WaTER/Hydrosphere Mapper: A wide swath altimetry mission for hydrology and oceanography Lee-Lueng Fu Douglas Alsdorf Nelly Mognard OSTST Meeting, March 12-15, 2007 Hobart, Australia

At the Venice meeting a year ago, Hydrosphere Mapper and WaTER were separately presented as potential future missions using wide- swath altimetry technique. Via a workshop held in last October, the oceanography and land hydrology communities discussed their respective requirements and found that a single mission was able to meet the needs of both communities ( A joint oceanography and hydrology session was held in last AGU, showing the wide range of science and application opportunities provided by a joint WaTER/Hydrosphere Mission. Such a mission has been recommended as Surface Water and Ocean Topography (SWOT) mission by the NRC Decadal Survey for launch in the time frame of Progress made since the Venice Meeting

Small-scale Variability of the Ocean Unresolved by Nadir-looking Altimeter 100 km ground tracks of Jason (thick) and T/P (thin) Tandem Mission 100 km scale eddies resolvable by WSOA 10 km scale eddies Resolvable by WaTER/HM

41.9º N 42.5º N < 10 km Coastal currents have scales less than 10 km < 10 km Observations made by ADCP offshore from the US West Coast T. Strub, 2006  h ~ 5 cm  v ~ 50 cm/sec

Scott (2005) McWilliams (2006) Sub-mesoscale variability Sub-mesoscale processes are poorly observed but important to the understanding of the dissipation mechanism of ocean circulation. Radius of deformation

Altimetry SSH wavenumber spectrum Wavenumber (cycles/km) Power density (cm 2 /cycles/km) Noise level of WaTER/HM for 2 cm measurement noise at 2 km resolution Jason pass 132 (147 cycle average) Much reduced noise floor will enable the study of the spectrum at sub-mesoscales which have not been well resolved from existing data.

USGS Coverage: ~7000 gauges Gauging from Space ≠ WaTER/HM Birkett, C.M., L.A.K. Mertes, T. Dunne, M.H. Costa, and M.J. Jasinski,Journal of Geophysical Research, 107, Hirsch, R.M., and J.E. Costa, EOS Transactions AGU, 85, , Amazon: 6 M km 2, ~175,000 m 3 /s U.S.: 7.9 M km 2, Mississippi ~17,500 m 3 /s OSTP 2004: “Does the United States have enough water? We do not know.” “What should we do? Use modern science and technology to determine how much water is currently available …”  Using a radar altimeter, 32% of the rivers and 72% of the world’s lakes are not sampled Topex/POSEIDON: ~70 points

Amazonian wetlands are 750,000 km 2 Alaskan braided river K. Douce Photo Problems with 1D Stream Gauge Measurements

Measurements Required to Address Science & Applications : h,  h/  x,  h/  t, and area, globally, on a ~weekly basis ∆S =   h/  t Photos: B. Kiel, K. Frey Siberian Arctic Muskingum R., Ohio  q – Q  x =  h  t L  h ( ) Q= k  x R n 2/3 1/2 Continuity Equation A Typical Flow Law R = Hydraulic radius

Targets are Global Matthews, E. and I. Fung, GBC, 1, 61-86, Peace-Athabasca Delta: 3200 km 2 Coastal Zones Braided Rivers Reservoirs Worldwide Floods Congo River Basin: 3.7M km 2 New Orleans Ohio R. Costa Rica Canada Alaska N. Korea

WaTER/Hydrosphere Mapper A SAR interferometry radar altimeter Near-global coverage with 16-day repeat orbit Same technique as WSOA – radar interferometry Use of SAR to enhance the along-track resolution 0.5 cm measurement precision at 2 km resolution 1 micro-radian precision in mean sea surface slope at 2 km resolution No data gap near the coast Number of Observation

Tidal aliasing of sun-synchronous orbit S1 and S2 aliased to zero frequency and hence contaminate mean circulation K1 and P1 aliased to annual and K2 to semiannual periods annual semiannual Chambers et al, 2000

Internal tides from altimetry Wavenumber spectrum 100 km Ray & Mitchum (1997) Besides the intrinsic science of internal tides, they introduce 1-5 cm/sec error in ocean current velocity at mesoscales.

Two key issues for OSTST to consider The choice of orbit: Although a sun-synchronous orbit has significant cost and technical benefits, the tidal aliasing problem will significantly reduce the value of the mission to oceanographic applications. Do we need any further studies before putting this issue to bed? (to be discussed in the Tide Splinter) The wet tropospheric correction in coastal oceans: As the interferometry technique is able to provide SSH measurements next to the coast line, the land contamination of the microwave radiometer observations will limit their utility for high-resolution coastal applications.