| 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 a
strong increase in stability. Accordingly, the buoyancy frequency often undergoes a jump
at the tropopause. Analysis of radiosounding data also shows the existence of a
strong inversion layer (tropopause inversion layer, TIL) characterised by a strong
maximum in buoyancy frequency just above the tropopause, 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 (for a recent
review: 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 (Eulerian/semi-Lagrangian fluid solver, see e.g.
Smolarkiewicz and Margolin, 1997) model is used to investigate the impact of the tropopause
on vertically propagating gravity waves excited by flows over topography. The choice of
topography (sine-shaped mountains, bell-shaped mountain) 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 tropsphere. In order to analyse how these
spectra change for several topographies when a tropopause is present, we investigate different
idealized cases in a two-dimensional domain. By varying the vertical profiles of buoyancy
frequency (step-wise vs. continuos change, including TIL) and wind shear, the
tropopause characteristics are changed and the impact on vertically propagating gravity
waves, such as change in wavelength, partial reflection or wave trapping can be
studied.
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.
Smolarkiewicz, P. and L. Margolin, 1997.: On forward-in-time differencing for fluids: an
Eulerian/Semi- Lagrangian non-hydrostatic model for stratified flows, Atmos.-Ocean., 35,
127-152. |