Lessons in Boundary Layer Meteorology

The boundary layer is the lower portion of the atmosphere that is affected by friction and the transfer of heat from the earth's surface into the atmosphere.  It's depth varies depending on the time of day, the environmental conditions, and the local land surface and topographic characteristics.

Typically the evolution of the boundary layer is conceptualized as shown below.  During the morning, sunlight heats the Earth's surface, resulting in a transfer of heat into the atmosphere.  This erodes the shallow stable layer that typically forms overnight from radiational cooling.  The boundary layer then continues to grow, eventually reaching maximum depth later in the day (the time varies depending on conditions).  Within the boundary layer, turbulence driven by wind shear and surface heating results in considerable mixing.  Concentrations of gasses like water vapor and carbon dioxide are often nearly constant with height in the boundary layer.  Hence, the "mixed layer" label in the schematic.  Often there is a stable layer or inversion at the top of the mixed layer. 

Source: COMET/METED

Near and after sunset, the Earth's surface cools rapidly.  Heat is transferred from the atmosphere, which cools rapidly near the Earth's surface.  This forms a shallow nocturnal stable boundary layer, that may be tens or perhaps one or two hundred meters deep.  Above this layer, the old remnants of the boundary layer remain.  This layer is called the residual layer.

In quiescent weather conditions, this pattern repeats itself daily: 1. The sun rises;  2. The nocturnal stable boundary layer is "burned off";  3. The boundary layer grows rapidly into the residual layer and constituents (including pollutants) are mixed through its depth;  4. The sun sets and the nocturnal boundary layer forms and strengthens, leaving a residual layer aloft.

Evidence of these processes was very apparent on my ride above Ensign Peak this morning.  At the time, the top of the pollution layer was perhaps 1 km above the valley floor.  There was a very clear discontinuity in the pollution at that level.  It was early enough that I suspect that discontinuity did not mark the top of today's boundary layer, but instead the top of the residual layer, which was loaded with pollutants from emissions from yesterday.

I've added by eye yesterday's sounding (red=temperature, green=dewpoint) from the airport.  In this case, the temperature or dewpoint decrease if the line slopes to the left and increase if it slopes to the right.  The top of the residual layer was very near the elevation of an inversion in yesterday's sounding, as one might expect.  Below that inversion, the atmosphere yesterday afternoon was relatively well mixed.  For example, temperature decreased rapidly with height at a rate of about 10ºC per kilometer, which is consistent the density being constant with height.  Dewpoint decreased with height at a rate close to that expected if the concentration of water vapor is constant with height.  However, the decrease with height near the surface was more rapid than one might expect if the atmosphere is well mixed.  This isn't unusual as the turbulence typically can't mix the atmosphere fast enough right near the ground to make the water vapor concentration constant with height if there is evaporation or transpiration occurring.

Expect views like this most days this week due to the presence of high pressure.  Note that the sharp top of the residual layer is most apparent in the morning and evening if you are at an elevation somewhat above the valley floor and you face somewhat toward the sun (but perhaps not right at it).  This is an optical effect related to how pollution scatters sunlight.

Announcement

I'm pleased to announce that we will be exhibiting the Doppler on Wheels mobile radar (pictured below) at the Natural History Museum of Utah this coming Saturday, November 4, from 10–5 PM.  Bring yourself, the family, and friends.  The exhibit is also one of their Behind the Scenes days when they open the place up for a public viewing of their collections.  Come and geek out!