Altimeter and scatterometer seminar SMHI, March 2012 Introduction to altimetry and altimetry applications Julia Figa Saldaña with special contribution from Doug Vandemark and other members of the OSTST

Altimeter and scatterometer seminar SMHI, March 2012 Outline Background Missions Measuring concept and accuracy Applications Orbits and coverage Resources

Altimeter and scatterometer seminar SMHI, March 2012 Coupling of ocean and atmosphere

Altimeter and scatterometer seminar SMHI, March 2012 Need for all weather observations - MW Radars Active microwave sensors such as altimeters and scatterometers provide all-weather Earth surface information, crucial to understand air-sea(/land) interaction processes driving surface water and heat exchange, with direct implications in seasonal and forecasting and climate modelling, as well as in environmental monitoring Specially important in high latitude areas Lecture Notes in Earth Sciences - Satellite Altimetry in Geodesy and Oceanography Eds. R.Rummel and F. Sanso, Springer Verlag, 1993

Altimeter and scatterometer seminar SMHI, March 2012 Background The altimeter is an established ocean remote sensor providing sea surface height (SSH), near surface winds, and significant wave height (SWH), globally All weather – microwave (14 GHz) Dedicated to ocean global observations Main objective – ocean dynamic topography A narrow swath, not an imager or synoptic scale mapping system Database since 1978 (SeaSAT) – climate applications Topex/Poseidon advanced the state-of-the art in many facets (since 1993, now Jason-1 and 2) Large constellation – similar issues and apps Continued work on measurement product accuracy

Altimeter and scatterometer seminar SMHI, March 2012 SSH accuracy evolution over time Oceanic signal orbit error RA error Ionosphere Troposphere EM Bias 100 Centimeters Geos 3SEASATGEOSAT ERS T/P (before launch) T/P (after launch) 843 km 115° various repeat cycles 800 km 108° 3 days 800 km 108° 17 days (ERM) 780 km 98.5° 35 days (3/168) 1336 km 66° 9.95 days EMR PRARE TMR GPS/DORIS Jason-1/2 ENVISAT Courtesy of N.Picot, CNES

Altimeter and scatterometer seminar SMHI, March 2012

How altimetry works Altimetry satellites basically determine the distance from the satellite to a target surface (altimeter range) by measuring the satellite-to- surface round-trip time of a radar pulse. Any interference with the radar response signal also needs to be taken into account. Water vapour and electrons in the atmosphere, sea state and a range of other parameters can affect the signal round-trip time, thus distorting range measurements. We can correct for these interference effects on the altimeter signal by measuring them with supporting instruments, or at several different frequencies, or by modelling them. Sea surface height measurements accurate to within a few centimetres over a range of several hundred kilometres requires an extremely precise knowledge of the satellite's orbital position SSH = (Orbit altitude - Range - corrections) Additionally, by looking at the return signal's amplitude and waveform, we can also obtain information about the surface roughness, from which we can derive wave height and wind speed over the oceans

Altimeter and scatterometer seminar SMHI, March 2012 Reference surfaces relevant to altimetry The reference ellipsoid The sea surface height (SSH) and the mean sea surface (MSS) The marine geoid is the shape of the sea surface assuming a complete absence of perturbing forces and reflects the Earth's gravitational field The dynamic topography (also called absolute dynamic topography (ADT) is the sea surface heights associated with ocean dynamics. It excludes surface ‘roughness’ such as waves and water ripples The mean dynamic topography (MDT) is the permanent stationary component of ocean dynamic topography. The sea level anomalies (SLA) (also called sea surface height anomalies (SSHA)) are the non-permanent component of ocean dynamic topography. SSH = geoid + ADT = MSS + SLA/SSHA = geoid + MDT + SLA/SSHA

Altimeter and scatterometer seminar SMHI, March 2012 Altimeter range data – global tide gauge Ocean dynamic topography – global and basin-scale (3-5 cm precision) : Ocean circulation (model assimilation) – geostrophic currents Rossby waves, Kelvin waves, Mesoscale eddies,Tides, Internal Tides Sea level rise ( mm/year accuracy) Ocean dynamic topography often corresponds to the amount of heat stored in the upper layers of the ocean. This information is useful in predicting hurricane season severity and forecasting individual storm severity, as well as in predicting basin scale events like ENSO

Altimeter and scatterometer seminar SMHI, March 2012 SSH computation – the different corrections List of main corrections to range and their source -Wet tropospheric (from on board MW radiometer) -Dry tropospheric (from NWP analyses) -SSB (from altimeter SWH) -Ionospheric (from instrument dual-frequency) The orbit correction -In near real time: On board DORIS navigation -Off-line: GPS, Laser Reflectors and DORIS The tides -Tide effects on SSH are provided in altimeter products but not derived from them. They are considered a geophysical correction, to be applied or not depending on the application

