Soundings, Clouds, and Convection

Page 8 of 11

Superadiabatic Layers

To assess stability, you don't need to be lifting air parcels all over the place. All you need to do is look for layers in which the atmosphere is superadiabatic, which is where the lapse rate is steeper than the dry adiabats. Actually, such layers are somewhat rare, and are often erroneous. None of the soundings we've seen so far include an unstable layer. The closest they come are layers in which the environmental lapse rate just matches the dry adiabatic lapse rate, such as the 650-800 mb layer at 00Z in Salt Lake City.

Superadiabatic layers are rare precisely because they are unstable. If there's any turbulence whatsoever, the warm air at the bottom of the layer will rapidly be lifted upward, and the cold air will sink downward. The temperature distribution becomes neutral or stable, and the turbulence dies down.

The most common location for superadiabatic layers is right at the ground on a sunny day. The ground heats up the air, which becomes unstable and rises. Colder air from aloft is brought down, which comes in contact with the ground and in turn heats up. The result is a shallow superadiabatic layer, topped by a deep layer of neutral stability in which the environmental lapse rate is very nearly dry adiabatic. The air is continually being mixed throughout these layers, and both the potential temperature and the mixing ratio tend to be constant within them.

Questions or Comments

E-mail John Fulton < jdfult@nimbus.tamu.edu >