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Wide-Swath Altimetric Measurement of Water Elevation on Earth: The Surface Water and Ocean Topography (SWOT) Mission|
2. Science objectives
2.1 Oceanography: Mesoscale and Submesoscale Processes
2.1.1 Eddies, fronts, and filaments (R. Morrow, D. Chelton)
2.1.2 3-D Upper Ocean Circulation (P. Klein, R. Ferrari)
2.1.3 Eddy-mean flow interaction (B. Qiu)
2.1.4 Cascade and Dissipation of ocean kinetic energy (J. McWilliams)
2.1.5 Biogeochemical processes (M. Levy)
2.1.6 Coastal Processes (T. Strub)
2.2 Terrestrial Hydrology: Storage Change and Discharge
2.2.1 Global water cycle (Aaron Boone)
2.2.2 Lakes (Yongwei Sheng)
2.2.3 Reservoirs and human impacts (Faisal Hossain)
2.2.4 Floodplain processes (Paul Bates)
2.2.5 Arctic Hydrology (Tamlin Pavelsky)
2.2.6 Floods and flood modelling ( Paul Bates)
2.3 Additional objectives
2.3.1 Ocean tides (R. Ray, B. Arbic)
2.3.2 Ocean bathymetry (D. Sandwell)
2.3.3 Ice sheets (W. Abdalati)
2.3.4 Sea ice (R. Kwok)
2.3.5 Hurricanes and cyclones (G. Goni)
2.3.6 Sea level change (CK. Shum)
3. Measurement of water elevation
3.1 Radar interferometry
3.2 Media effects
3.3 Ocean tides
3.4 Ocean waves
3.5 Vegetation effects
3.6 Target layover
3.7 Waterbody delineation
3.8 Systematic errors and calibration
4. A mission to map the water elevation on Earth
4.1 Science requirements
4.2 Mission design issues
4.2.1 Orbit and sampling issues
4.2.2 Precision orbit determination
4.2.3 Science payload
4.2.4 Data products
4.2.5 Data downlink and processing
4.2.6 Calibration and validation
4.2.7 Expected measurement performance
Satellite radar altimetry has revolutionized oceanography by providing, for now 18 years, global measurements of ocean surface topography [e.g., Fu and Cazenave, 2001]). The long-term observations of large-scale circulation and heat storage of the global oceans have led to significant advances in our understanding of the interaction of ocean circulation with climate (e.g., El Niño and La Niña). Radar altimetry has also provided the first high-precision sea level measurements with global coverage (e.g, Nerem et al., 2006). Coupled with space gravity measurement and in-situ observations, satellite altimetry observations were analyzed for separating natural climate variability effects from human-induced changes in sea level, identifying most vulnerable coastal areas and improving climate models used for sea level projections. In the late 1990s, radar altimetry has also been used to measure surface waters levels on land (i.e., lakes, rivers and floodplains), extending its applications to land hydrology (e.g., Milly et al., 2004, Alsdorf et al., 2006, 2007). However, a critical limitation for both ocean dynamics and land hydrology, however, is the 200- to 300-kilometer spacing between satellite orbital tracks, causing the missing of small-scale features in oceanography (e.g., currents and oceanic mesoscale processes) and a large number of surface water bodies on land (small lakes, and reservoirs, wetlands, most and rivers, etc.).
In physical oceanography, several issues cannot be addressed from currently available radar altimetry measurements. First, the strongest currents of the ocean (e.g., Gulf Stream and Kuroshio) have spatial scales on the order of 100 km in the cross-stream direction. These currents and their meanders and eddies carry most of the kinetic energy of the ocean. The present generation of satellite altimeters can resolve part, but not all, of this eddy energy. Without this information, questions are left open, regarding the effects of ocean currents and eddies on global climate. Second, the stirring and mixing of ocean properties at scales of 10-100 km, which are presently not sampled globally, is an important process of the transport of mass, heat, salt and nutrients in the ocean. Third, substantial vertical transfers of the ocean properties mentioned above also take place at these scales. This vertical exchange is an important process in many parts of the global oceans. In the coastal oceans, upwelling and cross-shelf circulation have a strong effect on marine life, ecosystems, and waste disposal. In the open ocean, it is estimated that about 50 % of the vertical transfer of nutrients in the ocean takes place at the submesoscales of 10-100 km (Lapeyre and Klein, 2006). This is of critical importance to understanding the role of the oceanic circulation and ecosystem in climate change.
