Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 132

Fig. 4.37 Time-height section of the horizontal wind vector in an Alpine valley from SODAR
observations. An upward pointing arrow designates a southerly wind, a rightward pointing a west-
erly wind, and so on. The length of the arrows is proportional to the wind speed. The figure shows
half-hourly data for one day (from midnight to midnight) for a height interval from 50 m to 800 m
above ground with a vertical resolution of 30 m
the European Alps. This plot clearly documents the diurnal variation of the chan-
nelled wind regime in such a valley during undisturbed synoptic weather conditions.
At night-time out-valley (southerly) flow can be observed over the whole height
range while during daytime in-valley (northerly) flow prevails. The changes between
the two regimes roughly take place at 9 a.m. and 9 p.m. There is some indication
that the change occurs first in lower layers. Furthermore, from the directional scatter
of the wind arrows, it is obvious that the daytime flow is much more turbulent than
the nocturnal flow.
While Fig. 4.37 gives an example for a vertically well-mixed convective daytime
valley boundary layer, the example plotted in Fig. 4.1 presents the opposite case: a
very stable valley boundary with several persisting inversions within a few hundreds
of metres above the valley flow. Statistical evaluations of the SODAR soundings
in Emeis et al. ( 2007b ) show that such layering with multiple inversions occurs
quite regularly in Alpine valleys. Figure 4.38 shows the frequency distribution of
the height of the lowest inversion (the mixing layer height) and Fig. 4.39 shows the
frequency of multiple inversions from hourly data in January. It turns out that in a
wintry Alpine valley the mixing layer height is most frequent at about 100 m above
ground or even lower. In 83% of all cases at least one inversion was present, and
in 56% of all cases multiple inversions were observed. This multiple layering is a