Characteristics of atmospheric attenuation events for Ka-band altimetry

Bruno Picard (CLS, France)


Marie-Laure Frery (CLS, France); Nicolas Picot (CNES, France); Gérald Dibarboure (CNES, France); Nathalie Steunou (CNES, France)

Event: 2018 Ocean Surface Topography Science Team Meeting

Session: Instrument Processing: Propagation, Wind Speed and Sea State Bias

Presentation type: Type Poster

Satellite altimetry missions measure the sea surface height (SSH) at a global scale and an increasing accuracy since 1992. The altimeter signal is traditionally emitted in the Ku-band at around 13.5 GHz as used by Topex (13.6 GHz) and Poseidon (13.65 GHz) altimeters up to the latest Jason-3 (13.6 GHz) and Sentinel-3 (13.575 GHz) series.
Atmospheric attenuation (∆σ0 ) at Ku-band has been specifically characterized since the very beginning of the spatial altimetry era, Ku-band being mostly affected by rain.
The CNES/ISRO SARAL/AltiKa mission launched in 2013 was the first attempt to use a central frequency in the Ka-band at 35.75 GHz, and the future CNES/NASA Surface Water and Ocean Topography (SWOT) mission to be launched in 2021 will rely on the Ka-band radar interferometer (KaRin).
Using a higher frequency allowed to reduce the footprint of the altimeter from about 10 km at Ku-band to about 5 km at Ka-band, but also to improve the range noise and the impact of the ionosphere. The major counterpart is a larger sensitivity to the water in the atmosphere: the altimeter backscattering coefficient atmospheric attenuation is 7 times larger at Ka-band than at Ku-band.

A paper covering the following topics is about to be submitted.
The present paper aims at anticipating as accurately as possible what could be expected in terms of data availability regarding atmospheric attenuation for the future Ka-band missions.
A complete study on the sensitivity of the Ka-band atmospheric attenuation to atmospheric components is performed. Indeed, the work by Lillibridge et al. in 2009 on this subject missing the impact of rain events when the works of Quartly and Tournadre focus only on the liquid and rain components. As for Tournadre et al. 2015, our analysis is also based on SARAL/AltiKa measurements. But on the contrary to this study, the present one is not dedicated to develop a new rain flag. Without any assumption on the signal-to-noise ratio of the future instruments, the objective here is to provide information that could be extrapolated to their imaging or multi-incidence capability and to serve as guidelines adaptable to any future configurations.

The first part of this paper is dedicated to the sensitivity of Ka-band atmospheric attenuation to all the four atmospheric components, dry, wet, liquid and rain. It is conducted at global scale for a typical day in order to anticipate what could be the statistic of the attenuation without any consideration on the orbit, the swath or the resolution of the altimetry missions. The use of ECMWF forecast insures the consistency between these sources of attenuation and the limits of the representation of clouds and rain in the model are discussed by comparison to satellite-based observations. The occurrences of atmospheric attenuations events and their global distribution are discussed.
In a second part, a simple detection algorithm of atmospheric attenuation cell based on the analysis of SARAL/AltiKa altimeter backscattering coefficient is described. It is then compared to existing clouds/rain flag in order to validate the representativeness of this approach.
Finally, the amplitude of atmospheric attenuation events and their respective sized are discussed in terms of occurrences and geographical distribution.

Poster show times:

Room Start Date End Date
Foyer, Salao Nobre & tent Thu, Sep 27 2018,18:00 Thu, Sep 27 2018,20:00
Foyer, Salao Nobre & tent Fri, Sep 28 2018,14:00 Fri, Sep 28 2018,15:00
Bruno Picard