GCOM: observe water and climate with complementary sensors

The Global Change Observation Mission, GCOM, links observations of the atmosphere, oceans, land, ecosystems and frozen regions. Its two main observing branches use different instruments: GCOM-W, called Shizuku, launched in May 2012, while GCOM-C, called Shikisai, launched in December 2017. Their measurements address related parts of Earth’s water, energy and carbon cycles.

Shizuku carries the Advanced Microwave Scanning Radiometer 2, AMSR2. It detects naturally emitted microwave radiation rather than transmitting a radar pulse. Several frequency channels respond differently to atmospheric and surface conditions. Those measurements support retrievals of quantities such as sea-surface temperature and sea-ice conditions, using models and assumptions appropriate to the selected product.

Shikisai carries the Second-generation Global Imager, SGLI, which observes optical wavelengths extending into the infrared. Its products address clouds, aerosols, vegetation, ocean color and snow and ice. Optical and passive microwave instruments do not supply interchangeable pictures: their spatial detail, atmospheric sensitivity and physical measurements differ.

A JAXA case study combined the two GCOM missions to examine Greenland conditions in June 2019. The comparison illustrates how different measurements can contribute to one environmental question. It does not mean that a short event alone establishes a long-term trend, or that every sensor measures identical melt conditions at the same time and resolution.

For a fictional study of seasonal sea-ice change, identify the product, geographic area, dates and quality flags before calculating a difference. A measurement footprint — open definition may include several surface types, and a missing or rejected observation is not the same as zero ice. The method used to turn radiation measurements into a physical estimate belongs with the result.

Long-term comparisons also need consistent calibration and attention to processing changes. JAXA’s GCOM-C version-release documentation describes changes to data products, providing a practical reason to retain version information. If an apparent step occurs when processing changes, investigate that possibility before attributing the whole difference to nature. A climate record is built through sustained, comparable observations and analysis, not by collecting attractive images alone.