H43F-01 INVITED
Estimating continental hydrology parameters from existing space missions: the need for a dedicated surface water mission
Different instruments on board Earth observing satellite missions that were designed either for ocean missions or land surface classification have been used to retrieve continental surface hydrology parameters. Conventional altimeter profilers that have been designed for measuring the ocean surface topography provide limited use for surface hydrology. Analysis of conventional altimeter time series over lakes and rivers clearly indicates superimposed seasonal and interannual variabilities while the synergy of altimeter water height estimate with the water extent provided by radiometers is a means of estimating water volume variations. The synergy with the GRACE gravimetry mission, which estimates the variations of the integrated water mass, can provide estimates of the underground water mass variability. However, profiling altimetric methods of measuring water surface elevations and their changes are incapable of capturing the inherent dynamics of all continental surface waters. For example, using a profiling altimeter and a 16-day orbital repeat cycle, like that of Terra, misses about 30 percent of the rivers and 70 percent of the lakes in the global data bases. An international team is proposing the Water Elevation Recovery mission (WatER), a high-resolution, image-based approach with two-dimensional acquisitions of water surface elevations h, dh/dt, and dh/dx required to answer important hydrologic questions. A key technology of the WatER mission is a Ka-band Radar INterferometer (KaRIN) which is a near-nadir viewing, 120 km wideswath based instrument that uses interferometric SAR processing of the returned pulses to yield single-look 5m azimuth and 10m to 70m range resolution, with an elevation accuracy of approximately 50 cm. Polynomial based averaging of heights along the water body increases the height accuracy to about 3 cm. The entire globe is covered twice every 16 days and orbit subcycles allow the average visit to be about half this time at low to mid-latitudes, and almost daily at high latitudes missing very few lakes or rivers: 1 percent for 16-day repeat and 7 percent for 10-day repeat. KaRIN has a rich heritage based on (1) the many highly successful ocean observing radar altimeters, (2) the Shuttle Radar Topography Mission (SRTM), and (3) the development effort of the Wide Swath Ocean Altimeter (WSOA). The WatER concept has been selected but not funded by ESA in the Earth Explorer program but will be jointly submitted to upcoming NASA opportunities. WatER is designed to meet high priority targets for all nations and will provide essential data for the EU Water Framework Directive and the European Flood Alert System. WatER will meet the United Nations call for a "greater focus on water related issues", responds to the hydroclimatological needs of the International Working Group on Earth Observations, and answers the U.S. federal government call to focus on our "ability to measure, monitor, and forecast U.S. and global supplies of fresh water".
H43F-02 INVITED
Satellite Altimeter Measurements of Eddy Variability in the Ocean
The importance of mesoscale and sub-mesoscale eddies to the dynamics and thermodynamics of ocean circulation is well established. While a traditional satellite altimeter in an appropriately chosen orbit is well suited to studies of large-scale ocean circulation, it can provide only a crude statistical characterization of the eddy variability (e.g., the standard deviation of sea surface height variability along the satellite ground track, or the velocity variability at the crossovers of ascending and descending ground tracks). The ocean variability that can be studied by satellite altimetry can be extended to smaller scales with a tandem or triplet altimeter mission, but much of the eddy variability still cannot be resolved. The limitations of traditional altimetry will be illustrated from previously published applications of single and multiple altimeter missions and from analysis of the high- resolution Parallel Ocean Program (POP) ocean general circulation model. Examples will be given of new oceanographic applications that will become possible with accurate, high-resolution measurements from a wide- swath satellite altimeter.
