Twilight Irradiance Reflected by the Earth Estimated from Clouds and the Earth's Radiant Energy System (CERES) Measurements

The upward shortwave irradiance at the top of the atmosphere when the solar zenith angle is greater than 90° (twilight irradiance) is estimated from radiance measurements by the Clouds and the Earth's Radiant Energy System (CERES) instrument on the Tropical Rainfall Measuring Mission (TRMM) satellite. The irradiance decreases with solar zenith angle from 7.5 W m−2 at 90.5° to 0.6 W m−2 at 95.5°. The global and daily average twilight irradiance is 0.2 W m−2, which is three orders of magnitude smaller than the daily and global average reflected irradiance at the top of the atmosphere. Therefore, the twilight irradiance can be neglected in global radiation budget estimate. The daily average twilight irradiance, however, can be more than 1 W m−2 at polar regions during seasons when the sun stays just below the horizon for a long period of time.