Part II
Mission Summaries

In Chapter 2, the committee describes the observational portion of a strategy for obtaining an integrated set of space-based measurements in the decade 2010-2020. The 171 missions listed in Tables 11.1 and II.2 form the centerpiece of this strategy. In Part IIChapter 4—the committee summarizes in alphabetical order the 17 recommended missions, providing a more detailed discussion of each. Each mission summary also contains references to the particular sections in the panel reports in Part III (Chapters 5-11) in which the missions are discussed, as well as index numbers that point to related responses to the committee's request for information.2

1

Note that CLARREO is listed twice because its instruments are recommended for support by both NASA and NOAA.

2

The request for information is reprinted in Appendix D. A complete index to the responses is provided in Appendix E. Full-text versions of the responses are included on the compact disk that contains this report.



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Part II Mission Summaries In Chapter 2, the committee describes the observational portion of a strategy for obtaining an integrated set of space-based measurements in the decade 2010-2020. The 171 missions listed in Tables 11.1 and II.2 form the centerpiece of this strategy. In Part II—Chapter 4—the committee summarizes in alphabetical order the 17 recommended missions, providing a more detailed discussion of each. Each mission summary also contains references to the particular sections in the panel reports in Part III (Chapters 5-11) in which the missions are discussed, as well as index numbers that point to related responses to the committee's request for information.2 1 Note that CLARREO is listed twice because its instruments are recommended for support by both NASA and NOAA. 2 The request for information is reprinted in Appendix D. A complete index to the responses is provided in Appendix E. Full-text versions of the responses are included on the compact disk that contains this report.

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TABLE II.1 Launch, Orbit, and Instrument Specifications for Missions Recommended to NOAA Decadal Survey Mission Mission Description Orbita Instruments Rough Cost Estimate (FY 06 $million) 2010–2013         CLARREO (instrument reflight components) Solar and Earth radiation characteristics for understanding climate forcing LEO, SSO Broadband radiometer 65 GPSRO High-accuracy, all-weather temperature, water vapor, and electron density profiles for weather, climate,and space weather LEO GPS receiver 150 2013–2016         XOVWM Sea-surface wind vectors for weather and ocean ecosystems LEO, SSO Backscatter radar 350 NOTE: Missions are listed by cost. Colors denote mission cost categories as estimated by the committee. Green and blue shading indicates medium-cost ($300 million to $600 million) and small-cost (<$300 million) missions, respectively. The missions are described in detail in Part II, and Part III provides the foundation for selection. aLEO, low Earth orbit; SSO, Sun-synchronous orbit.

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TABLE II.2 Launch, Orbit, and Instrument Specifications for Missions Recommended to NASA Decadal Survey Mission Mission Description Orbita Instruments Rough Cost Estimate (FY 06 $million) 2010–2013         CLARREO (NASA portion) Solar and Earth radiation; spectrally resolved forcing and response of the climate system LEO, Precessing Absolute, spectrally resolved interferometer 200 SMAP Soil moisture and freeze-thaw for weather and water cycle processes LEO, SSO L-band radar L-band radiometer 300 ICESat-II Ice sheet height changes for climate change diagnosis LEO, Non-SSO Laser altimeter 300 DESDynl Surface and ice sheet deformation for understanding natural hazards and climate; vegetation structure for ecosystem health LEO, SSO L-band InSAR Laser altimeter 700 2013–2016         HyspIRI Land surface composition for agriculture and mineral characterization; vegetation types for ecosystem health LEO, SSO Hyperspectral spectrometer 300 ASCENDS Day/night, all-latitude, all-season CO2 column integrals for climate emissions LEO, SSO Multifrequency laser 400 SWOT Ocean, lake, and river water levels for ocean and inland water dynamics LEO, SSO Ka- or Ku-band radar Ku-band altimeter Microwave radiometer 450 GEO-CAPE Atmospheric gas columns for air quality forecasts;ocean color for coastal ecosystem health and climate emissions GEO High-spatial-resolution hyperspectral spectrometer Low-spatial-resolution imaging spectrometer IR correlation radiometer 550 ACE Aerosol and cloud profiles for climate and water cycle; ocean color for open ocean biogeochemistry LEO, SSO Backscatter lidar Multiangle polarimeter Doppler radar 800 2016–2020         LIST Land surface topography for landslide hazards and water runoff LEO, SSO Laser altimeter 300 PATH High-frequency, all-weather temperature and humidity soundings for weather forecasting and sea-surface temperatureb GEO Microwave array spectrometer 450 GRACE-II High-temporal-resolution gravity fields for tracking large-scale water movement LEO, SSO Microwave or laser ranging system 450 SCLP Snow accumulation for freshwater availability LEO, SSO Ku- and X-band radars K- and Ka-band radiometers 500 GACM Ozone and related gases for intercontinental air quality and stratospheric ozone layer prediction LEO, SSO UV spectrometer IR spectrometer Microwave limb sounder 600 3D-Winds (Demo) Tropospheric winds for weather forecasting and pollution transport LEO, SSO Doppler lidar 650 NOTE: Missions are listed by cost. Colors denote mission cost categories as estimated by the committee. Pink, green, and blue shading indicates large-cost ($600 million to $900 million), medium-cost ($300 million to $600 million), and small-cost (<$300 million) missions, respectively. Detailed descriptions of the missions are given in Part II, and Part III provides the foundation for their selection. aLEO, low Earth orbit; SSO, Sun-synchronous orbit; GEO, geostationary Earth orbit. bCloud-independent, high-temporal-resolution, lower-accuracy sea-surface temperature measurement to complement, not replace, global operational high-accuracy sea-surface temperature measurement.