H43F-03 INVITED
KaRIN: an Instrument for Measuring High-Resolution Sea-Surface Topography and Fresh Water Extent, Stage, and Slope
Traditional nadir profiling altimeters, such as Topex, Jason, or IceSat, are incaple of fully sampling the space-time signatures of both ocean mesoscale and submesoscale phenomena and changes in river discharge. To overcome this limitation, we present an instrument concept, the Ka-band Radar Interferometer (KaRIN), which is able to provide the appropriate space-time sampling to sample these phenomena with a height and slope accuracy suitable to resolve topographic signatures for both ocean and land hydrology applications. Although ocean and hydrlogic applications are quite different, the required sampling characteristics are similar. Both applications require global coverage up to high latitudes (78deg). Measurement of ocean mesoscale and submesoscale phenomena requires a temporal revisit time on the order of 10 days and a height accuracy of about 2cm over a spatial scale of 2km. The sampling of river discharge requires an approximately weekly revisit time, an ability to image water bodies (to determine extent) with a spatial resolution of 100m, a height accuracy better than 10cm and a slope accuracy of 1cm/1km, after averaging over a river area equivalent to 1km x 1km. The similarity in measurement requirements allows for the possibility of meeting both ocean and hydrology requirements with a single instrument. The KaRIN instrument builds on the interferometric SAR concept demonstrated by the NASA Shuttle Radar Topography Mission (SRTM), and the Wide-Swath Ocean Altimeter concept, which was studied by NASA as a potential complement to the Ocean Surface Topography Mission (OSTM). Two major modifications are made to these systems to achieve the desired performance: the spatial sampling requirement implies that full synthetic aperture must be used. Second, achieving the desired height and slope accuracy with a realizable spaceborne instrument requires using a Ka-band (0.8 cm wavelength) radar at near nadir incidence. To validate the science performance of the proposed system we have implemented virtual mission simulators to examine the performance over rivers, lakes, and the ocean. The river virtual mission results which will presented include simulations over a variety of river types, including the Ohio, Amazon, Lena, and Meuse. We also present virtual mission results which demonstrate the ability of the system to appropriately sample ocean mesoscale, submesoscale, tidal, and coastal phenomena.
H43F-04
Coastal Applications for High-Resolution Altimetry
The scales of mesoscale features in sea surface height (SSH) that can be resolved by fields constructed by 2-4 altimeters are of the order of 50-100km. These scales are much larger than the features considered by traditional coastal oceanography, which examines the region over the continental shelves, which can be 10-200km wide. Even over wide shelves, however, circulation features occur across narrow (10km or less) fronts, such as upwelling fronts, shelf break fronts, etc. For upwelling fronts over narrow (10-40km) shelves, as found off the western U.S. and other upwelling regions, an additional problem is the gap in data from standard altimeters next to the coast, which can also be 10-40km. Here we illustrate several examples of these structures and apply two modifications to the normal altimeter analyses. First, we combine multiple altimeters with coastal tide gauge data to bridge the coastal gap in data. Second, we use the high-frequency 10-Hz (T/P) and 20-Hz (Jason-1) data in the near-coast region off Oregon to attempt to resolve narrow SSH gradients that correspond to the SST gradients caused by upwelling. The along-track resolution of these high-frequency SSH data is similar to the highest 2-D resolution that is expected from future Wide-Swath Altimeters, providing an initial indication of what we might expect from the future sensors.
H43F-05
Mapping Ageostrophic Ekman Currents with Imagery and Altimetry
Three 7-year data sets of sea-surface velocities in the California Current are compared. The velocities are calculated from merged altimetry data, Lagrangian surface drifter data, and data extracted from thermal satellite imagery using the maximum cross-correlation (MCC) technique. The main goal of this analysis is to determine the characteristics of the velocities that the MCC method produces; mainly at what depth the current is being measured. This question will be answered by using the ageostophic signal that is evident in the MCC data set. The existence of the ageostrophic velocity component in the MCC data set can be demonstrated by a comparison of a gridded and time-averaged merged absolute altimetry product and a similarly gridded and averaged product from MCC. The MCC velocities are found to be consistently displaced to the right of the purely geostrophic altimetric currents. This demonstrates, consistent with Ekman theory, that an ageostrophic velocity component is being measured by the MCC method. Two time dependent ageostrophic velocity data sets will be created by subtracting a merged absolute geostrophic altimetry product from the MCC and drifter data sets. These data sets will be compared with an ageostrophic velocity data set calculated from scatterometer wind stress data using a physical model of a weakly stratified upper ocean in which the Richardson number remains near unity. The ageostrophic drifter data set, which has a known depth, will be used to estimate the gradient of vertical turbulent stress. Using this information we will estimate the depth of MCC velocity measurements by comparing the rotation of the ageostrophic MCC product with the depth dependent rotation of the wind-driven scatterometer velocities. A current hypothesis of MCC measurement depth is that the method measures the average flow above the thermocline. This will be tested by comparing the estimated MCC depth from the ageostrophic analysis with the depth given by XBT data. If this hypothesis is correct, the MCC product could be used to test the validity of the physical model used to calculate wind driven ocean currents and to better quantify the vertical exchange coefficient. The employment of a wide-swath altimeter would further this analysis by removing the intrinsic one dimensional measuring capabilities of the current altimeters. This would allow for a more reliable analysis of the time dependent characteristics of wind-driven ocean currents.
