Radar Altimetry Johnny A Johannessen Nansen Environmental and

Radar Altimetry Johnny A. Johannessen Nansen Environmental and Remote Sensing Center, Bergen, Norway GEOF 334 – Spring 2010

Radar Altimetry OUTLINE PRINCIPLES OF ALTIMETRY FROM SATELLITE HEIGHT TO SURFACE HEIGHT GEOPHYSICAL PARAMETERS AND APPLICATIONS FUTURE ALITMETRY GEOF 334 – Spring 2010

Radar Altimetry OCEAN SURFACE QUANTITIES MEASURED FROM SPACE? WAVES SEA ICE CHLOROPHYLL SEA ICE THICKNESS GEOID & MDT SURFACE SALINITY SURFACE CURRENT NEAR SURFACE WIND SURFACE TEMPERATURE SEA LEVEL ICEBERG GEOF 334 – Spring 2010

Radar Altimetry Satellite altimetry coverage • Spatial coverage : Exact repeat orbits (to within 1 km) - global - homogeneous - Nadir (not swath) • Temporal coverage : - repeat period 10 days, T/P-Jason-1 35 days ERS/ENVISAT 1 measure/1 s (every 7 km) all weather (radar) TOPEX/Poseidon Sampling GEOF 334 – Spring 2010

Radar Altimetry Centimeters 100 Error Budget for altimetric missions 90 80 orbit error RA error Ionosphere Troposphere EM Bias 70 60 50 40 EMR 30 PRARE 20 TMR GPS/DORIS 10 0 Geos 3 SEASAT GEOSAT ERS T/P Jason GEOF 334 – Spring 2010

Radar Altimetry Principles of radar altimetry. h Active radar sends a microwave pulse towards the ocean surface, f = 13. 5 Ghz h Precise clock onboard mesures the return time of the pulse, t t = 2 d/c Centimetre Precision (10 -8) from an altitude of 800 – 1350 km h Measures the backscatter power (wind speed) h Measures ocean wave height GEOF 334 – Spring 2010

Radar Altimetry Physical parameters from the waveform t : Time to reach mid-power point : Distance, R x : Back slope : antenna mispointing Pu : Energy of the pulse : backscatter Coefficient, s SWH : Leading edge slope : Pb: Instrument noise Wave height t t = 2 d/c GEOF 334 – Spring 2010

Radar Altimetry Pulse Limited Footprint Full Area illuminated stays constant The full area has a radius R=(2 hcp)1/2 Position of pulse t=T t=T+p t=T+2 p t=T+3 p GEOF 334 – Spring 2010

Radar Altimetry Pulse Limited Footprint GEOF 334 – Spring 2010

Radar Altimetry Pulse Limited Footprint GEOF 334 – Spring 2010

Radar Altimetry Pulse Limited Footprint GEOF 334 – 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 GEOF 334 – 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 H 1/3). . GEOF 334 – Spring 2010

Radar Altimetry Atmospheric Pressure Forcing Evolving atmospheric pressure field with highs and lows leads to spatial and temporal variation of the sea level pressure lows high low ++ --- +++ -Sea surface Bottom pressure 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 GEOF 334 – Spring 2010

Radar Altimetry GEOF 334 – Spring 2010

Radar Altimetry SINGLE PULSE STRUCTURE GEOF 334 – Spring 2010

Radar Altimetry MULTIPLE PULSE AVERAGING GEOF 334 – Spring 2010

Radar Altimetry FROM SATELLITE HEIGHT TO SURFACE HEIGHT SSH = Orbit – Range – S Corr Orbit errors in position of satellite 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, …) GEOF 334 – Spring 2010

Radar Altimetry SSH = Geoid + dynamic topography + «noise» • hg : geoid 100 m • hd : dynamic topography 2 m • h. T : tides 1 -20 m • ha : inverse barometer 1 cm/mbar GEOF 334 – Spring 2010

