A supplemental illustration of the generalized effect of turbulent
diffusivity diurnal variation is depicted in Fig. 4, where frictional
decoupling of the 950–800-hPa wind after sunset cause the simultaneous
streamwise acceleration of the flow over large areas across the domain
(Fig. 4d), whereas the activation of turbulent mixing during daytime
induce the overall deceleration of the wind (Fig. 4a, b). In a like
manner, the diurnal evolution of the potential temperature contours and
mean wind speeds at different locations (Fig. 6, 8, and 10) show that
once the OLLJ has developed during nighttime (Fig. 6d–f, 8d–f, and
10d–f), the growth of the daytime convective boundary layer erodes the
LLJ from below diminishing its mean wind speed (Fig. 6a–c, 8a–c, and
10a–c).
Interestingly, despite the turbulent diffusivity diurnal variation
triggering the inertial oscillation, so causing the clockwise rotation
of the horizontal wind at each of the core locations (not shown), the
Blackadar mechanism does not strictly apply to all of them. The
950–800-hPa streamwise and crosswise diurnal cycle of the mean
horizontal, geostrophic, and ageostrophic wind (Fig. 15), for the same
locations as in Fig. 14, show that the maximum streamwise wind speed is
the result of the combined streamwise geostrophic and ageostrophic winds
at the C1 and C3 regions only (Fig. 15a, c), whereas the maximum wind
speed in the C2 and C4 regions (Fig. 15b, d) results from a balance
between the geostrophic and ageostrophic winds. In general, the
horizontal wind in the C2 and C4 regions reflects such balance during
the entire diurnal cycle. Moreover, the minimum wind speed is
subgeostrophic at the C1 and C2 regions only, whereas it is
supergeostrophic in the C3 and C4 regions. Strictly speaking, the minima
wind speeds should be subgeostrophic everywhere, according to
Blackadar’s theoretical predictions.
Hence, the diurnal variation of turbulent diffusivity and subsequent
inertial oscillation contribute to the OLLJ formation, although it plays
a secondary role, especially close to the equator where the Coriolis
parameter is so small that ageostrophic effects (e.g., changes in PGF
due to terrain heating) drive the wind. The streamwise ageostrophic wind
is, in general, of higher magnitude than the streamwise geostrophic wind
in the C3 and C4 regions (Fig. 15c, d).