Geoscience Reference
In-Depth Information
of convoys and constellations is also expected to strengthen the value and use of the
satellite data for understanding and monitoring of the hydrological cycle.
The state-of-the-art overview of currently available EO products for hydrological
monitoring and modelling is provided in the following. Importantly, it is not aimed for
listing methodologies on how to implement such products in models, as this have been
extensively summarized elsewhere (Van Dijk and Renzullo
2011
), but it clearly empha-
sizes the multidisciplinary aspects and complexity that jointly with the wide range of
spatial and temporal scales establish a significant demand on the observing system. The
individual papers presented in this issue provide further quantitative evidence of the
capability and limitation of the observing system.
2 Clouds and Precipitation
Cloud and precipitation systems tend to be somewhat random in character, and they are
usually small scale and also evolve very rapidly, especially during the summer in con-
vection regimes. These factors make clouds and precipitation difficult to quantify. Reliable
ground-based precipitation measurements are difficult to obtain over regional and global
scales because more than 70 % of the Earth's surface is covered by ocean and lakes, and
additionally many countries are not equipped with precise rain measuring sensors (i.e., rain
gauges and/or radars). In such regions, regional and global scale precipitation measure-
ments from Earth Observation satellite systems are extremely valuable.
Over its lifetime of more than 15 years, the Tropical Rain Monitoring Mission (TRMM)
satellite has provided a wealth of information on tropical cyclones and short-duration
climate shifts such as El Ni
˜
o (Curtis et al.
2007
) and has proved to be an essential tool for
the measurement of precipitation. Current operational and research platforms form a
constellation that can be used for the routine generation of precipitation with nominal 3-h
temporal and 0.25-degree spatial resolution. Estimates derived at full resolution (4 km) are
available up to instantaneously, albeit more sporadically. However, TRMM misses low-
rate precipitation, e.g., drizzle, which is expected to contribute significantly to the total
precipitation in regions with low rainfall amounts. The upcoming Global Precipitation
Measurement (GPM) mission [e.g.,
http://pmm.nasa.gov/GPM
] is a network of satellites
and will provide solid and liquid precipitation, including light precipitation. Full vertical
profiles of clouds and light and solid precipitation are being observed by CloudSat [
http://
cloudsat.atmos.colostate.edu/
]
and will be further improved by EarthCARE (see, e.g.,
Explorers/EarthCARE/ESA_s_cloud_aerosol_and_radiation_mission
)
. EarthCARE will provide
extended vertical cloud profiles with significantly higher sensitivity compared to CloudSat and
added Doppler capability for observation of vertical motion within clouds. Solid precipitation
and light precipitation will be measured together with full cloud profiles, and heavy precipitation
will be detected. In contrast to TRMM and GPM radar satellites, CloudSat and EarthCARE
which travel on polar orbits provide full global coverage.
The usefulness of radar systems capable of measuring precipitation (e.g., TRMM
Precipitation Radar and CloudSat Cloud Profiling Radar) has been demonstrated (Lonfat
et al.
2004
). They provide a unique and crucial addition to our observational capabilities
for precipitation. The retrieval of precipitation at higher latitudes remains an open chal-
lenge. Problems include contamination by the surface background and low-level, frozen
precipitation.
