Geoscience Reference
In-Depth Information
temperature with time that is almost independent of height, consistent with the
fact that there is a well-mixed atmosphere over this height range. The heat flux
changes sign through the mixed layer revealing that this ABL warming is supported
partly by upward heat from the ground at lower levels and partly by downward
heat in the warm air entrained from above the inversion layer at higher levels. The
heat flux remains negative (i.e., downward) through and just above the inversion,
but the rate of change in heat flux becomes positive. Equation (18.2) therefore
implies that at these levels the air that is being entrained from the overlying free
atmosphere is cooling as it merges with the cooler air in the growing mixed layer.
The behavior of the moisture flux is less well-illustrated by the data, but is clearly
very different. The moisture flux remains positive through the mixed layer and
then falls off progressively through and just above the inversion layer. The change
in moisture flux with height through the mixed layer is small (slightly negative on
one day and slightly positive on the second day), so Equation (18.1) implies there is
only modest change in moisture content with time. Through and just above the
inversion layer, the moisture flux remains positive, but it falls rapidly. At these levels
moisture from the surface, which has largely passed straight through the mixed
layer, is used to moisten the drier free atmosphere air as it is entrained downward
to become part of the growing mixed layer.
In summary, the behavior of observed fluxes through and above the evolving
daytime boundary layer shows boundary layer growth involves inputs from both
below and above the inversion layer. Growth is achieved by entraining air from the
free atmosphere which is warmer and drier than the air already in the mixed layer.
Moisture that entered the mixed layer from below is largely used to moisten the
dry entrained air. Hence, the humidity of the air in the ABL is often reasonably
constant through the day. On the other hand, sensible heat is brought into the ABL
from both below and above, and there is a significant diurnal cycle in ABL
temperature during the day. This comparatively simple model of boundary layer
growth over flat uniform terrain can be modeled quite well. Figure 18.4a, for
example, shows the modeled time evolution of profiles of potential temperature,
which can be compared with observed profiles measured at Wangara, Australia on
a day in 1967, shown in Fig. 18.4b. Figure 18.4c shows modeled profiles of humidity
on the same day which can be compared with the observed profiles shown in
Fig. 18.4d.
Nighttime ABL profiles
At night, turbulence in the ABL declines and other terms in the equations
describing the mean flow remain significant in comparison. Over uniform flat
terrain the rate of change of mean values or turbulent fluxes with distance along
the X and Y axes can still be assumed to be small, but subsidence (the mean wind
speed along the Z axis) cannot necessarily be assumed to be zero. Also, because
the net radiation is now all longwave radiation, temperature variation with height
