A NEW CLIMATOLOGY OF SEA SURFACE WIND SPEED UNDER CLOUD FREE CONDITIONS FOR THE ESTIMATION OF THE IMPACT OF SUNGLINT IN GMES PHASE A SENTINEL 3

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1 A NEW CLIMATOLOGY OF SEA SURFACE WIND SPEED UNDER CLOUD FREE CONDITIONS FOR THE ESTIMATION OF THE IMPACT OF SUNGLINT IN GMES PHASE A SENTINEL 3 Mounir Lekouara 1, Bernardo Carnicero Dominguez 2 and Miguel Aguirre 2 1 National Oceanography Centre, Southampton, UK 2 EO Future missions, ESTEC-ESA, Noordwijk, The Netherlands

2 Introduction MERIS L2 RR 15/07/ h01 pink : medium_glint/high_glint yellow: haze

3 Introduction Sun glint: specular reflection of sun light by suitably tilted facets of the water surface into the sensor Extent and radiance of sunglint = f (imaging geometry, surface roughness) = f (lat, time) f ( surface wind speed ) To be significative, sun glint needs moderate to high winds = f (lat, time) MERIS L2 products: detection and correction for ocean pixels that are contaminated by sunglint, are based on the Cox Munk model (1954). It uses external knowledge of the wind speed and direction, the illumination and observation geometry. Traditionally coverage taking into account sun glint has been estimated using a constant value of wind speed (typically 7.5 m/s) identical for the whole Earth and for the whole year.

4 Introduction Optical instruments do not see through clouds. For any optical mission, we need an estimation of the value of the surface wind speed under cloud free conditions for the global determination of the impact of wind speed in the sun glint extent the proper, accurate, and optimum coverage estimation and design of the mission and instruments We expect a lower value of wind speed to simulate the sun glint, looking at cloud free areas. Phase A Sentinel 3 benefit from the outputs of this study by decreasing the degree of uncertainty on this topic. The study should replace the 7.5 m/s value with spatial / temporal climatology of wind speed wind speed under cloud free conditions

5 Data set MERIS L2 Reduced Resolution The whole archive until April years from May 2002 to April 2007 Cloud Coverage MERIS L2 cloud flag, 1.2 km resolution Instantaneous Wind Speed ECMWF analysis, auxiliary data, 19.2 km resolution Archives and Computational Capabilities The entire MERIS_RR_2 archive is accessed and processed using the Grid Processing on Demand Services and Architecture in ESRIN.

6 G POD

7 Climatology of wind speed under all type of cloud conditions Global monthly average wind speed varies from 1 2 m/s up to 15 m/s ESMWF wind speed (m/s)

8 Climatology of wind speed under cloud and haze free conditions Global monthly average wind speed varies from 1 2 m/s up to 15 m/s ESMWF wind speed (m/s)

9 Wind speed selection for sun glint extent estimation. First level of precision: global wind speed value Mean Wind Speed Standard deviation up to now 7.5 m/s global value from the study 7.15 m/s 3.38 m/s global value from the study UNDER CLOUD_HAZE FREE CONDITIONS 6.72 m/s 3.22 m/s

10 Wind speed selection for sun glint extent estimation. Second level of precision: Average wind speed over latitudinal bands

11 Wind speed selection for sun glint extent estimation. Second level of precision: Average wind speed over latitudinal bands

12 Wind speed selection for sun glint extent estimation. Monthly averaged wind speed under cloud free conditions appears to be lower than in all kind of cloud conditions from 50 south to 50 North

13 Wind speed selection for sun glint extent estimation. Third level of precision: 50x50 km monthly averaged wind speed under cloud_haze_free conditions ESMWF wind speed (m/s)

14 Wind speed selection for sun glint extent estimation. A coverage analysis is carried out, based on real wind speed data (cloud free conditions) satellite/ Sun geometry simulations Cox Munk model Estimation of the phase-a Sentinel 3 swath decrease sensitivity to wind speed Equator, September 21 WS = 7 m/s WS= 6 m/s WS= 5 m/s

15 Coverage simulations for Phase A Sentinel 3 Cox-Munk model Real WS data + Satellite/Sun geometry

16 Coverage simulations for Phase A Sentinel 3

17 Coverage simulations for Phase A Sentinel 3

18 Coverage simulations for Phase A Sentinel 3

19 Coverage simulations for Phase A Sentinel 3 geometrical coverage simulations for Sentinel 3 Number of valid observations per month

20 Coverage simulations for Phase A Sentinel 3 Effective coverage simulations for Sentinel 3: (clouds + sun glint)

21 Coverage simulations for Phase A Sentinel 3

22 Conclusions A new climatology of global ocean wind speed under cloud free and all kind of cloud conditions has been processed. This can be used for: Improvement of accuracy on the estimation of sun glint extent impact during the early definition of Remote Sensing ocean optical instruments and missions. Improvement on the worldwide estimation the coverage taking into account sun glint and clouds. Early assessment on temporal expected distribution of products. More accurate information to optimize instrument and mission design during the very early project phases

23 Acknowledgments We would like to thank the G POD team in ESRIN, in particular to Olivier Colin, Miguel Angel Rubio and Emmanuel Mathot for providing us with a well appreciated amount of support and flexibility. Marc Bouvet, Jean Paul Huot, Steven Delwart, and Jean Loup Bézy for their consistent support and expertise on a wide range of technical aspects.

24 THANK YOU