Setting of Cloud Albedo in the Atmospheric Correction Procedure to Generate the Ocean Colour Data Products from OCM-2
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SHORT NOTE
Setting of Cloud Albedo in the Atmospheric Correction Procedure to Generate the Ocean Colour Data Products from OCM-2 P. V. Nagamani & T. Preethi Latha & K. H. Rao & T. Suresh & S. B. Choudhury & C. B. S. Dutt & V. K. Dadhwal
Received: 11 March 2014 / Accepted: 29 September 2014 / Published online: 20 January 2015 # Indian Society of Remote Sensing 2015
Abstract Cloud masking is one of the primary and important steps in the atmospheric correction procedure in particular to coastal ocean waters. Cloud masking for ocean colour data processing is based on the assumption that the water reflectance is close to zero in the nearinfrared (NIR). This is valid over the open ocean, but coastal waters may have a higher NIR reflectance due to the presence of suspended sediment concentration and non-maritime aerosols. As the result, more scattering is observed in the water leaving radiance and hence sometimes the cloud-free pixels are also classified as clouds, leading to data loss. In this present study, we have analysed cloud albedo value for masking the clouds using the default cloud masking threshold (0.027) set for processing the global ocean colour sensors like SeaWiFS and MODIS and now OCM-2 in the SeaWiFS Data Analysis System (SeaDAS). The default cloud masking value of SeaDAS over masks the clouds and sometimes the turbid coastal water pixels are also getting masked when implemented over the COM-2 imageries. Hence, an exercise has been carried out to set the cloud albedo value for OCM-2 sensor and the albedo values were tested with the lower and higher values ranging from 0.015 to 5.0 as the threshold and from the analysis it is found that 0.07 is the optimum value for Ocean Colour Monitor-2 (OCM-2) sensor for masking the clouds. Using this threshold, maximum P. V. Nagamani (*) : T. P. Latha : K. H. Rao : S. B. Choudhury : C. B. S. Dutt : V. K. Dadhwal Ocean Sciences Division, OSG, Earth and Climate Sciences Area (ECSA), National Remote Sensing Center, ISRO, Balanagar, Hyderabad 500625, India e-mail: [email protected] T. Suresh National Institute of Oceanography, Goa, India
valid ocean pixels are achieved compared to the default cloud albedo mask with the value 0.027. This has been checked with both the Local Area Coverage (LAC) and Global Area Coverage (GAC) products of OCM-2 and the results were also compared with the MODIS data. Hence, it is recommended that 0.07 is the optimum cloud masking threshold for OCM-2 sensor. Keywords Oceansat-2 OCM . Atmospheric correction . Cloud albedo
8Introduction The Indian Space Research Organisation launched its first GLOBAL ocean colour sensor “Ocean Colour Monitor” on board Oceansat-2 satellite on 23rd September 2009. Since then the ocean colour data with 1 km resolution is available for the global community for free of cost. The purpose of the OCEANSAT-2, Ocean Colour Monitor-2(OCM-2) mission is to provide continuity to the OCEANSAT-1, OCM-1mission and to obtain quantitative information on the ocean colour variables e.g., (i) chlorophyll-a concentration, (ii) ver
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