Basic Principles of Surface-Based Remote Sensing - Surface Based Remote Sensing of the Atmospheric Boundary Layer - page 61

Major emission bands in the microwave region between 20 and 200 GHz are two
hydrogen bands at 22 GHz and 183 GHz, a very broad oxygen band at 60 GHz,
and an isolated oxygen line at 119 GHz (Cimini et al. 2006 ). Height attribution of
the received signal is made from the pressure-related broadening of these lines and
bands. In optically thick bands such as the oxygen band at 60 GHz, measurements
may be made in different distances from the band centre. The transparency of the
atmosphere increases with increasing distance from the band centre, so that the radi-
ation received at a satellite stems from deeper and deeper layers. The exact location
of the band centres is temperature-dependent.
A radiometer that is sensitive in the spectral range of visible light is called pho-
tometer. It is designed to measure the intensity of light sources. The detection is
made with photo-diodes. The spectral sensitivity of these diodes is adapted to the
sensitivity of the human eye by filters brought into the light beam. Photo-resistors
made from cadmium sulphide can also be used (Strangeways 2003 ).
A novel development is HATPRO (Humidity And Temperature PROfiler
HATPRO, Rose et al. ( 2005 )) with high temporal resolution. Compared to other
microwave radiometers, HATPRO is able to observe temperature profiles with high
vertical resolution in the atmospheric boundary layer in addition to the standard
products IWV (integrated water vapour), LWP (cloud liquid water path), and full
troposphere temperature and humidity profiles. The enhanced vertical resolution in
the boundary layer is achieved by conical scanning at six different elevation angles
at a frequency where the atmosphere is quite opaque. HATPRO continuously mea-
sures thermal emission by atmospheric components (water vapour, oxygen, liquid
water) simultaneously in 14 channels distributed over two frequency bands. Seven
channels are located along the high-frequency wing of the water vapour absorp-
tion line at 22.235 GHz (22.24-31.4 GHz), and another 7 channels are located
along the low-frequency wing of the 60 GHz oxygen absorption complex (51.26-
58 GHz). HATPRO is operating in zenith pointing mode most of the time (except
when performing the scans at six different elevation angles) giving a set of 14
brightness temperatures every 2 seconds. Statistical algorithms are used in order
to retrieve atmospheric parameters from brightness temperatures. These algorithms
are developed on the basis of synthetic observations generated from a representative
long-term data set (Pospichal and Crewell 2007 ).

3.6.1 Infrared Interferometer

The Atmospheric Emitted Radiance Interferometer (AERI) is a high spectral reso-
lution interferometer sounder, which records spectra in the infrared (3.3-18.2 m)

with a spectral resolution of better than 1 cm − 1 at less than 10-min temporal resolu-
tion (Feltz et al. 2003 ). These radiance spectra are then converted into temperature
and humidity profiles by an inversion of the radiation transfer equation (Knuteson
et al. 2004a , b ). This inversion requires a first guess, which may be taken from a
radiosonde measurement or from climatology. A typical measurement cycle con-
sists of a 3-min sky dwell period followed by 2-min dwell periods for each of the

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