Ground Validation for Global Precipitation Measurement

Not all GPM measurements will originate from satellites. A key component of GPM, called “Ground Validation”(GV) will have its foundation firmly upon Earth. Its purpose is to provide an independent means of evaluation, diagnosis, and improvement of the GPM spaceborne measurements and precipitation retrievals.

The GPM satellites’ sensor measurements will be used as input to a suite of algorithms that generate a wide variety of intermediate and final data products. Error estimates attached to satellite products will provide increased value and credibility to users of these products. Error characterization is so important to GPM that it will be addressed in two parallel ways: through the satellite algorithms, and through the ground validation program. Satellite algorithms will have an associated error model where the impact of uncertainties within input data, physical assumptions, and mathematical approximations are propagated through the algorithm. Ground validation will provide an independent assessment of errors by comparing the satellite products to independent measurements.

The primary goals of GPM GV are threefold:

· Evaluation to estimate the quality of satellite precipitation products in terms of systematic and random error and their spatial correlation
· Diagnosis to ascertain the causes of errors within satellite products
· Improvement of satellite products by refining some of the physical and scaling assumptions within the satellite algorithms, the underlying cloud models, and the underlying radiative transfer calculations

Errors in satellite products can result from any number of reasons. In particular, GV will focus on diagnosing sources of error associated with sensor calibration, algorithm assumptions, and algorithm applicability.

In contrast to the Tropical Rainfall Measuring Mission (TRMM), which focused on tropical precipitation, GPM will address both tropical and mid-latitude precipitation. Mid-latitude precipitation presents some unique challenges. At mid-latitudes, storm structure characteristics rapidly change across weather fronts, and differentiation of surface rain, mixed rain and snow, and snow can be difficult. An algorithm initially designed for tall storms in the tropics may not work well in shallow storms associated with winter precipitation in the mid-latitudes of the United States, Europe, and Asia. An important challenge for GPM is to modify TRMM’s algorithms so the resulting GPM algorithms are applicable to both tropical and mid-latitude storms.

Like the constellation of GPM spacecraft, GV will rely upon a diverse partnership of domestic agencies and international scientific collaboration. Countries with and without space programs are anticipated to participate in GPM through the GV element.

GV requires that storms within distinct geographic and climatic regimes be observed on a continuous basis through mission life. These regimes include tropical oceanic, tropical continental, mid-latitude oceanic, and mid-latitude continental. Fixed sites around the globe (termed supersites) will be established in these distinct regimes. For its part NASA will provide, maintain, and manage two supersites: a tropical oceanic site at Kwajalein Atoll in the Republic of the Marshall Islands (Figure 1), and a mid-latitude site within the continental United States.

Figure 1: Sunrise over a meteorological research radar on Kwajalein Island in the southwestern Pacific Ocean.

Supersites will be well-instrumented with research radars, rain measurement networks, remote sensing instruments similar to those on the satellites, and a variety of surface-based instruments to aid in physical interpretation of the other measurements. Data collected on a 24-hour basis and coincident with satellite overpasses will be analyzed by a dedicated research arm of the supersite. These scientists and engineers will be responsible for providing a variety of GV products on a routine and continuous basis throughout GPM mission life. Error characterization (based on input and algorithms from the supersites) will be run as part of the centralized data processing at the Precipitation Processing System, where GPM products will be generated as the satellite data streams arrive.

Figure 2: A tipping bucket rain gauge located at the NASA/Goddard Space Flight Center Wallops Island Flight Facility, VA

Geography and climatology will preclude addressing the full set of critical GPM GV measurement objectives at the small number of supersites. Focused measurement programs will fill in these gaps. These programs will include long-term monitoring and field programs with aircraft. These focused measurement programs will have two main goals:

· Refine specific physical and scaling assumptions within satellite algorithms, cloud models, and/or radiative transfer calculations
· Provide observations to support cloud and regional modeling and/or radiative transfer calculations at climatologically important locations distinct from the supersites

GPM GV customers will be interested in the physical processes of precipitation and its role in weather and climate. Specific customers may include satellite and GV algorithm specialists, data assimilation specialists working to use GPM products as input to weather prediction models, and climate and hydrologic researchers.

Satellite and GV algorithm specialists will want to improve the accuracy of the retrievals and error estimates through refinement of underlying physical assumptions, and will want to widen the algorithm’s application. Data assimilation specialists will require removal of known biases in the satellite retrievals, and measures of the spatial correlation and random error magnitude to employ satellite observations in their models. Climate and hydrologic researchers will seek to improve understanding of the interaction of physical processes in the atmosphere, ocean, and land, and to discern trends from the obscuring effect of error. The GV component of GPM will work closely with its diverse customers to provide products that address their needs.

Those wishing to learn more about the GPM ground validation effort can contact Dr. Steven Bidwell at bidwell@agnes.gsfc.nasa.gov or Dr. Sandra Yuter at yuter@atmos.washington.edu.

By Steven W. Bidwell/NASA Goddard Space Flight Center, Greenbelt, MD and
Sandra Yuter/Department of Atmospheric Sciences, University of Washington, Seattle, WA

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