The Orinoco Low‐Level Jet: An Investigation of Its Mechanisms of Formation Using the WRF Model

Abstract

AbstractThe 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 to March 2014) performed with the Weather Research and Forecasting (WRF) model, with initial and boundary conditions provided by the Global Forecast System (GFS) analysis. Dynamical downscaling is demonstrated to be an effective method not only to better resolve the horizontal and vertical characteristics of the OLLJ 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 (2,100 km × 450 km) in January. The maximum diurnal mean wind speeds (13–17 m s−1) at each core location occur at different times during the night and early morning (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.