Basic Principles of Surface-Based Remote Sensing - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 43

quiet atmosphere at rest without any temperature gradients, no backscattered signal
can be received with a SODAR. For optimal backscatter, the spatial size of the tem-
perature gradients in the atmosphere should be about half the acoustic wavelength
(Bragg condition). This condition can be fulfilled by turbulent fluctuations as well
as by temperature inversions. When deducing information on the atmospheric state
from the backscattered signal, one has to differentiate between these two possibili-
ties. This can be done either from an assessment of the general weather conditions
or (if the SODAR is a Doppler-SODAR) from an analysis of the variance of the
simultaneously recorded vertical velocity. High variances indicate thermal forcing,
and the backscatter intensity is proportional to the turbulence intensity; low vari-
ances indicate stable layering and the backscatter intensity should be proportional
to mean vertical temperature gradients (i.e. inversions).
Depending on the technology how the sound beams are focussed, a difference is
made between “classical” SODARs, which had been developed first, and phased-
array SODARs. Classical SODARs use three large tiltable antennas to focus the
beams (Fig. 3.4 ). Several large sound transducers are fixed at the bottom of each of
these antennas, and within one antenna they are operated simultaneously. The sound
transducers serve as emitters as well as receivers. Phased-array SODARs have a
larger number of smaller sound transducers (often 32 or 64), which are regularly
arranged on a quadratic plate with an area of about 1 m 2 . By using the interference
principle (enhancement and extinction), slanted and vertical beams are formed by
operating the various sound transducers with specified time delays among each other
(Fig. 3.5 ). While with classical SODARs, the antennas simultaneously serve as a
shield to protect the environment from the sound pulses as well as the SODAR
from disturbing ambient noise, phased-array SODARs need a shield (baffles) around
them. Nevertheless, even if optimal lateral shields are erected, the sound pulse is still
Fig. 3.4 Three-antenna sodar for wind and turbulence profile measurements up to about 1000m
above ground