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4 Summaries of Recommended Missions

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ACTIVE SENSING OF CO2 EMISSIONS OVER NIGHTS, DAYS, AND SEASONS (ASCENDS) MISSION The primary human activities contributing to the nearly 40 percent rise in atmospheric CO2 since the middle of the 20th century are fossil-fuel combustion and land-use change, primarily the clearing of forests for agricultural land. More than 50 percent of the CO2 from fossil-fuel combustion and land-use change has remained in the atmosphere; land and oceans have sequestered the nonairborne fraction in roughly equal proportions. However, the balance between land and oceans varies in time and space. The current state of the science cannot account with confidence for the growth rate and interannual variations of atmospheric CO2. The variability in the rate of increase in the concentration of CO2 in the atmosphere cannot be explained by the variability in fossil-fuel use; rather, it appears to reflect primarily changes in terrestrial ecosystems that are connected with large-scale weather and climate modes. The overall pattern is important and is not understood. The geographic distribution of the land and ocean sources and sinks of CO2 has likewise remained elusive, an uncertainty that is also important. As nations seek to develop

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strategies to manage their carbon emissions and sequestration, the capacity to quantify current regional carbon sources and sinks and to understand the underlying mechanisms is central to prediction of future levels of CO2 and therefore to informed policy decisions, sequestration monitoring, and carbon trading (Dilling et al., 2003; IGBP, 2003; CCSP, 2003, 2004). Background: Direct oceanic and terrestrial measurements of carbon and of the flux of CO2 are important but are resource-intensive and hence sparse and are difficult to extrapolate in space and time. Space-based measurements of primary production and biomass are valuable and needed, and the problem of source-sink determination of CO2 will be aided greatly by such measurements and studies, but it will not be resolved by this approach. There is, however, a different complementary approach. The atmosphere is a fast but incomplete mixer and integrator of spatially and temporally varying surface fluxes, and so the geographic distribution (such as spatial gradient) and temporal evolution of CO2 in the atmosphere can be used to quantify surface fluxes (Tans et al., 1990; Plummer et al., 2005). The current set of direct in situ atmospheric observations is far too sparse for this determination; however, long-term accurate measurements of atmospheric CO2 columns with global coverage would allow the determination and localization of CO2 fluxes in time and space (Baker et al., 2006; Crisp et al., 2004). What is needed for space-based measurements is a highly precise global data set for atmospheric CO2-column measurements without seasonal, latitudinal, or diurnal bias, and it is possible with current technology to acquire such a data set with a sensor that uses multiwavelength laser-absorption spectroscopy. The first step in inferring ecosystem processes from atmospheric data is to separate photosynthesis and respiration; this requires diurnal sampling to observe nighttime concentrations resulting from respiration. Analyses of flux data show that there is a vast difference in the process information obtained from one measurement per day versus two (i.e., one measurement per day plus one per night), with a much smaller gain attributable to many observations per day (Sacks et al., 2007). It is also essential to separate physiological fluxes from biomass burning and fossil-fuel use, a distinction that requires simultaneous measurement of an additional tracer, ideally carbon monoxide (CO). A laser-based CO2 mission—the logical next step after the launch of NASA’s Orbiting Carbon Observatory (OCO),1 which uses reflected sunlight—will benefit directly from the data-assimilation procedures and calibration and validation infrastructure that will handle OCO data. In addition, because it will be important to overlap the new measurements with those made by OCO, the ASCENDS mission should be launched in the 2013–2016 time frame at the latest. Science Objectives: The goal of the ASCENDS mission is to enhance understanding of the role of CO2 in the global carbon cycle. The three science objectives are to (1) quantify global spatial distribution of atmospheric CO2 on scales of weather models in the 2010–2020 era, (2) quantify current global spatial distribution of terrestrial and oceanic sources and sinks of CO2 on 1-degree grids at weekly resolution; and (3) provide a scientific basis for future projections of CO2 sources and sinks through data-driven enhancements of Earth-system process modeling. Mission and Payload: The ASCENDS mission consists of simultaneous laser remote sensing of CO2 and O2, which is needed to convert CO2 concentrations to mixing ratios. The mixing ratio needs to be measured to a precision of 0.5 percent of background (slightly less than 2 ppm) at 100-km horizontal length scale over land and at 200-km scale over open oceans. Such a mission can provide full seasonal sampling to 1 The Orbiting Carbon Observatory (OCO) is a NASA Earth System Science Pathfinder (ESSP) project mission designed to make precise, time-dependent global measurements of atmospheric CO2 from an Earth-orbiting satellite. OCO should begin operations in 2009. See description at http://oco.jpl.nasa.gov/.

