Radiative effect of cirrus with different optical properties over the tropics in MODIS and CERES observations

The radiative effects of cirrus clouds are evaluated based on observations extending over a period of six years (2000–2005) from the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Clouds and the Earth's Radiant Energy System (CERES) over the Tropics (25°S–25°N). The cloud radiative effect (CRE) shows a positive sign for optically thin cirrus (total‐column cloud optical depth: τ < 10) and a negative sign for thicker clouds ( τ ≥ 10), regardless of the region and season. Since a considerable portion of tropical ice clouds consists in thin cirrus clouds (>60%), their net effective CRE (net CRE × cloud amount) is roughly twice as strong as that of thicker clouds; the modulation of the net radiative flux by variations in thin cirrus (particularly with 1 ≤ τ < 9) dominates—up to 15 W m −2 —that by thicker clouds. These results suggest that thin cirrus has a significant effect on the tropical energy balance.