A heuristic model is
used to study the synoptic response to mountain Gravity Waves (GWs)
absorbed at directional critical levels. The
model is a Semi-Geostrophic version of the Eady model for baroclinic
instability adapted by Smith (1984) to study lee cyclogenesis. The GWs
exert a force on the large-scale flow where they encounter directional
critical levels (Shutts 1995). This force is taken into account in our
model, and produces Potential Vorticity (PV) anomalies in the
mid-troposphere.
First, we consider
the case of an idealized mountain range such that the orographic
variance is well separated between small-scale and large-scale
contributions. In the absence of tropopause, the PV produced by the GWs
force has a surface impact that is significant compared to the surface
response due to the large scales. For a cold front, the GWs force
produces a trough over the mountain and a larger amplitude ridge
immediately downstream. It opposes somehow to the response due to the
large scales of the mountain range, which is anticyclonic aloft and
cyclonic downstream. For a warm front, the GWs force produces a ridge
over the mountain and a trough downstream, hence it reinforces the
response due to the large scales.
Second, we verify the robustness of the previous results, by a series
of sensitivity tests. We change the specifications of the mountain
range, and of the background flow. We also repeat some experiments by
including baroclinic instabilities, or by using the Quasi-Geostrophic
approximation. Finally, we consider the case of a small-scale
orographic spectrum representative of the Alps.
The significance of our results is discussed in the context of GWs
parameterization in the General Circulation Models. Our results may
also help to interpret the complex PV structures occurring when
mountain gravity waves break in a baroclinic environment.
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