H43F-06
Prospects for river discharge and depth estimation through assimilation of swath-altimetry into a raster-based hydraulics model
New methods of measuring surface water elevations from space have the potential to revolutionize the type, frequency, and spatial scale of observations that are available globally. One complication is that satellite altimeters can measure surface water elevation directly with high accuracy, but streamflow of river discharge is not amenable to direct measurement. The most effective use of satellite-based methods for this purpose may be through data assimilation. Hydrology and river hydraulics models can be used to model surface water profiles, then adjusted as satellite altimetry observations become available, and subsequently fed forward to estimate discharge. We describe our use of a synthetic data assimilation framework to evaluate the potential for such a strategy. Simulated surface water elevation profiles for a reach of the Ohio River were ingested by the Jet Propulsion Laboratory Instrument Simulator to represent satellite swath measurements of surface water altimetry fields with errors representative of those that would be inherent in observations from a dual-sensor Ka-band wide swath altimeter that is being considered jointly by U.S. and European agencies. We used the Ensemble Kalman filter, with a raster-based river hydraulics model, LISFLOOD-FP, as its dynamical core, to assimilate the synthetic observations. The filter was able to recover water depth and discharge successfully from a corrupted LISFLOOD-FP simulation by assimilation of the synthetic water surface observations. A simple autoregressive error model was used to correct boundary inflows, and which increased the persistence of the assimilation's benefits in time. In addition, the sensitivity of water depth and discharge estimation errors with assumed observation errors and assimilation frequency (i.e. satellite overpass frequency) was examined. Results showed modest sensitivity to the observation error magnitude within the range explored (which was dictated by plausible instrument performance); while as expected the shorter update frequency simulation gave the best overall results.
H43F-07 INVITED
The Peace-Athabasca Delta: A Potential Testbed for Hydrologic Altimetry
The Peace-Athabasca Delta of Northern Alberta is, at 6000 sq km, among the world's largest inland freshwater deltas. It is internationally recognized as a global center of biodiversity and is both a RAMSAR convention wetland and a United Nations world heritage site. The delta is composed of a series of wide and extremely shallow ( < 2 m deep) lakes interconnected by a network of distributaries from the Peace and Athabasca Rivers and interspersed with disconnected wetland basins. Topographic slope is very low in most of the delta, with water surface elevations ranging between 213 and 206 m.a.s.l. The delta is largely ice-covered from October through April, but during the open water season water levels are quite variable, with influences coming through changing inputs from the major delta rivers as well as from within the delta. Most notably, wind- driven seiche events on Lake Athabasca can result in rapid increases or decreases in water level of up to 1 m in much of the delta. Because of its extensive open water area, large temporal variability in water surface elevation, and international prominence as a pristine biodiversity hotspot, the Peace-Athabasca Delta represents an ideal location to test future advances in wide-swath altimetry. Ground-based water level monitoring in the Delta has improved in recent years, with a network of 32 pressure-transducer water level monitors with cm-scale accuracy deployed during the summer of 2006 along with two meteorological stations. Such a network, encompassing the majority of the delta, would allow for extensive validation of wide swath altimetry instruments in a variety of different hydrologic environments including river channels, disconnected floodplain basins, and large connected lakes.