4
Anticipated Contributions of TRMM

INTRODUCTION

The Tropical Rainfall Measuring Mission (TRMM) has exceeded its original mission goals. The TRMM objectives and goals, however, evolve as more is learned about how to make use of TRMM data—on their own as well as integrated with other observations and numerical weather prediction and cloud-resolving models. For example, learning from peculiar differences in how TRMM Microwave Imager (TMI) and precipitation radar (PR) observe weather systems and estimate rainfall around the globe and during various phases of the El Niño-Southern Oscillation (ENSO) cycle have yielded new ideas about the physical processes of precipitation formation, but many more observations need to be gathered to fully sample the many possible conditions and dependencies of cloud and weather systems on external conditions. These include large-scale atmospheric circulation and sea surface temperature patterns of warm and cold water, monsoon and ENSO structures, and orographic effects to name a few.

Extending TRMM operations beyond 2004 offers broad opportunities for advancing the operational and scientific uses of TRMM data. The value of acquiring new data for future operational applications and learning from existing TRMM data is obvious. This is also the case for many research applications.

This chapter is organized to address two of the committee’s tasks: (1) to consider the expected operational and research contributions if TRMM is continued and (2) to assess the expected benefits of continuing TRMM operations until (a) the fuel is depleted to the level still needed for a controlled reentry, and (b) until all fuel is depleted. The key benefits of an extension of TRMM are summarized in Table 4-1. The table is divided into two columns. The left-hand column



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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report 4 Anticipated Contributions of TRMM INTRODUCTION The Tropical Rainfall Measuring Mission (TRMM) has exceeded its original mission goals. The TRMM objectives and goals, however, evolve as more is learned about how to make use of TRMM data—on their own as well as integrated with other observations and numerical weather prediction and cloud-resolving models. For example, learning from peculiar differences in how TRMM Microwave Imager (TMI) and precipitation radar (PR) observe weather systems and estimate rainfall around the globe and during various phases of the El Niño-Southern Oscillation (ENSO) cycle have yielded new ideas about the physical processes of precipitation formation, but many more observations need to be gathered to fully sample the many possible conditions and dependencies of cloud and weather systems on external conditions. These include large-scale atmospheric circulation and sea surface temperature patterns of warm and cold water, monsoon and ENSO structures, and orographic effects to name a few. Extending TRMM operations beyond 2004 offers broad opportunities for advancing the operational and scientific uses of TRMM data. The value of acquiring new data for future operational applications and learning from existing TRMM data is obvious. This is also the case for many research applications. This chapter is organized to address two of the committee’s tasks: (1) to consider the expected operational and research contributions if TRMM is continued and (2) to assess the expected benefits of continuing TRMM operations until (a) the fuel is depleted to the level still needed for a controlled reentry, and (b) until all fuel is depleted. The key benefits of an extension of TRMM are summarized in Table 4-1. The table is divided into two columns. The left-hand column

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report summarizes anticipated contributions from extending TRMM up to the fuel point when a controlled reentry is still possible. The right-hand column summarizes the anticipated additional contributions of extending TRMM beyond the fuel point (i.e., supplementing what is gained up to the fuel point). In addition, the expected contributions are separated into anticipated research and operations contributions. The elaborations in this chapter, working from the specific to the general, mirror the outline of Table 4-1. ANTICIPATED CONTRIBUTIONS WHEN CONTROLLED REENTRY IS STILL POSSIBLE Anticipated Operational Contributions Another Year of PR and TMI Data for Tropical Storm Monitoring and Forecasting Another year’s worth of TMI and PR data would be valuable to the National Oceanic and Atmospheric Administration (NOAA) and recipients of its forecasts for the same reasons expressed by Vice Admiral Lautenbacher to Dr. Sean O’Keefe in 2004 (see Appendix H). Similarly, Department of Defense agencies, other agencies around the world (refer to Chapter 3), and their users will continue to benefit from the data. TABLE 4-1 Anticipated Operational and Research Contributions due to Extending TRMM to the Fuel Point (approximately December 2005) and Beyond Anticipated Contributions of TRMM Up to the Fuel Point (when controlled reentry is still possible) Additional Anticipated Contributions of TRMM Beyond the Fuel Point (i.e., in addition to what is gained up to the fuel point) OPERATIONS Another year of TMI and PR data for tropical storm monitoring and forecasting** Another year of TMI data for numerical weather prediction** Another year of PR and TMI data for enhancing near-real-time rainfall products** Another year of lightning data for air traffic advisories* Realizing the potential to use PR as a global rainfall reference standard* OPERATIONS Technology demonstration of the endurance of the first precipitation radar inspace* Improved forecasts from the operational assimilation of PR and TMI data into weather and climate prediction models**

