Energy and Water Cycles in the Climate System ...
23
an almost nationwide network of digital Doppler-radar which allows a continentalwide picture
on precipitation events and intensities, their convective activities and horizontal movements.
Such information is of enormous economic importance, for instance to minimize the damage by
strong events, such as thunderstorms or tornadoes or to manage the water resources. A similar
network already exists also over Western Europe and the Nordic countries.
But does even such a dense network, even over plain and uniform land "measure" the amount
of water reaching the ground?? Collier and Knowles (1995), Joss and Waldvogel (1990), Sauvageot (1994) and Zawadski (1982) and others point out that the inversion of even such multiinformative signals is dependent on various assumptions on the properties of the falling rain, in
particular on the droplet size distributions and on the falling speed. Ground glitter and other
sources may deteriate the signals and their information contents. Further, more distant rain
events can only be seen several hundred meters above ground, where the falling rain has still
some chance to evaporate while falling to the ground.
Nevertheless such information is of vital importance for weather analyses and short range forecasts (in particular of hazardeous events) and also for many hydrological applications. To
interpolate between the ranges of validity of the various radar stations, there are also attempts
to "blend" the radar data with measurements of the geostationary satellite Meteosat. Such
products and their values are discussed by Browning and Collier (1989).
Despite of these still existing uncertainties a new satellite (TRMM: Tropical Rainfall Monitoring Mission) will soon be launched into a near-equatorial orbit to measure with radar and
passive multispectral radiometers precipitation events and their intensity over the near equatorial regions. It is an urgent challenge for the numerical modellers to assimilate such information
into their analysis schemes and forecast models to improve our knowledge on the quantitative
precipitation over oceans and continents. Further details on data assimilation and modelling
problems are given by Courtier and DelGenio, respectively (this volume).
1. 7 Evaporation and Evapotranspiration
(see also contributions by Bastiaansen, Hallikainen, Katsaros, Kiely, Paloscia,
Schliissel)
Regional values of the evaporation over sea and the evapotranspiration over continental surfaces
are required to estimate and to validate model results on the energy and water cycles over
regions or to derive within dedicated process studies parametrization schemes for use in global
models. A quite complete account of the complexity of such research is given in a recent review
by Parlange et al. (1995).
The evaporation from water surfaces or even just from flat and bare soil cannot be measured
directly, rather it is derived from vertical flux (of momentum, sensible and latent heat) measurements, and then related to easier measurable quantities like the "moisture deficit" above
ground, wind speed and the net radiation budget at ground levels. These principles led to
reliable values over many areas, provided the soil moisture over the continents was known.
Valuable assistance came here from modelling of the atmospheric boundary layer over various
surface types.
The evaporation estimates over oceans, as reported in details by Schliissel (this volume) make
use of such relations and derive the required input information from satelite measurements
of the sea surface temperature, near surface winds (from roughness of the sea) and moisture.
A simplified formulation of the boundary layer dynamics helps to estimate the vertical water
vapour fluxes. Unfortunately no recent "ground-truth" experiment is known to validate such
23
an almost nationwide network of digital Doppler-radar which allows a continentalwide picture
on precipitation events and intensities, their convective activities and horizontal movements.
Such information is of enormous economic importance, for instance to minimize the damage by
strong events, such as thunderstorms or tornadoes or to manage the water resources. A similar
network already exists also over Western Europe and the Nordic countries.
But does even such a dense network, even over plain and uniform land "measure" the amount
of water reaching the ground?? Collier and Knowles (1995), Joss and Waldvogel (1990), Sauvageot (1994) and Zawadski (1982) and others point out that the inversion of even such multiinformative signals is dependent on various assumptions on the properties of the falling rain, in
particular on the droplet size distributions and on the falling speed. Ground glitter and other
sources may deteriate the signals and their information contents. Further, more distant rain
events can only be seen several hundred meters above ground, where the falling rain has still
some chance to evaporate while falling to the ground.
Nevertheless such information is of vital importance for weather analyses and short range forecasts (in particular of hazardeous events) and also for many hydrological applications. To
interpolate between the ranges of validity of the various radar stations, there are also attempts
to "blend" the radar data with measurements of the geostationary satellite Meteosat. Such
products and their values are discussed by Browning and Collier (1989).
Despite of these still existing uncertainties a new satellite (TRMM: Tropical Rainfall Monitoring Mission) will soon be launched into a near-equatorial orbit to measure with radar and
passive multispectral radiometers precipitation events and their intensity over the near equatorial regions. It is an urgent challenge for the numerical modellers to assimilate such information
into their analysis schemes and forecast models to improve our knowledge on the quantitative
precipitation over oceans and continents. Further details on data assimilation and modelling
problems are given by Courtier and DelGenio, respectively (this volume).
1. 7 Evaporation and Evapotranspiration
(see also contributions by Bastiaansen, Hallikainen, Katsaros, Kiely, Paloscia,
Schliissel)
Regional values of the evaporation over sea and the evapotranspiration over continental surfaces
are required to estimate and to validate model results on the energy and water cycles over
regions or to derive within dedicated process studies parametrization schemes for use in global
models. A quite complete account of the complexity of such research is given in a recent review
by Parlange et al. (1995).
The evaporation from water surfaces or even just from flat and bare soil cannot be measured
directly, rather it is derived from vertical flux (of momentum, sensible and latent heat) measurements, and then related to easier measurable quantities like the "moisture deficit" above
ground, wind speed and the net radiation budget at ground levels. These principles led to
reliable values over many areas, provided the soil moisture over the continents was known.
Valuable assistance came here from modelling of the atmospheric boundary layer over various
surface types.
The evaporation estimates over oceans, as reported in details by Schliissel (this volume) make
use of such relations and derive the required input information from satelite measurements
of the sea surface temperature, near surface winds (from roughness of the sea) and moisture.
A simplified formulation of the boundary layer dynamics helps to estimate the vertical water
vapour fluxes. Unfortunately no recent "ground-truth" experiment is known to validate such