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

The National Aeronautics and Space Administration (NASA), in cooperation with the Japan Aerospace Exploration Agency (JAXA), launched the Tropical Rainfall Measuring Mission (TRMM) in 1997. Designed as a minimum three-year mission with the goal of five years duration, TRMM has been collecting data for seven years, in large part due to the reliability of its sensors and the high quality of their measurements. Although initially intended as a purely research-oriented mission, TRMM now is used in operational applications such as hurricane forecasting because data from its suite of complementary sensors are unique and available in near real time. In the United States TRMM data are used operationally by the Joint Typhoon Warning Center, the National Center for Environmental Prediction, and the National Hurricane Center, among others. Internationally the data are used operationally by entities such as JAXA, the European Centre for Medium-Range Weather Forecasts, and the World Meteorological Organization tropical cyclone warning centers.

In July 2004 NASA announced that it would finally terminate TRMM in August 2004. At the request of the National Oceanic and Atmospheric Administration (NOAA), and with additional urging from others in the scientific and operational user community and the White House (see Appendixes E through H), NASA agreed to continue TRMM operations through the end of the hurricane season and until the end of 2004 (see Appendix I). But many users hope that the mission will be extended even longer, setting the stage for a difficult decision.

A further extension of TRMM beyond 2004 will pit financial constraints against the operational and scientific benefits of continuing; that is, are the benefits greater than the costs and can the necessary funds be secured? The scenario

becomes more complicated if the mission is extended beyond late 2005. The TRMM spacecraft is sufficiently large that it will not burn up completely on reentry. Thus NASA will face a second decision point in roughly December 2005 when it must weigh in the element of the additional risk to life and property. For this second decision NASA must choose whether to (1) use TRMM’s remaining fuel to conduct a controlled reentry into the atmosphere that directs the remains of the satellite into the ocean far from human settlements¹ or (2) continue TRMM operation until the fuel runs out in 2010 or 2011 and accept the added risk of an uncontrolled reentry because of the operational and scientific benefits of doing so.

In August 2004 NASA Administrator Sean O’Keefe requested that the National Academies provide advice on the anticipated scientific and operational contributions from extending TRMM beyond 2004. The charge to the Committee on the Future of the Tropical Rainfall Measuring Mission in the first phase of its work was 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. The Academies were able to begin the study process in October 2004, and immediately assembled the committee and planned an information-gathering workshop. 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 second phase of the committee’s work will focus on needs for satellite-based measurements of tropical rainfall beyond TRMM (see Appendix B).

The committee hosted its phase I workshop in Washington, D.C., on November 8, 2004,² and subsequently drew on information and discussions from the meeting and other written inputs and information sources to address its task for phase I.

NASA and JAXA are to be commended on the highly successful TRMM satellite whose achievements and longevity have exceeded even the optimistic expectations at the time of launch. These agencies are also to be commended for their visionary actions to extend the lifetime of TRMM beyond the original anticipated maximum length of the mission thereby enhancing the value of the data from TRMM to science and operations. The committee found the following:

• TRMM has two unique attributes that make it ideal for observing tropical rainfall systems: (1) its suite of complementary observing instruments, and (2) its orbital characteristics.

  1. The Precipitation Radar (PR) is the only precipitation radar in space and provides direct, fine-scale observations of the three-dimensional structure of precipitation systems. The combination of PR and Lightning Imaging Sensor (LIS) observations provides a measure of convective intensity.³ The combination of PR, TRMM Microwave Imager (TMI), and Visible and Infrared Scanner observations from the same platform serves, in effect, as a “Rosetta Stone” for cross-calibration of the indirect estimates of precipitation from microwave, visible, and infrared observations.

  2. The TRMM orbit (low altitude, non-sunsynchronous, precessing, 35-degree tropical inclination) provides sampling in the tropics that is far more frequent, and far more spatially comprehensive than that obtained from standard polar orbiter satellites.