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high latitudes, day-night sampling, and some ability to resolve (or weight) the altitude distribution of the CO2-column measurement, particularly across the middle to lower troposphere. CO2 lines are available in the 1.57- and 2.06-µm bands, which minimize the effects of temperature errors. Lines near 1.57 µm are identified as potential candidates because of their relative insensitivity to temperature errors, relative freedom from interfering water-vapor bands, good weighting functions for column measurements across the lower troposphere, and the high technology readiness of lasers. To further reduce residual temperature errors in the CO2 measurement, a concurrent passive measurement of temperature along the satellite ground track with an accuracy of better than 2 K is required. Atmospheric pressure and density effects on deriving the mixing ratio of CO2 columns can be addressed with a combination of simultaneous CO2 and O2 column density measurements at the surface or cloud tops, or possibly with surface-cloud-top altimetry measurements from a lidar in conjunction with advanced meteorological analysis for determining the atmospheric-pressure profile across the measured CO2 density column. The concurrent on-board O2 measurements are preferred and can be based on measurements that use an O2 absorption line in the 0.76- or 1.27-µm band. The mission requires a Sun-synchronous polar orbit at an altitude of about 450 km and with a lifetime of at least 3 years. The mission does not have strict requirements for specific temporal revisit or map revisit times, because the data will be assimilated on each pass and the large-scale nature of the surface sources and sinks will emerge from the geographic gradients of the column integrals. The important coverage is day and night measurements at nearly all latitudes and surfaces to separate the effects of photosynthesis and respiration. The maximal power required would be about 500 W, with a 100 percent duty cycle. Swath size would be about 200 m. Ideally, a CO sensor should complement the lidar CO2 measurement. The two measurements are highly synergistic and should be coordinated for time and space sampling, with the minimal requirement that the two experiments be launched close together in time to sample the same area. Cost: About $400 million. Schedule: ASCENDS should be launched to overlap with OCO and hence in the 2013–2016 (the middle) time frame. Technology development must include extensive aircraft flights demonstrating not only the CO2 measurement in a variety of surface and atmospheric conditions but also the O2-based pressure measurement. Further Discussion: See in Chapter 7 the section “Carbon Budget Mission (CO2 and CO).” Related Responses to Committee’s RFI: 4 and 20. References: Baker, D.F., S.Doney, and D.S.Schimel. 2006. Variational data assimilation for atmospheric CO2. Tellus B 58(5):359–365. CCSP (Climate Change Science Program). 2003. Strategic Plan for the U.S. Climate Change Science Program. Final report by the Climate Change Science Program and the Subcommittee on Global Change Research, Washington, D.C., July, 202 pp., available at http://www.climatescience.gov/Library/stratplan2003/final/ccspstratplan2003. CCSP. 2004. Our Changing Planet: The U.S. Climate Change Science Program for Fiscal Years 2004 and 2005. A Supplement to the President’s Fiscal Year 2004 and 2005 Budgets, Washington, D.C., August, available at http://www.usgcrp.gov/usgcrp/Library/ocp2004–5/ocp2004–5.pdf. Crisp, D., R.M.Atlas, F.M.Breon, L.R.Brown, J.P.Burrows, P.Ciais, B.J.Connor, S.C.Doney, I.Y.Fung, D.J.Jacob, C.E.Miller, D.O’Brien, S.Pawson, J.T.Randerson, P.Rayner, R.J.Salawitch, S.P.Sander, B.Sen, G.L.Stephens, P.P.Tans, G.C.Toon, P.O.Wennberg, S.C.Wofsy, Y.L.Yung, Z.Kuang, B.Chudasama, G.Sprague, B.Weiss, R.Pollock, D.Kenyon, and S.Schroll. 2004. The Orbiting Carbon Observatory (OCO) Mission. Adv. Space Res. 34(4):700–709. Dilling L., S.C.Doney, J.Edmonds, K.R.Gurney, R.Harriss, D.Schimel, B.Stephens, and G.Stokes. 2003. The role of carbon cycle observations and knowledge in carbon management. Annu. Rev. Env. Resour. 28:521–558.

