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.