• TRMM has achieved its original science goals and produced a greater than expected range of scientific results in

  1. climate and weather research (e.g., a reliable seven-year climatology of the mean annual tropical rainfall and its interannual and diurnal cycles; fundamental new information on the synoptic climatology of tropical weather systems, e.g., the first detailed precipitation and latent heating profiles throughout the tropics and subtropics, first detailed convective and stratiform rainfall structure, and a description from space of the fine-scale structure of rainfall systems that can only be determined from the PR data; understanding of how sea surface temperature patterns modify precipitation through air-sea interaction; quantitative documentation of precipitation patterns; mapping sea surface temperature through clouds for improved climate

records; demonstrating the effect of pollution and other human influences on precipitation formation); and

1. applied research (e.g., a wealth of climatological and diagnostic information on tropical rainfall; insight on the physical processes of precipitation formation; unique, fine-scale information on hurricane and typhoon structure linked to rapid intensification; calibration of a long-term satellite precipitation dataset and multisatellite three-hour analyses; experimental tropical cyclone forecast methods; enhanced sea surface temperature now-casting applications using TMI data; integration of TRMM data into forecast model initialization procedures; enhanced understanding of tropical cyclone inner eyewall dynamics and tropical cyclone intensity).

• Since 1998, TRMM has provided near-real-time information for operational purposes. Data from the TMI sensor are most often used. There are four principal applications: (1) monitoring and predicting the future behavior of tropical cyclones, (2) estimating rainfall, (3) predicting weather, and (4) monitoring of climate variability (precipitation and sea surface temperature). Many organizations and individuals have invested in bringing TRMM data into the operational environment because of the unique aspects of TRMM’s orbit and sensor suite. This reflects their professional judgment of the value of doing so based on their experiences of improvements in such things as accuracy of center fixes for tropical cyclones and prediction of storm intensity. Nonetheless, the effect of TRMM data on operational applications has not been widely quantified because the data record is too short for meaningful statistical analysis and no one has done control experiments wherein the TRMM data are eliminated and the analysis is rerun. Further, the socioeconomic effects on end-users of improved forecasts have not been quantified.

  In the United States TRMM data are used in operations by NOAA organizations (e.g., Tropical Prediction Center, National Centers for Environmental Prediction, Satellite Analysis Branch, Aviation Weather Center, Climate Prediction Center, National Climate Data Center) and the Department of Defense (e.g., Joint Typhoon Warning Center, Air Force Weather Agency, Fleet Numerical Meteorology and Oceanography Center, Naval Research Laboratory). TRMM data are used internationally for operations by World Meteorological Organization centers throughout the tropics for monitoring and forecasting tropical cyclone activity. Groups in Japan and Europe have begun using TRMM data in numerical weather prediction models. Because TRMM data are already being used operationally (which does not fit with NASA’s primary focus of research and exploration) NASA has sought partnerships with other agencies to fund extension of TRMM. Thus, determining the future of TRMM has become a multi-agency issue.

• TRMM’s potential to help improve forecasts—especially through increased use of PR data in models—has not been fully realized because of

1. the PR data having only recently become available in near real time to the broader community outside of NASA and JAXA,

2. the new and unique nature of the PR data and the learning required to exploit them,

3. the perceived experimental nature and finite lifetime of the PR, and

4. the lack of sophistication in the representation of cloud and precipitation physics in current operational forecast models and global climate models such that they cannot yet take advantage of the native resolution of the PR data.

• The TRMM satellite and its sensors remain in excellent condition. There is every reason to believe that they will continue to operate well for the next few years.

• NASA will incur costs for operating the TRMM satellite through 2007 even if the mission is terminated in December 2004, because of the time it takes for the spacecraft to drift down to an appropriate altitude for controlled reentry. These costs exceed the amount currently in NASA’s budget for TRMM. The additional cost of extending TRMM from December 2004 to November 2005 is approximately $4 million. It is NASA’s practiced policy to try to recover its costs of mission extension from related research programs. In the case of TRMM, these extra costs would likely have to be borne by NASA’s precipitation research budget, which is around $16 million per year. However, it is outside this committee’s charge to assess the effects on other satellite operations, missions, and research budgets of NASA bearing the entire cost of extending TRMM, though such an assessment is part of the overall decision context.

• The most recent analyses of the risks from uncontrolled reentry are those reported in 2001 (Pielke et al., 2001) and 2002 (Martin, 2002) (see Chapter 2), and the committee is unaware of any subsequent changes to the conclusions. Although the risk from uncontrolled reentry is part of the overall decision context, the committee is neither tasked to assess this risk nor does it have the expertise to do so.

