THERMODYNAMIC MODEL FOR THE
DEVELOPMENT OF A CONVECTIVELY
UNSTABLE BOUNDARY LAYER
D. J. CARSON AND F. B. SMITH
Boundur! Luyer Rtwtrrch Brunch. Af~rrorolugicul Ofice. Brucknell. Brrkshire. Enqbind
1. INTRO1)lJCTION
At the present time there is a great deal of interest being shown in the
study of the nonstationary aspects of the atmospheric boundary layer. One
reason for this is the desire to incorporate the boundary layer realistically
into numerical forecasting and general circulation models. Another is the
need to provide reliable estimates of the depth and character of the mixing
layer for use in schemes dealing with the dispersion of concentrations of
atmospheric pollutants where steady-state theories have proved to be totally
inadequate.
The stability of the mixing layer is of primary importance in short-range
pollution studies; however its depth h becomes increasingly more important
in determining ground-level concentrations when the distance of travel is
greater than about 1011 from the source. The inclusion of such nonsteady
features in a practical scheme for estimating the vertical dispersion of pollutants has been outlined by Smith (1972). Also, the ability to specify the
diurnal cycle of boundary-layer evolution becomes important when dealing
with pollutants tracked for several days on a regional scale, and such effects
are discussed elsewhere in this Symposium (Pasquill, Vol. 18B, p.1).
Although intricate parameterizations and numerical models are being
developed for the study of evolving boundary layers (Deardorff. 1972a,b,
1973), there remains a need to provide relatively simple parametrizations for
general use and a start would be to consider the development of the important dry, convectively unstable layer capped by a stable layer.
Observations of the boundary layer in convective situations show that in
general it is a diurnally evolving system with discontinuities around sunrise
and sunset. The discontinuities arise because we distinguish between the
relatively shallow nocturnal inversion layer in which buoyancy and viscous
effects suppress any mechanically generated turbulent motions and the more
111
DEVELOPMENT OF A CONVECTIVELY
UNSTABLE BOUNDARY LAYER
D. J. CARSON AND F. B. SMITH
Boundur! Luyer Rtwtrrch Brunch. Af~rrorolugicul Ofice. Brucknell. Brrkshire. Enqbind
1. INTRO1)lJCTION
At the present time there is a great deal of interest being shown in the
study of the nonstationary aspects of the atmospheric boundary layer. One
reason for this is the desire to incorporate the boundary layer realistically
into numerical forecasting and general circulation models. Another is the
need to provide reliable estimates of the depth and character of the mixing
layer for use in schemes dealing with the dispersion of concentrations of
atmospheric pollutants where steady-state theories have proved to be totally
inadequate.
The stability of the mixing layer is of primary importance in short-range
pollution studies; however its depth h becomes increasingly more important
in determining ground-level concentrations when the distance of travel is
greater than about 1011 from the source. The inclusion of such nonsteady
features in a practical scheme for estimating the vertical dispersion of pollutants has been outlined by Smith (1972). Also, the ability to specify the
diurnal cycle of boundary-layer evolution becomes important when dealing
with pollutants tracked for several days on a regional scale, and such effects
are discussed elsewhere in this Symposium (Pasquill, Vol. 18B, p.1).
Although intricate parameterizations and numerical models are being
developed for the study of evolving boundary layers (Deardorff. 1972a,b,
1973), there remains a need to provide relatively simple parametrizations for
general use and a start would be to consider the development of the important dry, convectively unstable layer capped by a stable layer.
Observations of the boundary layer in convective situations show that in
general it is a diurnally evolving system with discontinuities around sunrise
and sunset. The discontinuities arise because we distinguish between the
relatively shallow nocturnal inversion layer in which buoyancy and viscous
effects suppress any mechanically generated turbulent motions and the more
111