Broadband radiometry

Broadband radiometry measures the total reflected shortwave and emitted longwave radiative flux integrated across broad spectral bands, providing the basis for retrieving the top-of-atmosphere Earth radiation budget. Rather than resolving individual spectral channels, the method integrates radiance over wide wavelength windows to capture the balance between incoming solar radiation and outgoing thermal emission.

Implementations typically use two or three broadband channels covering the shortwave and total (shortwave plus longwave) portions of the spectrum, with the longwave contribution either measured directly or derived by subtracting the shortwave channel from the total channel. The Clouds and the Earth's Radiant Energy System (CERES) instrument uses three channels: a shortwave channel spanning 0.3 to 5 micrometers, a longwave window channel spanning 8 to 12 micrometers (5 to 35 micrometers on the unit flown on NOAA-20), and a total channel spanning 0.3 to 200 micrometers ^[1]. CERES has flown on TRMM, Terra, Aqua, Suomi NPP, and NOAA-20, with reported radiometric accuracy of about 1 percent for the shortwave channel and 0.5 percent for the longwave and total channels ^[1].

The Geostationary Earth Radiation Budget (GERB) instrument takes a two-channel approach: a shortwave channel spanning 0.32 to 4.0 micrometers and a total channel spanning 0.32 to 30 micrometers, with the longwave component derived by subtracting the shortwave channel from the total ^[2]. Flown on Meteosat Second Generation, GERB is calibrated on-board to about 1 percent accuracy and delivers measurements at the 15-minute cadence typical of geostationary observation ^[2].

Broadband radiometry is cloud-tolerant: because it measures integrated flux rather than resolving cloud structure, it can operate continuously across varying atmospheric conditions. The method requires a minimum of two broad spectral bands, and implementations span wavelengths from the near-ultraviolet through the far infrared.