3.2 Katabatic flow
The contribution to the OLLJ streamwise acceleration coming from the
Coastal Cordillera (i.e., mesoscale area of positive acceleration at
~9ºN– 68.5ºW in Fig. 4d) has its origin in two factors:
the radiative cooling of the southern slope near sunset, which produces
shallow density currents; and the advection of cool maritime air over
the top of the ridge. The diurnal variations of potential temperature
and mean wind speed along the axis of propagation K (Fig. 10), show that
the combination of these two factors starting around 1500 LST (Fig.
10b), cools down the hillside enough to generate a fast, cold downslope
flow similar to that of a Bora phenomenon (Stull, 2015b; Fig. 10c–f).
The maximum wind speed over the lee slope (13 m s-1)
is attained at 2300 LST (Fig. 10e). Because the flow is supercritical
(Froude Number = 2.4), once it reaches the lowlands it creates a
nocturnal hydraulic jump, as suggested by the curved isentropes over the
southern flank of the mountain (Fig. 10d–f).
As cool air continues flowing downslope the Coastal Cordillera, it acts
as a gravity current with a leading edge around 9.2ºN at 1900 LST (Fig.
10d) that propagates up-valley during the night, weakening near 6ºN–
72ºW at 0200 LST (not shown) once it encounters a point wake in the
Eastern Cordillera. Hence, this density current exhibits a ground speed
of ~14 m s-1, which results from the
combination of the depth and magnitude of the negatively buoyant air
(\(C_{\text{gc\ }}\sim\)5 m s-1), and the background
wind speed (~9 m s-1). The
streamwise-acceleration area that follows this gravity-current leading
edge ends up merging at 0300 LST around 7ºN– 70ºW with the trailing
acceleration pocket originated by the sea breezes (Fig. 4f).
Evidence for this katabatic flow having the characteristics of a gravity
current proposed by Simpson (1987), Koch & Clark (1999), and Koch et
al. (2005), is given in Fig. 11, which shows the diurnal cycle of the
wind field, potential temperature, surface pressure, and mixing ratio at
a selected surface location (7.5ºN– 69.7ºW) along the axis of
propagation K (Fig. 5). With the arrival of the gravity current at
around 2200 LST, there is an increase in wind speed and a small change
in its direction (Fig. 11a); meanwhile, a change in the rate of
nocturnal cooling with a corresponding change in the increasing surface
pressure rate is produced (Fig. 11b). The sustained increase in mixing
ratio indicates the maritime origin of the gravity current.
In a similar way to the Unare and Orinoco-delta sea breezes, as the
katabatic flow propagates over the Llanos and the near-surface layer
ahead of it stably stratifies (via nocturnal radiative cooling), the
intrusion of this denser current could generate waves in the form of
bores or solitary waves. However, the evaluation of the ratio \(\mu\)[Eq. (3)] yields a value of 0.12, so indicating that flow is
supercritical, and no waves can be spawned.