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Tanja C. Portele, Andreas Dörnbrack, Johannes S. Wagner, Sonja Gisinger, Benedikt Ehard, Pierre-Dominique Pautet, and Markus Rapp

1. Introduction Mountain waves under transient tropospheric forcing conditions were frequently observed during the Deep Propagating Gravity Wave Experiment (DEEPWAVE) in austral winter 2014 ( Fritts et al. 2016 ). These events occurred episodically and were associated with migratory low pressure systems impinging the South Island (SI) of New Zealand (NZ; Gisinger et al. 2017 ). During these events, the conditions for wave excitation and propagation varied temporally. Continuous ground

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Sonja Gisinger, Andreas Dörnbrack, Vivien Matthias, James D. Doyle, Stephen D. Eckermann, Benedikt Ehard, Lars Hoffmann, Bernd Kaifler, Christopher G. Kruse, and Markus Rapp

wave forcing over SI, typically located upstream of New Zealand. Altogether, the DEEPWAVE area of operations encompassed a region from 65° to 30°S and from 145°E to 180°. The field phase of DEEPWAVE was conducted during May–July 2014. Measurements taken on board the two research aircraft, the NSF/NCAR GV and the DLR Falcon, provided gravity wave data from the lower troposphere to the mesosphere using a variety of in situ and remote sensing instruments ( Fritts et al. 2016 ). The aircraft

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David C. Fritts, Ronald B. Smith, Michael J. Taylor, James D. Doyle, Stephen D. Eckermann, Andreas Dörnbrack, Markus Rapp, Bifford P. Williams, P.-Dominique Pautet, Katrina Bossert, Neal R. Criddle, Carolyn A. Reynolds, P. Alex Reinecke, Michael Uddstrom, Michael J. Revell, Richard Turner, Bernd Kaifler, Johannes S. Wagner, Tyler Mixa, Christopher G. Kruse, Alison D. Nugent, Campbell D. Watson, Sonja Gisinger, Steven M. Smith, Ruth S. Lieberman, Brian Laughman, James J. Moore, William O. Brown, Julie A. Haggerty, Alison Rockwell, Gregory J. Stossmeister, Steven F. Williams, Gonzalo Hernandez, Damian J. Murphy, Andrew R. Klekociuk, Iain M. Reid, and Jun Ma

numerical simulation and NCEP GFS = National Centers for Environmental Prediction Global Forecast System. MOTIVATIONS. GWs, or buoyancy waves, for which the restoring force is due to negatively (positively) buoyant air for upward (downward) displacements, play major roles in atmospheric dynamics, spanning a wide range of spatial and temporal scales. Vertical and horizontal wavelengths, λ z and λ h , respectively, for vertically propagating GWs are dictated by their sources and propagation conditions

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Benedikt Ehard, Peggy Achtert, Andreas Dörnbrack, Sonja Gisinger, Jörg Gumbel, Mikhail Khaplanov, Markus Rapp, and Johannes Wagner

respective periods are characterized by strong-to-moderate tropospheric forcing, which excited mountain waves over northern Scandinavia. Forcing conditions are considered to be moderate if the component of the wind at 700 hPa perpendicular to the Scandinavian mountain ridge is smaller than 15 m s −1 , whereas strong forcing occurs if the wind component is larger than 15 m s −1 . Ambient westerly winds in the stratosphere favored the propagation of mountain waves in both cases. The gravity wave analysis

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Tyler Mixa, Andreas Dörnbrack, and Markus Rapp

can reach the winter MLT ( Pautet et al. 2019 ), where high-frequency gravity wave responses transported the most momentum and eastward (upwind) gravity wave propagation correlated with elevated tropospheric forcing below. Given that small-scale gravity waves have been observed above the PNJ in the winter MLT, and given that several orographic and nonorographic sources have been shown to generate these scales of mesospheric gravity wave responses, it is essential to evaluate how tropospheric

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