Chapter 11
Retrieval of Precipitation from
Satellites
c. Simmer
Institut fiir Meereskunde
Diisternbrooker Weg 20, 24105 Kiel
Germany
11.1 Introduction
For land areas, precipitation is one of the most important processes for the biosphere, including
mankind. For the oceans, precipitation is a source for freshwater and thus an important forcing
term for the thermohaline circulation. In the atmosphere, precipitation liberates latent energy,
which is a main forcing term for atmospheric motions, especially in the tropics. The global
average precipitation is equivalent to a water column of about 1 m height. This amounts to
a continuous energy supply to the atmosphere of about 80 Wm- 2 , which is roughly 30% of
the incoming solar energy. Mainly caused by its strong coupling to the vertical velocity field,
precipitation is extremely variable in space and time.
Over most areas of the globe, especially over sea, there are not enough direct measurements
for the needs of an adequate understanding of the hydrological cycle. In addition, measurements over land are hampered by orographic effects. Over sea, errors are caused by the air-flow
obstruction caused by the ship. Remote sensing from the surface, e.g. by radar, enable high
temporal sampling and larger areal coverage, but cannot be applied globally due to high costs
for equipment and personnel. In addition, the inversion of radar measurements is not straightforward and has problems of its own like uncertainy of dropsize spectra and the increased
elevation of the radar beam from the earth surface with increasing distance. Remote sensing
from satellites is left as the only practible method to achieve global coverage of precipitation
estimates.
11.2 General problems related to remote sensing of
rain
Remote sensing of any parameter is always based on the interpretation of the intensity field
of electromagnetic radiation (radiance) emanating from the raining cloud and its environment.
NATO ASI Series. Vol. 145
Radiation and Water in the Climate System:
Remote Measurements
Edited by Ehrhard Raschke
© Springer-Verlag Berlin Heidelberg 1996
Retrieval of Precipitation from
Satellites
c. Simmer
Institut fiir Meereskunde
Diisternbrooker Weg 20, 24105 Kiel
Germany
11.1 Introduction
For land areas, precipitation is one of the most important processes for the biosphere, including
mankind. For the oceans, precipitation is a source for freshwater and thus an important forcing
term for the thermohaline circulation. In the atmosphere, precipitation liberates latent energy,
which is a main forcing term for atmospheric motions, especially in the tropics. The global
average precipitation is equivalent to a water column of about 1 m height. This amounts to
a continuous energy supply to the atmosphere of about 80 Wm- 2 , which is roughly 30% of
the incoming solar energy. Mainly caused by its strong coupling to the vertical velocity field,
precipitation is extremely variable in space and time.
Over most areas of the globe, especially over sea, there are not enough direct measurements
for the needs of an adequate understanding of the hydrological cycle. In addition, measurements over land are hampered by orographic effects. Over sea, errors are caused by the air-flow
obstruction caused by the ship. Remote sensing from the surface, e.g. by radar, enable high
temporal sampling and larger areal coverage, but cannot be applied globally due to high costs
for equipment and personnel. In addition, the inversion of radar measurements is not straightforward and has problems of its own like uncertainy of dropsize spectra and the increased
elevation of the radar beam from the earth surface with increasing distance. Remote sensing
from satellites is left as the only practible method to achieve global coverage of precipitation
estimates.
11.2 General problems related to remote sensing of
rain
Remote sensing of any parameter is always based on the interpretation of the intensity field
of electromagnetic radiation (radiance) emanating from the raining cloud and its environment.
NATO ASI Series. Vol. 145
Radiation and Water in the Climate System:
Remote Measurements
Edited by Ehrhard Raschke
© Springer-Verlag Berlin Heidelberg 1996