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H.-J. Bolle, S. I. Rasool, P. Try
networks. Energy and water fluxes cannot be measured directly, they have to be
computed using other measurable quantities which determine these fluxes. The
global models used for this task are those which simulate the General Circulation
(GCM's) of the atmosphere and the world ocean. They exist also in coupled versions
and as climate models which are GCM's that can be integrated over long time
periods. Global measuring networks to be considered here must be operational ones.
Only this guaranties continuous operation and replacement of instruments in case
of failure. Two of these systems exist, the meteorological network at the ground
including aerological stations and the operational meteorological satellite system.
The classical investigations of energy and water fluxes one can think of are those of
Oort (1971), Vonder Haar and Suomi (1971), Oort and Vonder Haar (1976), Ellis
and Vonder Haar (1978), and Peixoto and Oort (1982), and the more recently the
NOAA Pathfinder Project.
Both the modelling as well as the measuring approach have deficits in assessing
climate variability and trends. In the models some of the important processes are
only marginally represented - in other words parameterized - especially if one comes
down to the smaller scales. The operational ground based and aerological
measurements are not as dense as one may wish and are afflicted with instrumental
errors. Only standard parameters are measured here. The observations made from
space is the only data source continuous in space and time but the remotely
measured signals have to be converted into the required information. The two
methods applied to determine the global exchange of water and energy consequently
have to be validated and to be improved stepwise. On a global scale this is an
impossible task. The potential to install a dense network equipped with sophisticated
instrumentation just does not exist. Therefore these tasks have to be done at smaller
scales in representative or critical areas where the most essential problems can be
treated with great accuracy.
GEWEX experiments have been structured in a way that the area under
investigation is large enough to compare the results with those obtained by global
climate models. Since one grid cell is by far not enough for such a comparison the
area must be ofthe order of 10 grid sizes but regional differences should as well be
assessed at much smaller regional scales. Because hydrological processes at the land
surfaces have response times of more than a year such an experiment should ideally
expand over a number of years. These requirements are already incompatible with
the available resources. Therefore a strategy to further reduce the requirements was
developed. Only those comparative measurements are extended over two, three
years, which are necessary to assess the long term processes like the cycles of soil
moisture and aquifer replenishment. On the other hand the investigations needed to
improve the parametrization of regional and local processes are only carried out in
specific areas nested into the larger experimental area. Satellite data are used to
bridge the results obtained in these selected areas over to the whole area and to
provide continuous information at least about a few state parameters. The
experiments need thorough information on land-use, land-cover, soils, geohydrology and vegetation which integrates many disciplines into this effort.
H.-J. Bolle, S. I. Rasool, P. Try
networks. Energy and water fluxes cannot be measured directly, they have to be
computed using other measurable quantities which determine these fluxes. The
global models used for this task are those which simulate the General Circulation
(GCM's) of the atmosphere and the world ocean. They exist also in coupled versions
and as climate models which are GCM's that can be integrated over long time
periods. Global measuring networks to be considered here must be operational ones.
Only this guaranties continuous operation and replacement of instruments in case
of failure. Two of these systems exist, the meteorological network at the ground
including aerological stations and the operational meteorological satellite system.
The classical investigations of energy and water fluxes one can think of are those of
Oort (1971), Vonder Haar and Suomi (1971), Oort and Vonder Haar (1976), Ellis
and Vonder Haar (1978), and Peixoto and Oort (1982), and the more recently the
NOAA Pathfinder Project.
Both the modelling as well as the measuring approach have deficits in assessing
climate variability and trends. In the models some of the important processes are
only marginally represented - in other words parameterized - especially if one comes
down to the smaller scales. The operational ground based and aerological
measurements are not as dense as one may wish and are afflicted with instrumental
errors. Only standard parameters are measured here. The observations made from
space is the only data source continuous in space and time but the remotely
measured signals have to be converted into the required information. The two
methods applied to determine the global exchange of water and energy consequently
have to be validated and to be improved stepwise. On a global scale this is an
impossible task. The potential to install a dense network equipped with sophisticated
instrumentation just does not exist. Therefore these tasks have to be done at smaller
scales in representative or critical areas where the most essential problems can be
treated with great accuracy.
GEWEX experiments have been structured in a way that the area under
investigation is large enough to compare the results with those obtained by global
climate models. Since one grid cell is by far not enough for such a comparison the
area must be ofthe order of 10 grid sizes but regional differences should as well be
assessed at much smaller regional scales. Because hydrological processes at the land
surfaces have response times of more than a year such an experiment should ideally
expand over a number of years. These requirements are already incompatible with
the available resources. Therefore a strategy to further reduce the requirements was
developed. Only those comparative measurements are extended over two, three
years, which are necessary to assess the long term processes like the cycles of soil
moisture and aquifer replenishment. On the other hand the investigations needed to
improve the parametrization of regional and local processes are only carried out in
specific areas nested into the larger experimental area. Satellite data are used to
bridge the results obtained in these selected areas over to the whole area and to
provide continuous information at least about a few state parameters. The
experiments need thorough information on land-use, land-cover, soils, geohydrology and vegetation which integrates many disciplines into this effort.