likely due to a combination of a very weak shortwave trough passage and possible radiative effects discussed above.
Heading into Wednesday and beyond, the NAM continues to predict moist conditions from the surface up to a lowering inversion near 750-800 hPa through the duration of this event. It appears that it will dry out a bit above the inversion top, at least in the next 24-36 hours, with a very weak short wave ridge passing overhead.
We have gone back to 6 hourly launches at the ISS with possible operations planned tomorrow afternoon/evening to capture the potential onset of dense fog in the valley. Conditional graw launches are planned for Wednesday afternoon/evening as well as Thursday morning, most likely near the lake. Mobile weather stations will also be operated during this time.