Energy and Water Cycles in the Climate System ...
9
Figure 1.4: Total atmospheric precipitable water in vertical columns (in units of 10 kg m- 2 ),
from Peixoto and Gort (1992). These values are primarily based on conventional analyses.
About 80% of the total vertical water column is contained in the lowest 2000 meters of the
atmosphere, and less than 1 % in the strato- and mesosphere.
Recent airborne comparisons of various water vapor sensors have been described by Strom et
al. (1993). Their results will urge for improvements of the present water vapor sensors used in
the operational radio-wind sondes.
Space-borne soundings can make use of various principle which are described below:
• Measurements of the upward spectral emission to space within strong absorption bands
provide global coverage but very low vertical resolution due to the half-width of its weighting functions in the troposphere of about 5 km. They have firstly been done from the
very first American meteorological satellites of the TIROS-series (e.g. Moller, 1961)
• The limb emission (as discussed by Read et al., 1995) or absorption (occultation measurements), provides an excellently high vertical resolution, but only relatively few data
points are obtained over the globe during each day.
• The attenuation of radio signals which is transmitted from the Global Positioning Systems
(GPS) in earth satellites and had been considered often as noise for geodetic purposes
(e.g.: Ware et al., 1996). The attenuation of these signals is proportional to changes of
the index of refraction of air, and thus of the temperature and humidity, where the latter
in the lowest 5 km-layers can be determined.
• Attenuation occurs also along the path of lidar rays, where however the technology is
not yet available for use in space. Measurements with Raman-Lidar, proven as an excellent tool for water vapor soundings (see Weitkamp, this volume), require relatively long
integration times.
9
Figure 1.4: Total atmospheric precipitable water in vertical columns (in units of 10 kg m- 2 ),
from Peixoto and Gort (1992). These values are primarily based on conventional analyses.
About 80% of the total vertical water column is contained in the lowest 2000 meters of the
atmosphere, and less than 1 % in the strato- and mesosphere.
Recent airborne comparisons of various water vapor sensors have been described by Strom et
al. (1993). Their results will urge for improvements of the present water vapor sensors used in
the operational radio-wind sondes.
Space-borne soundings can make use of various principle which are described below:
• Measurements of the upward spectral emission to space within strong absorption bands
provide global coverage but very low vertical resolution due to the half-width of its weighting functions in the troposphere of about 5 km. They have firstly been done from the
very first American meteorological satellites of the TIROS-series (e.g. Moller, 1961)
• The limb emission (as discussed by Read et al., 1995) or absorption (occultation measurements), provides an excellently high vertical resolution, but only relatively few data
points are obtained over the globe during each day.
• The attenuation of radio signals which is transmitted from the Global Positioning Systems
(GPS) in earth satellites and had been considered often as noise for geodetic purposes
(e.g.: Ware et al., 1996). The attenuation of these signals is proportional to changes of
the index of refraction of air, and thus of the temperature and humidity, where the latter
in the lowest 5 km-layers can be determined.
• Attenuation occurs also along the path of lidar rays, where however the technology is
not yet available for use in space. Measurements with Raman-Lidar, proven as an excellent tool for water vapor soundings (see Weitkamp, this volume), require relatively long
integration times.