Radar Altimetry OCEAN DYNAMICS FROM ALTIMETRY LARGE SCALE SSH ANOMALIES MESOSCALE VARIABILITY PLANETARY WAVES SEA LEVEL CHANGE GEOF 334 – Spring 2010

Radar Altimetry Coverage, interpolation and gridding to SSH anomalies GEOF 334 – Spring 2010

Radar Altimetry Mesoscale variability Jason-1 T/P ENVISAT Jason-1 + ENVISAT GEOF 334 – Spring 2010

Radar Altimetry Along track SSH GEOF 334 – Spring 2010

Radar Altimetry Geostrophic Currents rface Sea su West -- +++ East Pressure force horizontal plane Vertical plane North 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. Pressure force Coriolis force East West South GEOF 334 – Spring 2010

Radar Altimetry Geostrophic Currents from altimetry A With altimetry, we measure the sea surface height along a groundtrack. Geostrophic currents calculated from the alongtrack slope will be perpendicular to the groundtrack. Groundtrack A B Groundtrack B h’ v’ Groundtrack A perpendicular to slope : strong currents h’ v’ Groundtrack B parallel to slope : weak currents GEOF 334 – Spring 2010

Radar Altimetry Global Observations – Geostrophic Current GEOF 334 – Spring 2010

Radar Altimetry IMPORTANCE OF MAPPING FREQUENCY AND COVERAGE EKE estimated with 4 satellites missions (Jason-1, T/Pi, ERS-2/ENVISAT, GFO) Units are in cm 2/s 2 0 800 EKE differences between 4 and 2 satellites missions Units are in cm 2/s 2 Courtesy of CLS 0 400 GEOF 334 – Spring 2010

Radar Altimetry IMPORTANCE OF MAPPING FREQUENCY AND COVERAGE Cyclonic eddy of the Gulf Stream. 2 ALTIMETERS LEFT 4 ALTIMETERS RIGHT Courtesy of CLS GEOF 334 – Spring 2010

Radar Altimetry PLANETARY WAVES Surface Layer (warmer, lighter) Deep Layer (cooler, denser) GEOF 334 – Spring 2010

Radar Altimetry Hovmuller diagrams and propagating Rossby waves Sea Level Variance Courtesy of Remko Scharroo, DEOS, TU Delft, NL GEOF 334 – Spring 2010

Radar Altimetry Global Sea Level Change GEOF 334 – Spring 2010

Radar Altimetry SPATIAL TRENDS GEOF 334 – Spring 2010

Radar Altimetry EL NINO 1997 GEOF 334 – Spring 2010

Radar Altimetry Waveform and SWH GEOF 334 – Spring 2010

Radar Altimetry GEOF 334 – Spring 2010

Radar Altimetry Wind speed retrievals GEOF 334 – Spring 2010

Radar Altimetry Timeline GEOF 334 – Spring 2010

Radar Altimetry Computation of Mean Dynamic Topography (MSS - Geoid) Compute the geoid relative to the TP ellipsoid and in the mean tide system Substract the geoid from the mean sea surface Geoid Apply a Gaussian filter with a 400 km width MDTS MSS CLS 01 -EIGENGL 04 S cm m geoid From GUTS Study, Courtesy of Rio, 2007 GEOF 334 – Spring 2010

Radar Altimetry CRYOSAT 2 - Altimeter Thickness Observations after Laxon GEOF 334 – Spring 2010

Radar Altimetry GEOF 334 – Spring 2010

Radar Altimetry Summary GEOF 334 – Spring 2010

Radar Altimetry T H A N K Y O U GEOF 334 – Spring 2010

Radar Altimetry Principles of radar altimetry Beam limited footprint < pulse limited footprint Pulse limited Beam limited L L: antenna size : wavelength d d L (P/2)2 + d 2 = = B/2 d (d+ c 2 B=d /L P = 2(2 c d)1/2 B Pulse length = c P GEOF 334 – Spring 2010