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Background
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| One of the most important potential effects of climate change is a change in the extremes of
the hydrological cycle manifest in changing extremes of precipitation. Precipitation rate is so highly variable
in time and space that it is one of the most difficult of the essential climate variables to measure and monitor
precisely, particularly the more extreme (rarer) events. Current-day surface precipitation gauge arrays are much
too sparse in spatial coverage and (usually) report too infrequently to resolve precipitation variations and can
easily miss extreme events. Although indirect satellite precipitation products do exists at relatively high space
and time resolution, the indirect nature of these methods may not be able to properly capture the extreme events
that represent non-average conditions by their very nature. Spaceborne radars such as the TRMM PR, and GPM DPR in
the future, have good spatial resolution and very good calibration stability but have very low time sampling
frequencies compared with the variations of precipitation.
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| Weather radars, specifically designed to observe precipitation at high temporal and spatial
resolutions, are becoming ubiquitous in many nations around the world and are currently the only
precipitation-measuring system with the requisite space-time sampling. However, until now these systems have also
been generally sparsely distributed and operated and analyzed separately in a case-study approach. The recent
significant growth in the number of radars being operated, now makes possible high-resolution determinations of
precipitation over extensive land areas and over long time periods. Such datasets, when merged with very stable
spaceborne radar reflectivities would provide the observational basis for learning how the small-scale extreme
events are connected with the large-scale atmospheric circulation and how this may be changing in time. To provide
this type of data requires systematic collection and analysis of data from these radar networks in as many different
climate regimes as possible, but efforts to coordinate the exchange of precipitation radar data are only just
beginning. For example, within Europe, the OPERA (Operational Programme
for the Exchange of weather RAdar information) Program of EUMETNET, the
Network of European meteorological
services, has been organized to "harmonize European radar data and products, raise their quality, facilitate their
exchange, and support their application." Most of the precipitation radar applications are directed at real-time
monitoring of rainfall for detection and warning of flood events and not the retrospective analysis needed for
appropriate "climate-scale" statistics. The potential of these radar networks for climate studies is high but
has not been realized.
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Purpose
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| The GEWEX Radiation Panel has proposed to organize an
international Working Group to foster
systematic and collective use of comprehensive precipitation radar products, especially from extensive networks
that provide large-area coverage for long time periods with very high space-time resolutions, to allow for the
study of the coupling of small-scale and large-scale variability and the examination of the statistics of extreme
events. Such studies are needed to advance understanding of the precipitation process.
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Tasks
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The working group's tasks are to:
- solicit expressions of interest to participate from operators of the major precipitation radar networks and
from research scientists using these products,
- collect descriptions of the attributes of the available weather radar networks [including the number of
stations, average separation distance, area covered, type of radar (frequencies, polarization, doppler),
space-time sampling interval of standard products],
- find out what data are currently archived [radiances, precipitation and other products] and discuss what
products could/should be archived and disseminated,
- find out whether radiances and products can be shared, and
- make plans for producing common products (using common analysis methods) for common periods of time so that
a comprehensive collection of precipitation products could be produced for global-scale studies.
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| Ackowledgments
| Pages hosted by the Institute of Atmospheric Sciences and Climate (ISAC) of the National Research Council (CNR).
WGPRN is a working group of the GEWEX Radiation Panel (GRP).
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