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:
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.