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report Anticipated Contributions of TRMM Up to the Fuel Point (when controlled reentry is still possible) Additional Anticipated Contributions of TRMM Beyond the Fuel Point (i.e., in addition to what is gained up to the fuel point) RESEARCH Overlap with CloudSat radar operations and the A-Train satellite series** Overlap with the Coriolis WindSat sensor** Unique opportunities to enhance field experiments (TCSP, TEXMEX-II)** Unique opportunities to enhance international research programs (GEWEX, THORPEX, Hurricane Field Program)** TRMM’s Precipitation Radar provides a calibration reference for the current GPM mission-like constellation of microwave satellite sensors** TRMM is a catalyst for tropical cyclone research (e.g., research on convective bursts, tropical cyclone eyewall replacement cycles, improved forecasting of inland flooding during hurricanes)** Longer TRMM record needed for tropical cyclone forecasting* Longer TRMM record needed for climate research* Foster improving moist physics parameterization for climate models, numerical weather prediction, and related assimilation systems by evaluating models of clouds and precipitation physics* RESEARCH Unique opportunities to enhance field experiment (AMMA)** Developing the next generation hurricane forecast model** Seamless transition into the Global Precipitation Measurement (GPM) mission* Realization of a prototype GPM-like operation* Avoiding researchers being ill-prepared for GPM** Better characterization of interannual variability and the El Niño-Southern Oscillation cycle* NOTE: See Appendix J for acronym definitions for field experiments and programs. We use a single asterisk to differentiate applications that use TRMM data only, or as the primary component of a research or operational activity, from those that use TRMM data as a complementary component of an operational or research activity (marked with a double asterisk). There is a gray area between these two categories, but the distinction serves as a first-order attempt to differentiate between essentially stand-alone contributions and complementary but still unique contributions of TRMM.

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report Another Year of TMI Data for Numerical Weather Prediction In addition to the focused benefits of TMI data for tropical cyclone intensity forecasting and center fixing, the radiances detected by the TMI sensor are used to initialize tropical precipitation in global models. Furthermore, information from TMI on sea surface temperature and precipitation is used for numerical weather prediction. Another Year of PR and TMI Data for Enhancing Near-Real-Time Rainfall Products Continued availability of TRMM data will enable continued production of high-quality near-real-time and research three-hour precipitation analyses. TRMM data are used to calibrate other satellite information, with PR data as a key building block and TMI data to fill in temporal gaps. The fact that TMI is coincident with PR data gives an excellent time and space match, and correlations between TMI brightness temperatures and PR retrievals are better than matches with any other sensor due to the fast temporal changes in rain characteristics. Another Year of Lightning Data for Air Traffic Advisories Lightning data from the Lightning Imaging System on TRMM are used for aircraft routing over the world’s oceans. Realizing the Potential to Use PR as a Global Rainfall Reference Standard The use of TRMM data as the global reference against which data from spaceborne microwave sensors are adjusted is the subject of ongoing research. The combination of TRMM’s unique sensor suite and orbit underpins this approach. Anticipated Research Contributions Overlap with CloudSat Radar Operations and the A-Train Satellite Series CloudSat is an experimental satellite that will use radar to measure the vertical structure of clouds and cloud properties from space including some characteristics of precipitation.1 CloudSat will fly in formation with other satellites referred to as the “A-Train.” This constellation comes into formation with the launch of CloudSat and CALIPSO in mid-2005. Combining the observations of 1 See http//www.cloudsat.atmos.colostate.edu.

