Wednesday, September 7, 2011

Lessons in Land Surface Contrasts

I spent the day today at Dugway Proving Grounds where we will be running a major field program this coming year to improve weather prediction in complex terrain.  It was a long trip, but fascinating meteorologically because it offered up a nice perspective on how land-surface contrasts affect minimum temperatures.

The plotted temperatures below are the 24-h minimum temperature reported to MesoWest (the winds are from 2300 UTC so ignore them).  Notice that the minimum temperatures in the Salt Lake Valley vary widely (ignore a few as they are clearly bad), with a 57F at KSLC, a few 60s where the stations are a bit higher, and some 50s scattered about elsewhere.

Now, take a look to the west.  There is a more coherent pattern.  Over the Salt Flats, the minimum temperatures range from 56-62F, but if one moves south and east of the Salt Flats, where there is a higher density of stations near Dugway, the minimum temperatures are much lower, ranging from 43–50F.  Stations above 50F are at somewhat higher elevations and either in or above the morning inversion.

The contrast between the Salt Flats and surrounding desert illustrates nicely how the land-surface affects the surface energy balance.  The Salt Flats have a larger thermal inertia than the surrounding desert land surface.  This means it takes more energy compared to the surrounding desert to cause the same temperature change.  As a result, they cool off more slowly at night and typically see a higher minimum temperature than the surrounding desert.  They also heat up more slowly during the day and typically see a lower maximum temperature.  The site in the Salt Flats along I-80 was even warmer than KSLC this morning.

Topography strongly affects suface temperatures in northern Utah, but land-surface contrasts are quite important as well.  

1 comment:

  1. I have noticed that convection usually dies when it moves over these salt flat areas during the day, but often favors these areas at night (similar to the lake itself). I wonder if the salt conducts heat effectively, or if it is due to the higher moisture content in these areas.