Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 87

coincidence was good in most cases, except for a very low MLH at or even below
the first range gate of the SODAR and the RASS.
4.2.1.4 Other Algorithms Using More Than One Instrument
Using more than one instrument for sounding can help to overcome some of
the above described deficiencies (limited vertical range, limited data availability)
of the individual instruments. Possible combinations are listed in the following
subsections.
Combined Deployment of SODAR and Wind Profiler
Beyrich and Görsdorf ( 1995 ) have reported on the simultaneous usage of a SODAR
and a wind profiler for the determination of MLH. For the SODAR data the ARE
method was used. From the wind profiler data MLH was likewise determined from
the height of the elevated signal intensity maximum (see also Angevine et al. 1994 ;
Grimsdell and Angevine 1998 , White et al. 1999 ). Good agreement between both
algorithms was found for evolving convective boundary layers. The vertical ranges
of the two instruments (50-800 m for the SODAR and 200-3000 m for the wind
profiler) allowed following the complete diurnal cycle of MLH.
Combined Deployment of SODAR and Ceilometer
There is an interesting difference between the schemes for the determination of
MLH from acoustic and optical backscatter intensities, which should be noted care-
fully. While the acoustic backscatter intensity itself is taken for the detection of H 1
and H 3(see( 4.1 ) and ( 4.3 )) and the first derivative of this backscatter intensity for
the determination of H 2(see( 4.2 )), the first and the second derivative of the opti-
cal backscatter intensity (but not the optical backscatter intensity itself) is used to
determine H 4(see( 4.5 )). This discrepancy in the processing of the two backscat-
ter intensities is due to the different scattering processes for acoustic and optical
waves: Acoustic waves are scattered at atmospheric refractivity gradients and thus
at temperature gradients (Neff and Coulter 1986 ) while optical waves are scattered
at small particles. Therefore, the optical backscatter intensity is proportional to the
aerosol concentration itself. The MLH, on the other hand, which we desire to derive
from these backscatter intensities, is in both cases found in heights where we have
vertical gradients of the temperature and of the aerosol concentration. Therefore, in
principle, the vertical distribution of the acoustic backscatter intensity should look
very much alike to the negative of the vertical distribution of the vertical gradient of
the optical backscatter intensity. The schematic diagrams in Fig. 4.6 are illustrating
this fact.
Simultaneous measurements with different remote-sensing devices have mainly
been made in order to evaluate one remote-sensing method against the other (Devara
et al. 1995 ). But one could also think of combining the results two or more remote-
sensing devices for determining the structure of the ABL. Direct detection of MLH
from acoustic backscatter intensities is limited to the order of about 1 km due to the