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report the different sensors of the A-Train will provide an unprecedented view of clouds, aerosol, and precipitation (to a lesser degree) and the relationships between them. TRMM will add value to the A-Train observations of clouds and precipitation and to the science of the A-Train which seeks to address the effect of pollution and aerosols on precipitation. Because the TRMM and CloudSat mission satellites both carry a radar, albeit with differing wavelength, the opportunity to have measurements by both radars simultaneously over a substantial amount of time will provide a basis for statistical comparison and cross-referencing. Statistical comparison offers a way of calibrating the information on tropical precipitation provided by CloudSat on the one hand and the possibility on the other hand of extending the capability of the PR to the extratropics and higher latitudes through the CloudSat radar. In addition, such a combined dataset would yield a direct measure of the percentage of the light precipitation that has been below TRMM’s measurement threshold. This is important for Global Precipitation Measurement algorithm development, in particular through validation of these algorithms for higher-latitude precipitation. Overlap with Coriolis WindSat Mission The Coriolis WindSat polarimetric radiometer’s launch in January 2003 produced the next step in spaceborne passive microwave radiometers. The WindSat sensor measures multiple polarizations not captured by previous microwave imagers such as SSM/I and TMI that enable the retrieval of all four Stokes vectors. This capability permits the extraction of the ocean surface wind speed and direction (vector), a major parameter for both meteorological and oceanographic now-casting and forecasting (Gaiser et al., 2004). The WindSat wind vector calibration and validation team has just completed their initial studies and a six-month digital dataset has been released to the scientific user community. WindSat is the risk reduction pathfinder for the upcoming (2009) National Polar-orbiting Operational Environmental Satellite System’s Conical Scanning Microwave Imager/Sounder (CMIS) sensor that will be the main wind vector instrument for the next several decades. Using the PR data to help understand rain effect on WindSat wind vector retrievals will further CMIS efforts, since no planned satellite will have joint WindSat, CMIS, and PR capabilities on one platform. Numerous cross-validation efforts are envisioned since WindSat and TMI channels are nearly identical except for the enhanced polarization on WindSat. WindSat and CMIS rainrate algorithms will directly benefit from having PR data available.

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report Unique Opportunities to Enhance Field Experiments Unique opportunities exist in 2005 (and beyond) for the TRMM mission to enhance atmospheric field campaigns. For example, the National Aeronautics and Space Administration (NASA) is planning a major field experiment to support the Tropical Cloud Systems and Processes (TCSP) initiative in 2005. The TCSP campaign will include the NASA DC-8 and ER-2 aircraft in addition to many other facilities supported by NOAA and the National Science Foundation. This program will investigate tropical cloud systems and their environmental feedback. The initiative seeks innovative approaches that use NASA’s observational data for investigations of tropical cyclones, the effect of cirrus clouds on atmospheric cycles of water and energy, and related feedbacks on the radiative, compositional, and dynamic attributes of the upper troposphere and lower stratosphere. The initiative also addresses the use of this knowledge in facilitating the development and evaluation of models and data assimilation systems that include representations of tropical cloud processes and their effect on Earth’s climate system. NASA’s Earth Science Enterprise has sponsored a series of field experiments under the title CAMEX (Convection and Moisture Experiment) during the last several years to study rainfall and water vapor properties of the atmosphere.The third and fourth field missions of the series, conducted in 1998 and 2001 respectively, were focused more specifically on the study of tropical cyclone (hurricane) development, tracking, intensification, and landfalling effects using NASA’s aircraft, space, and surface remote-sensing instrumentation. A noteworthy accomplishment of the most recent CAMEX has been a successful collaboration with NOAA and the U.S. Weather Research Program to address common research goals. The next experiment is planned as part of TCSP in 2005 and will continue to address the key research questions of the NASA Earth Science Enterprise while also making the best use of that program’s assets. The emphasis of this experiment will be hurricane genesis and associated environmental processes.Other elements will include satellite calibration activities and collaborative research with the radiation physics community. There will be several opportunities for close coordination of the field campaign observations with direct overpasses if TRMM is still operational. Unique Opportunities to Enhance International Research Programs TRMM data enhance international programs such as the World Climate Research Program’s Global Energy and Water Cycle Experiment (GEWEX) and World Meteorological Organization’s THORPEX (The Observing-System Research and Predictability Experiment). GEWEX and THORPEX are existing international programs that aim to better observe and understand Earth’s atmospheric system processes. The primary objective of GEWEX is to observe and model the hydrologic cycle and energy fluxes in the atmosphere. The primary