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IGBP (International Geosphere-Biosphere Programme). 2003. Integrated Global Carbon Observation Theme: A Strategy to Realize a Coordinated System of Integrated Global Carbon Cycle Observations. Integrated Global Carbon Observing Strategy (IGOS) Carbon Theme Report. Available at http://www.igospartners.org/Carbon.htm. Plummer, S., P.Rayner, M.Raupach, P.Ciais, and R.Dargaville. 2005. Monitoring carbon from space. EOS Trans. AGU 86(41):384– 385. Sacks, W., D.Schimel, and R.Monson. 2007. Coupling between carbon cycling and climate in a high-elevation, subalpine forest: A model-data fusion analysis. Oecologia 151(1):54–68, doi:10.1007/s00442–006–0565–2. Tans, P.P., I.Y.Fung, and T.Takahashi. 1990. Observational constraints on the global atmospheric CO2 budget. Science 247(4949):1431–1438.

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AEROSOL-CLOUD-ECOSYSTEMS (ACE) MISSION The primary goal of the Aerosol-Cloud-Ecosystems (ACE) mission is to reduce uncertainty about climate forcing in aerosol-cloud interactions and ocean ecosystem carbon dioxide (CO2) uptake. Aerosol-cloud interaction is the largest uncertainty in current climate models. Aerosols can make clouds brighter and affect their formation. Aerosols can also affect cloud precipitation and have been linked to decreased rainfall in the Mediterranean. Results from the ACE mission would narrow the uncertainty in climate predictions and improve the capability of models to provide more precise predictions of local climate change, including changes in rainfall. ACE aerosol measurements could also be assimilated into air-quality models to improve air-quality forecasts. Ocean ecosystem measurements would provide information on uptake of CO2 by phytoplankton and improve estimates of the ocean CO2 sink. As CO2 increases, the oceans will acidify, and this will affect the whole food chain, including coral-reef formation. The ACE mission could assess changes in the productivity of pelagic fishing zones and provide for early detection of harmful algal blooms. Benefits of the mission would include enabling the development of strategies for adaptation to climate change, evaluation of the consequences of increases in greenhouse gases, enabling of improved

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SOIL MOISTURE ACTIVE-PASSIVE (SMAP) MISSION Soil moisture is a key control on evaporation and transpiration at the land-atmosphere boundary. Large amounts of energy are required to vaporize water, and so soil control on evaporation and transpiration also influences surface energy fluxes. Hence, variations in soil moisture affect the evolution of weather and climate over continental regions. Initialization of numerical weather prediction (NWP) models and seasonal climate models with correct information on soil moisture enhances their prediction skill and extends their lead times. Soil moisture strongly affects plant growth and therefore agricultural productivity, especially during conditions of water shortage, the most severe of which is drought. There is no global in situ network for measuring soil moisture, and global estimates of soil moisture, and, in turn, plant water stress, must be derived from models. The model predictions (and hence drought monitoring) could be greatly enhanced through assimilation of soil-moisture observations. Soil moisture and its freeze-thaw state are also key determinants of the global carbon cycle. Carbon uptake and release in boreal landscapes are a major source of uncertainty in assessing the carbon budget of the Earth system (the so-called missing carbon sink). Soil moisture also is a key variable in water-related natural hazards, such as floods and landslides. High-

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resolution observations of soil moisture would help to improve flood forecasts, especially for intermediate to large watersheds, where most flood damage occurs, and thus improve the capability to protect downstream resources. Soil moisture in mountainous areas is one of the most important determinants of landslides, a hazard that could be better predicted with consistent observations, which are currently lacking. Background: Global mapping of soil moisture and its freeze-thaw state at high resolution has long been of interest because these variables link the terrestrial water, energy, and carbon cycle. Such measurements also have important applications in predicting natural hazards, such as severe rainfall, floods, and droughts. The spatial variations in soil-moisture fields are determined by precipitation and radiation forcing, vegetation distribution, soil-texture heterogeneity, and topographic redistribution processes. The spatial variations lead to the need for high-resolution soil-moisture mapping (Entekhabi et al., 1999). Numerous airborne and tower-based field experiments have shown that low-frequency L-band microwave measurements are reliable indicators of soil-moisture changes across the landscape. Only by combining high-resolution active radar and high-accuracy passive radiometer L-band measurements is it possible to produce data that meet the science and application requirements. The proposed SMAP mission builds on the risk-reduction performed for the AO-3 ESSP called the Hydrosphere State (Hydros) mission (Entekhabi et al., 2004). The SMAP radar makes overlapping measurements, which can be processed to yield resolution enhancement and 1- to 3-km resolution mapped data. The SMAP radar and radiometer share a large deployable light-weight mesh reflector that is spun to make conical scans across a wide (1,000-km) swath. This measurement approach allows global mapping at 3- to 10-km resolution with 2- to 3-day revisit. Science Objectives: Soil moisture and its freeze-thaw state are primary controls on the exchange fluxes of water, energy and carbon at the land-atmosphere interface. More important, those variables are what link the water, energy, and carbon cycles over land. The availability of soil-moisture data will remove existing stovepiping in the water, energy, and biogeochemistry communities by directly characterizing the link between the cycles over land regions. The data will also enable the Earth system science community to address the question of how perturbations in one cycle (radiative forcing) affect the rates of the other cycles. The spatial variability that is due to the influences of intermittent precipitation, patchy cloudiness, soil and vegetation heterogeneity, and topographic factors leads to the requirement for high-resolution mapping of soil moisture and its freeze-thaw state. Currently there are no in situ networks to support the data needs of Earth system scientists. Forthcoming satellite missions do not have the active-sensor and passive-sensor combination needed to meet the resolution requirements to characterize the heterogeneous fields. Soil moisture serves as the memory at the land surface in the same way as sea-surface temperature does at the ocean surface. The use of sea-surface temperature observations to initialize and constrain coupled ocean-atmosphere models has led to important advances in long-range weather and seasonal prediction. In the same way, high-resolution soil-moisture mapping will have transformative effects on Earth system science and applications (Entekhabi et al., 1999; Leese et al., 2001). As the ocean and atmosphere community synergies have led to substantial advances in Earth system understanding and improved prediction services, the availability of high-resolution mapping of surface soil moisture will be the link between the hydrology and atmospheric communities that share interest in the land interface. The availability of such observations will enable the emergence of a new generation of hydrologic models for applications in Earth system understanding and operational severe-weather and flood forecasting. Mission and Payload: The SMAP mission, based on one flight system in a low-Earth, Sun-synchronous orbit, includes a capability for active radar and passive radiometer measurements. The two sensors share a single feedhorn and mesh reflector to form a beam offset from nadir with the surface of 39°. This beam is rotated

