| Abstract: |
The development of the L-band missions SMOS (ESA) and Aquarius (NASA) occurred in the last decade has offered the opportunity to map the sea surface salinity from satellite-borne sensors with both spatial and temporal accuracy unattained before. Indeed, L-band is the most suitable frequency band available to the remote sensing community for this specific topic. Recent studies showed that the availability of microwave radiometers operating at even lower frequencies (e.g. down to P-band such as UWBRAD [1]) could improve the current performances [2]. As reported in [3], the sensitivity of the brightness temperature Tb to the sea surface salinity has a dynamic range of about 5K; thus every further contribution to the Tb in the observed scenario must be taken into account accurately. Among these contributions, one may recall the cosmic microwave background, the sun-emitted and the moon-reflected radiation, and the self-emission of the atmosphere [4]. Whereas the first three contributions are well assessed and they can be estimated with the required accuracy, the last one is variable in itself as it depends on the continuously changing atmospheric conditions in term of water content, in both vapor or liquid phase, and occurrence of precipitation. A fluctuation of 0.5K due to the atmosphere can have an appreciable impact on the Sea Surface Salinity (SSS) estimations. In order to assess the impact of the atmospheric contamination on the current SSS retrieval and to contribute to the phase 0 studies of the Ultra Wide Band satellite radiometer mission Cryorad [2], an analysis has been carried out by using experimental data and theoretical models. The atmospheric status is taken from radiosonde profiles collected during 2015 and 2016 on the test site of Lihue airport, Kauai Island, HI-USA (WMO id. 91165 PHLI, 21° 58’ N, 159° 20’ W, 399m a.s.l.) [5]. The profiles database includes 804 dry and 673 with non-precipitating clouds atmospheric scenarios. Integrated Water Vapor content IWV shows an almost Gaussian distribution with average value equal to 30.2 mm and S.D. of about 8 mm; the maximum integrated Liquid Water Path (LWP) in cloudy profiles is 0.5 mm. These data were used as input to the radiative transfer model known as Tbmodel [6] to estimate the atmospheric contribution to satellite Tb measurements under well-controlled conditions. The database of predicted brightness temperature values computed at 1.4 GHz restitutes an average value of about 4.7 K with S.D. = 0.02 K as contribution of the atmospheric thermal emission along zenithal path over the two years. It should be noticed that the predicted Tb values at this frequency show only a slight variation with the IWV. Details about these simulation results and the comparison against predictions obtained with empirical laws applied to meteorological surface parameters will be shown at the conference.
[1] J. T. Johnson, “The ultrawideband softwaredefined radiometer (UWBRAD) for ice sheet internal temperature sensing: results from recent observations”, 2016 IEEE Int. Geosci and Rem. Sens. Symposium, pp. 7085-7087.
[2] G. Macelloni et al., “Preliminary study for a spaceborne ultrawideband microwave radiometer for the monitoring of cryosphere elements: the Cryorad project”, 2017 IEEE Int. Geosci and Rem. Sens. Symposium
[3] D. M. Le Vine, G. S. E. Lagerloef, and S. E. Torrusio, ” Aquarius and Remote Sensing of Sea Surface Salinity from Space”, Proc. of the IEEE, Vol. 98, No. 5, May 2010
[4] F. J. Wentz and D. M. Le Vine, “Version 2: ATBD Aquarius Salinity Retrieval Algorithm”, RSS Technical Report 082912
[5] available at http://weather.uwyo.edu/upperair/sounding.html
[6] J. A. Schroeder and E. R. Westwater, Users’ guide to WPL Microwave Radiative Transfer Software, NOOA Technical Memorandum ERL WPL-213, WPL, Boulder, CO, Oct. 1991. |