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report objective of THORPEX, a global atmospheric research program, is to accelerate improvements in the accuracy of 1- to 14-day weather forecasts for the benefit of society and the economy. The observational components of both GEWEX and THORPEX will rely heavily on satellite-based measurements. TRMM, with its unique capabilities, could make an important contribution to such aspects as calibration of the 25-year record of satellite passive microwave and infrared data that is part of GEWEX. Many other potential contributions are listed in a July 6, 2004 letter from the World Climate Research Program to NASA and JAXA (see Appendix E). TRMM’s Precipitation Radar Provides a Calibration Reference for the Current GPM-like Constellation The combination of TRMM’s TMI and PR with the microwave sensors on other satellites provides many of the key elements of a Global Precipitation Measurement constellation concept. Now is the golden era of passive microwave satellite instruments, with nine currently in orbit (see Figure 4-1). The scientific FIGURE 4-1 Duration of primary missions of spaceborne passive microwave imagers and their potential extensions (see Appendix J for explanations of program name abbreviations). For an updated version of this figure, see the companion report on GPM (NRC, in review).

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report and operational weather and climate communities have a fortuitous confluence of missions that have placed or will soon place microwave sensors in space. Data from the PR provide the calibration reference point for the entire constellation thereby enhancing the data products from all constellation sensors. Extending TRMM would continue its role as a reference instrument. Extension also would allow NASA, NOAA, Department of Defense agencies, and foreign weather and space agencies to further experiment with integrating and merging data from these sensors with data from terrestrial sensors. TRMM is a Catalyst for Tropical Cyclone Research Data from TRMM have stimulated advances in tropical cyclone research and understanding. Summarized below are three areas of ongoing research that will benefit from extending TRMM. Convective bursts. TRMM’s ability to reveal the characteristics of a cloud or cloud system with active precipitation, and to highlight these differences from location to location throughout the tropics, has underscored our lack of understanding of cloud physics on a number of scales.2 The connection between convective bursts and tropical cyclogenesis and intensity changes is an active research topic in cloud physics. The PR is the only space-based method for quantifying the vertical structure of these convective systems. Fully characterizing and understanding the relationship between these convective bursts and tropical cyclone variability will require a longer record than is currently available, given the temporal and spatial coverage of TRMM.3 Without PR no space-based near-real-time observations of the convective bursts (and the applicability of these observations to forecasting) will be available for the rest of the decade. Furthermore, the lightning sensor on TRMM adds insights into the intensity of convection, especially over land, where lightning is more often associated with deep clouds. This is a new resource for convection and storm research that would also benefit from a longer record. Tropical cyclone eyewall replacement cycles. Passive microwave imagers such as TMI can view tropical cyclone inner-core structure often obscured by upper-level clouds and thus masked in visible and infrared imagery. Intense tropical cyclones typically have a central dense overcast expanding from the central eyewall outward, sometimes clouding over the eye as well. These clouds make it difficult to view the eyewall replacement cycle, which is a key factor in storms with intensities at or above 120 knots (Hawkins and Helveston, 2004). 2 Presentation by Ed Zipser, University of Utah, at November 8 workshop. 3 Ibid.

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report Microwave imagers, especially TMI, provide the means to observe the eyewall shrink as the storm intensifies, the formation of an outer eyewall, the collapse of the inner eye as moisture and momentum flux is constricted by the outer eyewall, and the shrinking diameter of the outer eyewall as the process begins anew (Velden et al., 2003). The TMI’s unique qualities of temporal gap filling and high spatial resolution have enabled an improved understanding of this basic tropical cyclone dynamical feature that is now the subject of intense study.4 Improved forecasting of inland flooding during hurricanes. Models that forecast hurricane-induced inland flooding are still in their infancy. Data from the PR and TMI sensors would help improve these forecasts and assist public officials to mitigate the effects of damage from high winds, storm surges, and inland flooding resulting from hurricanes (Williamson et al., 2002). This is especially true for countries with little observational infrastructure or coastal areas with little upstream information. Furthermore, continuing the flow of data from TRMM would allow policymakers to assess the socioeconomic value of these data in saving lives and improving the U.S. economy. Extending TRMM’s mission will also allow weather modelers and forecasters to quantify the operational benefits of using PR and TMI data. Longer TRMM Record Needed for Tropical Cyclone Forecasting A longer record is required to collect enough examples to cover the parameter space of synoptic variability of tropical cyclones more fully. 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.5 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. The PR on TRMM has already provided more data on the vertical structure of precipitation in tropical cyclones than a quarter century of aircraft penetrations into hurricanes in the Atlantic and the Caribbean.6 The vertical structure information from the PR indicates the distribution of latent heat release in tropical cyclones. This information is used in mesoscale tropical cyclone models. Additionally, PR data are being used to verify the cloud and precipitation structure in such models.7 Joint Hurricane Testbed projects relate the TRMM data to 4 For example, there were several dozen papers at the 26th American Meteorological Society Hurricane Conference on this subject in May 2004. 5 Frank Marks, NOAA, personal communication, December 2004. 6 Frank Marks, NOAA, personal communication, November 2004. 7 Ibid.