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conically about the nadir axis to make a wide-swath measurement. The reflector is composed of lightweight mesh material that can be stowed for launch. The feed and reflector components shared between the two sensors lead to cost savings. The SMAP hardware is derived from the Hydros design and has therefore been subject to substantial study and risk reduction. Similarly, the spacecraft dynamics, ground data system, and science algorithms have been tested to a great extent. Field experiments have been used to validate the science algorithms, and scale models have been constructed to test the antenna performance. As a result of the Hydros risk-reduction investments and activities, all the components of the proposed SMAP are at technology readiness level 7 and higher. Cost: About $300 million. Schedule: As a pathfinder, SMAP is conceptualized as being built on the foundations of the earlier AO-3 concept (Hydros) that has undergone risk reduction marked by rigorous reviews. As a result, SMAP is ready to move on a fast-track toward launch as early as 2012, when there are few scheduled Earth missions. SMAP’s readiness also gives a capability for gap-filling observations to meet key NPOESS community needs; soil moisture is the key parameter (see Section 4.1.6.1.6 in Joint Requirements Oversight Council, 2002). In addition, SMAP will yield continuity measurements for the Aquarius mission community. Further Discussion: See in Chapter 11 the section “Soil Moisture and Freeze-Thaw State.” Related Responses to Committee’s RFI: Similar to those of the Hydros mission proposed for ESSP. References: Entekhabi, D., G.R.Asrar, A.K.Betts, K.J.Beven, R.L.Bras, C.J.Duffy, T.Dunne, R.D.Koster, D.P.Lettenmaier, D.B.McLaughlin, W.J.Shuttleworth, M.T.van Genuchten, M.-Y.Wei, and E.F.Wood. 1999. An agenda for land-surface hydrology research and a call for the second International Hydrological Decade. Bull. Am. Meteorol. Soc. 80(10):2043–2058. Entekhabi, D., E.Njoku, P.Houser, M.Spencer, T.Doiron, J.Smith, R.Girard, S.Belair, W.Crow, T.Jackson, Y.Kerr, J.Kimball, R.Koster, K.McDonald, P.O’Neill, T.Pultz, S.Running, J.C.Shi, E.Wood, and J.van Zyl. 2004. The Hydrosphere State (HYDROS) mission concept: An Earth system pathfinder for global mapping of soil moisture and land freeze/thaw. IEEE Trans. Geosci. Remote Sens. 42(10):2184–2195. Joint Requirements Oversight Council. 2002. Joint DOD-NOAA-NASA Integrated Operational Requirements Document II (IORD-II). Available at http://www.osd.noaa.gov/rpsi/IORDII_011402.pdf. Leese, J., T.Jackson, A.Pitman, and P.Dirmeyer. 2001. GEWEX/BAHC international workshop on soil moisture monitoring, analysis, and prediction for hydrometeorological and hydroclimatological applications. Bull. Am. Meteorol. Soc. 82:1423–1430.

