Calibration and Validation Studies for Aquarius Salinity Retrieval

PI: Shannon BrownCo-Is: Shailen Desai and Anthony Scodary

Jet Propulsion Laboratory, California Institute of Technology

Project Objectives

• Perform a rigorous on-orbit calibration of the Aquarius and MWR radiometers and retrieval validation– Our objective is to identify the cause of any observed calibration or

retrieval error or instability in order to apply a suitable correction at the appropriate level of processing

– Ensures well calibrated brightness temperatures for other applications (e.g. soil moisture)

– Follows approach developed for altimeter radiometers (Topex, Jason)

• Develop an empirical L-band rough sea surface emission model

• Develop a sea surface roughness correction algorithm to improve salinity retrieval

• Analyses will begin with SMOS data

Calibration Approach for Aquarius and MWR

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-4

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0 20 40 60 80 100 120 140 160

JMR(VA)-GPSJMR(VA)-SSMIJMR(VA)-TMIJMR(VA)-Model

Jason Repeat Cycle

Cycle 31

Cycle 68

Monitor instrument level parameters and diagnostics

Compare brightness temperatures to natural on-Earth reference targets

Compare retrievals to in

situ ground truth or models

Instrument Level Monitoring

• Track metrics of instrument stability to detect and diagnose calibration anomalies – relative noise diode ratios

– reference load counts

– receiver gain

– relative brightness temperature stability between the different polarizations and feed horns

• Plots continuously updated and viewable via a web interface

AMR 34-18.7 GHz Ocean TBs

AMR 34-18.7 GHz Land TBs

Discontinuities indicate calibration

shifts

AMR 18.7 GHz ND Ratios

AMR 18.7 GHz Gain

Developing On-Earth TB Calibration References at L-band

• Assess natural targets for L-band radiometer calibration over on-Earth dynamic range– Calm, flat ocean scenes

– Land areas: flat, dry deserts; homogeneous heavily vegetated regions

– Ice sheets: Antarctica (e.g. Dome-C), Greenland

• Use to assess absolute calibration, monitor stability and assess residual instrument calibration errors

Compare over time

Compare versus instrument front end temperature

37 V-H

23 V-H

18 V-H

10 V-H

6 V-H

AMSR-E De-polarization

Salinity Retrieval Validation

• Form database of Aquarius co-locations with in situ data– Argo float array, the Shipboard Sensor Database

(SSD) and the Global Temperature-Salinity Profile Program (GTSPP).

• Analyze global mean differences between Aquarius retrieved salinity and in-situ measurements – Assess over time and instrument temperature

• Analyze regional differences– Assess antenna pattern correction, faraday

rotation correction and atmospheric and roughness corrections

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Ocean Rough Surface Emission Model

• Develop empirical rough surface emission/backscatter model– Function of significant wave height and surface wind speed

• Form co-located database of SMOS/Aquarius TB data and Aquarius σ0 with satellite altimeter derived SWH and WS and in situ SST and SSS measurements– Remove atmospheric contribution to determine emissivity, sigma0

– Evaluate model function for emissivity and sigma0 both globally and regionally

Significant Wave Height Surface Wind Speed

Plots from Aviso: Topex/Poseidon data

Roughness Correction Algorithm

• Accurate roughness correction algorithm is vital to ensure high-quality salinity retrievals

• Explore two types of algorithms

– Characterize σ0 to excess TB relationship for different regions classified by the statistics of the sea state.

– Algorithms that directly use ancillary wind speed and significant wave height data as inputs

• Algorithm types– Radar backscatter only

– Radar backscatter and ancillary surface wind speed

– Radar backscatter and ancillary significant wave height

– Radar backscatter, wind speed and wave height

– Only wind speed and wave height 8

• Match-ups distributed over global oceans (+/- 66 degrees)

• Significant number of match-ups with minimal temporal and spatial difference– ~170,000 match-ups per horn

for 28-day simulation9

Match-ups With Simulated Data

Number of match-ups per 1o bin – all horns

Faraday Rotation Correction

• Dual-frequency altimeter match-ups also useful for assessing Faraday rotation correction

• Match-ups cover large range of TEC values

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Summary

• We will perform studies to assess the calibration of the Aquarius and MWR radiometers and validate the retrievals

• Develop an empirical L-band rough surface emission model and use it to develop roughness correction algorithms

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