3.4 Turbulent diffusivity
diurnal variation
The Blackadar mechanism relies on the diurnal variation of turbulent
diffusivity to produce an inertial oscillation of the horizontal wind,
which eventually will generate the LLJ. Such inertial oscillation starts
at sunset when turbulence diffusion in the boundary layer is reduced by
static stabilization of the surface layer (via radiative cooling) so
that frictionally retarded air is no longer transported upward, and the
wind above becomes frictionless and accelerates. As the magnitude of the
horizontal wind increases so does the Coriolis force, which makes the
wind turn clockwise (counterclockwise in the Southern Hemisphere) and
describe an oscillation around the geostrophic wind. At sunrise,
radiative heating activates the turbulent mixing again, so friction adds
to the force balance and the horizontal wind speed reduces to
subgeostrophic values.
From a different perspective, this oscillation can be seen as the
rotation of the ageostrophic wind in a circle centered on the
geostrophic wind so that at some time during the night—which varies
depending on the value of the Coriolis parameter f for each
latitude—the ageostrophic wind has the same direction as the
geostrophic wind. With both geostrophic and ageostrophic winds pointing
in the same direction, the horizontal wind becomes supergeostrophic thus
producing the LLJ.
In the hourly 950–800-hPa streamwise momentum balance at C1–C4 OLLJ
cores (Fig. 14a–d), obtained by averaging a 45 km \(\times\) 45 km box
at the maximum wind speed location along 64ºW, 66.9ºW, 5ºN, and 1.5ºN
respectively, the residual term (i.e., frictional effects) is the
primary driving term causing total deceleration of the streamwise
horizontal wind during daytime. On the other hand, during nighttime,
when its value approaches close to zero (i.e., no frictional effects),
the other forces in balance cause the positive streamwise acceleration
of the wind. The latter is particularly true for the C3–C4 regions
(Fig. 14c–d), whereas for the C1–C2 regions (Fig. 14a–b) the negative
advection term shortens the total streamwise nocturnal acceleration
cycle. The streamwise deceleration, initiated by the advection term at
the C1–C2 regions, contributes to the early appearance of the maxima
wind speeds, in comparison to the C3–C4 cores (Table 1), and is an
indication of drainage flow (down valley) opposing the large-scale
motions.