MODIS Land Bands for Ocean Remote Sensing Applications

The Ocean Biology Processing Group (OBPG) at NASA's Goddard Space Flight Center provides the global processing and distribution of ocean color products from MODIS (Esaias et al. 1998), SeaWiFS (McClain et al. 1998), and other ocean color capable sensors. The fundamental measurement in ocean color remote sensing is the spectral distribution of radiance upwelling from the ocean, or water-leaving radiance, as this information can be used to derive various geophysical parameters such as chlorophyll concentration (Clark et al. 1970). However, space borne ocean color sensors actually observe the total radiance exiting the top of the atmosphere (TOA), of which at least 90% is scattered Sun light from aerosols and air molecules that never penetrated the ocean surface. The retrieval of water-leaving radiance from observed TOA radiance requires a process generally referred to as atmospheric correction. The OBPG developed the Multi-Sensor Level-1 to Level-2 code (MSL12, Franz 2006) to standardize the atmospheric correction and production of ocean color products from various space borne sensors (e.g., Franz et al. 2005). The MODIS instrument was designed with 36 spectral channels to support observation of clouds and land as well as oceans. The traditional channels used for ocean color observation are the 9 bands in the visible to near infrared (NIR) spectral regime from 412-869 nm, which have a spatial resolution of approximately 1 km at nadir. These ocean bands were designed with high sensitivity over the range of reflectance typical of open ocean observations with maritime atmospheric conditions. Over highly turbid coastal and inland waters it is possible for this dynamic range to be exceeded, such that the bands saturate and the true signal is unknown. Other bands on MODIS were specifically designed for land and cloud observations, with both increased spatial resolution and reduced sensitivity over a broader dynamic range. These land/ cloud bands overlap the spectral range of the ocean bands and extend into the short-wave infrared (SWIR), from 469 to 2130 nm, with a spatial resolution of 250 to 500-meters at nadir. A number of investigators have looked to exploit this additional information for ocean application. For example, Gao et al. (2000) developed a generalized processing code that was able to utilize the full spectral range of MODIS, and Li et al. (2003) proposed a method to use the expanded dynamic range of the land/cloud channels for the detection of suspended sediments and ocean bottom reflectance. Arnone et al. (2002) developed a technique for enhancing the resolution of inherent optical property retrievals (water absorption and backscatter) in coastal areas using the 250-meter channel at 645 nm, while other investigators have used the higher resolution bands to assess estuarine water quality (Hu et al. 2004) or identify harmful algal blooms (Kahru et al. 2004). Recently, Wang & Shi (2005) demonstrated an approach for utilizing the SWIR bands to improve the performance of the Gordon & Wang (1994) atmospheric correction algorithm over turbid or highly productive waters typically found in coastal environments.