S. Khanna et Jg. Brasseur, 3-DIMENSIONAL BUOYANCY-INDUCED AND SHEAR-INDUCED LOCAL-STRUCTURE OF THE ATMOSPHERIC BOUNDARY-LAYER, Journal of the atmospheric sciences, 55(5), 1998, pp. 710-743
Three-dimensional visualization together with statistical measures are
used to describe the instantaneous local structure of the atmospheric
boundary layer (ABL) under various stability states using large-eddy
simulation (LES) data. To explore the relative roles of buoyancy and s
hear in ABL structure, a wide range of -z(i)/L ABL states, from 0.44 t
o 730, is analyzed. It is known that buoyancy-induced updrafts and dow
ndrafts are primarily responsible for the upward flux of momentum, hea
t, and passive scalar, and strongly influence near-ground horizontal m
otions. These buoyancy-induced features of the convective boundary lay
er (CBL) are presented here in clearly observable 3D visual images of
vertical velocity and temperature, showing large turbulent cell-like s
tructure several z(i) in horizontal extent. The horizontal length scal
es of the temperature field near the ground are found to be of the ord
er of the horizontal velocity length scales. It is noted by comparing
visual structure with spectra that the disparity in the near-ground ho
rizontal scale between temperature and vertical velocity reflects the
structure of more localized thermals within the large-scale cells. By
contrast, the structure of the near-neutral atmospheric boundary layer
is quite different. Recent LES studies have shown that, like the flat
-plate boundary layer, the dominant energy-containing motions in a nea
r-neutral atmospheric boundary layer are near-ground shear-induced reg
ions of high-and low-speed flow. Several features of the low-speed str
eaks are examined. Most importantly, there exists an influence by z(i)
-scale outer eddies on the structure of near-ground streaks, which, it
is argued, strengthens at higher -z(i)/L. Warm fluid accumulates in t
hese low-speed streaks, localizing buoyancy forces there that, at suff
iciently high -z(i)/L, drive the warm fluid vertically within sheets a
ligned with the mean wind. These coherent sheetlike updrafts turn at t
he capping inversion to form the often-observed large-scale streamwise
roll vortices. In this way, it is argued, shear-induced near-ground s
tructure of the surface layer directly influences the global structure
of the moderately convective ABL. It follows, therefore, that inadequ
acies in subgrid-scale parameterization near the ground can influence
the structure of the entire ABL. In particular, the well-known overpre
diction of mean shear near the ground by standard Smagorinsky closures
increases the streamwise coherence of the shear-induced low-speed str
eaks, thereby increasing the overall streamwise coherence of the verti
cal velocity field.