Energy and Water Cycles in the Climate System ...
5
In the following we will, as it had been planned for this summer school, concentrate only on the
description of the present knowledge of the components of the radiation budget at the top and
bottom of the atmosphere and of the availability and transport of water in the atmosphere, at
ground and in the uppermost soil layers. Here, we will describe briefly how these quantities can
be inferred from remote satellite measurements and to some degree also from those at ground.
More details on the modelling (Del Genio, this volume), on data analysis and assimilation schemes (Courtier, this volume) and in particular on special space-borne measurements and some
principle arguments of the radiative transfer theory (Fouquart and Vesperini, this volume) are
given in the papers to follow. The same holds also for some of the more recent "climatologies"
on the radiation budget components and clouds, and the precipitation and evaporation over
the oceans. We also will stress the need for accurate and reliable ground-based informations,
which in many cases are urgently required to validate the quantities which are estimated from
the satellite data.
It is one of the major goals of the Global Energy and Water Cycle Experiment (GEWEX;
WCRP, 1988) to develop and use operationally spaceborne techniques and appropriate analysis
schemes to measure these quantities with high spatial detail over the entire globe.
Very detailed accounts of our present knowledge on energy and water cycles are given in more
recent textbooks, e.g. by Hartmann (1995) or Peixoto and Oort (1992), in some other papers of
these and many other authors, and also in the publications of earlier NATO-Advanced Study
Institutes (e.g. Raschke and Jacob, 1992; Oliver and Oliver, 1994). They, therefore, will not
be repeated in this book, although they have been presented and discussed during the various
lectures.
1.2 Water in the Climate System
Our climate system (see e.g. summary by Chahine, 1992) contains more than 1.4 Mill. x 10 12 t
water, of which only a very small fraction (about 0.0001 %) is abundant within the atmosphere,
and another but larger fraction (about 4%) at the land surfaces. Fig. 1.1, from Chahine (1992),
displays these distributions and also the annual mean fluxes between the different reservoirs
which are formed by the atmosphere, the continents and the oceans. This schematic shows that
the atmosphere in general transports, in general, water from the oceans over the continents,
from where it flows back into the oceans.
This freshwater supply into the oceans is believed to obey a strong influence on the formation
of deep sea currents in the Arctics, which in turn may effect the mean position of warmer
near-surface branches of the world-wide conveyor belt and lead to changes of the continental
climate (e.g. Schmitz, 1995).
Over the oceans the atmosphere recycles its water content about 40 times during each year
(precipitation divided by atmospheric storage), over continents only about 24 times. Thus the
mean residence time of water within the atmosphere is between about 9 to 15 days, while in
the oceans it may remain more than 3000 years. This recycling over land becomes important in
precipitation forecasts, when frontal systems move inward over a continent and the soil moisture
might be predicted with too low values, then also too small amounts of precipitation are often
predicted.
These immensely large water reservoirs slow down all climate processes. A "fast" regime is
found in the system atmosphere, land surfaces and upper ocean layers, which directly controls
the amplitude and regional patterns of climate variations. A "slow" regime is caused by large
5
In the following we will, as it had been planned for this summer school, concentrate only on the
description of the present knowledge of the components of the radiation budget at the top and
bottom of the atmosphere and of the availability and transport of water in the atmosphere, at
ground and in the uppermost soil layers. Here, we will describe briefly how these quantities can
be inferred from remote satellite measurements and to some degree also from those at ground.
More details on the modelling (Del Genio, this volume), on data analysis and assimilation schemes (Courtier, this volume) and in particular on special space-borne measurements and some
principle arguments of the radiative transfer theory (Fouquart and Vesperini, this volume) are
given in the papers to follow. The same holds also for some of the more recent "climatologies"
on the radiation budget components and clouds, and the precipitation and evaporation over
the oceans. We also will stress the need for accurate and reliable ground-based informations,
which in many cases are urgently required to validate the quantities which are estimated from
the satellite data.
It is one of the major goals of the Global Energy and Water Cycle Experiment (GEWEX;
WCRP, 1988) to develop and use operationally spaceborne techniques and appropriate analysis
schemes to measure these quantities with high spatial detail over the entire globe.
Very detailed accounts of our present knowledge on energy and water cycles are given in more
recent textbooks, e.g. by Hartmann (1995) or Peixoto and Oort (1992), in some other papers of
these and many other authors, and also in the publications of earlier NATO-Advanced Study
Institutes (e.g. Raschke and Jacob, 1992; Oliver and Oliver, 1994). They, therefore, will not
be repeated in this book, although they have been presented and discussed during the various
lectures.
1.2 Water in the Climate System
Our climate system (see e.g. summary by Chahine, 1992) contains more than 1.4 Mill. x 10 12 t
water, of which only a very small fraction (about 0.0001 %) is abundant within the atmosphere,
and another but larger fraction (about 4%) at the land surfaces. Fig. 1.1, from Chahine (1992),
displays these distributions and also the annual mean fluxes between the different reservoirs
which are formed by the atmosphere, the continents and the oceans. This schematic shows that
the atmosphere in general transports, in general, water from the oceans over the continents,
from where it flows back into the oceans.
This freshwater supply into the oceans is believed to obey a strong influence on the formation
of deep sea currents in the Arctics, which in turn may effect the mean position of warmer
near-surface branches of the world-wide conveyor belt and lead to changes of the continental
climate (e.g. Schmitz, 1995).
Over the oceans the atmosphere recycles its water content about 40 times during each year
(precipitation divided by atmospheric storage), over continents only about 24 times. Thus the
mean residence time of water within the atmosphere is between about 9 to 15 days, while in
the oceans it may remain more than 3000 years. This recycling over land becomes important in
precipitation forecasts, when frontal systems move inward over a continent and the soil moisture
might be predicted with too low values, then also too small amounts of precipitation are often
predicted.
These immensely large water reservoirs slow down all climate processes. A "fast" regime is
found in the system atmosphere, land surfaces and upper ocean layers, which directly controls
the amplitude and regional patterns of climate variations. A "slow" regime is caused by large