The Orinoco Low-Level Jet: An Investigation of its Mechanisms of
Formation Using the WRF Model
Abstract
The Orinoco low-level jet (OLLJ) is characterized using finer
horizontal, vertical, and temporal resolution than possible in previous
studies via dynamical downscaling. The investigation relies on a
5-month-long simulation (November 2013-March 2014) performed with the
WRF model, with initial and boundary conditions provided by the GFS
analysis. Dynamical downscaling is demonstrated to be an effective
method not only to better resolve the horizontal and vertical
characteristics of the Orinoco low-level jet but also to determine the
mechanisms leading to its formation. The OLLJ is a single stream tube
over Colombia and Venezuela with wind speeds greater than 8 m s-1 , and
four distinctive cores of higher wind speeds varying in height under the
influence of sloping terrain. It is an austral summer phenomenon that
exhibits its seasonal maximum wind speed and largest spatial extent
(2100 km × 450 km) in January. The maxima diurnal mean wind speeds
(13–17 m s-1) at each core location occur at different times during the
night (2300–0900 LST). The momentum balance analysis in a natural
coordinate system reveals that the OLLJ results from four phenomena
acting together to accelerate the wind: a sea-breeze penetration,
katabatic flow, three expansion fans, and diurnal variation of turbulent
diffusivity. The latter, in contrast to the heavily studied nocturnal
low-level jet in the U.S. Great Plains region, plays a secondary role in
OLLJ acceleration. These results imply that LLJs near the equator may
originate from processes other than the inertial oscillation and
topographic thermal forcing.