NOAA's Role in Space-Based Global Precipitation Estimation and Application (2007)

The National Oceanic and Atmospheric Administration (NOAA) will play a significant role in the Global Precipitation Measurement (GPM) mission and the GPM follow-on mission. The committee recommends a three-phase strategic plan to formalize NOAA’s preparations for the GPM mission and recommends a steering group for coordinating partnership activities and overseeing the implementation of the GPM strategic plan (Recommendations 2.1 and 4.1). This chapter presents a model for NOAA’s GPM strategic plan. Next, NOAA’s activities for the strategic plan are listed in the context of the three phases of the GPM mission (pre-launch, post-launch, and potential NOAA takeover phases). Then, the committee offers a recommendation for NOAA’s role in the transition of the GPM mission from research to operations. Finally, five challenges for future space-based precipitation measuring missions are discussed.

NOAA’s strategic plan for GPM could draw from the NOAA Geostationary Operational Environmental Satellite-R (GOES-R) Risk Reduction Plan. Even though GOES-R will not be launched until 2012-2015 (similar to the time frame for launch of the GPM core satellite), NOAA has developed research funding instruments and supported efforts to reduce post-launch risk.¹ For example, fund-

ing has been set aside for NOAA research offices and cooperative institutes to create simulated GOES-R data sets as proxies for algorithm testing and development by an Algorithm Working Group. These preparations serve two purposes: (1) they reduce the risk of not being ready to use data once GOES-R is launched and fully functional, and (2) they provide industry developing the GOES-R instruments with state-of-the-art software and algorithms to derive products from the data once these begin flowing. Collectively, these efforts should ensure a quick and effective transition to operations.

Recommendation 2.1 emphasizes both intra- and interagency strategic planning. Of the activities recommended earlier in this report, examples of interagency activities include the following:

• Formalizing and supporting existing coordinating mechanisms between the National Aeronautics and Space Administration (NASA) and NOAA and establishing new mechanisms as needed

• Implementing an efficient NOAA-NASA interface for data producer and user feedback, quality assessment, operational readiness, and product distribution

• Assigning and supporting GPM readiness as a priority effort at the Joint Center for Satellite Data Assimilation (JCSDA)

• Developing a strategy for participation in the GPM validation and calibration effort

• Strengthening partnerships with international agencies (e.g., European Space Agency, Japan Meteorological Agency, World Climate Research Program) and international science working groups (e.g., the International Precipitation Working Group [IPWG])

Examples of recommended intraagency activities include the following:

• Formalizing a NOAA steering group (as successor to NOAA’s ad hoc working group) to provide internal NOAA communication and coordination, help develop user requirements, set up data management, participate in inter-

agency partnership activities, and ensure readiness of prototype data and product algorithms for smooth transition into NOAA operations

• Continuing to develop the NOAA data and product requirements from the GPM post-processing system

The main thrust of Recommendation 2.1 is the three phases of effort. For each of these three phases, activities can be differentiated by whether they are participation and initiation activities or planning and preparatory activities (Table 5.1). In addition to the concepts and actions that NOAA has already expressed interest in pursuing (e.g., GPM follow-on activities), the committee has recommended additional, detailed guidance for activities in preparation for the GPM mission. NOAA will need to start as soon as possible with its GPM preparations during the pre-launch phase.

Beginning during the pre-launch phase and continuing throughout the post-launch phase, NOAA will have to develop a comprehensive implementation plan for the transition of GPM from a research mission to an operational mission as part of its three-phase strategic plan. The implementation plan will need to be designed to ensure continued acquisition and application of high-quality, intercalibrated global satellite-based precipitation observations in support of NOAA’s mission-oriented forecasting operations and climate services. To accomplish this, operational versions of the four basic components of the GPM research program (see Chapter 1) will be needed and budget planning will have to begin during the GPM post-launch phase.

The array of satellites required for operational global precipitation measurement and mapping can be viewed in an international context as a major component of the World Weather Watch Global Observing System and the Global Earth Observation System of Systems. International collaboration and NOAA leadership are essential to maintain the constellation of GPM satellites since no nation can fully fund the total operational system. Consequently, the operational strategy for such a constellation will have to account for the international aspects of the program and the continually evolving mix of sensors.² The GPM concept can accommodate these international and uncertain aspects as long as the GPM data structure is flexible and there is a common baseline for intercalibration of sensors (i.e., the GPM core satellite radiometer) (Kummerow, 2006).