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SURFACE WATER AND OCEAN TOPOGRAPHY (SWOT) MISSION More than 75 percent of the world’s population depends on surface water as its primary source of drinking water, but there is no coordinated global observing system for surface water. Furthermore, in the case of transboundary rivers, information is often not freely available about water storage, discharge, and diversions in one country that affect the availability of water in its downstream neighbors. For rivers, the surface stage, or water level, is the most critical observation that allows estimation of river discharge, but the global network of in situ river discharge observations is extremely nonuniform; generally, the observation density is much higher in the densely populated portions of developed countries than in the developing world. The SWOT mission would produce swath (image) altimetry of water surfaces over both the lands and oceans globally at much higher spatial resolution than is now available. That information would extend the successes of ocean altimeters to inland and coastal waters and would provide a basis for directly measuring the storage of water in lakes, reservoirs, and wetlands globally. River discharge would be estimated as a derived variable. River discharge is a key variable not only for water management but also for flood forecasting, which is the main tool for mitigation of property damage and loss of life related

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to one of the most devastating natural hazards. Moreover, major health issues, such as malaria, are linked to freshwater storage and discharge. In addition to providing information about the distribution of surface water and its movement over land, the SWOT swath altimeter would also provide precision measurements to continue a climate record of sea level and to extend the record to coastal regions (including estuaries), where continued population growth and development pressures threaten marine resources. Bathymetry from a swath altimeter would improve navigation and marine rescue operations, planning for resource management, prediction of tsunami heights, and mixing rates in the deep ocean. The swath altimeter would help to improve climate and weather forecasts as well by providing essential information on changes in ocean circulation and the contributions of ocean eddies to the changes. Changes in ocean circulation are related in large part to changes in wind forcing such that the coordination of sea-level measurements with improvements in the observations of ocean-vector winds will greatly enhance the measurements of either mission. Coastal ecosystems are greatly affected by changes in wind-forced coastal circulation, and high resolution in both measurements will contribute to improved fisheries management. Hurricanes in the Gulf of Mexico have been shown to intensify over the warm Loop Current and its eddies, a system not well resolved by the current nadir altimeters. A similar issue of insufficient measurement detail confronts ocean-climate models. Improving such models could result in improved forecasts and, in turn, mitigation of storm effects on health and property. Background: SWOT will address science and applications questions related to the storage and movement of inland waters, the circulation of the oceans and coastal waters, and the fine-scale bathymetry and roughness of the ocean floor. SWOT will consist of a swath altimeter that will produce measurements of water-surface elevations over inland waters, as well as near-coastal regions and the open ocean. Over land, it will provide observations of water stored in rivers, lakes, reservoirs, and wetlands, with river discharge estimated as a derived variable. Surface-water storage change and river discharge are major terms in the terrestrial water balance that are now observed only at points with highly varied density. Spatial mapping of water-surface elevations will capture the dynamics of wetlands and flooding rivers, which exert important controls on the fluxes of biogeochemical and trace gases between the land, atmosphere, and oceans. Over the ocean, the scientific value of past altimetry missions is well documented for ocean circulation, tides, waves, sea-level change, geodesy, and marine geophysics. However, spatial-resolution issues have precluded the use of ocean altimeters in near-coastal waters. With the much higher spatial resolution that is facilitated by swath altimetry, SWOT is expected to produce information about bathymetry, tidal variations, and currents in near-coastal and estuarine areas. Science Objectives: The wide-swath altimeter will measure spatial fields of surface elevations for both inland waters and the ocean. Those will lead to new information about the dynamics of water stored at the land surface (in lakes, reservoirs, wetlands, and river channels) and improved estimates of deep-ocean and near-coastal marine circulation. These observations will provide the basis for estimation of the dynamics of water-storage and river-discharge variations. The SWOT altimeter will have a vertical precision of a few centimeters (averaged over areas of less than 1 km2) and the ability to estimate surface water slopes to a precision of 1 microradian over areas of less than 1 km2. The latter will lead to an improvement in the spatial resolution of global estimates of ocean bathymetry by a factor of 20, which is expected to result in the mapping of ~50,000 additional seamounts. The altimeter requires a precise (non-Sun-synchronous) orbit for measurement accuracy, with a likely repeat cycle of about 21 days (combining ascending and descending orbits results in a revisit of about 10.5 days) and coverage to latitudes up to 78°. For rivers, the goal is to recover channel cross-sectional profiles to within 1-m accuracy at low water, which will allow estimation

