SURFACE LAYER OF THE ATMOSPHERE
IN UNSTABLE CONDITIONS
ROBERT R. LONG
Drpnrtrtirnr of Mrchunics (inti Mclrt*riul.v Science
7 h c Johns Hopkins Unicersity. Balriniorr. Murylund 2I?IX. L'.S..L
1. INTRODI ~CTION
When the sun heats the ground or when cold air flows over a warm
surface, the lower portion of the atmosphere becomes unstable and turbulent convection results in which the potential energy created by the heating is
converted into kinetic energy of the turbulence. In addition, the shear provides another source of turbulent kinetic energy. The accompanying energy
loss is accomplished by a cascade of energy from the larger eddies to the
smaller eddies where the energy is ultimately dissipated. The temperature
fluctuations also tend to be reduced by the action of molecular heat conduction in the small eddies.
Many investigators have explored the problem of the surface layer from
an observational and theoretical viewpoint. A valuable reference in English
is a recent translation of a book by Monin and Yaglom (1971) which emphasizes contributions by scientists in the U.S.S.R. Many experiments have been
constructed to study convection between two stationary horizontal surfaces,
heated below and cooled above (Malkus, 1954a,b; Thomas and Townsend,
1957: Silveston, 1958; Croft, 1958; Townsend, 1959; Globe and Dropkin.
1959; Somerscales and Dropkin, 1966; Deardorff and Willis, 1967; Somerscales and Gazda, 1969). and one recent experiment also involves shear
(Townsend. 1972).
If one divides the scientists interested in turbulent convection into two
groups, one finds that the experimentalists all attach major importance to
the molecular coefficients of viscosity and conduction, v and K, whereas the
atmospheric scientists believe that these quantities are unimportant in the
surface layer.
There are two reasons for this. One is that laboratory experiments are on
much smaller scales than those characteristic of the atmosphere, so that the
relevant Reynolds numbers or Rayleigh numbers in the laboratory are much
131
IN UNSTABLE CONDITIONS
ROBERT R. LONG
Drpnrtrtirnr of Mrchunics (inti Mclrt*riul.v Science
7 h c Johns Hopkins Unicersity. Balriniorr. Murylund 2I?IX. L'.S..L
1. INTRODI ~CTION
When the sun heats the ground or when cold air flows over a warm
surface, the lower portion of the atmosphere becomes unstable and turbulent convection results in which the potential energy created by the heating is
converted into kinetic energy of the turbulence. In addition, the shear provides another source of turbulent kinetic energy. The accompanying energy
loss is accomplished by a cascade of energy from the larger eddies to the
smaller eddies where the energy is ultimately dissipated. The temperature
fluctuations also tend to be reduced by the action of molecular heat conduction in the small eddies.
Many investigators have explored the problem of the surface layer from
an observational and theoretical viewpoint. A valuable reference in English
is a recent translation of a book by Monin and Yaglom (1971) which emphasizes contributions by scientists in the U.S.S.R. Many experiments have been
constructed to study convection between two stationary horizontal surfaces,
heated below and cooled above (Malkus, 1954a,b; Thomas and Townsend,
1957: Silveston, 1958; Croft, 1958; Townsend, 1959; Globe and Dropkin.
1959; Somerscales and Dropkin, 1966; Deardorff and Willis, 1967; Somerscales and Gazda, 1969). and one recent experiment also involves shear
(Townsend. 1972).
If one divides the scientists interested in turbulent convection into two
groups, one finds that the experimentalists all attach major importance to
the molecular coefficients of viscosity and conduction, v and K, whereas the
atmospheric scientists believe that these quantities are unimportant in the
surface layer.
There are two reasons for this. One is that laboratory experiments are on
much smaller scales than those characteristic of the atmosphere, so that the
relevant Reynolds numbers or Rayleigh numbers in the laboratory are much
131