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Lessons from TRMM Applicable to the GPM Mission

The Tropical Rainfall Measuring Mission (TRMM) has provided important lessons that are relevant to the observational and research aspects of the Global Precipitation Measurement (GPM) mission. Much was also learned from TRMM that is relevant to the operational use of GPM data and to an effective partnership between the National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (NASA). This chapter addresses the first question of the committee’s task: What lessons were learned from the TRMM mission with respect to operational uses of the data and how can these lessons enhance the use of GPM mission data and other NASA research mission data in NOAA operational forecasts?

As context to discussing the lessons learned from TRMM, it is useful to contrast the different initial conditions when TRMM was launched in 1997 from those conditions when the GPM core satellite is launched (proposed for 2013 as of publication) (Table 2.1). The difference in the conditions for these precipitation satellites reflects to some extent the lessons that were learned from TRMM. Before discussing the two areas of lessons that directly address the committee’s first task, this chapter includes a set of related, but general lessons for space-based precipitation measurement.

LESSONS FOR SPACE-BASED MEASUREMENT OF PRECIPITATION

The overarching technological achievement of TRMM was its demonstration that the mission approach to inferring precipitation from space was sound. TRMM’s scientific accomplishments establish its unique role as a “flying rain



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Noaa's Role in Space-Based Global Precipitation Estimation and Application 2 Lessons from TRMM Applicable to the GPM Mission The Tropical Rainfall Measuring Mission (TRMM) has provided important lessons that are relevant to the observational and research aspects of the Global Precipitation Measurement (GPM) mission. Much was also learned from TRMM that is relevant to the operational use of GPM data and to an effective partnership between the National Oceanic and Atmospheric Administration (NOAA) and the National Aeronautics and Space Administration (NASA). This chapter addresses the first question of the committee’s task: What lessons were learned from the TRMM mission with respect to operational uses of the data and how can these lessons enhance the use of GPM mission data and other NASA research mission data in NOAA operational forecasts? As context to discussing the lessons learned from TRMM, it is useful to contrast the different initial conditions when TRMM was launched in 1997 from those conditions when the GPM core satellite is launched (proposed for 2013 as of publication) (Table 2.1). The difference in the conditions for these precipitation satellites reflects to some extent the lessons that were learned from TRMM. Before discussing the two areas of lessons that directly address the committee’s first task, this chapter includes a set of related, but general lessons for space-based precipitation measurement. LESSONS FOR SPACE-BASED MEASUREMENT OF PRECIPITATION The overarching technological achievement of TRMM was its demonstration that the mission approach to inferring precipitation from space was sound. TRMM’s scientific accomplishments establish its unique role as a “flying rain

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Noaa's Role in Space-Based Global Precipitation Estimation and Application TABLE 2.1 Differences in the Context for Launch of TRMM Versus the GPM Core Satellite Context for Launch of TRMM Context for Launch of the GPM Core Satellite Experimental sensors on board (the first deployment of quantitative weather radar in space), including multiple sensors on one spacecraft (e.g., precipitation radar and passive microwave imager). Proven sensor technology; potential for the GPM dual-frequency precipitation radar to fly with other radars still in orbit (TRMM, CloudSat). No long data sets to which sensor data could be attached. Decade-long record of precipitation radar and other TRMM data. No operational experience with data. Operational experience with data since 1998. NOAA scientist involvement in NASA’s 1986 workshop on TRMM, and NOAA scientist participation on the TRMM science team. However, there was no expectation (and therefore no planned activities) of operational application of TRMM data at NOAA. Collaboration among NASA and operational agencies since 2001. NOAA involvement through attendance at annual GPM planning workshops, input on operational requirements for GPM, and participation on the Precipitation Measurement Missions science team. The GPM research plan has operational objectives, and efforts are under way to establish an effective NASA-NOAA partnership for the GPM post-launch phase. The TRMM ground validation approach followed the traditional lines of rain rate-oriented intercomparisons with classical ground validation site set-ups. The GPM ground validation program will include quantitative assessment of the distribution and the nature of retrieval errors. Moist physics in operational models was not well developed. Moist physics is evolving away from purely parameterized physics toward more explicitly resolved physics in the form of cloud-resolving models. This evolution is removing the artificial distinction between clouds and precipitation. Data assimilation of moist physics was in its infancy. Data assimilation of moist physics, while still in its formative stages, is progressing, and will help treat observations of clouds and precipitation as part of one combined system. Diverse and active community of researchers experimenting with a wide variety of evolving algorithms for retrieving rainfall and related information from passive microwave radiometers. Dedicated funding and available human resources have diminished.