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of the discharge of about 100-m-wide rivers via assimilation into hydrodynamic models. The resulting discharge estimates will constitute fundamentally new measurements for many parts of the globe where there is no in situ stream-gauge network or where the network is too sparse to estimate surface-water dynamics at large scales. For the ocean, the mission will map sea level with a precision of a few centimeters and a spatial resolution of less than 1 km2, extending the sea-level measurements to the ocean-eddy field and into the coastal zones. With a nadir-looking altimeter, a non-Sun-synchronous orbit, and precise tracking, the mission can extend the climate record of sea level beyond the current Jason series of altimeters. Mission and Payload: A suite of instruments will be flown on the same platform: a Ku-band near-nadir SAR interferometer; a 3-frequency microwave radiometer; a nadir-looking Ku-band radar altimeter;7 and a GPS receiver. The Ku-band SAR interferometer draws heavily from the heritage of the Wide Swath Ocean Altimeter (WSOA) and the Shuttle Radar Topography Mission (SRTM). The Ku-band synthetic aperture interferometer would provide vertical precision of a few centimeters over areas of less than 1 km2 with a swath of 120 km (including a nadir gap). The nadir gap would be filled with a Ku-band nadir altimeter similar to the Jason-1 altimeter, with the capability of doing synthetic aperture processing to improve the along-track spatial resolution. Because the open ocean lacks fixed elevation points, a microwave radiometer will be used to estimate the tropospheric water-vapor range delay and the GPS receiver for a precise orbit. A potential side benefit is that the GPS receiver could in principle also be used to provide radio-occultation soundings. Orbit selection is a compromise between the need for high temporal sampling for surface-water applications, near-global coverage, and the swath capabilities of the Ku-band interferometer. A swath instrument is essential for surface-water applications because a nadir instrument would miss most of even the largest global rivers and lakes. To achieve the required precision over water, a few changes will be incorporated into the SRTM design. The major one would be reduction of the maximal look angle to about 4.3°, which would reduce the outer swath error by a factor of about 14 compared with SRTM. A key aspect of the data-acquisition strategy is reduction of height noise by averaging neighboring image pixels, which requires an increase in the intrinsic range resolution of the instrument. A 200-MHz bandwidth system (0.75-m range resolution) would be used to achieve ground resolutions varying from about 10 m in the far swath to about 70 m in the near swath. A resolution of about 5 m (after onboard data reduction) in the along-track direction can be achieved with synthetic aperture processing. To achieve the required vertical and spatial resolution, SAR processing must be performed. Raw data would be stored on board (after being passed through an averaging filter) and downlinked to the ground. The data-downlink requirements (for both ocean and inland waters) can be met with eight 300-Mbps X-band stations globally. Cost: About $450 million. Schedule: As a practical matter, the scheduling of SWOT may be dictated by the need for continuing ocean altimeter observations. SWOT could satisfy the operational requirements of the Jason series (meaning that SWOT would essentially become Jason-3). Depending on the longevity of Jason-2 (currently scheduled for launch in mid-2008), this would suggest a SWOT launch date in the 2013–2015 range. Given the heritage of SWOT in WSOA and SRTM, the technology is sufficiently mature that such a schedule should be feasible. An overlap with XOVWM to measure winds is highly desirable for ocean applications. Further Discussion: See in Chapter 11 the section “Surface Water and Ocean Topography.” 7 The assumption is that the swath altimeter would use the Ku band. However, as discussed in Chapter 11 (in the section “Surface Water and Ocean Topography”), studies of tradeoffs will be required to decide between the Ka and the Ku band, the primary tradeoff being precision (higher for the shorter Ka wavelength) and data loss rates during precipitation (lower for the Ku band).

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Related Responses to Committee’s RFI: 79 and 108. Supporting Documents: Alsdorf, D.E., and D.P.Lettenmaier, 2003. Tracking fresh water from space. Science 301:1491–1494. Alsdorf, D., D.Lettenmaier, and C.Vörösmarty. 2003. The need for global, satellite-based observations of terrestrial surface waters. EOS Trans. AGU 84(29):275–276. Alsdorf, D., E.Rodriguez, and D.P.Lettemaier. 2007. Measuring surface water from space. Rev. Geophys. 45:RG2002, doi:10.1029/ 2006RG000197. Fu, L.-L, and A.Cazenave, eds. 2001. Satellite Altimetry and the Earth Sciences: A Handbook of Techniques and Applications. Academic Press, San Diego, Calif. Fu, L.-L, and E.Rodriguez. 2004. High-resolution measurement of ocean surface topography by radar interferometry for oceanographic and geophysical applications. Pp. 209–224 in State of the Planet: Frontiers and Challenges (R.S.J.Sparks and C.J.Hawkesworth, eds.). AGU Geophysical Monograph 150, IUGG Vol. 19. American Geophysical Union, Washington, D.C. Goni, G., and J.Trinanes. 2003. Ocean thermal structure monitoring could aid in the intensity forecast of tropical cyclones. EOS Trans. AGU 84:573–580. Smith, W.H.F., ed. 2004. Special issue: Bathymetry from space. Oceanography 17(1):6–82. Available at http://www.los.org/oceanography/issues/issue_archive/1 7_1 .html. Smith, W.H.F., R.K.Raney, and the ABYSS team. 2003. Altimetric Bathymetry from Surface Slopes (ABYSS): Seafloor geophysics from space for ocean climate. Proceedings of the Weikko A. Heiskanen Symposium in Geodesy (C. Jekeli, ed.). Ohio State University, October 1–5, 2002, Columbus, Ohio. Available at http://www.ceegs.ohio-state.edu/~cjekeli/Proc_PC.pdf