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report several of these models such as those of Geophysical Fluid Dynamics Laboratory, the Eta vertical coordinate model, and the Global Forecast System (Marchok et al., 2004). Further research with TRMM PR data will give a better understanding of the vertical structure of precipitation in tropical cyclones. In addition to developing mesoscale tropical cyclone models, forecasters are developing models that predict precipitation as a function of the development stage of the storm (from weak tropical depressions on up in intensity to Category 5 hurricanes or Super Typhoons).8 With improved accuracy of forecast track and storm propagation speed, rain accumulation forecasts have recently become operational based on the published climatology using the early TRMM data (Lonfat et al., 2004). However, the relatively short TRMM record has not sampled a wide variety of extreme events. More hurricane seasons of TRMM data would allow researchers to reduce uncertainty in this forecast tool since more details of the many influences from intensity, location, track speed, and other factors on the rainfall could then be included.9 Longer TRMM Record Needed for Climate Research The value of the relatively short (in climate terms) TRMM dataset for climate research—in particular the interannual to multiyear variability and the interaction of regional with global climate—rises exponentially with every year of added observations.10 The original motivation for TRMM and its low-Earth, inclined orbit was to collect a benchmark climatology of tropical rainfall averages. This goal has been achieved to a large extent. The more sophisticated climate definition, in terms of the variability of precipitation, requires a longer record to fully characterize and understand the nature of seasonal and interannual precipitation variability. Research using TMI and PR has provided information on the variations in vertical structure of precipitating clouds, the distribution of stratiform and convective rain, diurnal variations in rainfall, and regional and seasonal variability. Even with a seven-year record, the long-term statistics of this variability are not yet well established. Precipitation is an episodic process with small-scale structure, and is therefore much more difficult to characterize than a continuous field such as temperature. The sampling problem becomes particularly severe when attempting to characterize such features as the fine-scale struc- 8 Ibid. 9 For example, research is showing that the often-observed asymmetry of the rainfall between left and right sides of a storm (with respect to the storm’s direction of propagation) is related to the wind shear in the lower atmosphere. Forecast wind shear is available from the intensity forecast tool SHIPS, and research to combine this tool with the model R-CLIPER (Rainfall Climate and Persistance) is under way through the Joint Hurricane Testbed. 10 Testimony of Joanne Simpson, NASA (retired), at the November 8 workshop.

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report ture of precipitation systems (which requires PR data) and the convective and stratiform structure (Steiner and Houze, 1998). A comprehensive climatology of these features must consider their variability in space by season with type of disturbance and with other factors, which requires that the individual observations be stratified into a number of categories if the results are to be meaningful. Consequently, the existing TRMM database is inadequate for establishing stable and useful statistics for many of these characteristic features of the tropical hydroclimate. Extension of TRMM for a single year would provide important but limited additional sampling and could nonetheless benefit all applications of TRMM data. Even better, an extension to 2010-2011 would almost double the record length and would undoubtedly result in major improvements in the comprehensiveness and robustness of a variety of hydroclimatic statistics, including their year-to-year variability. This extension would become even more valuable if it interfaced with the implementation of the Global Precipitation Measurement mission, thus providing a continuous climate time series beginning in late 1997. Foster Improving Moist Physics Parameterization for Climate Models, Numerical Weather Prediction, and Related Assimilation Systems by Evaluating Models of Clouds and Precipitation Physics Global cloud models are advancing and will become a reality in the next 5 to 10 years (Randall et al., 2003; Stephens, 2004). These cloud models will resolve processes on scales that more closely match the native resolution of the TRMM sensors and in a way that more directly links to the observations of TRMM and future satellites like CloudSat (Stephens et al., 2002, 2003). The PR provides an essential source of data for evaluating these cloud process models globally. Spatial and system dependencies mean that longer records are needed, because researchers cannot rely solely on the existing dataset. ANTICIPATED CONTRIBUTIONS BEYOND THE FUEL POINT (IN ADDITION TO WHAT IS GAINED WHILE CONTROLLED REENTRY IS STILL POSSIBLE) Anticipated Operational Contributions Technology Demonstration of First Precipitation Radar in Space Evaluation of the PR’s potential contribution to future radar development, especially with regard to reliability, longevity, and stability, would be enhanced by continuing its operation. Among other things it would help in the design of future spaceborne weather radar technologies in the pipeline.