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Noaa's Role in Space-Based Global Precipitation Estimation and Application gauge” and are detailed in reports by the National Research Council (NRC, 2004) and NASA (Adler et al., 2005). The committee identified eight lessons learned from TRMM with respect to space-based measurements of precipitation. 1. TRMM Was a Model for International Cooperation in Pursuing an Earth Remote-Sensing Initiative TRMM is a bilateral cooperative project between NASA and the Japan Aerospace Exploration Agency (JAXA). The project constitutes a significant investment by both agencies. JAXA contributed the precipitation radar to the satellite and provided the integration and launch aboard the Japanese H-II launcher in 1997. In addition, JAXA collected and preprocessed the precipitation radar data. NASA contributed the spacecraft bus and the remaining sensors. It also contributed integration of the instruments with the TRMM spacecraft and all shipping and handling of the completed spacecraft. Both countries maintain their own data downlinks and processing systems, but they share data and results. For example, “NASA and NASDA [JAXA’s precursor, the National Space Development Agency of Japan] shall share all TRMM data and make such data available to other users for research, operations and other uses under the terms of the IEOS DEP [International Earth Observation System Data Exchange Principles] (contained in the Appendix to this MOU [memorandum of understanding]). The shared data shall include all products from the NASA-provided instruments, the NASDA-provided instrument, and ground truth data used to validate the TRMM products” (NASA, 1995). In addition to NASA and NASDA, the IEOS included the European Space Agency (ESA), the European Organisation for the Exploitation of Meteorological Satellites (EUMETSAT), NOAA, the Japanese Science and Technology Agency (STA), the Ministry of International Trade and Industry of Japan (MITI), the Japan Environment Agency (JEA), the Japan Meteorological Agency (JMA), and the Canadian Space Agency (CSA). In addition to the partnership on data collection, the mission contributed to the U.S. Global Change Research Program and to related international efforts. Precipitation measurement from space is an area of great international collaborative potential. The lessons learned from the TRMM bilateral cooperation can be applied in establishing effective international partnerships on GPM and, ultimately, to the transition of GPM from a research mission to a viable operational system. 2. Robustness and High Endurance of the TRMM Microwave Imager and Precipitation Radar Systems in Space TRMM was designed as a minimum 3-year research mission with a goal of 5 years’ duration. The precipitation sensors have now been operating for more

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Noaa's Role in Space-Based Global Precipitation Estimation and Application than 8 years, and continue to function as designed (Adler et al., 2005). An independent technical assessment of the impact of extended life on spacecraft systems by the Goddard Space Flight Center (Adler et al., 2005) indicated that the “change in risk is minimal.” There is a high probability of each of TRMM’s four instruments operating successfully for another 5 years from the date of the assessment (Adler et al., 2005). As an example, the robustness of TRMM’s measurements has lead to a benchmark 7-year rain climatology, narrowing considerably the range of uncertainty in previous space-based rainfall estimates (Adler et al., 2003; Nesbitt et al., 2004). 3. Value of Precipitation Radar for Observing the Fine-Scale, Three-Dimensional Structure of Precipitation Systems from Space Discriminating precipitating scenes from cloudy scenes and estimating precipitation over land surfaces are two of the challenges of using passive microwave observations. Precipitation radar provides a more direct observation of precipitation in both cases and provides a far more detailed and potentially accurate measurement of precipitation than is obtained from passive microwave (and visible or infrared) data. Precipitation radar observations have revealed the fine-scale, three-dimensional structure of tropical storm systems (e.g., Nesbitt et al., 2000; Kelley et al., 2004) and provided new insights into the microphysical dynamics of the formation of precipitation (Schumacher and Houze, 2003; Schumacher et al., 2004; Chandrasekar et al., 2005) and the vertical profile of latent heat release (Olson et al., 1999; Tao et al., 2004). The precipitation radar has also exposed issues relating to passive microwave and visible-infrared methods of inferring precipitation and how the accuracy of these methods varies with atmospheric conditions and the space and time scales of interest (Berg et al., 2006). As an example of a rainfall-related feature not previously well-described and understood before TRMM, the TRMM precipitation radar has enabled the quantification of the diurnal cycle of precipitation and the convective intensity over land and oceans in the tropics (e.g., Sorooshian et al., 2002; Nesbitt and Zipser, 2003; Hong et al., 2005). TRMM’s single-frequency precipitation radar (13.8 GHz) represented a significant advance in observation technology. However, due to sensitivity limitations, the TRMM precipitation radar can only detect moderate to high rainfall rates. The major scientific and technological leap forward with the GPM mission is the dual-frequency precipitation radar in the core satellite, which will have several advantages over the TRMM precipitation radar. The two frequencies of the GPM precipitation radar are the “Ku band” frequency (13.6 GHz; similar to the 13.8 GHz frequency of TRMM’s precipitation radar) and the “Ka band” frequency (35.55 GHz); the Ka-band frequency has greater sensitivity to low precipitation rates (light rain, drizzle) and snow, and therefore will be able to measure lower rain rates than TRMM. In addition, both frequencies on the GPM

