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.