CASES-99: Boundary Layer Depth
The following file includes our latest estimates of the boundary-layer
depths at the main tower. Here we provide a single estimate for each
record, as opposed to our previous file, which contained multiple
estimates of the boundary-layer depth for each record.
The boundary-layer depth is estimated from profiles of the buoyancy flux
with consultation of the momentum flux and turbulence energy profiles.
The
boundary-layer depth is more easily defined in terms of the buoyancy flux
because the small stratification above the surface inversion layer often
forces the buoyancy flux to small values even when the turbulence energy
and momentum flux do not decrease with height. In this sense, the height
dependence of the buoyancy flux is more regular than that for the
turbulence energy and momentum flux. Even so, the buoyancy flux still
increases with height for part or all of the tower layer in many cases due
to near collapsed turbulence close to the surface and significant
turbulence at higher levels with some stratification. The evaluation of
the boundary-layer depth based on the 160 buoyancy flux profiles where
data is available at all levels yields:
1. 44 cases where the buoyancy flux decreases with height to small values
and then remains relatively small at the higher levels in the tower layer,
allowing relative clear definition of the boundary-layer depth in terms of
the buoyancy flux
2. 40 cases where a boundary-layer depth is definable but the buoyancy
flux does not remain small above the boundary layer
3. 86 cases where the turbulence generally increases with height across
the tower layer although some of these cases may include a very shallow
layer near the surface where the buoyancy flux decreases with height,
belonging to class 2 as well.
4. 6 cases where the buoyancy flux was relatively independent of height,
implying a deep boundary layer and 7 cases where the buoyancy flux varied
erratically with height, sometimes corresponding to layering.
Thus, a traditional boundary layer could be defined only about 25% of the
time, even less often when posed in terms of the momentum flux or
turbulence energy. The buoyancy flux increased with height for much of
the tower layer for more than half of the cases.
EXPLANATION FOR THE FILE
columns: Idex Jday Hour Hbl Qual UD
Idex dummy index 1,2,..151
Jday julian day of 1999 for start of 1-hour period
Hour of day (Central Standard Time) for start of 1-hour period
Hbl boundary layer depth estimate (m). When the boundary layer depth
exceeds
the tower level, it is set to 99 m.
Qual 1 means that there is some confidence in the estimate of Hbl.
Many cases of bl-depth are poorly defined. About 15% of the cases
are
considered too ambiguous to provide even a tentative estimate of the
boundary layer depth and the records are not in this file. When Qual
= 1 the estimates of the bounday layer
depth may still contain some amiguity due to the flux loss in the
lowest 5 m (fluxes increase with height). The main
criteria is that the heat flux is small above the boundary layer.
Qual=0 means that the boundary-layer depth is computed as the level where
the buoyancy flux is a minimum and that the buoyancy flux and
turbulence may be substantially larger at higher levels, or, the boundary
layer depth seems semi-definable the height variation of the buoyancy
flux is somewhat erratic.
UD 1 means the heat flux at higher levels is comparable to or greater
than
that near the surface AND the turbulence energy increases with height
across the tower layer.