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Ocean Measurements from Space in 2025

A. Freeman*

OVERVIEW

Ocean measurements from space have advanced significantly since the first sensors were flown on NASA satellites such as Seasat in the 1970s. New technologies have opened the door to new, unforeseen scientific questions and practical applications, which, in turn, have guided the next generation of technology development—a fruitful, mutual coupling between science and technology. Recent advances in modeling of ocean circulation and biochemistry are now also linked to improved measurement capabilities.

Since the ocean is largely opaque over much of the usable electromagnetic spectrum, ocean measurements from space are largely confined to surface properties such as SSH, SST, surface wind vectors, sea surface salinity (SSS), ocean color, and surface currents. In some cases properties of the ocean beneath the surface can be inferred from such measurements, the most striking example being the determination of bathymetry from sea surface height measurements made by altimeters. Measurements of variations in the Earth’s gravity fields (e.g., by NASA’s Gravity Recovery and Climate Experiment [GRACE]) mission are somewhat a special case, and have been used to infer ocean bottom pressure, for example.

With the release of the 2007 decadal survey for Earth Science and

*

California Institute of Technology



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Ocean Measurements from Space in 2025 A. Freeman* OvERvIEW Ocean measurements from space have advanced significantly since the first sensors were flown on NASA satellites such as Seasat in the 1970s. New technologies have opened the door to new, unforeseen scien- tific questions and practical applications, which, in turn, have guided the next generation of technology development—a fruitful, mutual coupling between science and technology. Recent advances in modeling of ocean circulation and biochemistry are now also linked to improved measure- ment capabilities. Since the ocean is largely opaque over much of the usable electro- magnetic spectrum, ocean measurements from space are largely confined to surface properties such as SSH, SST, surface wind vectors, sea surface salinity (SSS), ocean color, and surface currents. In some cases properties of the ocean beneath the surface can be inferred from such measurements, the most striking example being the determination of bathymetry from sea surface height measurements made by altimeters. Measurements of variations in the Earth’s gravity fields (e.g., by NASA’s Gravity Recovery and Climate Experiment [GRACE]) mission are somewhat a special case, and have been used to infer ocean bottom pressure, for example. With the release of the 2007 decadal survey for Earth Science and * California Institute of Technology 92

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93 A. FREEMAN Applications from Space (NRC 2007), we are poised on the brink of a series of improvements in ocean measurements from space that will revo- lutionize oceanography from space in the next decade—as big an advance or bigger than the advent of ocean altimetry with TOPEX/Poseidon. This white paper looks forward to that timeframe and beyond, towards the type of measurements we should expect in 2025, and the science questions that we should be able to ask. OvERARCHING SCIENCE qUESTIONS The scientific and practical questions that are likely to drive develop- ments in the 2025 timeframe are: • Oceans as part of the coupled ocean-atmosphere-ice-land- biogeochemical system – Most current ocean models that assimilate data are presently run in ‘forced’ mode; they do not affect the atmosphere. Most atmospheric models that assimilate data use an overly sim- plified representation of the oceans (a mixed layer) or worse, only sea surface temperature. These are due to computational expense, a barrier that is fast receding. – CO2 uptake, hurricanes, ENSO, ice shelf disintegration and ice sheet advance are all examples where the coupling between the ocean and in these cases either the atmosphere or the cryo- sphere are critical. • Description and prediction of the global water cycle in the con- text of global climate change can only be fully realized when the marine branch of the hydrological cycle is considered. • Increased spatial and temporal resolution in ocean observations, ocean models, and climate models. – Spin up / spin down time scales in the oceans depend on eddy (~ 100 km or less) parameterization. These time scales are essen- tial for climate forecasts. Thus climate models need to resolve or parameterize properly ocean eddies for realistic climate simula- tions (Marshall, personal communication, 2008). – In ocean models, dissipation of momentum is achieved through enhanced vertical viscosities and drag laws with little physical validation. Turbulent transport of tracers like heat, salt, carbon and nutrients is represented with unphysical constant eddy diffusivities in numerical ocean models. Ocean models run- ning at sufficient resolutions to address submesoscale (1-100 km) dynamics have just begun to emerge (Capet et al. 2008).

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9 OCEANOGRAPHY IN 2025 Global observations at these scales are needed to constrain the models. – For coastal work, forecasting for navigation, inundation, and marine resources critically depends on short length scales, con- trolled by the shallow ocean depth. • Need to forecast with increasing accuracy and shorter time delays both short time scales (navigation, harmful algal blooms) and long ones (climate) for societal benefit. To address these questions, we believe a progressive improvement in measurement capability is necessary, across a broad range of parameters, as outlined in Table 1 and the discussion below. TECHNOLOGICAL ADvANCES The kind of technology advances that will enable the improvements in ocean measurements from space described above include: • Miniaturized, more efficient radar components to reduce mass/ power needs of radar electronics • Efficient, high-power transmitters at shorter wavelengths (espe- cially Ku- and Ka-Band) • Increased onboard processing and/or downlink capability, allow- ing data acquisition at higher spatial and temporal resolution. • Larger deployable antennas in the 6-12 m range, particularly employed in a conical scan mode, enabling higher resolution radiometry and scatterometry. • A scanning interferometer pair of antennas, rotating through an azimuth scan of 360 degrees to provide along-track interferom- etry measurements of surface currents at high resolution. • Precision formation flying, to enable bistatic wide-swath sea sur- face height measurements from two platforms flying in forma- tion, and gravity measurements from multiple platforms. • Laser interferometry to improve the accuracy of gravity measure- ments from future GRACE-like missions. • Wide field of view imaging spectrometers with improved stability and signal to noise (SNR) and atmospheric correction capabili- ties, enabling global ocean biosphere measurements at moderate resolution (~1 km) on a daily basis and on fine resolution (60 m) on synoptic basis. • Deployment of ocean color imagers on geostationary platforms to sample the highly dynamic processes of coastal ecosystems. • The spaceborne implementation of active remote sensing of bio-

