GEOF334 – Spring 2010 Radar Altimetry Johnny A. Johannessen Nansen Environmental and Remote Sensing Center, Bergen, Norway

GEOF334 – Spring 2010 Radar Altimetry OUTLINE PRINCIPLES OF ALTIMETRY FROM SATELLITE HEIGHT TO SURFACE HEIGHT GEOPHYSICAL PARAMETERS AND APPLICATIONS FUTURE ALITMETRY

GEOF334 – Spring 2010 Radar Altimetry OCEAN SURFACE QUANTITIES MEASURED FROM SPACE? SEA LEVEL CHLOROPHYLL NEAR SURFACE WIND WAVES SURFACE TEMPERATURE ICEBERG SEA ICE SURFACE CURRENT SURFACE SALINITY SEA ICE THICKNESS GEOID & MDT

GEOF334 – Spring 2010 Radar Altimetry Spatial coverage : TOPEX/Poseidon Sampling - global - homogeneous Temporal coverage : - repeat period 10 days, T/P-Jason-1 35 days ERS/ENVISAT 1 measure/1 s (every 7 km) all weather (radar) Satellite altimetry coverage - Nadir (not swath) Exact repeat orbits (to within 1 km)

GEOF334 – Spring 2010 Radar Altimetry Error Budget for altimetric missions orbit error RA error Ionosphere Troposphere EM Bias 100 Centimeters Geos 3SEASATGEOSAT ERST/P Jason EMR PRARE TMR GPS/DORIS

GEOF334 – Spring 2010 Radar Altimetry  Measures the backscatter power (wind speed)  Measures ocean wave height  Active radar sends a microwave pulse towards the ocean surface, f = 13.5 Ghz  Precise clock onboard mesures the return time of the pulse, t t = 2d/c Centimetre Precision (10 -8 ) from an altitude of 800 – 1350 km Centimetre Precision (10 -8 ) from an altitude of 800 – 1350 km Principles of radar altimetry.

GEOF334 – Spring 2010 Radar Altimetry Energy of the pulse : backscatter Coefficient,  Pu  Back slope : antenna mispointing   Leading edge slope : Wave height SWH : Time to reach mid-power point : Distance, R  Instrument noise Pb: t Physical parameters from the waveform t = 2d/c

GEOF334 – Spring 2010 Radar Altimetry Pulse Limited Footprint t=T t=T+p t=T+2p t=T+3p t=T t=T+p t=T+2p t=T+3p Position of pulse Full Area illuminated stays constant The full area has a radius R=(2hcp) 1/2

GEOF334 – Spring 2010 Radar Altimetry Pulse Limited Footprint

GEOF334 – Spring 2010 Radar Altimetry Pulse Limited Footprint

GEOF334 – Spring 2010 Radar Altimetry Pulse Limited Footprint

GEOF334 – Spring 2010 Radar Altimetry Sea State Effects Electromagnetic bias The concave form of wave troughs tends to concentrate and better reflect the altimetric pulse. Wave crests tend to disperse the pulse. So the mean reflecting surface is shifted away from mean sea level toward the troughs. Mean Sea Level Mean Reflecting Surface

GEOF334 – Spring 2010 Radar Altimetry Sea State Bias Skewness bias For wind waves, wave troughs tend to have a larger surface area than the pointy crests – the difference leads to a skewness bias. Again, the mean reflecting surface is shifted away from mean sea level toward the troughs The EM Bias and skewness bias (= Sea State Bias or SSB) vary with increasing wind speed and wave height, but in a non-linear way. SSB is estimated using empirical formulas derived from altimeter data analysis (crossover, repeat-track differences and parametric/non-parametric methods). The range correction varies from a few to 30 cm. EM bias accuracy is ~2 cm, skewness bias accuracy is ~1.2 cm. Empirical estimation of the SSB also includes tracker bias (depends on H1/3)..

GEOF334 – Spring 2010 Radar Altimetry Atmospheric Pressure Forcing Sea level rises (falls) as the low (high) pressure systems pass. The inverse barometer effect implies that 1 mbar of relative pressure change leads to a 1 cm sea level change Evolving atmospheric pressure field with highs and lows leads to spatial and temporal variation of the sea level pressure lows low Sea surface Bottom pressure high

GEOF334 – Spring 2010 Radar Altimetry

GEOF334 – Spring 2010 Radar Altimetry SINGLE PULSE STRUCTURE

GEOF334 – Spring 2010 Radar Altimetry MULTIPLE PULSE AVERAGING

GEOF334 – Spring 2010 Radar Altimetry FROM SATELLITE HEIGHT TO SURFACE HEIGHT Precision of the SSH : Orbit error Errors on the range Instrumental noise Various instrument errors Various geophysical errors (e.g., atmospheric attenuation, tides, inverse barometer effects, …) SSH = Orbit – Range –  Corr Orbit errors in position of satellite

