Cloud Masking for Ocean Color Data Processing in the Coastal Regions

Abstract

The Sea-viewing Wide Field-of-view Sensor (SeaWiFS) and Moderate Resolution Imaging Spectroradiometer (MODIS) use the near-infrared (NIR) reflectance threshold at 865 nm (869 nm for MODIS) to discriminate clear sky from clouds for processing of the ocean color products. Such a simple scheme generally works well over the open oceans where Case-1 waters and maritime aerosols are usually the case. However, in coastal regions, there are often cases with significant ocean contributions at the NIR wavelengths from the turbid waters. In addition, aerosols are likely to be dominated with small particles (large Aringngstrom exponent). In these cases, the cloud-masking scheme using the NIR reflectance threshold often mistakenly identifies these scenes as clouds, leading to significant loss of coverage in coastal regions. In this paper, we propose to use the MODIS short wave infrared (SWIR) bands at either 1240 or 1640 nm for detecting clouds. Ocean is black for turbid waters at SWIR wavelengths due to much stronger water absorption. The aerosol contribution in the SWIR bands is also significantly lower for nonabsorbing and weakly absorbing aerosols with small aerosol particle size. Thus, using the SWIR reflectance threshold, the performance of the cloud-masking algorithm in the coastal region is much better than that of using the NIR band. For sensors that do not have SWIR bands (e.g., SeaWiFS), we propose to use the Rayleigh-corrected (RC) reflectance ratio value from two NIR bands in addition to the reflectance threshold at 865 nm. The clouds are spectrally flat and have lower reflectance ratio values from two NIR measurements than cases with reflectance contributions from ocean and aerosols. It was found that, corresponding to the RC reflectance threshold of 2.7% at 869 nm, the RC threshold reflectances for 1240 and 1640 nm are 2.35% and 2.15%, respectively. The cloud-masking performance with the SWIR bands in the coastal region can usually be achieved using the RC reflectance ratio value (ges 1.15 as clear atmosphere) between two NIR bands in addition to the reflectance threshold at 869 nm