MMM SEMINAR NCAR
Turbulence Structure and Exchange
Processes
in an Alpine Valley
Mathias W. Rotach
Federal Office for Meteorology and
Climatology
MeteoSwiss,
The
MAP-Riviera project was probably one of the most detailed field studies related
to turbulence in highly complex terrain and its numerical modeling. In the
seminar a few key characteristics of the flow and turbulence structure in the
As
an example for the exchange between valley atmosphere and free troposphere
moisture exchange is considered. Typical numerical weather and climate
prediction models apply parameterizations to describe the sub-grid scale
exchange of moisture, heat and momentum between the surface and the free
atmosphere. To a large degree, the underlying assumptions are based on
empirical knowledge obtained from measurements in the atmospheric boundary
layer over flat and homogeneous topography. Yet, it is still unclear what
happens if topography is complex and steep. Not only is the applicability of
classical turbulence schemes questionable in principle over such terrain.
Mountains additionally induce fluxes on the meso-gamma-scale,
such as thermally or mechanically driven valley winds, which are neither
resolved nor parameterized by climate models but nevertheless contribute to
vertical exchange. The present simulations show that moisture exchange with the
free atmosphere is indeed not governed by turbulent motions alone. Other
mechanisms become important, such as mass export due to topographic narrowing
or the interaction of thermally driven cross-valley circulations. Under certain
atmospheric conditions, these topography-related mechanisms exceed the
“classical” turbulent contributions a coarse model would see by several times.
The study shows that conventional sub-grid scale parameterizations can be far
off reality if applied over complex topography, and that large-eddy simulations
could provide a helpful tool for their improvement.
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