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Mountain Wave Propagation under Transient Tropospheric Forcing : A DEEPWAVE Case Study
AuthorPortele, Tanja Christina
Thesis advisorGohm, Alexander ; Dörnbrack, Andreas
Description189 Seiten
Institutional NoteInnsbruck, Univ., Masterarb., 2016
Arbeit an der Bibliothek noch nicht eingelangt - Daten nicht geprüft
Date of SubmissionMarch 2016
Document typeMaster Thesis
Keywords (EN)Transient Forcing / Transient Gravity Wave Event / Changing Propagation Conditions / DEEPWAVE / New Zealand
URNurn:nbn:at:at-ubi:1-3721 Persistent Identifier (URN)
 The work is publicly available
Mountain Wave Propagation under Transient Tropospheric Forcing [61.08 mb]
Abstract (English)

During the deep-propagating gravity wave experiment (DEEPWAVE) on the South Island of New Zealand, for the first time a gravity wave event under transient tropospheric forcing was observed in detail. The corresponding intensive observing period (IOP) 9 took place from 28 June till 01 July 2014. The objective of this thesis is to explore the gravity wave response to such a transient forcing, how waves propagate from their excitation over the Southern Alps up to the middle atmosphere or are dissipated on their way. For this purpose, a comprehensive set of airborne and ground-based measurements are combined with ECMWF (European Centre of Medium-Range Forecasts) analyses and forecasts, as well as with high-resolution simulations with the Weather Research and Forecasting (WRF) model.

It is found that the entire IOP 9 is subject to a clear transience with increasing and decreasing upstream low-level cross-mountain wind speeds with a relative change of about 20 m s1. Comparisons to published simulations of transient forcing gravity wave events reveal several similarities to this case study. However, differences to the simulations appear, as the IOP 9 is not only characterized by a transient tropospheric forcing and therefore transient excitation of orographically induced gravity waves, but also by synoptically changing propagation conditions for the excited waves. The wave response observed at flight level during the different forcing phases is thus strongly altered by processes like wave breaking at self-induced critical levels, partial wave reflection and wave attenuation. Also the relative importance of the different wave scales changes among the flux carrying waves at flight level: During the maximum forcing phase long waves (> 60 km) dominate, during the early decelerating forcing phase intermediate (20 to 60 km) and long waves are of equal importance, and during the late decelerating forcing phase the intermediate-scale waves are dominant. Showing leg-integrated absolute values of the total momentum and vertical energy flux up to around 2600 kW m1 at the end of the maximum and early decelerating forcing phase, the IOP 9 is further characterized as a moderate to strong wave event. Moreover, the results demonstrate, that during this transient event at least part of the wave spectrum is able to propagate deeply up to the mesosphere.