Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities: Interim Report (2006)

This is the interim report of the Committee on the Future of the Tropical Rainfall Measuring Mission (TRMM) (see Appendix A). The charge to the committee in this first phase of its work is to conduct a workshop and prepare an interim report to be delivered in December 2004 on how best to use the remaining TRMM spacecraft life. In particular, the committee and workshop participants were asked to consider:

• scientific and research contributions of TRMM to date and those expected if TRMM is continued;

• operational contributions of TRMM to date and those expected if TRMM is continued;

• assessment of expected benefits of continuing TRMM operation until (1) fuel is depleted to the level needed for a controlled reentry (around December 2005) and (2) all fuel is depleted (estimated to be 2010-2011).

A later phase of the committee’s work will focus on needs for satellite-based measurements of tropical rainfall beyond TRMM (see Appendix B).

The decision context for mission extension has three major components: the operational and research benefits of extension, financial constraints, and risk to life and property during satellite reentry. In December 2004 the National Aeronatuics and Space Administration (NASA) will weigh the research and operational benefits against the cost of extending the mission for approximately one year. If NASA decides to continue TRMM, a second decision point will be

forced in approximately December 2005. At that point the agency will have to weigh the societal benefits of TRMM against both the additional cost and the risk of an uncontrolled reentry if the remaining fuel is used to maintain the satellite’s orbit rather than to guide the spacecraft into the ocean.

The committee’s task focused on the research and operational benefits component of the decision. The committee was not tasked to analyze the risk or cost components, although these components are part of the overall context and thus laid out in Chapter 2. Chapter 3 describes the achievements of TRMM in research and operations to date and Chapter 4 looks to the potential future research and operational applications of TRMM. Chapter 4 differentiates between the potential contributions from a TRMM extension until fuel is depleted to a level that is still sufficient for a controlled reentry or until all fuel is depleted. A series of appendixes provides supporting information that is referenced in the body of the report.

The audience for this report includes NASA and the Japan Aerospace Exploration Agency (JAXA), operational agencies (e.g., the National Oceanic and Atmospheric Administration [NOAA], Department of Defense agencies), Congress (e.g., House Science Committee), the White House Office of Science and Technology Policy and Office of Management and Budget, the scientific community in general, and users in particular.

The committee gathered information using four approaches.

1. inviting presentations at a workshop on November 8, 2004 (see Appendix C);

2. promoting and tracking discussion at the workshop (from participants in the room and on the phone [see Appendix D]);

3. reviewing existing written materials, such as peer-reviewed papers, reports, information from Websites, and letters; and

4. soliciting input to a Website that posed questions relating to the committee’s phase I task.

The workshop had four sessions (Appendix C) with formal input from 14 people. Approximately 45 people attended the workshop and participated in general discussion. Participants from Japan attended on an open phone line. The formal presentations are available on the Web.¹

The TRMM was motivated by recognition of the fundamental importance of precipitation in Earth’s climate system, the inadequate network of surface precipitation measurements over much of Earth, and recognition of the crucial role of tropical rainfall in global climate dynamics and the global hydrologic cycle.

The TRMM science plan was developed in the late 1980s (Simpson et al., 1988). The overarching mission science goal was to advance knowledge of global water and energy cycles by observing from space the temporal and spatial distribution of tropical rainfall and associated latent heating. TRMM thus carried a suite of five sensors (Box 1-1 and Figure 1-1) designed to address seven key science questions (Box 1-2).

TRMM was launched in November 1997 as a joint effort of NASA and JAXA. It has a low-inclination, precessing orbit between 35°north and 35°south. The mission was designed as a science path-finding experiment that would last a minimum of three years with an initial goal of five years. Based on TRMM’s excellent performance and promise, NASA and JAXA decided in 2001 to extend the mission. TRMM’s orbit was boosted from 350 km to 402.5 km to reduce drag and conserve fuel.

TRMM has now been operating successfully for seven years. Initially the TRMM data were provided to users in a delayed mode because of the mission’s original focus on research. Since the data became available in near real time in Fall 1998, they have been increasingly used in operational applications, especially for monitoring and predicting tropical cyclones.

As a consequence of TRMM’s success the research and operational communities have created a demand for the continuation of TRMM data (see, for example, Appendixes E through H). To date, however, no party outside NASA has stepped forward to contribute to the cost of continued TRMM operations. Although NASA and JAXA agreed to decommission TRMM in July 2004, TRMM is being kept alive through 2004 because of pressure from the research and operational community that ultimately led to a direct request from the NOAA administrator to the NASA administrator (Appendix H and I).

Since 2001, NASA, JAXA, and other international partners have been planning the Global Precipitation Measurement (GPM) mission, a follow-up on TRMM to measure precipitation on a global scale (NASA, 2004). GPM is envisioned as a constellation of approximately eight satellites, with a core satellite similar to TRMM. All of these satellites will carry passive microwave sensors for rainfall estimation. Key sensors onboard the core spacecraft will be a dual-frequency precipitation radar and a multichannel microwave imager. GPM goes beyond TRMM in several important ways: The number of satellites in the con-

FIGURE 1-1 Schematic illustration of the TRMM satellite sensors and how they collect data. SOURCE: Presentation by Jack Kaye, NASA, at November 8, 2004, workshop.

stellation will facilitate a significant reduction in revisiting time (a three-hour sampling interval is anticipated) over TRMM, the core satellite will cover a broader latitudinal band (up to approximately 50 degrees), and the dual-frequency precipitation radar will assist in reducing uncertainty due to raindrop size variability and also measure lighter precipitation than TRMM. The GPM launch is currently projected for 2010 at the earliest.²