|BOUNDARY LAYER DECOUPLING
METEOROLOGIST JEFF HABY
Boundary layer decoupling is the term used for the change in weather conditions that occurs in the
vertical due to significant radiational cooling of the earth's surface. During the afternoon the
boundary layer (lowest portion of troposphere) tends to be well mixed. This is
because solar radiation warms the surface and this air rises like bubbles in a pot of
boiling water. These rising thermals of air mix with the surrounding air to produce a fairly
constant lapse rate of cooling with height within the boundary layer. The boundary layer is well mixed
and at its deepest during the day.
At night there is no more solar radiation to
initiate this mixing process. The earth's surface cools by emitting longwave radiation. The
air closest to the surface will cool the most. The air does not mix as well at night since a stable situation
of colder air under warmer air develops. Eventually the boundary layer will decouple. A layer
of air that is much cooler than it was during the day will develop nearest the surface with warmer
air aloft in a layer known as the residual layer. The atmosphere has the best chance to decouple
at night when winds are light, the air is dry and skies are clear.
If the winds in the boundary layer are brisk, a front is moving through or storms are influencing
the area then the boundary layer will tend to not decouple at night or it will not decouple as significantly.
Strong winds will keep the air well mixed and the shallow layer of much cooler air near
the surface will not be able to develop. Mixing of air from thunderstorm outflow or brisk
winds in the vicinity of a frontal boundary will also keep the air well mixed.
Being able to forecast how much the boundary layer will decouple is important. It is important
since the overnight low will be much cooler when the boundary layer decouples as compared
to when it does not. If the winds are brisk or there is some process mixing the air then
the low temperature will often be above the model MOS guidance. If complete decoupling occurs
and the skies are clear with very light winds then the low temperature will often fall below
model MOS guidance.