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THREE-DIMENSIONAL TROPOSPHERIC WINDS FROM SPACE-BASED LIDAR (3D-WINDS) MISSION More accurate, more reliable, and longer-term weather forecasts, driven by fundamentally improved tropospheric wind observations from space, would have direct and measurable societal and economic effects. Tropospheric winds are the number-one unmet measurement objective for improving weather forecasts. Improved forecasts of extreme-weather events would also benefit public safety through disaster mitigation. Hurricanes, for example, are generally steered by tropospheric winds whose vertical shear is often responsible for increasing a hurricane’s intensity. Public confidence in hurricane warnings will increase as forecasts get better, and a superior description of hurricane wind fields should result in substantial numbers of lives saved. Similar benefits of improved three-dimensional tropospheric wind observations from space should accrue with improved predictions of severe weather outbreaks, tornadic storms, floods, and coastal high-wind events.

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Background: The proper specification and analysis of tropospheric winds are important prerequisites of accurate numerical weather prediction (NWP). Even with the recent advances in the assimilation of radiances, wind is still a critical parameter for data assimilation and NWP because of its unique role in specifying the initial potential vorticity, required for accurate forecasting. Scientific applications are severely limited by the lack of directly measured three-dimensional wind information over the oceans, the tropics, the polar regions, and the Southern Hemisphere, where other meteorological observations are scarce. Large analysis uncertainties remain over wide areas of the globe, especially for the three-dimensional tropospheric wind field. Science Objectives: The space-based 3D-Winds mission is designed to characterize three-dimensional tropospheric winds on a global scale under a variety of aerosol loading conditions. Because wind is ultimately related to the transport of all atmospheric constituents, its measurement is crucial for understanding sources and sinks of constituents, such as atmospheric water. The transport of water vapor is essential to closing regional hydrologic cycles, and its measurement should enable scientific advances in understanding El Niño, monsoons, and the flow of tropical moisture to the United States. Reliable global analyses of three-dimensional tropospheric winds are needed to improve the depiction of atmospheric dynamics, the transport of air pollution, and climate processes. Finally, the value of accurate wind measurements in day-to-day weather forecasting is well-known; for example, the tracks of tropical cyclones are modulated by environmental wind fields that will be better analyzed and forecasted with the assimilation of newly available wind profiles. Mission and Payload: A hybrid Doppler wind lidar (HDWL) in LEO could have a transforming effect on global tropospheric-wind analyses. The HDWL is a combination of two DWL systems (coherent and noncoherent) operating in different wavelength ranges that have distinctly different but complementary measurement advantages and disadvantages. One DWL system would be based on a coherent Doppler lidar using a 2-µm laser transmitter and a coherent detection system, a type of system used extensively in ground-based Doppler lidars and more recently in a few airborne lidar systems. Because the operational wavelength of the system is in the near-infrared, it is particularly sensitive to wind in the presence of aerosols, such as in the planetary boundary layer or in aerosol-rich layers in the free troposphere resulting from biomass-burning plumes or clouds. It has low sensitivity in regions with low aerosol loading frequently found in the free troposphere and above the tropopause. The second type of DWL that would be part of the HDWL operates at ultraviolet wavelengths and uses the noncoherent detection of molecular Doppler shifts to enable wind measurements in the “clean” air regions. Combining the two DWL systems into an HDWL would allow measurements of wind across most tropospheric and stratospheric conditions. Because of the complexity of the technology associated with an HDWL, an aggressive program is needed early on to address the high-risk components of the instrument package and then to design, build, aircraft-test, and ultimately conduct space-based flights of a prototype HDWL. The program should also complement and, when possible, leverage the work being performed by the European Space Agency (ESA) with a noncoherent lidar system. Phased development of the 3D-Winds mission would proceed as follows: Stage 1 would be the design, development, and demonstration of a prototype HDWL system capable of global wind measurements to meet demonstration requirements that are somewhat reduced from operational threshold requirements. All the critical laser, receiver, detector, and control technologies would be tested in the demonstration HDWL mission. Stage II would entail the launch of an HDWL system that would meet fully operational threshold tropospheric wind measurement requirements. The 3D-Winds mission would transform how global wind data are obtained for assimilation into the latest NWP models.

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Cost: About $650 million (Stage I demonstration HDWL mission). Schedule: Stage I, space demonstration of a prototype HDWL in LEO, could take place as early as 2016. Stage II, launch of a fully operational HDWL system, could take place as early as 2022. Further Discussion: See in Chapter 10 the section “Space-based Measurements of Tropospheric Winds.” Related Responses to Committee’s RFI: 28, 29, and 78.

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