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Noaa's Role in Space-Based Global Precipitation Estimation and Application core satellite in principle can be used to more accurately estimate precipitation types and rates than with a single-frequency precipitation radar. Research still remains to be performed on both the microphysical inferences and improved precipitation retrieval using spaceborne dual-frequency techniques. The dual-frequency precipitation radar offers excellent potential for inferring precipitation microphysics (Chandrasekar et al., 2003a). The challenges in microphysical inferences and application to precipitation retrieval algorithms are discussed in papers by Chandrasekar et al. (2003b), Iguchi (2006), and Meneghini et al. (1992). 4. Feasibility of Co-located, High-Grade Precipitation Measurements from Space TRMM has provided important lessons on the optimal approach to measuring precipitation from space (e.g., Adler et al., 2003; Nesbitt et al., 2004). The accurate, scientifically robust measurement of precipitation from multiple sensors on a single platform (e.g., co-located precipitation radar and passive microwave radiometer) has yielded insight into the limitations of different methods and how to improve them (e.g., Berg et al., 2006). 5. Value of a Multi-Sensor Reference Satellite for Calibration of Data from Other Space-Based Observational Systems TRMM has two unique attributes that make it an ideal “flying rain gauge” for cross-calibrating passive microwave data from other satellites: its suite of complementary sensors and its low, non-sun-synchronous orbit that permits high-spatial resolution measurements. Co-location of the precipitation radar, the microwave imager, and the visible-infrared sensor on TRMM allows the use of high-precision precipitation radar measurements to calibrate radiometric measurements made by the TRMM Microwave Imager (Adler et al., 2003; Nesbitt et al., 2004). Subsequently, the non-sun-synchronous orbit provides orbital intersections between 35 degrees latitude North and South for intercalibration between the TRMM Microwave Imager and other passive microwave measurements from polar-orbiting satellites. For example, TRMM data are used to calibrate rain estimates from other satellites to provide analyses at higher time resolution than available from any single satellite (Adler et al., 2000). With the co-location of the precipitation radar and the GPM Microwave Imager on the GPM core satellite, the GPM mission will also realize these benefits demonstrated by TRMM. Despite the uncertain status of other satellites in orbit or under development (see Chapter 3), the combination of the precipitation radar and the GPM Microwave Imager will provide a unique reference tool for

