Calibration and Validation Studies for Aquarius Salinity Retrieval Shannon Brown and Sidharth Misra Jet Propulsion Laboratory, California Institute of Technology 7th Aquarius SAC-D Science Meeting 11-13, April 2012 Buenos Aires, Argentina

Project Overview Developing methods to track Aquarius calibration over Antarctica and rainforest regions –Enables characterization of drift into gain and offset components –Ensures well calibrated brightness temperatures over full TB range Important for other applications (e.g. soil moisture) –Follows approach developed for altimeter radiometers (Topex, Jason) Investigation of roughness correction algorithms –Evaluating v1.2.3 roughness correction –Assessing dependence on sea state through match-ups with radar altimeters

Aquarius Ocean Drift Represents drift only at one TB – need another reference to determine whether it is a gain or offset drift –Offset drift means constant drift at all TBs (e.g. front end path loss drift) –Gain drift means drift largest at cold TBs that approaches zero for warm scenes (e.g. ND drift) 3 Horn 1 V-pol Horn 1 H-pol Horn 2 V-pol Horn 2 H-pol Horn 3 V-pol Horn 3 H-pol

Natural On-Earth Calibration Targets for Stability Tracking Rainforest (warm end TB ~280K) –Select depolarized heavily vegetated areas within the Aquarius swath –Use TMI and WindSat to determine canopy temperature to track Aquarius calibration 4 6 V-H Antarctica (mid-range TB ~200K) –Select areas with stable temperature (V-pol TB) and snow structure (H-pol TB) –Radiative transfer model used to determine L-band TB over time using in-situ temperature and higher frequency microwave observations as input

Aquarius L-band Temporal Stability (Sep’11-Feb’12) 5  28.7 o  37.8 o  45.6 o V-pol H-pol V-pol H-pol V-pol H-pol

Ice Model Temperature profile determined from in-situ surface temperature data coupled with a heat transport model to determine T(z,t) AMSR-E GHz V&H-pol observations constrain ice structure, ice dielectric model and thermal diffusivity MEMLS model (Wiesmann and Matzler, 1999) used to compute upwelling TB 6

Drift over Antarctica 7 Horn 2 V pol Horn 2 H pol - Aquarius minus Model over Antarctica - Scaled ocean drift Aquarius – model TB shows drift that is approximately half the ocean drift for all channels which is consistent with an instrument gain drift Computed Aquarius – Model TB for each channel

Rainforest Regions Select regions in Amazon and Congo that exhibit small polarization signature at 6 and 10 GHz and contain Aquarius swath –Opaque canopy obscures the surface –Exhibit little polarization or incidence angle dependence –Brightness temperature closely tracks canopy physical temperature 5 regions identified Amazon Region TMI 10 GHz De- polarization Congo Region TMI 10 GHz De-polarization 6 V-H ~ Canopy Temperature Variations ~ Vegetation properties variations 0.05 K 2 K

Drift over Amazon Used simple parametric model to estimate Aquarius TBs from TMI/WindSat 10.7 GHz TB time series to estimate residual drift over warm rainforest regions –TMI and WindSat data filtered for rain using flag based on GHz TB difference –Applied 30-day smoothing Negligible drift observed over warm regions – consistent with gain drift Rainforest regions show potential to monitor drift at the warm end

Roughness Correction Investigation 10

Generating database of Aquarius match-ups with Jason-1/2 radar altimeters –Find altimeter observations that fall within Aquarius footprint separated by < 1 hour –Analyze roughness correction as a function of altimeter WS, SWH Database used to evaluate v1.2.3 roughness correction 11 Aquarius/Altimeter Match-ups Number of match-ups per 1 o bin – all horns

2-dimensional lookup table: [wind speed, σ 0 ] → ΔT B 2-dimensional lookup table: [wind speed, σ 0 ] → ΔT B V1.2.3 Surface Roughness Correction: Wind Speed + Scatterometer σ 0 Meissner and Wentz, Aquarius Cal/Val Workshop March 2012

V1.2.3 Excess TB Compared to Altimeter Winds Compared V1.2.3 specular TB (rad_TB_rc) to model using ancillary SST,SSS Generally unbiased with respect to altimeter wind speed –0.1K level biases observed in some channels for winds 15m/s 13 Excess TB-V Bias vs Alt. WS Excess TB-H Bias vs Alt. WS

V1.2.3 Excess TB Compared to Altimeter Winds Standard deviation typically between K (slightly higher for H- pol) Standard deviation minimum near 9m/s - larger for low and high winds –More pronounced for H-pol 14 Excess TB-V StdDev vs Alt. WS Excess TB-H StdDev vs Alt. WS

