Use of AVHRR Imagery to Study Sea Surface Reflectance in Turbid Coastal Waters

Sea surface reflectance is important to study trends and variability in turbid coastal waters. Trends and variability can be due to climate and episodic weather forcing. For example, heavy precipitation can result in strong runoff associated with turbid river outflow. Global climate events, such as, El Niño/Southern Oscillation (ENSO) have different effects over different areas of the U.S. — rain and flooding in the West Coast and Southwest, while precipitation is often deficient, although not quite a drought, along the East Coast. Storms along the coast can cause stirring, hence, re-suspension of the sediments particularly in shallow estuaries and coastal areas. AVHRR is suited to study sea surface reflectance in turbid coastal areas. Although it has only two reflected light bands (red at 580-680 nm and near-infrared, "nir," at 720-1100 nm), the red band is relatively sensitive to backscatter and relatively insensitive to absorption by various pigments, which absorbs mostly blue light. The near-infrared band allows a pixel-based atmospheric corrections, as we will show later. Since, the AVHRR was designed for land and atmospheric studies, it does not saturate over bright targets and can adjust faster to changes in target brightness. This sensor characteristic makes it useful in coastal measurements. Although AVHRR is less sensitive compared to other color sensors and is suitable only for turbid water environments, perhaps the most favorable aspect of AVHRR is that its longest record of satellite data.