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Noaa's Role in Space-Based Global Precipitation Estimation and Application intercalibration of other microwave imagers and thus enable better products from other microwave imagers. 6. Direct Validation of TRMM Precipitation Measurements Proved Difficult A comprehensive ground validation program encompasses four major elements: (1) reliable determination of precipitation from ground-based observations on time and space scales compatible with the satellite retrievals that are to be evaluated; (2) broad statistical comparisons of the ground-based observations and retrievals; (3) determination of the specific sources of retrieval errors, in terms of the physical and dynamical aspects of the precipitation environment; and (4) quantitative characterization of the distribution of retrieval errors as a function of meteorological conditions. Among the many notable aspects of TRMM was the application of substantial project resources to a ground validation program (NASA, 1988, Chapter 7). The stated goal of this pioneering effort was “to provide rainfall measurements which will allow the validity of the TRMM measurements to be established within specific limits” (NASA, 1988). The validation sites were selected to represent the different tropical rainfall regimes. Broadly speaking, the TRMM ground validation program was designed to focus primarily on the first two elements of a comprehensive ground validation effort. For example, the various observational sites were designed primarily to provide information on the typical retrieval bias for each of the major precipitation regimes of the tropics. While the program provided useful data for “placing bounds” on the acceptability of the TRMM retrievals (NASA, 1988), it generated a limited amount of information about the specific physical sources of retrieval errors and the distribution of errors—information that is important when using the retrievals in an operational setting. In light of the experience and insight gained from the TRMM ground validation program, GPM has developed a broad strategy for investigating and quantitatively assessing the distribution as well as the nature of retrieval errors. Bidwell et al. (2004) list two fundamental requirements of the GPM ground validation program. The first is to characterize the retrieval errors. The second is continued improvement of the retrieval algorithms. In addition, Bidwell et al. list the following as objectives of the GPM ground validation program: Quantitatively assess the error in spaceborne precipitation retrievals. This includes plans for estimating both the systematic and the random components of retrieval error and characterizing the spatial and temporal structure of the error. Diagnose the sources of error. Since the measurements extend beyond the tropics, GPM algorithms will experience new or modified sources of potential error (e.g., different distributions of precipitation types [stratiform, convective,

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Noaa's Role in Space-Based Global Precipitation Estimation and Application lighter rain rate, snowfall], different land-surface conditions and geographical effects, and large-scale forcing. Quantitatively evaluate and improve the retrieval algorithms. Plans for this aspect of the ground validation program include the active participation of algorithm developers. Advances in the understanding of precipitation physics are vital for satellite algorithm and data product improvements. Recognizing this need, GPM validation plans include both satellite products and cloud-resolving models run at some validation sites (Kummerow, 2006). Finally, the GPM validation strategy includes documentation of the percentage of time that an algorithm model meets specific accuracy levels for different observed meteorological conditions (Kummerow, 2006). This information is viewed as a first step in diagnosing, understanding, and improving precipitation products as well as numerical models. Further details of the GPM ground validation strategy are included in Chapter 1 (“The International Ground Validation Program”) and Chapter 4 (“Ground Validation Support”). 7. Feasibility of Obtaining Near-Real-Time Global Coverage of Precipitation Observations from Space The TRMM satellite was designed to sample precipitation between 35 degrees latitude North and South with sufficient frequency to provide accurate monthly rainfall estimates for five-by-five-degree latitude-longitude boxes. In doing so, TRMM also provided lessons about the optimum approach for obtaining time series of global precipitation measurements from space at higher spatial and temporal resolution. Specifically, TRMM has demonstrated that a satellite with a similar instrument compliment (precipitation radar, microwave imager, visible-infrared sensor) in a non-sun-synchronous orbit can serve as a reference system for calibrating passive microwave and visible-infrared observations from a constellation of other satellites. This approach to obtaining global precipitation measurements has been a focus of considerable applications research (e.g., Joyce et al., 2004; Huffman et al., 2005). These studies have demonstrated the utility of this approach when applied to the intercalibration of the existing constellation of operational and research satellites. NOAA’s Climate Prediction Center morphing technique (CMORPH) described by Joyce et al. (2004) is being used routinely to produce NOAA global precipitation analyses (Janowiak and Kousky, 2005). The TRMM multisatellite precipitation analysis at NASA (Huffman et al., 2005) also runs in real-time with results available from the TRMM web site1—not a re- 1 http://trmm.gsfc.nasa.gov.

