Mountain Wave Propagation under Transient Tropospheric Forcing - A DEEPWAVE case study

Abstract

The deep-propagating wave experiment (DEEPWAVE) was the first extensive measurement program observing especially orographically induced gravity waves from creation to destruction. The field campaign took place in and around New Zealand from 24 May till 27 July 2014. To gain new insights into gravity wave dynamics, comprehensive airborne and ground-based measurements were conducted. These observations provide different sensitivity to various gravity waves sources and their propagation up to higher altitudes. Among these are airborne measurements with the NSF/NCAR Gulfstream V (GV) and the DLR Falcon. The University of Innsbruck and the DLR operated two radiosonde stations at Lauder (45.038 S, 169.684 E) in the lee of the Southern Alps. There, the DLR Rayleigh lidar provided temperature profiles from 30 to 80 km MSL. In this study, we investigate one particular case which was observed during the intensive observing period (IOP) 9 from 29 June till 30 June 2014. The goal of this IOP was to explore the deep wave response to a transient tropospheric forcing. Flight-level in-situ data will be analysed and related to the ground-based lidar data. The lidar observed propagating gravity waves in an altitude range between about 40 to 60 km MSL from 10 UTC to 18 UTC on 30 June 2014. During this IOP, the DLR Falcon and the GV conducted four consecutive flights to cover the whole period of the transient tropospheric forcing. These airborne observations were complemented by 15 radiosondes launched at Lauder every 3 h. The first Falcon flight FF01 was flown under amplifying, the second flight FF02 under significant but decreasing orographic forcing. Here, particularly the horizontal wind at forcing level (700 hPa) was weakening and the wind direction was slightly turning from NW-NNW to WNW-NW from 29 June to 30 June. Due to this gradual change, amplitudes of vertical wind at flight level decreased from up to about 4 m/s on 29 June to up to about 2 m/s on 30 June. We concentrate on the impact of changing forcing conditions on the propagation of gravity waves across the tropopause into the stratosphere. The observations of this event are documented and analysed. In addition, the findings are compared with high-resolution (1-hourly) ECMWF analyses and forecasts. Furthermore, the potential of gravity waves to propagate into the mesosphere over the southern island of New Zealand is studied based on the observations.