Detection of Ice and Mixed Ice–Water Pixels for MODIS Ocean Color Data Processing

Current data processing for deriving ocean color products from the Moderate Resolution Imaging Spectroradiometer (MODIS) has no specific ice and mixed ice-water pixel detection procedure. The near-infrared (NIR) reflectance threshold at the MODIS 869-nm band, which has been used to discriminate clear sky from clouds (cloud masking) for standard ocean color data processing, can eliminate most of the ice pixels. However, there are still many cases for which the ice and mixed ice-water pixels have been misidentified as ocean waters in current ocean color data processing, leading to errors in the MODIS-derived ocean color product (e.g., chlorophyll-a concentration). This is particularly true for most of the mixed ice-water cases. For atmospheric correction using the short-wave infrared (SWIR) method, which also uses SWIR reflectance for cloud masking, the problem of ice misidentification is even worse. In this paper, we describe a method for detection of ice and mixed ice-water pixels for MODIS ocean color data processing. Using the MODIS-derived normalized water-leaving radiances at 412, 555, and 859 nm, a scheme for ice and mixed ice-water detection has been developed and tested for producing MODIS global ocean color products. In fact, the proposed algorithm is a by-product calculated from the MODIS-derived normalized water-leaving radiance spectra data. Thus, the MODIS-derived ice surface radiance data can be used to study sea ice physical and optical properties. With the new ice detection scheme, pixels with ice and/or mixed ice-water can be discriminated, flagged, or masked out. The ice detection results are compared with the MODIS ice map product produced from the MODIS land discipline team, as well as the ice product data obtained from the NOAA National Ice Center. We show improved results from the new masking algorithm for the purpose of MODIS ocean color data processing, particularly for detection of mixed ice-water pixels.