Cloud Profile and Rain Radars
Description
Cloud profile radars use very short
wavelength (mm) radar (typically 94 GHz) to
detect scattering from non-precipitating
cloud droplets or ice particles, thereby
yielding information on cloud
characteristics such as moisture content and
base height.
A 94 GHz cloud profiling radar has the
unique property of being able to penetrate
ice clouds with negligible attenuation and
providing a range-gated profile of cloud
characteristics.
Rain radars use microwave radiation
(centimetre wavelengths) to detect
backscatter from water drops and ice
particles in precipitating clouds, and to
measure the vertical profile of such
particles. One of the key challenges with
such radars is suppressing the return from
Earth’s surface (ground clutter), which is
inevitably much stronger than the rain echo.
Recent instruments however, can map the 3D
distribution of precipitating water and ice
in a relatively narrow swath (around 200 km)
along the track of a low-altitude satellite,
making it possible to infer more precise
estimates of instantaneous rainfall.
The Precipitation Radar (PR) on the Tropical
Rainfall Measuring Mission, launched in
1997, was the first radar in space with the
capability to measure rainfall. PR provided
3D maps of storm structure and invaluable
information on the intensity and
distribution of rain, rain type and storm
depth. NASA’s CloudSat uses an advanced 94
GHz radar to ‘slice’ through clouds to see
their vertical structure, providing a
completely new observational capability from
space. These instruments are the first to
study cloud profiles on a global basis, and
to look at their structure, composition and
effects. From early 2014, the Global
Precipitation Measurement mission – an
international cooperative programme – will
provide more frequent and complete sampling
of Earth’s precipitation using a
constellation of satellites. As of 2016, the
Japanese instrument on the ESA–JAXA
EarthCARE mission, the 94 GHz CPR, will
continue providing cloud profile
observations, with increased sensitivity and
additional Doppler capability to observe
vertical motion.
Applications
Measurements from cloud radar give
information on cloud type and amount, and,
more importantly, on cloud profile
(currently not measured). This information
is required both for improving numerical
weather prediction and for climate studies.
Scientists believe that some of the main
uncertainties in climate model simulations
are due to the difficulties in adequately
representing clouds and their radiative
properties. Satellite observations have now
started to address this issue.
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Current & planned
instruments
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Cloud radar (ACE)
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DPR
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CPR (CloudSat)
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PR
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CPR (EarthCARE)
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TRMM has demonstrated that spaceborne rain
radars can provide a unique source of
information on liquid water and precipitation
rate, since the ground-based rain radars used
at present have limited coverage over the
oceans. The global precipitation datasets
derived from TRMM have proved to be valuable
tools for climatologists. Information on
tropical rainfall and extreme events such as
hurricanes is of particular importance, since
more than two thirds of global rainfall is in
the Tropics, acting as a primary driver of
global atmospheric circulation.
The potential gap between termination of the
TRMM mission (currently operating well beyond
its planned lifetime) and availability of the
new information anticipated from GPM (no
earlier than 2014) is of concern to scientists
studying Earth’s global water cycle. CEOS has
initiated a precipitation virtual
constellation study team to address this and
related coordination issues.
Hurricane Julia (2010) AMSR-E microwave
data (top), with CloudSat reflectivity data
(bottom) profiles clouds through the eye of
the storm.
(Click image to view full size)
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