Saturday, January 1, 2011

IOP 5: Off to the races


The surface cooling has begun in earnest and now the question is how low will we go.The two key factors working in our favor are fresh snow cover and clear skies.

During the day on Saturday the high albedo of the fresh snow helped to limit the amount of solar heating throughout the Salt Lake Valley. This was especially true at the Playa ISFS site, where the reflection of incident solar radiation was so complete that the surface energy balance (the difference between the incoming and the outgoing radiation) remained negative throughout the day... which means that even in the peak of the solar day the surface was cooling not warming!

Now as night falls, the high emissivity of the fresh snow allows it to radiate in close approximation to a perfect black body... in other words it is really good at getting cold. And cold it is getting! In the hours following sunset the temperature at the Playa site plummetted to -5 F.

Most of the other sites around the valley are still much warmer (around 10 F), which may reflect the role that the urban landscape plays on altering the surface energy balance.

While it is tempting to think that the whole valley is covered in snow, a perspective from the foothills this evening revealed that within the urban/suburban portions of the valley the non-native trees, buildings, plowed road ways, etc all provide a decidedly darker and more absorptive surface than the snow cover found on the natively vegitated playa locales to the west. During the day this means that surfaces get warmer and heat the air above more effectively. At night the treed and developed landscape limits the sky view from the surface, which means more of the outgoing longwave radiation is "trapped" by the urban "canopy" which in turn diminishes the cooling potential.


The focus for tomorrow will turn to the potential for dense clouds and fog to develop within the boundary layer. Models are suggesting that sometime late Sunday that the lower levels will begin to significantly moisten eventually leading to clouds by Monday morning. We'll be increasing our temporal sampling during this period to try to quantify the moistening and to isolate where the additional water vapor is coming from... our guess is either influences of the Great Salt Lake or sublimation from the snow pack.

NAM Potential Temperature Time Height. Clouds are indicated in white.

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