Geoscience Reference
In-Depth Information
Figure 9.5 The time variation of the surface temperature flux observed in the
Kansas experiment. The scatter in these one-hour averages is due to the day-to-
day variations during the three weeks of midsummer observations. From
Wyngaard
(
1973
).
Because of its key role in determining the surface flux of virtual temperature,
which in turn determines the stability state, the surface energy balance is one of the
most important influences on the atmospheric boundary layer. Numerical models
for weather forecasting include a detailed treatment of it.
The
neutral
state of zero heat flux throughout the ABL is relatively rare (it
is sometimes said it exists only in the literature). Under overcast conditions the
surface and air temperatures can be very nearly equal so that the surface heat flux
is negligible and
z
i
/L
0. But if there is an overlying inversion within the first
kilometer or two of the atmosphere, entrainment frombelowcan diffuse this capping
inversion down into the boundary layer. Thus the vertical temperature flux vanishes
at the surface but is negative aloft, causing most of the ABL to be stably stratified.
The most likely place to find a neutral planetary boundary layer might be the
upper ocean. The state of boundary-layer observations in the ocean substantially
lags that in the atmosphere, however, because the ocean is a much more challenging
research environment.
9.3 Buoyancy effects
One of the important attributes of atmospheric turbulence is its strong sensitivity
to buoyancy forces. We'll explain this first through simple scaling arguments, and
then discuss the two limiting stability states.
