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Joseph Kidston, D. M. W. Frierson, J. A. Renwick, and G. K. Vallis

1. Introduction a. Background The leading mode of atmospheric variability in the extratropics of both hemispheres is the meridional vacillation of the equivalent barotropic eddy-driven jet streams and embedded storm tracks ( Kidson 1988 ; Mo and White 1985 ; Thompson and Wallace 2000 ; Baldwin 2001 ; Wallace 2000 ). This variability is variously referred to as the annular modes ( Limpasuvan and Hartmann 1999 ), the Antarctic or Arctic Oscillations ( Thompson and Wallace 1998 ; Gong and

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Zachary J. Handlos and Jonathan E. Martin

1. Introduction The genesis, evolution, and dissipation of extratropical weather systems and related midlatitude phenomena are often linked to the evolution of the Northern Hemisphere polar and subtropical jet streams, which have been researched extensively over the past several decades. The polar jet resides above regions of strong baroclinicity within the midlatitudes (usually poleward of 30° latitude), and its speed maxima is observed ~300 hPa. The polar jet is also referred to as the “eddy

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Mark W. Seefeldt and John J. Cassano

Liu 1996 ). The observations indicated the presence of a low-level jet (LLJ) approximately 200 m above ground level (AGL) with increasing intensity toward the Transantarctic Mountains. The increased use of numerical simulations has provided more insight into the low-level wind field. Seefeldt et al. (2003) discuss the complex structure of the low-level wind field in the widely varying topography of the northwest Ross Ice Shelf. Numerical simulations of LLJs near the northern west coast of the

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Loïc Robert, Gwendal Rivière, and Francis Codron

1. Introduction Midlatitude jets are also called eddy-driven jets because they are maintained against surface drag by the convergence of momentum by eddies ( Vallis 2006 ). These eddies in turn develop in regions of strong baroclinicity, which tend to follow the jet position through thermal wind balance. The eddies and the jet are thus tightly coupled and how their interaction influences the variability of the jet is still not fully understood. The convergence of eddy momentum fluxes, herein

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Joseph B. Olson and Brian A. Colle

1. Introduction The low-level flow from the Pacific Ocean interacts with the steep coastal terrain of Alaska to create strong (>25 m s −1 ) terrain-parallel winds ( Overland and Bond 1995 ; Loescher et al. 2006 ; Colle et al. 2006 ; Olson et al. 2007 ; among others) known as barrier jets ( Schwerdtfeger 1974 ; Overland and Bond 1993 , 1995 ). These jets can result in enhanced turbulence ( Smedman et al. 1995 ; Bond and Walter 2002 ) and wind stress forcing of local currents and storm

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Yu Du, Guixing Chen, Bin Han, Chuying Mai, Lanqiang Bai, and Minghua Li

2005 ; Keene and Schumacher 2013 ; Liu et al. 2018 ), and terrain effects ( Wang et al. 2014 ; Mulholland et al. 2019 ). In contrast, CI and UCG away from surface boundaries often occur during the night, and their mesoscale and small-scale features are poorly captured by numerical models ( Wilson and Roberts 2006 ; Davis et al. 2003 ). Nocturnal CI and UCG are typically associated with low-level jets ( Du and Chen 2019a ; Gebauer et al. 2018 ; Trier et al. 2017 ; Shapiro et al. 2018 ) and

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Man-Li C. Wu, Oreste Reale, Siegfried D. Schubert, Max J. Suarez, Randy D. Koster, and Philip J. Pegion

1. Introduction The African easterly jet (AEJ) is one of the most complex and intriguing dynamical features in tropical meteorology. It raises a number of questions, ranging from its formation mechanism to its maintenance and from the causes of its intensity fluctuations to its role in generating weather systems. The AEJ is a crucial element in global-, synoptic-, and mesoscale dynamics, and its representation in models is important for climate modeling, seasonal predictions, and weather

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Joshua G. Gebauer, Evgeni Fedorovich, and Alan Shapiro

1. Introduction Wind maxima in the lowest levels of the atmosphere have been an intensively studied meteorological phenomenon. Often referred to in the literature as low-level jets (LLJs), these wind maxima can occur as low as 90 m above ground ( Banta et al. 2002 ). Such LLJs can play a role in pollutant mixing and transport ( Zunckel et al. 1996 ; Banta et al. 1998 ; Darby et al. 2006 ; Bao et al. 2008 ; Klein et al. 2014 ) and can affect wind energy production ( Cosack et al. 2007

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Radu Herbei, Ian W. McKeague, and Kevin G. Speer

1. Introduction Directly observed flow at the depths of the North Atlantic Deep Water in the South Atlantic Ocean shows a system of alternating zonal jets ( Hogg and Owens 1999 ). Deep zonal flow has been explained, for example, using a coarse wind-driven circulation model in the Pacific Ocean ( Nakano and Suginohara 2002 ). Several numerical models of varying resolution of the South Atlantic Ocean have been used to study the origin of the zonal flows, leading to the conclusion that wind is the

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P. Baas, F. C. Bosveld, H. Klein Baltink, and A. A. M. Holtslag

1. Introduction Low-level jets (LLJs) are frequently observed phenomena in the nocturnal atmosphere in many parts of the world. They are characterized by a maximum in the wind speed profile, which is typically situated 100–500 m above the earth’s surface. In the literature, many studies can be found on the development and the characteristics of LLJs (e.g., Bonner 1968 ; Garratt 1985 ; Kraus et al. 1985 ; Whiteman et al. 1997 ; Andreas et al. 2000 ; Banta et al. 2002 ; Song et al. 2005

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