Comparison of Valid Ocean Observations Between MODIS Terra and Aqua Over the Global Oceans

Abstract

Ocean color satellite missions to measure the biophysical and geochemical properties of the surface ocean need to consider not only the spectral and spatial requirements of the sensors but also the satellite overpass time to maximize valid observations. The valid observations are impacted not only by cloud cover but also by other perturbations such as sun glint and stray light. Using Level-3 global composites of three ocean products (chlorophyll a or Chl-a, normalized florescence line height or nFLH, and sea surface temperature or SST), the daily percentage valid observations (DPVOs) over the global oceans were calculated, from which the differences between MODIS Aqua (afternoon pass) and MODIS Terra (morning pass) have been analyzed. For all three products, Aqua shows more valid observations than Terra over the Southern Ocean, the ocean near Peru and Chile, and the ocean around Angola and Namibia, with relatively >30% more valid observations in boreal winter months due to lower cloud coverage in the afternoon. In contrast, more than 20% of valid Chl-a and nFLH observations are obtained by Terra in the North Indian Ocean, and 10%–30% more valid observations by Terra are also found for the Equatorial Pacific and Atlantic oceans. These can be possibly linked to the lower presence of sun glint for Terra. Compared with Chl-a and nFLH, SST retrievals are more tolerant to sun glint and other perturbation factors, leading to much higher DPVOs. The implications of these findings to future satellite mission design and field campaigns are also discussed.