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report Operational Assimilation of TMI and PR Data for Weather and Climate Prediction Models A rapidly increasing number of studies have been demonstrating the value of assimilating passive microwave-retrieved humidity and precipitation information into mesoscale (e.g., Chang et al., 2001; Pu et al., 2002; Kato et al., 2003) and global numerical weather prediction models (e.g., Krishnamurti et al., 2001; Marecal et al., 2002; Aonashi et al., 2004; Hou et al., 2004). Typically, rainfall assimilation has been found to improve the analysis of clouds and radiation fields in areas of active convection, as well as latent heating and large-scale motions in the tropics, while total precipitable water assimilation leads to reduced moisture biases and improved radiative fluxes in clear-sky regions (Hou et al., 2001). A particularly strong case is made by Chang et al. (2001), who demonstrate how assimilation of TMI-retrieved rainfall, integrated water vapor, and sea surface temperature into a mesoscale numerical weather prediction model yielded improved nine-hour forecasts of radar reflectivity cross-sections compared with coincident observations from the PR. The effects of assimilating passive microwave-retrieved rainfall information are especially noticeable for tropical cyclones in terms of their path and intensity, and the kinematic and precipitation structures (e.g., Marecal and Mahfouf 2002; Pu et al., 2002; Hou et al., 2004). Kato et al. (2003) emphasize the importance of the vertical profile of moisture for successfully reproducing the structure and intensity of heavy rainfall over the Kyushu Islands, Japan. Krishnamurti et al. (2001) advocate the value of multi-analysis and multimodel superensembles for achieving higher skills than any of the participating member models. For reasons of familiarity, spatial coverage, and availability in real time, operational users of TRMM data have focused primarily on TMI and Visible Infrared Scanner observations. Another reason why PR data have not yet been assimilated into numerical weather prediction models is a lack of consistency between the observations and the model physics, which is still largely parameterized particularly in global-scale models. This situation will change as fine-scale, cloud-resolving models are further developed. At present the European Centre for Medium-Range Weather Forecasts (ECMWF) is using PR data primarily as an independent routine validation tool for precipitation analyses (Peter Bauer, personal communication, November 2004; Benedetti et al., 2004a). However, they are exploring the potential value of assimilating PR data into ECMWF’s numerical weather prediction model. Initial results with a variational assimilation scheme that adjusts model temperature and specific humidity based on PR reflectivity information, which are subsequently integrated vertically to yield total column water vapor, show a positive effect in the analysis and subsequent forecasts both on moist-related fields and on winds and surface pressure (Benedetti et al., 2004b). Moreover, the forecasted tracks of tropical cyclones more closely match the observations than the tracks obtained by control runs