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Noaa's Role in Space-Based Global Precipitation Estimation and Application quired function, but an activity to benefit users nonetheless. A research version of the 3-hour resolution of the multisatellite precipitation analysis is a standard TRMM product (3B42). The database now spans 8 years and forms a base for testing applications. Similar multisatellite-sensor precipitation techniques are being run in near real time by several other groups both in the United States and abroad. 8. Unexpected Bonuses Often Accrue from a Scientific Mission As with all scientific missions, there is the possibility of accruing additional, unexpected scientific results other than those for which the mission is designed. For example, TRMM achievements have surpassed mission goals in several ways. The scientific goals of TRMM were focused primarily on issues of climate and large-scale climate variability of tropical precipitation (e.g., the El Niño-Southern Oscillation cycle; NASA, 1988). However, TRMM data have also been fundamental for studying a broader and unanticipated range of topics. Such topics include better characterization and understanding of the nature and variability of tropical cyclones (NRC, 2004) and the value of TRMM data when used with coincident observations of other atmospheric parameters. The investigations of human impacts on precipitation by Rosenfeld (1999, 2000) demonstrate the benefits of coincident observations of cloud properties and precipitation, for example. In these studies, cloud microphysical information matched to TRMM precipitation observations provided new and unanticipated insights into the influence of aerosols on the formation or suppression of precipitation. LESSONS FOR OPERATIONAL APPLICATION OF RESEARCH MISSION DATA TRMM was planned as a purely research mission with no specific goals for operations, and this has affected the pace and nature of the subsequent development of operational applications of the data. The committee has identified four lessons from TRMM relating to the early operational use of GPM research data and the transition from GPM research to operations. Many of these lessons relate to past differences between research and operational missions. 1. TRMM Planning Did Not Anticipate the Broad Scope and High Degree of Interest That Developed for the Application of TRMM Data in Real-Time Operations Because TRMM was designed as a research mission, the timely availability of TRMM data was not a significant consideration in the initial planning for the mission. There was no strong motivation for making the TRMM Microwave

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Noaa's Role in Space-Based Global Precipitation Estimation and Application Imager data available to satisfy real-time applications and requirements. The precipitation radar data were not made available for a year after TRMM’s launch. 2. Application of TRMM Data to Operations Could Have Benefited from More Extensive Pre-launch Planning Within Operational Agencies The lack of pre-launch planning and budgeting by U.S. operational agencies for mission-oriented application of TRMM data influenced the nature and pace of applications. This was largely a consequence of TRMM planning not including specific application goals and having only limited involvement of operational personnel. With little if any planning prior to launch, it was, in effect, largely left to individual operational centers to recognize and respond to opportunities for near-real-time application of TRMM data. Many factors probably influenced the decision to invest human and financial resources in developing TRMM applications. These included data availability, ease of application, availability of needed resources, and the likelihood of a significant payoff. For example, since the assimilation of TRMM Microwave Imager precipitation retrievals into the global numerical weather prediction model at NOAA’s National Centers for Environmental Prediction resulted in only a small positive impact on the forecasts (Lord, 2004), there was no strong impetus to exploit the TRMM observations. Conversely, the United States has led the way in many applications where use of the TRMM data was straightforward, and the payoff clear, notably the extensive use of TRMM Microwave Imager and Precipitation Radar data for tropical cyclone monitoring (Chapter 3) and the use of TRMM Microwave Imager data for sea-surface temperature mapping. As mentioned in Table 2.1, rainfall data assimilation was in its infancy at the time TRMM was launched. As a consequence, this limited the effective use of TRMM data in numerical weather prediction. However, TRMM has generated the momentum for improving data assimilation systems and numerical weather prediction models, which may better prepare operational agencies for the launch of the GPM core satellite. The ultimate limiting factor in what can be achieved both operationally and for research with GPM may well not be hardware but rather human resources, which in turn depend on funding to government research centers as well as to partners in academic institutions. 3. TRMM Has Provided Important Lessons Relating to the Pace of Operational Application of Various Types of Research Data Since the operational community was already using passive microwave data from operational satellites, there was an early eagerness to apply the TRMM Microwave Imager data. In contrast, the operational community (both inside and outside the area of numerical weather prediction) has been much slower in ex-