Altimeter and scatterometer seminar SMHI, March 2012 Altimeter wave height and wind Weather and sea state forecasting: wave model validation and assimilation – especially SWH in enclosed seas, where fetch effects are sometimes not well modeled/understood Inter-annual to decadal climate studies Air-sea fluxes – gas transfer & wave breaking Altimeter range correction and inter-calibration of other satellite ocean sensors Wind accuracy (~1 m/s) and SWH accuracy (~0.2 m) Good spatial resolution ~10 km Main challenge for real time applications is the coverage

Altimeter and scatterometer seminar SMHI, March 2012 SWH coverage 10-d SWH coverage from reference altimeter orbit 6-h SWH coverage with reference and polar altimeter orbits

Altimeter and scatterometer seminar SMHI, March 2012

MF-WAM model, different flavours Validation around La Reunion (no in-situ data available) with ENVISAT altimeter SWH data (courtesy of J.M.Lefevre) SWH in wave model validation

Altimeter and scatterometer seminar SMHI, March 2012 Pulse-limited Ocean Altimetry Microwave altimeter sounding Eff. Radar Pulsewidth  = 3.1 nsec Plan view at surface Radar return P(t) Track point: range to the mean surface Amplitude Time P(t)

Altimeter and scatterometer seminar SMHI, March 2012 Pulse-limited Ocean Altimetry Microwave altimeter sounding Eff. Radar Pulsewidth  = 3.1 nsec Plan view at surface Radar return P(t) Amplitude Time P(t) Slope of leading edge  significant wave height (SWH) leading edge

Altimeter and scatterometer seminar SMHI, March 2012 Pulse-limited Ocean Altimetry Microwave altimeter sounding Eff. Radar Pulsewidth  = 3.1 nsec Plan view at surface Radar return P(t) Amplitude Time P(t) Maximum  inverse of wind speed (via short wave ‘speckle’)

Altimeter and scatterometer seminar SMHI, March 2012 SWH Data Quality Dropouts/Outliers Key factor explaining drop outs is non-oceanlike waveform Reasons for waveforms that are not typical rough ocean returns Slick or very smooth water patches Transitions from land to sea Sea ice Very heavy rain Data drop out determination Look for SWH data quality flag and/or wind speed data quality Look at altimeter wind speed and at RMS of SWH and wind speed averaging Each altimeter can differ in amount of NRT SWH dropouts Typically a very small percentage of the data

Altimeter and scatterometer seminar SMHI, March 2012 Dealing with SWH outliers – can almost filter by eye NRT product SWH NRT data & Reprocessed (3 day) product SWH Simple median filter SWH in blue

Altimeter and scatterometer seminar SMHI, March 2012 OSCAR currents - Ocean Surface Current Analyses (OSCAR) – Real time -Near real time Level 2 altimeter SSH an scatterometer winds are used to create gridded field at the analysis time -Currents as linear combination of geostrophic and wind-driven (Ekman) motion -Technique tuned to represent ageostrophic motion of WOCE/TOGA 15 m drogue drifters w.r.t surface wind stress -Validation against in situ data (drifters, moored current-meters, ship ADCP sections) and operational ocean general circulation models

Altimeter and scatterometer seminar SMHI, March 2012 OSCAR currents -

Altimeter and scatterometer seminar SMHI, March 2012 Main challenges Land contamination (re-tracking) Topography (on board tracking) Atmospheric corrections (low resolution radiometer) Knowledge of local geoid Hydrology -

Altimeter and scatterometer seminar SMHI, March 2012 Hydrology -

Altimeter and scatterometer seminar SMHI, March 2012 Monitoring water level in rivers and lakes Progress in the accuracy of inland surface water levels monitoring with altimeters (represented by average RMS in height) with state-of-the-art altimeter hydrology products analysed against in situ measurements (source CEMAGREF, France. Courtesy of N.Picot, CNES) )

Altimeter and scatterometer seminar SMHI, March 2012 Use of altimetry data at the coast Main challenges: Land contamination, topography, atmospheric corrections For SSH, local conditions affecting geophysical corrections of the range are not fully taken into account in global product (tides/bathymetry/atmospheric models, Sea State Bias) Ocean dynamic signal down to sub-mesoscale – lack of accurate validation data In order to address some of those: Yearly workshops, where a lot of progress has been made! State of the art product for coastal applications (experimental) PISTACH - height-products/global/coastal-and-hydrological-products/index.html

