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Ocean Surface Topography from Space
SCIENCE
Maintaining the Long Term Calibration of and Improving the Wet Tropospheric Correction from the Jason Radiometers for Climate Studies


Author:

Shannon Brown - (Jet Propulsion Laboratory)


Co-Investigator(s):
  Shailen Desai
Sidharth Misra
(California Institute of Technology - JPL, Tracking Systems and Applications Section)
(California Institute of Technology - JPL)

Abstract:


We propose to provide the best possible multi-decadal record of wet tropospheric path delay correction from the microwave radiometers on the Topex and Jason series altimeter missions through comprehensive calibration/validation analyses and algorithm improvements. This record will in turn contribute to a seamless record of altimetric global sea surface height (SSH) over the same period.

July 2012 will mark the dawn of the second decade of precision satellite ocean altimetry started by Topex/Poseidon and continued by the Jason series. As the altimeter record grows, long-term climate studies become feasible and the importance of identifying and reducing systematic mm-level errors in the data increases. The wet path delay measurement provided by the radiometers on the altimeter missions has been shown to be one of the largest sources of error in the global mean sea level time series requiring meticulous calibration and validation (Ablain et al., 2009). The first objective of the proposed work is to continue producing the climate quality long term wet path delay calibration for the Jason series radiometers (Jason 1/2 and 3 after launch) and making these data available to the science team on an annual basis. We will continue to apply and improve upon techniques developed by the proposal team to maintain mm-level stability of the retrieved path delay. In particular, we will focus on understanding and reducing the uncertainty of the radiometer calibration stability on shorter time scales (less than 3 years) to enable those studies evaluating accelerations or decelerations in regional and global mean sea level. This will be accomplished by developing additional independent methods to assess the time variability of the calibration which when combined with previously developed techniques will reduce the overall uncertainty in the long term calibration stability. Our goal is to ensure that the wet path delay measurements from the maintain accuracies of better than 1 cm per sample, less than 0.3 mm/year in global averages, and better than 5 mm per month in regional averages over a few hundred kilometers.

The second objective is to inter-calibrate the Jason-3 radiometer with the Jason-1 and Jason-2 radiometers after launch during the calibration phase of the Jason-3 mission. For this task, the proposal team will draw on their extensive experience performing the initial on-orbit calibration/cross-calibration between the radiometers on Topex and Jason-1 and between those on Jason-1 and Jason-2 (Brown et al., 2004; Desai and Haines, 2004; Zlotnicki and Desai, 2004; Brown et al., 2007, Brown et al., 2009; Sibthorpe et al., 2011; Brown et al., 2012). While a Jason-2/3 tandem mission greatly simplifies the cross-calibration process, the calibration techniques that we use do not require it to maintain a well inter-calibrated record. After the Jason-3 calibration phase, we will begin routine monitoring of its stability and apply the techniques that we have developed to maintain the long term calibration of the Jason-3 radiometer.

The third objective of the proposed work is to develop a second generation near land wet path delay algorithm, building upon the algorithms already developed by the proposal team (Brown, 2010). The algorithm previously developed, which was adopted for operational use by Jason-2, was tuned globally. Improvements can be made by tailoring the algorithm regionally. In this way, the algorithm can factor in regional relationships in the land emission. Our ultimate goal is to approach the same level of performance near land as is achieved in the open ocean.

Finally, we will leverage this well calibrated multi-decadal radiometer record to produce and distribute climate data records of integrated water vapor and cloud liquid water, both of which are important climate variables. This will contribute to studies evaluating climate feedback mechanisms between the ocean and atmosphere.



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