|BOUNDARY LAYER DECOUPLING
METEOROLOGIST JEFF HABY
The diagram below shows an example of the boundary layer being decoupled from the free atmosphere above it. The temperature is rapidly
increasing in temperature with height in the boundary layer. Above the boundary layer, the temperature decreases with height. This abrupt
transition is an indication that the characteristics and influences on the air are very different in the boundary layer as compared to
the free atmosphere.
Decoupling can occur from a strong influence of the Earth’s surface on the air above it or when a shallow air mass moves along the surface. Decoupling
is common at night especially in the winter season when the Earth’s surface cools off dramatically at night due to longwave energy
emission. This effect is maximized on long nights with light wind, clear skies and dry air. The Earth’s surface cools the air
above it by contact cooling. Going higher above the Earth’s surface in the boundary layer results in warming due to the reduced
influence of the cold ground on the air’s temperature. Also, with the temperature increasing with height, the air is very
stable and resists significant mixing. Thus a shallow region of cold air will develop in contact with the ground surface. These
types of temperature profiles can also occur when shallow polar or arctic air moves into a region. The polar or arctic air is
very dense and thus hugs along the ground surface. With the coldest air being at the surface, the temperature will increase
with height in the boundary layer.
The layer on the diagram labeled as residual layer occurs when the boundary layer undergoes a dramatic temperature structure shift while
the air within the layer higher aloft is not as impacted. The boundary layer is often deeper during the afternoon hours when there
is a better mixing of air due to rising thermals from solar heating of the ground surface. The boundary layer and free atmosphere
will have a more gradual transition and is thus coupled. At night, the surface can cool off dramatically and decouple
substantially from the free atmosphere, leaving a very stable shallow boundary layer with a residual layer of air
above this that is not nearly as influenced by the Earth’s surface.