In contrast to ocean observations, land surface water measurements are limited mostly to in situ networks of gauges that record water surface elevations at fixed points along river channels. Globally, the spatial and temporal distribution of water stored on land surface and moving through river channels is poorly known. Furthermore, water movement in wetlands and across floodplains throughout the world is essentially unmeasured, significantly limiting our understanding of flood processes. In situ networks measuring river flows are declining worldwide due to economic and political reasons, affecting not only developing countries but also developed countries. While radar altimetry over surface waters has demonstrated the potential of this technique in land hydrology, a number of limitations exist, however, because altimetry has been optimized for observing the ocean surface. Raw radar altimetry echoes reflected from the land surface are complex, with multiple peaks caused by multiple reflections from water, vegetation canopy and rough topography, resulting in much less valid data over land than over the ocean. Another major limitation is the large inter track distance preventing good coverage of rivers and other small water bodies.
To meet important new goals in physical oceanography and land surface hydrology, high-resolution measurement of the ocean surface topography and the elevation of water on land from space are urgently needed. Based on the heritage of the Shuttle Radar Topography Mission (SRTM), scientific and technical studies at JPL over the past 10 years have refined the wide-swath interferometric altimetry concept for high-resolution mapping of the ocean surface topography (Fu and Rodriguez, 2004). An instrument system called the Wide-Swath Ocean Altimeter (WSOA) was developed and prototyped for implementation on the Ocean Surface Topography Mission/Jason-2. Unfortunately the implementation of WSOA was canceled because of funding problems. More recently, the potential of this technique has been demonstrated for mapping terrestrial surface waters (e.g., LeFavour and Alsdorf, 2005; Kiel et al., 2006; Alsdorf et al., 2007).
In the U.S., two separate proposals –one in hydrology, one in oceanography- based on this concept were submitted in 2006 to the Decadal Survey conducted by the National Research Council (the National Academies, USA). In Europe (jointly with U.S. scientists), studies were conducted during the past 2-3 years to define a wide-swath altimetry mission for land hydrology. In response to the recommendation by the Decadal Survey, the physical oceanography and the land surface hydrology communities have now joined together in develpiongdeveloping a new mission named SWOT (Surface Water and Ocean Topography) based on the concept of wide-swath interferometric altimetry for high-resolution mapping of the elevation of water on Earth. The measurement of SWOT will address two key aspects on the problem of climate change: the role of the oceanic mesoscale and submesoscale processes in regulating climate change; the consequence of climate change on the distribution of water on land.
The purpose of this document is to provide an overall description of the science objectives and anticipated advances, the measurement approach and requirements, as well as mission design issues in relation to science requirements. The document will serve as the basis for the mission’s science requirements and mission design trade-offs.
2. Scientific Objectives
The primary science rationales for the development of SWOT are to make high-resolution, wide-swath altimetric measurement of the ocean surface topography and the elevation of water on land for making fundamental advances in the understanding of the oceanic mesoscale and submesoscale processes and the spatial and temporal distribution of the storage and discharge of water on land. In addition to the two primary objectives in oceanography and land hydrology, the measurement of SWOT will have applications to a host of other topics in science and application. These “additional objectives” are not considered main drivers for the mission. Significant resources will not be utilized in order to meet them. However, within the realm of mission design without incurring significant risk and cost, meeting these objectives are to be accommodated with best possible effort.