• The material in this National Research Council report provides science and operations information needed as input for a qualitative evaluation of the balance between the risk inherent with an uncontrolled reentry and the contribution through operations and research to the protection of life and property of an extension of the TRMM mission. Extension of the mission to at least late 2005 will provide time for further examination of the relevant issues.

• There are persuasive reasons to believe that significant contributions of TRMM to operations and science will continue if the mission is extended. The committee’s conclusions about operational and research benefits of extending

TRMM to the fuel point in approximately December 2005 and beyond are compiled in Table ES-1.

• From the perspective of anticipated research contributions, TRMM is worth continuing for six primary reasons.

  1. TRMM provides a unique complement of measurements. Specifically, the PR, the passive microwave imager, and the visible and infrared instruments provide a powerful overlap of precipitation, cloud, and water vapor measurements and the LIS helps isolate intense convective cells. The TMI permits sea surface temperature measurement through clouds at high spatial resolution. Continuation of the mission is vital to the future development of spaceborne PR technology, especially in the evaluation of radar technology life cycle.

  2. Mission extension creates the opportunity for cross-calibration, validation, and synergy with sensors on future missions, such as CloudSat and the A-Train satellite series, the National Polar-orbiting Operational Satellite System’s Conical Scanning Microwave Imager/Sounder, and the Global Precipitation Measurement core satellite and other constellation satellites.

  3. TRMM’s unique low-inclination, low-altitude, precessing orbit enhances science by providing unique spatial and temporal information that fills gaps in data from other current and upcoming polar-orbiting satellite sensors.

1. TRMM data will enhance field experiments and programs (e.g., TCSP, AMMA, GEWEX, THORPEX, TEXMEX-II [see Appendix J for explanations of program name abbreviations]), tropical cyclone research—including tropical cyclone forecasting—and development of cloud-resolving models.

2. A longer record is required to collect enough examples to cover the parameter space of synoptic variability more fully. For example, over the first six years of TRMM data, the TMI instrument passes within 750 km of storm centers during one of every eight orbits, whereas PR observes within 250 km of the center during one of every 25 orbits. The narrow swath of the PR and the rare occurrence and great variability of tropical cyclone structure, intensity, and precipitation amount strongly argues for mission extension to increase sample sizes for statistical analyses.

3. Longer TRMM data records will better characterize tropical seasonal-interannual climate variability in general and the El Niño-Southern Oscillation (ENSO) cycle in particular. ENSO is the dominant mode of global interannual climate variability. TRMM provides quantitative ENSO-related tropical rainfall anomalies that are needed to improve our understanding of both the local and remote effects of this phenomenon, and ultimately to make better predictions of its socioeconomic effects in both the tropics and extratropics.

• TRMM’s reliability combined with the value of TRMM data to operations shows the satellite’s potential as an operational system. From a perspective of anticipated operations contributions TRMM is worth continuing for three primary reasons.

  1. TRMM data from the TMI and PR sensors have a demonstrated capability (for TMI) or potential capability (for PR) to improve the weather forecasting process, especially for monitoring and forecasting the tracks and intensity of tropical cyclones and the intensity of rainfall they yield.

  2. Continuation of the TMI data stream would enable modelers and forecasters to continue to improve the overall numerical weather prediction process (i.e., model development and validation, forecast initialization, and forecast verification). This includes use of TMI in calibrating similar data from other microwave sensors and contributes to improved global as well as tropical precipitation monitoring and prediction.

  3. PR data are an underexploited yet unique resource. Having them available in near real time for an extensive period of time would foster investment of time and effort to make full use of PR data in the forecasting process.

• Considering the past and expected scientific and operational contributions presented in this report, important benefits would be obtained if TRMM were extended until it runs out of fuel. Although the scientific and operational arguments by themselves point toward maximum extension of the TRMM satellite life, the committee is concerned that there has not been proper consideration of all three elements of the decision (benefits, costs, and risk). The committee recognizes that consideration of the associated costs and reentry risks has to be part of the decision equation, which requires a solution acceptable to both the user and interagency communities.

The committee strongly recommends continued operation of TRMM, at least until such time as a decision on controlled reentry becomes unavoidable. The additional year can be used to more fully weigh the benefits, costs, and risks.

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