GEOF334 – Spring 2010 Radar Altimetry SSH = Geoid + dynamic topography + «noise» h g : geoid100 m h d : dynamic topography2 m h T : tides1-20 m h a : inverse barometer1 cm/mbar

GEOF334 – Spring 2010 Radar Altimetry OCEAN DYNAMICS FROM ALTIMETRY LARGE SCALE SSH ANOMALIES MESOSCALE VARIABILITY PLANETARY WAVES SEA LEVEL CHANGE

GEOF334 – Spring 2010 Radar Altimetry Coverage, interpolation and gridding to SSH anomalies

GEOF334 – Spring 2010 Radar Altimetry ENVISAT Jason-1 Jason-1 + ENVISAT T/P Mesoscale variability

GEOF334 – Spring 2010 Radar Altimetry Along track SSH

GEOF334 – Spring 2010 Radar Altimetry Sea surface Pressure force Vertical plane West East horizontal plane West Pressure force Coriolis force North South Geostrophic Balance Geostrophic Balance : Horizontal gradients in the pressure field create a downgradient force. On a rotating earth this is balanced by the Coriolis force. N Hemisphere : high P is to the right of the flow. S Hemisphere : high P is to the left of the flow.

GEOF334 – Spring 2010 Radar Altimetry Geostrophic Currents from altimetry With altimetry, we measure the sea surface height along a groundtrack. Geostrophic currents calculated from the alongtrack slope will be perpendicular to the groundtrack. A Groundtrack A h’ v’ B Groundtrack B h’ v’ Groundtrack A perpendicular to slope : strong currents Groundtrack B parallel to slope : weak currents

GEOF334 – Spring 2010 Radar Altimetry Global Observations – Geostrophic Current

GEOF334 – Spring 2010 Radar Altimetry EKE estimated with 4 satellites missions (Jason-1, T/Pi,ERS-2/ENVISAT,GFO) Units are in cm 2 /s 2 EKE differences between 4 and 2 satellites missions Units are in cm2/s Courtesy of CLS IMPORTANCE OF MAPPING FREQUENCY AND COVERAGE

GEOF334 – Spring 2010 Radar Altimetry Cyclonic eddy of the Gulf Stream. 2 ALTIMETERS LEFT 4 ALTIMETERS RIGHT Courtesy of CLS IMPORTANCE OF MAPPING FREQUENCY AND COVERAGE

GEOF334 – Spring 2010 Radar Altimetry Surface Layer (warmer, lighter) Deep Layer (cooler, denser) PLANETARY WAVES

GEOF334 – Spring 2010 Radar Altimetry Sea Level Variance Courtesy of Remko Scharroo, DEOS, TU Delft, NL

GEOF334 – Spring 2010 Radar Altimetry Global Sea Level Change

GEOF334 – Spring 2010 Radar Altimetry SPATIAL TRENDS

GEOF334 – Spring 2010 Radar Altimetry EL NINO 1997

GEOF334 – Spring 2010 Radar Altimetry Waveform and SWH

GEOF334 – Spring 2010 Radar Altimetry

GEOF334 – Spring 2010 Radar Altimetry Wind speed retrievals

GEOF334 – Spring 2010 Radar Altimetry Altimetry Timeline

GEOF334 – Spring 2010 Radar Altimetry Apply a Gaussian filter with a 400 km width MSS CLS01-EIGENGL04S Computation of Mean Dynamic Topography (MSS - Geoid) Substract the geoid from the mean sea surface MDTS m cm Compute the geoid relative to the TP ellipsoid and in the mean tide system Geoid From GUTS Study, Courtesy of Rio, 2007 geoid

GEOF334 – Spring 2010 Radar Altimetry CRYOSAT 2 - Altimeter Thickness Observations after Laxon

GEOF334 – Spring 2010 Radar Altimetry

GEOF334 – Spring 2010 Radar Altimetry Summary

GEOF334 – Spring 2010 Radar Altimetry THANKYOUTHANKYOU

GEOF334 – Spring 2010 Radar Altimetry Principles of radar altimetry Beam limited Pulse limited Beam limited footprint < pulse limited footprint  L L:antenna size :wavelength Pulse length = c   L = B/2d B=d /L BP d d (P/2) 2 + d 2 = (d+ c  2 P = 2(2c  d) 1/2