Geography Reference
In-Depth Information
static stability to more than 3 km in highly convective conditions. For average
midlatitude conditions the planetary boundary layer extends through the lowest
kilometer of the atmosphere and thus contains about 10% of the mass of the
atmosphere.
The dynamical structure of the flow in the planetary boundary layer is not pro-
duced directly by viscosity. Rather, it is largely determined by the fact that the
atmospheric flow is turbulent. In the
free atmosphere
(i.e., the region above the
planetary boundary layer), this turbulence can be ignored in an approximate treat-
ment of synoptic-scale motions, except perhaps in the vicinity of jet streams, fronts,
and convective clouds. However, in the boundary layer the dynamical equations
of the previous chapters must be modified to properly represent the effects of
turbulence.
5.1
ATMOSPHERIC TURBULENCE
Turbulent flow contains irregular quasi-random motions spanning a continuous
spectrum of spatial and temporal scales. Such eddies cause nearby air parcels to
drift apart and thus mix properties such as momentum and potential temperature
across the boundary layer. Unlike the large-scale rotational flows discussed in
earlier chapters, which have depth scales that are small compared to their horizontal
scales, the turbulent eddies of concern in the planetary boundary layer tend to have
similar scales in the horizontal and vertical. The maximum eddy length scale is thus
limited by the boundary layer depth to be about 10
3
m. The minimum length scale
(10
−
3
m) is that of the smallest eddies that can exist in the presence of diffusion
by molecular friction.
Even when observations are taken with very short temporal and spatial separa-
tions, a turbulent flow will always have scales that are unresolvable because they
have frequencies greater than the observation frequency and spatial scales smaller
than the scale of separation of the observations. Outside the boundary layer, in the
free atmosphere, the problem of unresolved scales of motion is usually not a seri-
ous one for the diagnosis or forecasting of synoptic and larger scale circulations
(although it is crucial for the mesoscale circulations discussed in Chapter 9). The
eddies that contain the bulk of the energy in the free atmosphere are resolved by
the synoptic network. However, in the boundary layer, unresolved turbulent eddies
are of critical importance. Through their transport of heat and moisture away from
the surface they maintain the surface energy balance, and through their transport of
momentum to the surface they maintain the momentum balance. The latter process
dramatically alters the momentum balance of the large-scale flow in the bound-
ary layer so that geostrophic balance is no longer an adequate approximation to
the large-scale wind field. It is this aspect of boundary layer dynamics that is of
primary importance for dynamical meteorology.
