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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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Christopher G. Kruse and Ronald B. Smith

exerted on the mountains by the atmosphere and momentum is transferred to Earth. An equal and opposite force is exerted by the mountain onto the lowest layers of the atmosphere, and MWs propagate this negative MF upward, depositing it wherever they dissipate. As much of Earth’s terrain and resulting MW spectra are unresolved in general circulation models and many weather prediction models, MW generation, propagation, and dissipation are parameterized. Including these parameterizations improves

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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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Stephen D. Eckermann, Jun Ma, Karl W. Hoppel, David D. Kuhl, Douglas R. Allen, James A. Doyle, Kevin C. Viner, Benjamin C. Ruston, Nancy L. Baker, Steven D. Swadley, Timothy R. Whitcomb, Carolyn A. Reynolds, Liang Xu, N. Kaifler, B. Kaifler, Iain M. Reid, Damian J. Murphy, and Peter T. Love

et al. 2017 ), we suggest that reemergence of this antiphased wave-1 anomaly structure in the MLT results from in situ MLT forcing. A plausible source of quasi-stationary wave-1 forcing in the MLT is zonally asymmetric GWD, resulting from zonal variations in stratospheric gravity wave filtering due to the zonally varying stratospheric zonal winds associated with the large-amplitude Rossby wave in Fig. 17a (e.g., Smith 2003 ). Fig . 18. Mean zonal-wind anomalies (departures from zonal mean) for

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Christopher G. Kruse, Ronald B. Smith, and Stephen D. Eckermann

little to nearly half of tropopause-level tropical upwelling among model members. Surprisingly, despite variable GWD contributions, the circulation strength was found to be relatively constant, implying changes in planetary wave driving compensate variations in GW forcing (e.g., Cohen et al. 2013 ) and that the mean transport circulation alone may not strongly constrain GWD parameterizations. A current common problem in chemistry–climate models is that the Southern Hemisphere pole is too cold in the

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Stephen D. Eckermann, James D. Doyle, P. Alex Reinecke, Carolyn A. Reynolds, Ronald B. Smith, David C. Fritts, and Andreas Dörnbrack

1. Introduction Gravity waves are ubiquitous features of the atmosphere. Although their major sources are tropospheric, some of these waves propagate into the stratosphere, mesosphere, and thermosphere where, in response to density decreases with height, amplitudes increase, leading to progressively larger impacts. Growth of amplitudes with height, for example, leads to wave breaking and deposition of energy and momentum into the flow as dynamical heating and body forcing, respectively

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Johnathan J. Metz, Dale R. Durran, and Peter N. Blossey

al. 2008 ). Three domains centered over a Lambert conformal conic projection of New Zealand were used with horizontal grid spacings of 18, 6, and 1 km, as shown in Fig. 1 . Terrain was specified using the USGS GTOPO30 dataset after filtering to minimize forcing at poorly resolved short wavelengths. The filtering was done using a nine-point approximation to the two-dimensional Laplacian operator given by (1) h ˜ i , j = h i , j + a ⁡ [ ∑ ⁡ ( ⁡ [ 1 6 2 3 1 6 2 3 − 10 3 2 3 1 6 2 3 1 6 ] ∘ ⁡ [ h i

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Stephen D. Eckermann, Dave Broutman, Jun Ma, James D. Doyle, Pierre-Dominique Pautet, Michael J. Taylor, Katrina Bossert, Bifford P. Williams, David C. Fritts, and Ronald B. Smith

value problem with “switch on” of surface forcing at t = 0. Since each harmonic ( k , l ) has an associated group propagation time to reach a given height z , then imposing a fixed cutoff time limit t prop ( k , l , z c ) = t c and solving numerically for z in (7) yields a spectrum of corresponding cutoff altitudes z c ( k , l , t c ). We can use this result to approximate the time-dependent wave field solution X ( x , y , z , t ) at time t = t c using the inverse Fourier

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