Tuesday, January 4, 2011

Chasing the Clouds/Fog

Monday began with clear skies in the Salt Lake Valley with models predicting a low level cloud/fog deck forming during the evening/overnight hours. Based on this, we began increased operations at the ISS beginning with a 21Z launch and continuing at 3 hour increments through 12Z this morning. The goal was to capture the evolution of the predicted low level (below 800 hPa) cloud/fog layer.

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.

-Chris Ander

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