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Yueh, S.H., Tang, W., Fore, A., Freedman, A.P., Neumann, G., Hayashi, A., and Lagerloef, G.S.E. (03-Dec-12)

The Aquarius/SACD satellite was launched on June 10, 2011, and the Aquarius instrument has been operational since August 25, 2012. Aquarius is a combined passive/active L-band microwave instrument developed to map the salinity field at the surface of the ocean from space. The primary science objective of the Aquarius mission is to monitor the seasonal and interannual variation of the large scale features of the sea surface salinity (SSS) field in the open ocean with a spatial resolution of 150 km and a retrieval accuracy of 0.2 psu globally on a monthly basis. The measurement principle is based on the response of the L-band (1.413 GHz) sea surface brightness temperatures to sea surface salinity. To achieve the required 0.2 psu accuracy, the impact of sea surface roughness (e.g. wind-generated ripples and waves), along with several additional factors impacting the observed brightness temperature, must be corrected to better than a few tenths of a degree Kelvin. To this end, Aquarius includes a scatterometer to help correct for this surface roughness effect.

In this paper we describe the characteristics and error estimates of the ocean surface wind vector and SSS retrieved using the Combined Active-Passive (CAP) algorithm, which does not use the NCEP wind for correcting Aquarius' brightness temperatures. Error estimates using the triple collocation analyses of SSM/I, NCEP and Aquarius-CAP winds indicate that the retrieved Aquarius wind speed accuracy is excellent with a random error of about 0.75 m s-1 for rain-free conditions. For rainy conditions when there were no SSM/I wind retrievals, we used the WindSAT-AW or ASCAT winds together with NCEP and CAP winds for triple collocation analysis. It is shown that Aquarius CAP wind speed is the most accurate under rainy conditions with RMS error below 2 m/s at 10 mm/h rain rate. In comparison with the NCEP wind direction, the Aquarius wind direction retrievals also appear accurate for above 10 m s-1. We further examine the data acquired from passes over storms and hurricanes. There is a good agreement between the Aquarius-CAP wind and the NOAA HWind analysis for hurricane Katia in 2011 with the maximum wind speeds differing by less than 3 m/s. The results suggest that Aquarius can provide excellent surface wind speed products for all weathers.

For the accuracy assessment of Aquarius-CAP SSS, comparison was made with the Hycom model and the in-situ SSS data collocated by the Aquarius Data Validation System. It is estimated that the monthly average accuracy is about 0.5 psu for the data produced using the current calibration algorithm. It is expected the accuracy will improve over time as the calibration algorithm becomes more mature. We have also examined statistics of the retrieved Aquarius SSS for numerous storms under rainy conditions; the results seem to suggest consistent lower Aquarius salinity than Hycom and in situ SSS.