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Noaa's Role in Space-Based Global Precipitation Estimation and Application ploiting the new precipitation radar data in part due to the small swath width (200 km), which limits utility of the precipitation radar data. This may also be due, at least in part, to lack of specific operational research and development plans for exploiting these data. Eight years after launch of TRMM, precipitation radar data are still used primarily by the research community and are still underutilized in operational applications. The lesson from TRMM is that digesting new types of data and applying them in operational contexts can take a significant amount of planning, time, and resources. 4. Uncertainties About TRMM’s Future Beyond Its NASA-Funded Research Mission Caused the Operational Modeling Community to Delay Allocating Resources to Fully Exploit the TRMM Data TRMM demonstrated that uncertainties about the future of a research mission can affect the pace and degree of operational use of the data. TRMM was initially funded as a minimum 3-year research mission with a goal of 5 years’ duration, without assurance of continuation beyond the research phase. This affected decisions to apply precipitation radar data to improve model physics and data assimilation procedures needed to fully exploit TRMM observations (NRC, 2004). There was generally little impetus to invest scarce operational resources to develop tools needed to exploit the research data. FINDING AND RECOMMENDATION TO APPLY THE LESSONS FROM TRMM TO ENHANCE THE OPERATIONAL USE OF GPM MISSION DATA The scientific and programmatic lessons learned from TRMM have fundamentally influenced the design of the GPM mission and NOAA planning for operational use of the GPM data. For example, because the pace of operational application of research data can be significantly enhanced if the mission has well-defined application goals, GPM has integrated application goals in addition to its scientific objectives. In addition, the GPM observational system is designed to be a prototype pre-operational research mission with commensurate scientific and technical requirements (Hou, 2005). To fulfill the broad application goals of the GPM mission as well as to satisfy NOAA requirements for real-time applications, the GPM data will have to be available for operational use in a more timely manner than was the case for the TRMM data (White, 2005). The reductions in data latency planned for GPM will broaden the range of potential operational applications. Recognizing that operational application of TRMM data could have benefited from participation of operational personnel in the implementation of mission application goals, NOAA has involved personnel in a number of GPM preparatory activities (see Chapter 4). The planning for a close partnership between

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Noaa's Role in Space-Based Global Precipitation Estimation and Application NASA and NOAA to support the post-launch phase of GPM and facilitate early NOAA operational use of GPM data represents another fundamental difference from TRMM. The operational application of TRMM data could also have benefited from pre-launch planning within operational agencies. It is important that NOAA develop and budget for comprehensive, coordinated, agency-wide preparatory activities prior to the post-launch phase of GPM to facilitate early and efficient exploitation of the data. Without comprehensive pre-launch planning, NOAA applications run the risk of being a collection of individual, ad hoc targets of opportunity—a situation similar to the early stages of TRMM—rather than a coherent, adequately funded agency effort that addresses the needs of the many NOAA centers that require space-based precipitation data. NOAA’s pre-launch planning would have to be aligned with and integrated into the basic structure of NOAA activities and make full use of the existing constellation of passive microwave satellites and the combination of the TRMM Precipitation Radar and the TRMM Microwave Imager. To avoid the potential negative effects on operational use of GPM data due to uncertainty regarding NASA’s continuation of the GPM mission, NOAA will have to plan early. This will involve NOAA-NASA long-term operational and budget planning for the transition of GPM from a research mission to an operational system. NOAA is not currently budgeted to take over any NASA satellite during the GPM research mission. However, NOAA is interested in taking over NASA-launched satellites that are a part of a long, continuous series designed for long-term measurements of parameters that NOAA needs (Dittberner, 2005). Since the NOAA takeover of a NASA satellite must be planned many years in advance, it is important that NOAA and NASA begin working together in earnest to determine agency roles and responsibilities beyond the post-launch phase of GPM. Based on its analysis, the committee offers the following finding and recommendation: Finding: Operational application of research data can be hampered if the mission has no specific application goals, no pre-launch planning for operational exploitation of the data, and uncertainty regarding the post-research phase. Lessons learned from the absence of such planning for TRMM have stimulated informal pre-launch planning by NOAA for operational exploitation of GPM data, and NOAA has expressed interest in the concept of operating the GPM mission after the NASA phase. Recommendation 2.1: As soon as possible, NOAA should formalize its GPM planning by developing a comprehensive, coordinated, agency-wide strategic plan for activities in all three phases of the GPM mission. In addition, NOAA and NASA should determine their respective roles and responsibilities in all three phases.

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Noaa's Role in Space-Based Global Precipitation Estimation and Application A long-term, strategic program of applied research will address many complex problems regarding NOAA’s use of space-based precipitation information to improve modeling, forecasting, and climate applications. To guide NOAA’s efforts in developing a GPM strategic plan, the following chapters identify operational uses for GPM data and recommend preparation activities.