Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 75

Fig. 4.1 Multiple layering of the atmosphere in a wintry Alpine valley (Inn valley near Schwaz,
Austria) detected from SODAR measurements on January 29, 2006 (time is given in GMT+1).
Left : acoustic backscatter intensity in dB ( green :low, red : high), right : horizontal wind direction
in degree ( blue : around 20 ◦ , greenish : 190 ◦ , reddish : 230 ◦ ). Black lines indicate layer boundaries
determined with the EARE algorithm. Only the lowest layer, present between 10 pm and midnight,
may be addressed as a mixing layer
may be an indication for the presence of temperature inversions, which can often be
found at the top of the mixing layer.
Beyrich ( 1997 ) listed possible analyses that can mainly be made from acoustic
backscatter intensities measured by a SODAR. Later, Asimakopoulos et al. ( 2004 )
summarized three different methods to derive MLH from SODAR data: (1) the hor-
izontal wind speed method (HWS), (2) the acoustic received echo method (ARE),
and (3) the vertical wind variance method (VWV). We will mainly follow this clas-
sification here and finally add a fourth method, the enhanced ARE method (EARE),
in section “Enhanced Acoustic Received Echo Method”.
Figure 4.1 , showing an acoustic sounding taken in an Alpine valley, gives an
impression what wealth of detailed vertical information can be derived from acous-
tic boundary layer sounding. The left-hand frame displays the acoustic backscatter
intensity and the right-hand frame the wind direction as time-height sections over
one day (from midnight to midnight) and over a height range of 700 m. The depicted
wintry situation from a day in January exhibits a multiple layering of the air in
that valley due to the very stable thermal stratification of the valley air over a
snow-covered valley floor. The multiple layering originated from an interlacing of
down-valley (wind direction around 190 ◦ ) and down-slope (wind direction around
230 ◦ ) flows. The layers are separated by temperature inversions, and each higher
layer is potentially warmer than the next lower layer. They persisted nearly the
whole day because no vertical mixing took place in the stably stratified valley
atmosphere.
Acoustic Received Echo Method
The ARE method is the oldest and most basic method of determining MLH from
acoustic remote sensing. Most of the methods listed in Beyrich ( 1997 ) belong to
this method. The method does not require an analysis of the Doppler shift of the
backscattered signals, but is based on the analysis of facsimile plots, i.e. time-height