4 Summary and conclusions
Dynamical downscaling of the large-scale atmospheric conditions over
northern South America, shows the presence of a low-level wind maximum
occurring during the austral summer in the 950–800-hPa layer. This
low-level wind maximum, herein identified as the Orinoco low-level jet
(OLLJ), is a single stream tube (2000 km long \(\times\) 300 km wide\(\times\) 3 km deep, approximately) over Colombia and Venezuela, with
mean wind speeds greater than 8 m s-1. The OLLJ
exhibits its seasonal maximum wind speed and largest spatial extent
(2100 km × 450 km) in January, and its interaction with the surrounding
topography produces four localized cores of higher wind speeds (C1–C4)
along its curved axis of propagation.
In the diurnal cycle, the altitude of the cores increases in the
streamwise direction (~500 m, 700 m, 700 m, and 1250 m
AGL, respectively) under the influence of the sloped terrain. The maxima
diurnal mean wind speeds (13–17 m s-1) at each core
location occur at different times during the night (2300, 0400, 0700,
0900 LST, respectively), thus showing an acceleration of the wind in the
streamwise direction that starts in the Orinoco Delta and ends over the
Amazon forest. The wind speeds are a minimum everywhere in the afternoon
(~8 m s-1, 1300–1600 LST).
Since acceleration of the flow is not uniform across the domain, as it
is somewhat expected from the inertial oscillation and the topographic
thermal forcing mechanisms, it is determined that the OLLJ is the result
of four phenomena acting together to accelerate the wind over the valley
of the Orinoco River basin, namely: the (i) sea breeze penetration over
the Orinoco River delta and Unare River depression, (ii) katabatic flow
down the Coastal Cordillera, (iii) three expansion fans from point wakes
in the topography, and (iv) diurnal variation of turbulent diffusivity.
The continuous nocturnal advection of relatively cool air through the
Orinoco delta region, Unare River depression (i.e., sea breezes
penetration), and downslope of the Coastal Cordillera (katabatic flow),
constitute a single density current that merges over the Llanos and
propagates up-valley, causing acceleration of the wind behind its
leading edge. Before combining over the Llanos, the Orinoco Delta sea
breeze generates the C1 core as the advected cool maritime air flows
downslope at the western limits of the Guanipa Mesa. Although density
currents are known to spawn bores or solitary waves as they propagate in
stably stratified environments, the size and propagation speed of the
overall gravity current inhibit such behavior.
As the merged density current and large-scale flow move along the
Orinoco River basin, the interaction with geographic points in the
surrounding topography create point wakes at three fixed locations: the
Guiana Highlands, Eastern Cordillera, and Macarena mountain range.
Supercritical-channel-flow theory explains the expansion fans generated
by these point wakes and properly predicts their final wind speeds and
PBL heights. Such expansion fans give origin to the C2–C4 core regions.
A momentum balance budget performed for each one of the core locations
shows that the diurnal variation of turbulent diffusivity decelerates
(day) and accelerates (night) the horizontal winds across the domain,
triggering the inertial oscillation mechanism and causing the clockwise
rotation of the wind. However, opposite to what happens in higher
latitudes, the role of the diurnal variation of turbulent diffusivity in
the OLLJ acceleration is secondary, given that the maxima wind speeds as
the result of combined geostrophic and ageostrophic winds occur in only
two of the four cores. The cores closer to the equator (C3–C4) are
driven by ageostrophic effects on the wind due to the large imbalance in
the geostrophic wind as the Coriolis parameter f becomes small.
Although dynamical downscaling and the momentum equation decomposition
improve the current OLLJ characterization and understanding about the
dynamics behind its formation, additional contributions are yet to be
resolved. For instance, what is the role of latent heat release over the
Amazon forest on the strength of the LLJ? Would some form of the OLLJ
still occur in the partial or complete absence of terrain? What is the
overall contribution of the radiative heating to its formation? Does the
OLLJ vary according to warm or cold ENSO phases? Answers to these
questions will improve our understanding of the OLLJ and will foster a
better grasp of the influence of the OLLJ on wind energy, aviation
safety, fire weather, oil-extraction pollutants transport, and public
health, among others.
Acknowledgments
The research was supported by the Colombian Air Force and Colfuturo –
Colciencias fellowships. Simulations were performed using the Advanced
Cyberinfrastructure (ACI) of Penn State’s Institute for Cyberscience,
which can be found at: https://doi.org/10.26208/jaky-kq22. The
assistance of Chuck Pavloski and the rest of the ICS staff was
indispensable. The GFS Analysis were provided from the website of
Research Data Archive (from
http://rda.ucar.edu/datasets/ds335.0/). We are also grateful for
the critiques provided by the anonymous reviewers; whose comments and
suggestions greatly improved the article. Furthermore, the lead author
thanks Prof. Andrew Carleton for a thorough review of an earlier version
of this work while serving on the lead author’s dissertation committee.
The lead author is an active member of the Colombian Air Force.