Aspects of flow characteristics and turbulence in complex terrain : results from the MAP-RIVIERA project

In this thesis turbulence measurements from
the MAP-RIVIERA project are analysed. During the
project a number of permanent turbulence stations
were installed on a cross section of the Riviera valley.
Furthermore measurements from e.g. radio sounding
systems and a passive microwave profiler were
carried out during intensive observation periods. As large fractions of the alps are forested, a
tower with amongst others six levels of ultrasonic
anemometer thermometers was installed in a mixed
forest at the slope (35°). This data set is compared to
studies carried out over forests in more ideal, flat
terrain. The analysis is carried out for 30 min mean
data, joint probability distributions, length scales and
spectral characteristics. Furthermore patterns of
coherent structures are determined and dominating
time scales as well as flux fractions are calculated. Thermally induced slope and valley winds
are interacting on different spatial and time scales
leading to complex patterns in momentum transport
which differ significantly from surface layer
characteristics. Directional shear causes lateral
momentum transports that are in the same order as the
longitudinal ones. In the canopy a sharp attenuation of
turbulence is observed. Skewed distributions of
velocity components indicate that intermittent
turbulent transports play an important role in the
energy distribution. In the absence of larger scale
waves energy is removed from the mean flow above
the canopy and injected into coherent eddies. Length
scales of these disturbances are comparable to those in
forests in flat terrain. In the canopy work is done
against pressure drag and against the viscous
component of canopy drag. Kinetic energy is
converted into fine scale wake turbulence and heat
leading to vanishing second moments. It is shown that wave like structures are a
very common feature of the stable boundary layer. In
the valley atmosphere temperature fluctuations are
observed which are in phase through the whole
measured layer indicating terrain generated waves.
These temperature oscillations are not confined to the
valley centre but they are also observed on stations at
the slope. Having a phase shift between valley and
slope oscillations it is proposed that terrain generated
waves back up or even enable the mechanism of
compressional warming on the valley slopes. It is
shown, that due to compressional warming a cycle is
initiated in which (relatively colder) air is advected
slope upwards above the canopy while in the canopy
(relatively warmer) air drains at low speed. When the
downslope winds occur above the canopy relatively
warmer air is mixed into the canopy. This effect is
strongest just before the onset of a new ´upslope
event´.