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report without the above PR-based adjustments. The continued and near-real-time availability of PR data will provide great impetus to move forward with these efforts and those of other groups such as the Joint Center for Satellite Data Assimilation (a NASA, NOAA, and Department of Defense joint center), which plans to focus on the use of cloud and precipitation information. Keeping the TRMM satellite in orbit for as long as possible, therefore, will contribute to the further development of advanced data assimilation schemes that will be needed for the Global Precipitation Measurement era. Anticipated Research Contributions Unique Opportunity to Enhance Field Experiment (African Monsoon Multi-Disciplinary Analysis) If TRMM were extended into at least 2006, it would overlap with and provide valuable input to the African Monsoon Multi-Disciplinary Analysis (AMMA), an international research project with a field campaign scheduled to take place in 2006. The campaign is focused on the eastern North Atlantic Ocean and West Africa. AMMA seeks to better understand the variability of the West African monsoon and to address practical issues related to prediction and applications. As stated in the AMMA research plan, “[s]atellite observations will strongly contribute to the objectives of the project by providing key variables of the surface-atmosphere system (e.g., Meteosat/MSG, ENVISAT, TRMM, AURA, AQUA-Train, TERRA, SMOS … [see Appendix J for explanations of abbreviations]). A challenge is to exploit this huge amount of data (20 years for Meteosat, for example) by optimizing the retrievals and data analysis for monitoring as well as validation of models and assimilation. The project will provide a unique set of integrated ground observations for validation of the coming satellites. It will also provide the framework to build a reliable monitoring strategy combining satellite and in situ network, to make up for the low density of routine observations in Africa.” Several ships, at least one equipped with a rain radar, are expected to be involved. AMMA also provides an opportunity for studies of Atlantic tropical cyclone formation, especially on the effect of the Saharan Air Layer (SAL) on cyclone formation. The TRMM data would be helpful for this experiment because of the uniqueness of having the PR co-located with the TMI to reveal the differences in vertical profiles of latent heat release in cases of SAL interaction with the tropical cyclone on the one hand and non-SAL interaction on the other. The TRMM data would help separate physical processes by which the aerosol layer is affecting storm formation and intensification.

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report Developing the Next Generation of Hurricane Forecast Model The Joint Hurricane Testbed is a component of the U.S. Weather Research Program that facilitates the transition from research to operations. After years of investment to develop and transition the next generation hurricane forecast model system (called the Hurricane Weather Research and Forecast model), it is scheduled to become operational in 2007. Continuation of the TRMM data streams is needed to enhance the benefits of this effort in terms of improved intensity and precipitation forecasts. For example, the model needs to be validated with high-resolution precipitation data such as PR data. Whereas research aircraft radar can provide some data of similar resolution, a satellite-based system is required to validate the precipitation during all stages in the hurricane life cycle and in regions where research aircraft are not available. Seamless Transition into the Global Precipitation Measurement Mission There are obvious merits to achieving this goal, for the many reasons stated in the previous section about extending datasets and, in particular, the cross-calibration value of overlap of TRMM sensors with the Global Precipitation Measurement sensors. Realization of a Prototype Global Precipitation Measurement-like Operation The Global Precipitation Measurement mission involves a virtual constellation of satellites served by a “core” satellite. Because of the dimensions of this undertaking, the mission is an across-the-board effort by an international collection of space agencies and partner agencies and organizations. Each member of the constellation is underwritten with its own unique experimental or operational purpose and agenda, but the collective set of platforms produces an integrated global rainfall measurement system. In such constellations the core satellite’s orbit is purposefully non-sunsynchronous in order to provide high-quality, diurnal-sampled calibration reference estimates. This assures coincident orbit intersections with all other sunsynchronous and non-sunsynchronous constellation members. Orbit intersections between the core satellite and each of the constellation’s remaining members are essential for producing closely coincident data in time needed to determine the systematic biases for the entire set of constellation members. The projected launch year for the core satellite is 2010 at the earliest.11 Currently there are two satellites in the constellation—Coriolis and AQUA—and there are planned launches for three other components in the near future: 11 For up-to-date GPM information, visit the mission Website at http://gpm.gsfc.nasa.gov.

