What Is Microwave SST, and Why Is It Blurry?

Microwave SST sensors measure the sea surface at a wavelength that passes through non-raining cloud (the one thing infrared sensors can't do), but the physics that makes that possible also caps their sharpness at roughly 25km per pixel, more than ten times coarser than an infrared pass.

The sensor: AMSR2 aboard GCOM-W

Chlorofishy's microwave SST comes from AMSR2, flying aboard Japan's GCOM-W satellite. Unlike VIIRS or MODIS, which read infrared radiation that cloud blocks almost completely, AMSR2 reads a microwave frequency band that clouds are largely transparent to. GHRSST's own product documentation classifies AMSR2's output as a subskin sea surface temperature, the temperature at the base of the ocean's thin conductive layer, a genuinely different physical quantity from the skin temperature an infrared sensor reads, though the two normally track closely.

Why the pixel is so much bigger

Seeing through cloud requires a longer wavelength, and a longer wavelength requires a physically larger antenna footprint to resolve any given ground area; there's no way around that tradeoff with current instruments. The result is a real, uncorrectable resolution ceiling around 25km, compared to roughly 1–2km for VIIRS or MODIS. AMSR2 also has a documented coastal exclusion: readings within roughly 75km of a coastline are contaminated by land in the antenna's field of view and aren't usable. Chlorofishy never stretches this coarser data to look sharper than it is. See native resolution, explained.

Where microwave SST actually helps

Because it isn't blocked by cloud, AMSR2 is one of the inputs Chlorofishy's SST composite blends in to fill gaps a cloud-covered infrared pass would otherwise leave. See how cloud-free SST charts work. It's the right layer for tracking a large-scale feature (a warm eddy, a broad temperature gradient) through several days of cloudy weather, and the wrong layer for pinpointing a sharp break, which its 25km footprint simply can't resolve.

See the sharper VIIRS and MODIS passes on the SST map, or the full sensor list at data sources. Back to Learn.