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.