Applications - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 105

The white areas in Fig. 4.5 , where temperature information is missing, provide
an indirect evidence for the presence of a low-level jet in each of the covered four
nights. A RASS is not able to observe any backscattered electromagnetic radiation if
the sound pulse from the SODAR part of the instrument has been blown away from
the focus of the electromagnetic antenna by too strong winds. With the instrument
which was used to obtain the data in Fig. 4.5 , this happened for horizontal wind
speeds greater than about 12 to 15 m s − 1 .
Figure 4.5 demonstrates the advantages of RASS as well as its disadvantages.
Advantageous are the low first range gate just a few tens of metres above ground and
the high vertical resolution. Disadvantageous are the limited height range, which
usually is not sufficient to cover the entire vertical extend of the boundary layer
and missing data for higher wind speeds. The determination of the temperature pro-
file is not hampered by fog, clouds, or precipitation. Thus, for low to moderate
wind speeds, a RASS offers a nearly one hundred percent data coverage. Therefore,
the instrument can be used for monitoring purposes, especially for shallow and
nocturnal boundary layers.
Due to the ability of a RASS to simultaneously monitor temperature and wind
profiles, profiles of derived quantities such as the Brunt-Vaisala frequency and the
Monin-Obukhov length can be analyzed as well. The Brunt-Vaisala frequency can
be computed from the temperature profile over the whole measurement range and
the Monin-Obukhov length from linear regressions of wind speed and potential
temperature differences of the four lowest levels (Pérez et al. 2009 ).
4.3.2.2 Raman LIDAR
Rotational Raman LIDAR measurements deliver temperature profiles throughout
the troposphere including the upper part of the ABL. The analysis is based on
Fig. 4.19 Temperature
profile from Raman LIDAR
sounding ( full line ) compared
to radiosonde data ( open
circles ). From Radlach et al.
( 2008 )