Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 133

Fig. 4.38 Frequency distribution of the height of the lowest inversion (mixing layer height) in a
wintry valley boundary layer from SODAR measurements (from Emeis et al. 2007b )
Fig. 4.39 Frequency of the
occurrence of multiple
inversions in a wintry valley
boundary layer from SODAR
measurements (from Emeis
et al. 2007b )
special feature of a valley boundary layer. It originates from an interlacing of down-
valley and down-slope flows. The layers are separated by temperature inversions,
and each higher layer is potentially warmer than the next lower layer.
The Mistral as an example for a gap flow has already been shown in Fig. 4.18 .
The Mistral is a wind blowing southward through the Rhone valley towards the
French Mediterranean coast. The river Rhone flows through a major gap between the
Massif Central to the West and the French Alps to the East. Mechanisms responsible
for the temporal evolution of the Mistral are related to the evolution of the upstream
synoptic wind speed and direction conditions during the event and the upstream
Froude number, calculated in the layer below the upstream inversion height. The
Froude number in the case shown in Fig. 4.18 has been found to be between 0.19
and 0.75, thus systematically smaller than 1 (subcritical incident flow). With such
Froude numbers, the ability for air masses to pass over the mountains is mainly gov-
erned by the comparison between the upstream inversion height and the mountain
elevations (Caccia et al. 2004 ).