The tropopause region is characterised by strong gradients in various atmospheric quantities that exhibit different properties in the troposphere compared to the stratosphere. The temperature lapse rate typically changes from negative to near-zero values resulting in an increase in stability, often characterised by a strong maximum in buoyancy frequency just above the tropopause (tropopause inversion layer, TIL), see e.g. Birner et al. (2002). Additionally, the magnitude of the vertical wind shear of the horizontal wind maximizes at the tropopause and the region also exhibits characteristical gradients of trace gases. Vertically propagating gravity waves can be excited in the troposphere by several mechanisms, e.g. by flow over topography (e.g. Durran, 1990), by jets and fronts (e.g. Plougonven and Zhang, 1990) or by convection (e.g. Clark et al., 1986). When these waves enter the tropopause region, their properties can be changed drastically by the changing stratification and strong wind shear.
Within this work, the EULAG (see e.g. Prusa et al., 2008) model is used to investigate the impact of the tropopause on vertically propagating gravity waves. The choice of topography along with horizontal wind speed and tropospheric value of buoyancy frequency determine the spectrum of waves (horizontal and vertical wavelengths) that is excited in the troposphere.
This contribution focuses on the analysis of several cases of atmospheric conditions that were found during the DEEPWAVE campaign in New Zealand. The reanalysed profiles of stability and approaching flow at the Southern Alps mountain range are used as initial conditions for the simulations. The effect of these tropopause structures on gravity wave propagation are elaborated in detail by varying the tropospheric spectrum of waves systematically. The results are then compared to the wave spectra retrieved from measurements (e.g. radiosoundings) during these conditions allowing a better understanding of the processes involved.
References:
Birner, T., A. Doernbrack, and U. Schumann, 2002: How sharp is the tropopause at midlatitudes?, Geophys. Res. Lett., 29, 1700, doi:10.1029/2002GL015142.
Durran, D.R., 1990: Mountain Waves and Downslope Winds, Atmospheric Processes over Complex Terrain. Meteorological Monographs, Vol 23, No. 45
Plougonven, R. and F. Zhang, 2013: Gravity Waves From Atmospheric Jets and Fronts. Rev. Geophys. doi:10.1002/2012RG000419
Clark, T., T. Hauf, and J. Kuettner, 1986: Convectively forced internal gravity waves: results from two- dimensional numerical experiments, Q.J.R. Meteorol. Soc., 112, 899-925.
Prusa, J. M., P. K. Smolarkiewicz, P. K. and A. A. Wyszogrodzki, 2008: EULAG, a computational model for mulstiscale flows, Computers & Fluids 37, 1193-1207
*email: v.bense@uni-mainz.de
*Preference: Oral