The stable boundary layer - Turbulence in the Atmosphere - page 273

the boundary-layer top. In their conclusions they pointed to the broad agreement
with the models of Nieuwstadt ( 1984 ), as adapted by Derbyshire ( 1990 ), and of
Brost and Wyngaard ( 1978 ).
The subgrid model used by Mason and Derbyshire is the standard one
(Chapter 6) in which the deviatoric subgrid stress is taken to be proportional to the
resolved strain rate. Thismakes the rate of transfer of kinetic energy from resolved to
subgrid motions, which is the subgrid stress contracted with this resolved strain rate
(Chapter 6) , positive definite. But this rate of transfer is known to be instantaneously
of either sign in turbulent flows - i.e., to or from the resolved motions. Subsequently
Kosovic and Curry ( 2000 ), contending that the standard subgrid model with one-
way energy transfer may not be appropriate for SBLs because it can cause spurious
laminarization of the flow, used LES with such two-way energy transfer to study
the SBL. Their results also agreed well with observations and with Nieuwstadt's
analytical model.
12.2 The late-afternoon ABL transition over land
In clear weather, the net shortwave radiative flux at the surface (Figure 9.4)
approaches zero as the sun nears the horizon in late afternoon. The rate of energy loss
through longwave emission becomes the dominant term in the surface energy bud-
get, changing its net effect from surface warming to surface cooling. This changes
the sign of the vertical gradient of temperature at the surface, which changes the
direction of the surface heat flux (Problem 12.3) . Above the diffusive sublayer on
the surface (Chapter 1) this now-downward heat flux continues to be carried by the
turbulence. Its divergence, supplemented by the divergence of radiative heat flux
( Garratt and Brost , 1981 ), causes the near-surface air to cool.
As time proceeds, turbulence diffuses this cooling upward in the boundary layer.
We'll see that the effects of the resulting stable stratification increasewith increasing
distance from the surface. Meanwhile the CBL turbulence well aloft decays as its
rate of input through buoyant production decreases to zero. This decay attenuates
the turbulent stress divergence term in the mean horizontal momentum budget,
which, as we'll see, triggers an inertial oscillation in the horizontal wind field aloft.
12.2.1 The near-surface response
We' l l define transition as the time when the surface temperature flux Q 0 ( Figure
9.5) changes sign. After transition the large, CBL-spanning eddies begin to decay,
and the emerging stable stratification also contributes to the decrease in surface
stress and near-surface mean wind speed ( Problems 12.10 , 12.11 ). Figures 12.2