2.1 Oceanic Mesoscale and Subesoscale Processes
Ocean surface topography
The oceans distribute heat, salt, nutrients, and other chemicals around the world. The circulation of the ocean is therefore vital to understanding climate change and other societal issues related to the oceans. Direct measurement of ocean current velocity is difficult owing to the turbulent nature of the flow. What is of interest for studying ocean circulation at scales larger than 10 km, however, is not the instantaneous velocity at every point of the ocean. Instead, a spatially and temporally averaged velocity field is required. Such smoothed flow has a special property called geostrophic balance; namely, the ocean velocity can be determined by the gradient of the pressure field at the ocean surface. This surface pressure field can be computed from the elevation of the ocean surface above the geoid, the ocean’s equi-geopotential surface. The sea surface elevation relative to the geoid is called the ocean surface topography, which provides a very effective (if not the best) approach to computing the large-scale, low-frequency surface current velocity of the ocean. This geostrophic component of ocean circulation varies vertically in relation to the density distribution in the ocean. Therefore, ocean surface topography is an important dynamic boundary condition for determining the three-dimensional structure of ocean circulation.
Ocean surface topography is only a minor departure from the geoid, which has a range of about 200 m relative to a reference ellipsoid. Therefore, to first order sea surface elevation essentially represents the geoid. Defined as the departure from the geoid, ocean surface topography varies by a range of about 3 meters, and contains information about ocean currents. The measurement of the shape of sea surface thus has profound applications to oceanography, geodesy, and geodynamics.
The new challenges for SWOT
The measurement of ocean surface topography by satellite radar altimeters over the past 18 years has made fundamental advances in our understanding of the large-scale ocean circulation and its role in climate change. However, as in the atmosphere, ocean circulation is dominated by turbulent eddies (Robinson, 1983). The most energetic ocean eddies have scales of ~100 km, the mesoscale, which is about 10 times less than the scale of atmospheric storms. Even with combined data from multiple altimeters, the ocean eddy field has not been well sampled by existing altimetry missions. Figure 1 shows the characteristics of spatial and temporal sampling by multiple conventional nadir altimeters in comparison to that by WSOA and SWOT.
Based on the observations from the TOPEX/Poseidon-Jason1 tandem mission, which provided only suboptimal sampling of the eddy field, Sharffenberg and Stammer (2009) reported that the eddy kinetic energy overwhelmingly dominated the total kinetic energy of ocean circulation (Figure 2). Note that the eddy energy has been underestimated from this data set. Ocean model simulations have suggested that only by including realistic eddies in the model can the simulated oceanic heat transport approach observations (Smith et al., 2000). We need observations that fully resolve ocean eddies for improving models for studying the effects of the ocean in climate change.
The sampling offered by SWOT down to 1 km scale is way beyond that by combinations of conventional altimeters, providing a unique opportunity for studying oceanic variability from the mesoscale to the submesoscale. While conventional altimetry has addressed the large-scale ocean variability associated with the density and mass distribution of the ocean, SWOT will address the small-scale energetic processes responsible for the maintenance and dissipation of the energy of ocean circulation. Figure 3 illustrates the features of sea surface temperature and ocean color at the various scales observed by infrared and visible channel sensors. While these sensors provide information about the sea surface, SWOT will provide SSH information that can be used to link surface observations to subsurface processes.
The current state of knowledge
Observations made by satellite altimeters since 1980s have provided progressively improved views of the global ocean mesoscale eddy field (Le Traon and Morrow, 2001). Shown in Figure 4 is a snapshot of global SSH anomalies from combined data from Jason-1 and Jason-2, revealing the ubiquitous ocean eddies of scales larger than about 200 km. In parallel to these observations, ocean models have also progressed from coarse-resolution, highly dissipative mesh grids to higher resolutions where mesoscale eddies dominate the solutions. We are now able to produce simulations of the present state of the ocean which compare increasingly well to observations. Figure 5 displays a comparison of the standard deviation of ocean surface topography variability between altimetry observation and simulation by an eddy-permitting model at resolution of 18 km produced by the ECCO-2 Project, showing reasonable agreement. Shown in Figure 6 is a snapshot of the speed of global ocean surface currents simulated by a model running at much higher resolution of 1/16 deg in latitude and longitude (~ 7 km at the equator and decreasing with latitude) as part of the ECCO-2 Project, showing the ubiquitous presence of mesoscale and submesoscale features – currents, eddies, fronts, and filaments. Most of the small eddies, fronts and filaments are not resolved by the available altimetry observations.