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TABLE 1. State of Ocean Measurements from Space in 2009, in 2017, and in 2025 Measurement from Space 2009 2017a 2025 Sea Surface Height (SSH) 100 km spatial scales; 10 day 10 km spatial scales; 10 day 10 km spatial scales; 1 day revisit revisit (TOPEX/Jason series) revisit (SWOT) (Multiple SWOT satellites) Ocean Vector Winds 25 km spatial scales; 1-2 day 3-25 km spatial scales; 6 hour 3-6 km spatial scales; 6 hour revisit (OVW) revisit (Quikscat/ASCAT) revisit (XOVWM/ASCAT/ (XOVWM and follow-ons) Oceans II) Sea Surface Salinity 0.2 psu; 150-200 km spatial 0.2 psu; 40 km spatial scales; 0.1 psu; 20 km spatial scales; 7 day (SSS) scales; 30 day time scale (SMOS 10-30 day time scale (SMAP) time scale (Aquarius follow-ons) 2009; Aquarius in 2010) Surface Currents Geostrophic only—100 km Geostrophic cf. SSH Geostrophic cf. SSH spatial scales; 10 day revisit Ageostrophic in coastal Ageostrophic globally—< 1 km; (Topex/Jason series) zones—< 1 km (DLR 1-2 day revisit (Scanning ATI) No ageostrophic. Tandem-X) Gravity 400 km spatial scales; monthly Improved precision; 400 km Improved precision; <400 km updates (GRACE) spatial scales; monthly updates spatial scales; weekly updates (GRACE II + GOCE) (GRACE follow-ons) 1000 km spatial scales; 0.4 100 km spatial scales; 0.4 gCm– 10 km spatial scales; 0.1 gCm–2yr–1 CO2 Flux at the Surface gCm–2yr–1 flux; monthly 2yr –1 flux; weekly updates; flux; daily updates (ASCENDS updates (AIRS/OCO) day/night (ASCENDS) follow-ons) 95 continued

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9 TABLE 1. Continued Measurement from Space 2009 2017a 2025 Sea Surface Temperature 1-2 km spatial scales; 1-day 1-2 km spatial scales; <1-day 1-2 km spatial scales; <1-day (SST) revisit; no visibility thru’ cloud revisit; no visibility thru’ cloud revisit; no visibility thru’ cloud (MODIS) (VIIRS on NPOESS) (VIIRS on NPOESS) 40 km spatial scales; 1-day 40 km spatial scales; 1-day 1-2 km spatial scales; 1-day revisit; revisit; all-weather (AMSR-E); revisit; all-weather (AMSR-E); all-weather (Next-gen Microwave DT = 0.3 – 0.7 K DT < 0.3 K radiometers); DT < 0.1 K Ocean Color/ < 1 km spatial scales; 1 day < 1km spatial; <1 day revisit .1–1 km spatial scales; 15 min Biogeochem revisit (CZCS/Seawifs/ (VIIRS on NPOESS) revisit globally (GEO network) MODIS/MERIS) 50 m spatial; 17-day revisit (HyspIRI) .25 km spatial; 15 min revisit (GEOCAPE) 1 km spatial; 1 day revisit (ACE) Sea Ice Area and Type 3 day revisit (Radarsat/ 1 day revisit (DESDynI) 1 day revisit (DESDynI follow-ons) Envisat) Sea Ice Thickness Freeboard @ < 1 km scales Freeboard @ < 1 km scales Ice thickness @ < 1 km scales (TBD) (Icesat II + Radarsat/Envisat) (Icesat II + DESDynI) Snow accumulation (Cryosat) Snow Accumulation (TBD) aThe projections for 2017 assume that the relevant missions in the National Research Council’s decadal survey for Earth Science and Applications from Space (NRC 2007) are implemented on schedule.

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9 A. FREEMAN chemical constituents of the ocean, including fluorescence spec- troscopy instruments (UV/visible) and lasers at the blue end of the visible spectrum to measure mixed layer depth, as input to biochemical models. • Increased computational power will allow more complex coupled models to be run at higher resolutions, and data assimilative models to assimilate data. ACKNOWLEDGEMENTS The work described in this paper was, in part, carried out by the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. Spe- cial thanks to V. Zlotnicki, T. Liu, L-L. Fu, B. Holt, R. Kwok, S. Yueh, I. Fukumori, J. Vazquez, D. Siegel, and G. Lagerloef for contributing to this white paper. REFERENCES NRC. 2007. Earth Science and Applications from Space: National Imperaties for the Next Decade and Beyond. National Academies Press, Washington, D.C. Capet, X., J.C. McWilliams, M.J. Molemaker, and A.F. Shchepetkin. 2008. Mesoscale to Submesoscale Transition in the California Current System. Part II: Frontal Processes. Journal of Physical Oceanography. 38: 44-64.