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Figure 74: Bragg condition for RASS sounding.
Figure 75: Sodar-RASS. The acoustic antenna is in the middle, the electro-magneticantennas
to the left and right.
The vertical air speed component w can be determined separately from the Doppler shift
of the backscattered electro-magnetic clear-air signal when operating a Bragg-RASS or from
the Doppler shift of the backscattered acoustic signal when operating a Doppler-RASS. Using
the definition of the acoustic temperature the height profile of cs can then be converted in a
height profile of the acoustic temperature Ta. For many purposes this acoustic temperature
is a sufficiently accurate approximation of the virtual air temperature. The maximum range
of a sodar-RASS is usually less than thousand metres so that such an instrument covers the
lower part of the boundary layer. Due to the high vertical resolution and the low minimum
range of about 30 to 40 m, which is comparable to the abilities of a sodar, a sodar-RASS is
well suited for the detection of shallow nocturnal boundary layers.
MLH detection algorithms MLH can be determined from the lowest height where the
vertical profile of potential temperature increases with height indicating stable thermal stratification
of the air. The great advantage of RASS measurements is that the magnitude of
132 DTU Wind Energy-E-Report-0029(EN)