However, the skill of the state-of-the-art models in making long range predictions of the ocean is still very limited, because they lack a physically-based representation of the submesoscales, i.e. scales of 10-100 km that are important for turbulent transport and energy dissipation. Ocean models running at sufficient resolutions (less than 1 km) to address submesoscale dynamics have just begun to emerge (e.g. Capet et al., 2008), but we need global observations at these scales to guide the model development.
Conventional nadir-looking radar altimeters have a footprint on the order of 2-10 km. Even with thousands of pulses averaged over 1 second, the noise level of the sea surface height (SSH) measurement is substantial, making ocean SSH signals at wavelengths less than 100 km not well observed. A typical wavenumber spectrum of SSH deviations from a time mean, sampled along a long satellite pass (from the Jason Mission) from Bering Sea to Drake Passage in the Southern Ocean, is shown in Figure 7a (from Fu and Ferrari, 2008). At wavelengths longer than 100 km, the spectrum shows a typical “redness” with power density increasing with wavelength. The spectral slope levels off at wavelengths shorter that 100 km, showing the dominance of measurement noise at the submesoscales. However, very high resolution models that resolve the submesoscale (Capet et al., 2008) show a cascade of energy from the mesoscale to the submesoscale, such that the ocean spectra remains “red” down to kilometric wavelengths (Fig. 7b).
When the noisy measurements along nadir tracks are smoothed and merged to produce two-dimensional maps (e.g., Figure 4), the spatial resolution is on the order of 200 km even with combined data from two altimeters (Ducet et al., 2000). This resolution is not even sufficient to resolve the details of the two-dimensional structure of ocean currents like the Gulf Stream and Kuroshio, whose cross-current dimension is on the order of 100 km. Although combined data from TOPEX/Poseidon and ERS have been used extensively to study the characteristics of ocean eddies (e.g., Chelton et al, 2007), the size of the eddies have been limited to diameters larger than 100 km.
Mesoscale eddies larger than about 100 km are effective in transporting ocean properties (nutrients, heat, salt) horizontally in the upper ocean, as illustrated by Figure 8. Ocean variability at the submesoscales in the form of fronts and filaments is most effective in vertical transport of ocean properties between the upper layers of the ocean and the deep ocean (Lapeyre and Klein, 2006). This vertical transport of ocean properties is important to understanding the ocean’s role in climate change, in terms of the rate of oceanic uptake of heat and CO2. The vertical transport of nutrients is important to the biogeochemical cycle of the ocean that also has important effects on climate.
The breakthrough of SWOT
To make an order of magnitude advance in resolution for resolving the submesoscales, the measurement noise must be less than signal at a wavelength of 10 km as shown by the horizontal dashed line in Figure 7, in which the SSH spectrum is extended from the power law to wavelengths of 10 km. The threshold of noise level corresponds to a power density of 1 cm/cycle/km, about two orders of magnitude less than that of the Jason-1 altimeter. This performance in SSH measurement translates to a geostrophic velocity error of 3 cm/sec at 10 km scale at 45 degree latitude. The two dimensional SSH maps from SWOT will then allow the study of the submesoscale ocean eddies, fronts, narrow currents, and even the vertical velocity at these scales. Described below are the science objectives enabled by SWOT in the oceanic mesoscale and submesoscale, including physical processes in the open ocean as well as the coastal zones, the high-latitude oceans, and biogeochemical-physical interactions.
Lapeyre, G, and P. Klein, 2006: Impact of the small-scale elongated filaments on the oceanic vertical pump. J. Mar. Res., Vol. 64, 835-851.
Figure 1. Sampling characteristics of satellite altimetry missions. The various straight lines represent the combination of different number of satellite altimeters. The ellipse is a schematic representation of the oceanic submesoscales.
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