The Orinoco Low-Level Jet: An Investigation of its Mechanisms of
Formation Using the WRF Model
Giovanni Jiménez-Sánchez1,2, Paul M.
Markowski1, George S. Young1, and
David J. Stensrud1
1Department of Meteorology and Atmospheric Science,
The Pennsylvania State University, University Park, Pennsylvania.
2Fuerza Aérea Colombiana, Bogotá, D.C., Colombia.
Corresponding author: Dr. Giovanni Jiménez-Sánchez
(jorge.jimenez@fac.mil.co)
Key Points:
- The Orinoco low-level jet is the result of four phenomena acting
together to accelerate the wind over the valley of the Orinoco River
basin.
- Opposite to what happens in higher latitudes, the role of the diurnal
variation of turbulent diffusivity in the Orinoco low-level jet
acceleration is secondary.
- Low-level jets near the equator may originate from processes other
than the inertial oscillation and topographic thermal forcing.
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.
1 Introduction
The occurrence of nocturnal low-level wind speed maxima—i.e.,
nocturnal low-level jets (LLJs)—is usually explained through the
inertial oscillation and topographic thermal forcing mechanisms. The
inertial oscillation mechanism, originally proposed by Blackadar (1957)
and subsequently modified by Van de Wiel et al. (2010), attributes the
acceleration of the wind to the imbalance of forces occurring at sunset
once turbulent mixing ceases, which effectively leads to a vanishing of
the drag force in the residual layer above the shallow stable nocturnal
boundary layer (Stull, 2015a). Without this frictional force, the
horizontal wind accelerates, which in turn increases the Coriolis
acceleration, ultimately causing the wind to oscillate about its
geostrophic velocity. The velocity becomes most supergeostrophic when
the ageostrophic wind component (its magnitude increases with an
increasing frictional force acting on the upper-boundary-layer wind
during the daytime period) rotates to an orientation aligned with the
geostrophic velocity. Although the rotation rate of the ageostrophic
velocity increases with latitude, the LLJ typically reaches its maximum
wind speed 4–6 hours after sunset.
The topographic thermal forcing mechanism, introduced by Holton (1967),
accounts for the effect that the diurnal oscillation of the horizontal
temperature gradient over sloping terrain has on the direction of the
thermal wind. At night, when the air adjacent to the slope cools more
than the surrounding air at the same level, the thermal wind reverses
its daytime direction, implying a poleward acceleration of the wind
above the slope.
Both mechanisms may act together to produce an enhanced wind speed
maximum, as is the case in the Great Plains LLJ (Bonner & Paegle, 1970;
Du & Rotunno, 2014; Shapiro et al., 2016), or they can independently
generate an LLJ (Du & Rotunno, 2014; Fedorovich et al., 2017; Stensrud,
1996); however, when only one of the mechanisms is considered, the LLJ
is usually less predictable in time, and/or weaker than observed (Du &
Rotunno, 2014; Shapiro et al., 2016). Recently, Parish (2017) stated
that the inertial oscillation is the most important forcing in the Great
Plains LLJ generation, giving only marginal relevance to the
sloping-terrain diurnal heating variation. The development of a strong
background pressure-gradient force (PGF), via long-term heating, is
proposed to be of greater importance than the daily heating
oscillations.
Although extensive research has been done on LLJs in the U.S. Great
Plains (e.g., Blackadar, 1957; Bonner, 1963, 1968; Bonner & Paegle,
1970; Du et al., 2014; Holton, 1967; Krishnamurthy et al., 2015; Parish,
2017; Shapiro et al., 2016; Song et al., 2005; Squitieri, 2014; Weaver
& Nigam, 2008; Wexler, 1961; Whiteman et al., 1997; Wu & Raman, 1993),
and in other parts of the world as well (e.g., Amador, 2008; Balmez &
Ştefan, 2014; G. T.-J. Chen & Hsu, 1997; R. Chen & Tomassini, 2015;
Cook & Vizy, 2010; Doyle & Warner, 1993; Du et al., 2014; Du, Chen, et
al., 2015; Du, Rotunno, et al., 2015; Findlater, 1969; Giannakopoulou &
Toumi, 2012; Hart, 1977; He et al., 2016; Hidalgo et al., 2015; Juliano
et al., 2017; Liu et al., 2000; Maldonado et al., 2016, 2017; Marengo et
al., 2004; Muñoz et al., 2008; Do Nascimento et al., 2016; Nicholson,
2016; Patricola & Chang, 2017; G Poveda & Mesa, 1999; Germán Poveda et
al., 2014; Germán Poveda & Mesa, 2000; Prabha et al., 2011; Rojas,
2008; O. Rueda & Poveda, 2006; Silva et al., 2009; Soares et al., 2014;
Vera et al., 2006; Virji, 1981; Wang et al., 2008; Wei et al., 2013;
Whyte et al., 2008; Zhao et al., 2003), just a few authors have
investigated the low-level wind maximum occurring during the Austral
summer in the savannas of the Orinoco River basin. Hereafter, this
phenomenon is referred to as the Orinoco low-level jet (OLLJ).
The OLLJ could have been noticed in the examination of the lower
tropospheric winds from satellite images performed by Virji (1981), or
in the reanalysis data over South America examined by Montoya et al.
(2001), but the existence of a larger-scale wind phenomenon—the South
American low-level jet (Montini et al., 2019)—prevented its detection
until recently. The identification and characterization of the OLLJ was
subsequently achieved using mesoscale simulations (Rife et al., 2010;
Vernekar et al., 2003) and a combination of pilot balloons, radiosondes,
surface data, and more recent versions of reanalysis (e.g., Douglas et
al., 2005; Labar et al., 2005; C. Rueda, 2015; Torrealba & Amador,
2010). However, the physical mechanisms of its formation have not yet
been established.
Vernekar et al. (2003), Labar et al. (2005), and Rueda (2015) have
hypothesized that the OLLJ formation is the result of the PGF between
the North Atlantic subtropical high and the low pressure over Amazonia,
when the Intertropical Convergence Zone (ITCZ)—also called the
Near-Equatorial Trough (NET)—migrates southward during austral summer
(Fig. 1a). As a result, the northeasterly trade winds strengthen while
undergoing topographic channeling between the Coastal Cordillera and the
Guiana Highlands. Diurnal heating and cooling of the basin, as well as
differential heating between the mountains and the plains, the land and
the ocean, produce changes in the vertical mixing of the air’s
horizontal momentum, modifying the OLLJ diurnal behavior. The studies
performed by Rueda (2015) using ERA-Interim (1983–2013) showed that the
OLLJ is only present from November to March, whereas the rest of the
year the northward migration of the ITCZ (Fig. 1b) weakens the LLJ and
introduces southeasterly trade winds into the basin.