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report Meteorological Operational Weather Satellite, December 2005 launch; NPOESS Preparatory Project, Fall 2006 launch; and Defense Meteorological Satellite Program, June 2005 launch. Until the core satellite is launched there is an excellent opportunity to prototype several aspects of the Global Precipitation Measurement-era constellation using TRMM as the effective core satellite. If TRMM were extended until the true core satellite launch there would be a seamless transition of radar-calibrated global precipitation measurements into the Global Precipitation Measurement era. Avoiding Researchers Being Ill Prepared for the Global Precipitation Measurement Mission The success of TRMM and its unique combination of sensors and special orbit has encouraged development of a cadre of graduate students and researchers now well versed in the intricacies of using a rain radar and a multichannel microwave radiometer in synergy with infrared and visible cloud sensors. If a hiatus of six to eight years in collection of this type of data occurred (between an end of TRMM and the eventual Global Precipitation Measurement mission), training opportunities would be lost. If TRMM continued with near-real-time availability of precipitation data (and with research funding opportunities intact), the community would be ready to take full advantage of the Global Precipitation Measurement constellation. Better Characterization of Interannual Variability and the El Niño-Southern Oscillation Cycle Seasonally averaged precipitation in the tropics exhibits pronounced year-to-year variability. The more significant regional-scale variations are associated with large-scale interactions between the atmosphere and land and ocean surface conditions (e.g., soil moisture, sea surface temperature). However, rainfall is not simply a passive response to these interactions. Rather, through the release of latent heat it plays a major role in the dynamics of the interactions. The effects of regional-scale latent heating propagate throughout the tropics and into the extra-tropics (through teleconnections) where the remote response results in significant climate variability. Consequently a full description and understanding of interannual rainfall variability in the tropics requires both a quantitative description of rainfall anomalies and a diagnostic understanding of the role of rainfall in the coupled processes of the Earth-ocean-atmosphere climate system. The most extreme and widespread year-to-year variations in tropical rainfall are associated with the ENSO phenomenon. In fact, interannual variability in tropical rainfall is dominated by the ENSO cycle, which arises from coupled ocean-atmosphere interactions in the equatorial Pacific (if above normal sea surface temperature, El Niño conditions result and if below normal sea surface

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report temperature, La Niña conditions). El Niño conditions are associated with massive regional shifts in the precipitation regimes and tropical storm activity, resulting in abnormally heavy rainfall and flooding in normally arid regions and wrenching drought and monsoon failure in other regions. These climate anomalies lead to severe human and socioeconomic effects over large regions of the tropics and affect midlatitude precipitation (e.g., flooding in California during the 1982-1983 and 1997-1998 El Niños). A meaningful characterization of interannual precipitation variability in the tropics must center on this phenomenon. Climate models have difficulty in realistically simulating the ENSO cycle and its global response. One of several likely reasons for this is an inadequate model representation of the rainfall processes. TRMM provides a reliable quantification of the evolving rainfall field and crucial information on latent heating profiles (only available from the PR). Both are key variables needed for validating the hydrometeorological subcomponents of climate models. The seven-year TRMM record that now exists includes the later stages of the major 1997-1998 El Niño as well as the weak 2002-2003 event. The differences between these two events illustrate their variability in intensity and character from event to event. While these two realizations are useful for preliminary studies, they are insufficient for a statistical characterization of the hydroclimatic aspects of the ENSO. Since El Niño recurs at irregular integrals of two to seven years, there is a high probability of one or more additional El Niño occurrences between 2005 and 2010. Continuous TRMM observations from 1997 into the Global Precipitation Measurement era would provide a unique and valuable continuous record for the ongoing study and characterization of the ENSO cycle. CONCLUSIONS CONCLUSION 4.1: The material in this report provides science and operations information needed as input for a qualitative evaluation of the balance between the risk inherent in 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. CONCLUSION 4.2: 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 either to the fuel point in approximately December 2005 or beyond are compiled in Table 4-1. CONCLUSION 4.3: From the perspective of anticipated research contributions, TRMM is worth continuing for six primary reasons:

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report TRMM provides a unique complement of measurements. Specifically, the precipitation radar, the passive microwave imager, and the visible and infrared instruments provide a powerful overlap of precipitation, cloud, and water vapor measurements and the lightning imaging sensor helps isolate intense convective cells. In addition, 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 precipitation radar technology, especially in the evaluation of the radar technology life cycle. 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, National Polar-orbiting Operational Environmental Satellite System’s Conical Scanning Microwave Imager/Sounder, and Global Precipitation Measurement core satellite and other constellation satellites. TRMM’s unique low-inclination, low-altitude, precessing orbit enhances science by providing unique spatial and temporal information that fills the gaps in data from other current and upcoming polar-orbiting satellite sensors. TRMM data will enhance field experiments and programs (e.g., TCSP, AMMA, GEWEX, THORPEX, TEXMEX-II), tropical cyclone research (including tropical cyclone forecasting), and development of cloud-resolving models. 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. Longer TRMM data records will better characterize tropical seasonal-interannual climate variability in general and the 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. CONCLUSION 4.4: 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: 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.

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Assessment of the Benefits of Extending the Tropical Rainfall Measuring Mission: A Perspective from the Research and Operations Communities - Interim Report 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. 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. CONCLUSION 4.5: 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. RECOMMENDATION 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.