First Frost of the Season is Revealing

Most of the region saw its first frost today.  

And it offers a lot to learn from.

Let's begin by looking at the surface low temperatures observed across the region (see below).  

Keep in mind that "surface" air temperatures reported at airports and other locations are NOT at ground level, but taken at 2 meters (about 6 feet) above the surface.

On cold nights, the temperature at the surface is usually colder than at 2 meters, often by 2-5 degrees F.  

Wow.  Substantial temperature range... from around 50F to below freezing!  

Warm near the water (around 50F near the coast), relatively warm near the Sound and the Strait (mid to upper 40s), but below freezing in the western interior (e.g., near Olympia Airport).    

30s in eastern Washington

Minimum temperatures were cooler at the surface than aloft (the opposite of the normal daytime situation), as shown by the temperature profiler at SeaTac Airport (see below at 8 AM).  

The warmest temperatures were at around 1600 ft.

The situation changes during the day, as the surface is warmed by the sun, as illustrated by the Seattle temperature profile at noon (see below).

As noted in previous blogs, the cold overnight temperatures are associated with longer nights and a lack of clouds aloft, which result in very effective cooling at the surface by the emission of infrared radiation from the surface (see below).

The visible satellite image from this morning shows the situation, with clear skies over much of the region and filaments of fog in a few valleys.

Now the problem.

 

In my "day job", I work to improve the skill of atmospheric forecast models.

A potent tool in doing so is comparing model simulations against observations. At Seattle-Tacoma Airport, the model simulation (colored lines) did a good job in matching reality (orange dots).

But at Olympia Airport, where temperatures got colder, the simulations were not as good, with models being too warm for the morning lows (but quite good for the daytime highs).

More work is needed, which is good. 😀