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.