Altimeter and scatterometer seminar SMHI, March 2012 Orbits and coverage Orbit choice considerations -Frequency of observations (revisit time) and coverage -Earth gravity potential effects (poles) -Aliasing of tides -Single/multi mission payloads -Orbit maintenance (measurement discontinuity) -Mission lifetime and de-orbiting considerations -On-board power and radar PRF Parameters: Altitude, inclination and precession, sun-synchronism

Altimeter and scatterometer seminar SMHI, March 2012 Altimeter orbits Reference altimeter orbit (T/P and the Jason series) Inclination 66 deg Altitude: ~1300 km Non-sun-synchronous Repeat cycle 10 days Polar orbit (ERS, ENVISAT, S-3) Inclination 98 deg Altitude: ~800 km Sun-synchronous Repeat cycle 35 days

Altimeter and scatterometer seminar SMHI, March 2012 Coverage – Reference Jason orbit

Altimeter and scatterometer seminar SMHI, March 2012 Coverage – Jason1 and 2 interleaved

Altimeter and scatterometer seminar SMHI, March 2012 Coverage – Jason 1, 2 and ENVISAT interleaved

Altimeter and scatterometer seminar SMHI, March 2012 Coverage

Altimeter and scatterometer seminar SMHI, March 2012 Altimeter references and contacts Principles of measurements: Chelton, D.B., J.C. Ries, B.J. Haines, L.L. Fu, P.S. Callahan, Satellite Altimetry, Satellite altimetry and Earth sciences, L.L. Fu and A. Cazenave Ed., Academic Press, 2001 Altimetry tutorial: Met. Center Data Users/Groups JCOMM ( GlobWave - NCEP – H. Tolman, J. Sienkewicz ECMWF – J. Bidlot, S. Abdalla Meteo-France – S. Lirola, J.-M. Lefevre LEGOS - COASTALT - AVISO -

Altimeter and scatterometer seminar SMHI, March 2012 Back-up

Altimeter and scatterometer seminar SMHI, March 2012 Altimeter wind speed – the energy reflected Flat river return 3-4 times greater than roughened ocean Altimeter normalized radar cross section =  0 roughly inverse with wind speed magnitude Good models up to U = 20 m/s, rms ~ 1 m/s. Gravity-capillary waves (10 cm- 5 mm)

Altimeter and scatterometer seminar SMHI, March 2012 SWH Data – waveform retracking Waveform averaging N=1 N=25 N=200 The perturbation to the altimeter “flat surface response” caused by ocean wave elevation alters the ‘shape’ of the reflected radar signal - > sea state elevation information If  = variance of the surface elevation within the footprint, then H(1/3) = SWH = Hs = 4 *  (by definition)

Altimeter and scatterometer seminar SMHI, March 2012 OSTM/Jason-2 payload. An Altimeter (Poseidon-3), provided by CNES – the main mission instrument. A Microwave Radiometer (AMR), provided by NASA – to correct the altimeter measurement for atmospheric range delays induced by water vapour. The radio positioning DORIS system, provided by CNES – for precision orbit determination using dedicated ground stations. A Laser Reflector Array, provided by NASA – to calibrate the orbit determination system. A precision GPS receiver (GPSP), provided by NASA - to provide supplementary positioning data to DORIS in support of the POD function and to enhance and/or improve gravity field models

Altimeter and scatterometer seminar SMHI, March 2012 Accuracy overview – Jason 2 Source: Jason 2 handbook

Altimeter and scatterometer seminar SMHI, March 2012 Pulse-limited Ocean Altimetry and SWH Highly accurate method of rms wave height measurement (10 cm bias, cm rms) Depends on radar receiver waveform sampling interval called range gates and stable short transmit pulse Minimum SWH tied to radar pulsewidth (order cm) Spatial resolution (footprint radius r) scales as r =c  h where h is distance to the ocean Precision gained by averaging (average from 2 Khz to 1 Hz ) Typical avgd. ocean and flat surface waveforms

Altimeter and scatterometer seminar SMHI, March 2012 SWH Data – waveform retracking and SWH estimation Deconvolve SWH from the waveform as related to a Gaussian spread (G) of leading edge Hs = SWH = 4 * Δh

Altimeter and scatterometer seminar SMHI, March 2012 Dec. 26, 2004 Tsunami observed from space

Altimeter and scatterometer seminar SMHI, March 2012 Satellite altimeter constellations

Altimeter and scatterometer seminar SMHI, March 2012 SWH Data Quality – Accuracy Long-term altimeter SWH calibration reference: see also JCOMM, ECMWF, NCEP, ESA GlobWave Summary of overall SWH data quality 1. SWH bias < 15 cm wrt buoys (usually lower) 2. Standard Deviation < cm wrt buoys < 3 cm wrt other satellite altimeters Tolman et al. NCEP 2009

Altimeter and scatterometer seminar SMHI, March 2012 Abdalla et al. – ECMWF 2009