Tuesday, February 7, 2017

Strong inversion and persistent cold air pool: January 2017

I'm looking at the inversion strength of the cold air pool in northern Utah between January 25 and February 5, 2017. This occurred during the Utah Winter Fine Particle Study (more about that study here: https://www.esrl.noaa.gov/csd/groups/csd7/measurements/2017uwfps/).

Inversions occur when the upper level air is warmer than the air below it. This makes it difficult for air to mix vertically and dilute pollutants from the surface. One way to determine the strength of an inversion is to compare the potential temperature at 700 mb (approximately the height of the mountain peaks) and the potential temperature at the surface. In the top figure in the graph below, I show potential temperature from the HRRR analyses at 700 mb (red) and the surface (blue). The 12-hr HRRR forecast is also shown in yellow. The observed potential temperature from the Salt Lake City radiosonde are plotted as black dots. The bottom figure is simply the difference between the upper level and surface level potential temperature, labeled "Surface Temperature Deficit."

For the most part, the HRRR analyses correspond well with the balloon observations, except the HRRR doesn't mix out as early as was observed on February 4th.

Below shows the potential temperature at the surface and 700 mb, as well as the differences from the HRRR model analyses for the course of the event. The right panel shows the difference between surface and 700 hPa potential temperature. Areas that turn dark red is where the inversion is strongest.

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