V1.2.3 Excess TB Compared to Altimeter WS/SWH Binned v1.2.3 model differences vs altimeter WS and SWH Roughness correction underestimated for low winds/high waves Over-estimated for high winds/low waves More pronounced for V-pol 15 Low winds/high waves Low winds/low waves High winds/low waves High winds/high waves Horn 1 V-pol Horn 3 V-pol

All Channels Largest residual correlation with SWH in Horn1 (V&H) and Horn 3(V) 16 Horn1 V-pol Horn1 H-pol Horn2 V-pol Horn2 H-pol Horn3 V-pol Horn3 H-pol

V1.2.3 SSS Bias and StdDev Lowest residuals when scatterometer is viewing surface in the along-wind direction Highest residuals in cross-wind direction for high wind speeds 17 Bias (psu)Std. Dev. (psu)

V1.2.3 Excess TB vs WS & Dir Highest residuals in excess TB when scatterometer viewing in cross-wind direction Suggests that hybrid approach may reduce errors –Weight observations for L3 product based on relative wind direction –Supplement scatterometer correction in cross-winds case with ancillary data 18 Bias (psu)Std. Dev. (psu)

Summary Method developed to track Aquarius radiometer TB drift over Antarctica and Amazon –References independent from ocean –Used to determine Aquarius drift is a gain drift Evaluation of v roughness correction shows little bias with respect to altimeter WS but some residual correlation with SWH Highest residuals from scatterometer correction when Aquarius is viewing cross wind –Suggest improvements can be gained from hybrid algorithm relying more on ancillary data in cross-wind cases 19

Level of gain vs offset drift will depend on which components are changing Example: Drift in noise diode brightness creates gain drift Largest drift for cold TBs, small drift at warmer TBs that are close to internal reference load temperature Separation of Gain vs Offset Drift 22 ND drift will cause TB drift over Antarctica which is ~0.5 x ocean drift TB drift will approach zero over warm regions

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Tracking Aquarius TB over Rainforest Ferrazzoli and Guerriero (1999) and other modeling studies show that for high biomass, optical depth is >>1 and emissivities (defined by scattering from the forest) are within few %) between L-band, C-band and X-band Track variation of L-band TB as a function of time from TMI and WindSat 10.7 GHz observations for morning passes (4-7 LST) –Annual surface temperature variations less than 2K in the morning over these regions so a few percent uncertainty in scaling factor not critical for estimating temporal variability 24

Drift over Rainforest Heavily vegetated regions act like pseudo-blackbodies –Opaque canopy obscures the surface –Exhibit little polarization or incidence angle dependence –Brightness temperature closely tracks canopy physical temperature Estimated canopy TB over rainforest regions as a function of time from TMI and WindSat 10.7 GHz observations for morning passes (4-7 LST) Negligible drift observed over warm regions – consistent with gain drift 25 5 Regions identified in Amazon and Congo which exhibit little polarization signature from GHz See Brown and Misra talk Thursday at 16:10 for details

Tracking Aquarius at Warm TBs Differenced Aquarius TBs from TMI/WindSat 10.7 GHz TB time series to estimate residual drift over warm rainforest regions –TMI and WindSat data filtered for rain using flag based on GHz TB difference –Applied 30-day smoothing Congo region 1 showed largest temporal variability (correlated in both TMI and Aquarius data) V-pol Horn 1 All Regions

Warm Reference Over warm regions (TB ~ 280K) we should see almost no drift in Aquarius TBs if it is a gain drift Heavily vegetated regions act like pseudo-blackbodies (Brown and Ruf, 2005) –Opaque canopy obscures the surface –Exhibit little polarization or incidence angle dependence –Brightness temperature closely tracks canopy physical temperature –TB near 280K weakly scattering canopy

Developing On-Earth T B Calibration References at L-band Natural targets for L-band radiometer calibration over on-Earth dynamic range –Calm, flat ocean scenes – Cold reference –Ice sheets: Antarctica, Greenland – Mid-range reference –Land areas: flat, dry deserts; homogeneous heavily vegetated regions – Hot reference Use to assess absolute calibration, monitor stability and assess residual instrument calibration errors 37 V-H 23 V-H 18 V-H 10 V-H 6 V-H AMSR-E De-polarization

Stability of Regions TBs stable to ~2K over these regions over several years –C-band to Ka-band highly correlated 6.9 GHz polarization difference stable to 0.05K –Vegetation microwave properties vary little with time AMSR-E TBs Amazon Region 1 April 2008 – October 2010 AMSR-E 6.9 GHz TBV-TBH April 2008 – October K

TB with Model – All Channels All channels in good agreement with model after applying gain drift correction based on ocean TA drift estimates 30 V1.1 TB not corrected for drift TB after applying gain drift correction (v1.2.3)