Since it now seems likely that the Global Space-Based Inter-Calibration System (GSICS) will be implemented long before launch of the GPM core satellite (see Chapter 3), links will develop during the GPM post-launch phase between the GSICS operational system and the GPM research intercalibration program that includes operational satellites. This intercalibration system will have to continue through the GPM operational mission.

Continuation and expansion of a ground validation program is essential for acquisition of high-quality, intercalibrated, and validated precipitation data from the operational constellation. An effective global ground validation program requires a high degree of international cooperation as well as substantial resources. The NOAA data and observational contributions to the GPM ground validation program (Chapter 3) will have to be developed with due consideration for possible long-term use as assets for an operational program.

The GPM operational mission would generate precipitation data and data products to meet agency operational needs and those of national and international customers. These products would be the operational equivalent of the research products from the GPM Precipitation Processing System, including intercalibrated radiances for model assimilation and timely high-resolution global and regional precipitation analyses (e.g., the NOAA Climate Prediction Center morphing technique [CMORPH] product [see Chapter 3]).

NOAA has presented the concept of an operational follow-on to GPM (Dittberner, 2006). Planning for such a system will have to be initiated during the post-launch phase of GPM (Table 5.1). Among many details, these plans will need to consider how to exploit developments that enhance the ability to measure rainfall from space and mitigate deficiencies in the GPM observational system. Five such challenges are described below.

There remain key gaps in the current GPM plans for measurement of global precipitation. One shortfall is in measurement of light precipitation in mid- and high latitudes. The GPM radar has questionable sensitivity for detection of light rain, thus, its value for validating passive microwave precipitation estimates of light precipitation is uncertain. The European GPM (EGPM) mission concept was aimed at addressing this measurement gap, but it appears that this mission will not materialize.

Satellite-based approaches for inferring solid precipitation are at an early stage of development, and observing strategies for advancing the measurement of solid precipitation have received little attention. Passive microwave methods are indirect and untested, and it remains unclear whether current and planned passive microwave measurements will overcome this deficiency. Compounded with the difficulty of measuring precipitation over land (see next section), estimating solid precipitation over mountains is of particular concern in many areas of the world where the population depends on snowmelt at high altitudes for fresh water.

Problems remain in inferring precipitation over land from passive microwave observations. The methods remain largely empirical; although the use of spaceborne radar for training these passive methods is helpful, the lack of a direct physical basis for overland passive microwave rainfall is a source of ambiguity that will require additional information. The type of information best suited and available remains unresolved. One promising development is a study demonstrating that precipitation retrievals from passive microwave sounders over

land are as good as (if not better than) scanning radiometers over land in the 1-10 mm per hour range (Hou, 2005; Bauer, 2005).

A passive microwave sensor is being considered as a component on the next generation of operational geostationary sensor suites (GOES-R) using higher frequencies to partially mitigate spatial resolution issues. This would allow continuous passive microwave viewing of the life cycles of low- and midlatitude precipitation systems, would provide valuable information for filling the time-space gaps in passive microwave observations from polar orbiters, and would provide another valuable tool for intercalibration. However, low spatial resolution remains a challenge for any space-based precipitation-measuring mission.

Advances in the understanding of precipitation processes (e.g., microphysics, the influence of aerosols) will likely contribute to the improvement of retrieval algorithms, which can be implemented for current precipitation retrievals and applied retrospectively to improve climate data records.

Although the future state of operational global precipitation measurements is unclear, NOAA has the opportunity to lead and catalyze development of an operational precipitation measurement network in addition to supporting and working on the next-generation observational efforts that are planned for the

GPM mission. NOAA’s resources could be directed at many activities that will help in this regard (Table 5.1), and the present constellation of passive microwave sensors in conjunction with TRMM’s unique suite of sensors provides an ideal source of global, intercalibrated data with which to refine operational applications in preparation for future precipitation-measuring missions.