10
E. Raschke
The GEWEX Water Vapor Project (GVaP) within GEWEX enhances our present activities to
approach higher accuracies in water vapor measurements and also spatially denser informations.
Several commercial aircraft in Europe and in the US carry already now automatic instrumental
packages to record atmospheric water vapor from take-off to the landing. These additional
airborne reports (AIREP) on profiles near the major airports will immediately be fed into the
operational analyses as also those measurements of the occasional research field campaigns.
1.4 Clouds in the atmosphere
(see also contributions by Ackerman, Rossow, Simmer, Stewart, Stuhlmann,
Weitkamp)
As many careful global analyses of, however, limited data sets have shown, clouds at present
occur daily over about 50 to 55% of the world, where highest ocurrences can be found in the
"belts" of stronger convective and synoptic-scale activities. On the other side the subtropical
branches of the Hadley-cell and, moreover, several regions over the continents contain very dry
air and consequently only very few clouds. Due to importance of clouds on the weather at
the surface, on the transport of precipitation and within the huge sequences of air-chemical
processes, world-wide observations began very early, being now complemented by space-borne
data. Fig. 1.5 displays results on the day-night difference of the fractional coverage by clouds
which was obtained from ground-based observations by Hahn et al. (1995). They analysed
most available groundbased cloud observations and obtained global cloud distributions which
are quite similar to those determined from satellite data (see Rossow, this volume).
Analyses of satellite data began as early as they could be visualized in images and manipulated
in computers. Such early overlay statistics led e.g. to the "discovery" of the "Double-ITC"
over the Pacific Ocean. Operational meteorologists also developed manual analysis techniques
(Nephanalyses).
The more modern analyses of satellite measurements can now make use
• of contrasts of cloud fields against the ambient background, the Earth's surface, in their
intensities of emitted and reflected radiation, respectively, and
• of the spectral properties of these quantities. For instance, ice clouds reflect slightly
less radiation than water clouds, thus "differences" between albedoes of simulataneous
measurements can be used to identify cirrus over bright lower level clouds, or low level
stratus over snow and ice fields.
• of up to 24 measurements per day from most geostationary satellites to meet the diurnal
variations.
Such data form the major source of informations within the quite straightforward algorithms
which at present are used in most cloud analysis schemes, also within the International Satellite
Cloud Climatology Project ( ISCCP; see papers by Rossow in this volume, and Rossow and
Schiffer,1991). Here the radiative transfer properties are estimated by comparison of measured
with calculated radiance data where additional ground-based observations of snow cover and
of the surface temperatures are used to "enhance" the contrasts. The ISCCP has at present
analysed worldwide data sets from more than 10 complete years, beginning in 1983. The diurnal
variation of cloud fields is observable over most regions of the world in the data of geostationary
satellites, which - with the exception of the INS AT-series - enter also the ISCCP analyses.
There are also several attempts to identify cloud species with pattern recognition methods (e.g.:
Lovejoy et al., 1992); but those methods have until present not reached an operational status
E. Raschke
The GEWEX Water Vapor Project (GVaP) within GEWEX enhances our present activities to
approach higher accuracies in water vapor measurements and also spatially denser informations.
Several commercial aircraft in Europe and in the US carry already now automatic instrumental
packages to record atmospheric water vapor from take-off to the landing. These additional
airborne reports (AIREP) on profiles near the major airports will immediately be fed into the
operational analyses as also those measurements of the occasional research field campaigns.
1.4 Clouds in the atmosphere
(see also contributions by Ackerman, Rossow, Simmer, Stewart, Stuhlmann,
Weitkamp)
As many careful global analyses of, however, limited data sets have shown, clouds at present
occur daily over about 50 to 55% of the world, where highest ocurrences can be found in the
"belts" of stronger convective and synoptic-scale activities. On the other side the subtropical
branches of the Hadley-cell and, moreover, several regions over the continents contain very dry
air and consequently only very few clouds. Due to importance of clouds on the weather at
the surface, on the transport of precipitation and within the huge sequences of air-chemical
processes, world-wide observations began very early, being now complemented by space-borne
data. Fig. 1.5 displays results on the day-night difference of the fractional coverage by clouds
which was obtained from ground-based observations by Hahn et al. (1995). They analysed
most available groundbased cloud observations and obtained global cloud distributions which
are quite similar to those determined from satellite data (see Rossow, this volume).
Analyses of satellite data began as early as they could be visualized in images and manipulated
in computers. Such early overlay statistics led e.g. to the "discovery" of the "Double-ITC"
over the Pacific Ocean. Operational meteorologists also developed manual analysis techniques
(Nephanalyses).
The more modern analyses of satellite measurements can now make use
• of contrasts of cloud fields against the ambient background, the Earth's surface, in their
intensities of emitted and reflected radiation, respectively, and
• of the spectral properties of these quantities. For instance, ice clouds reflect slightly
less radiation than water clouds, thus "differences" between albedoes of simulataneous
measurements can be used to identify cirrus over bright lower level clouds, or low level
stratus over snow and ice fields.
• of up to 24 measurements per day from most geostationary satellites to meet the diurnal
variations.
Such data form the major source of informations within the quite straightforward algorithms
which at present are used in most cloud analysis schemes, also within the International Satellite
Cloud Climatology Project ( ISCCP; see papers by Rossow in this volume, and Rossow and
Schiffer,1991). Here the radiative transfer properties are estimated by comparison of measured
with calculated radiance data where additional ground-based observations of snow cover and
of the surface temperatures are used to "enhance" the contrasts. The ISCCP has at present
analysed worldwide data sets from more than 10 complete years, beginning in 1983. The diurnal
variation of cloud fields is observable over most regions of the world in the data of geostationary
satellites, which - with the exception of the INS AT-series - enter also the ISCCP analyses.
There are also several attempts to identify cloud species with pattern recognition methods (e.g.:
Lovejoy et al., 1992); but those methods have until present not reached an operational status