High-Resolution Image

A new algorithm has been implemented for determining the uncertainty of cloud optical thickness and effective radius of liquid water and ice clouds using MODIS data. This algorithm, which estimates the uncertainty of cloud optical properties under the assumption of 5% uncertainty due to a combination of calibration and model error, 15% uncertainty in underlying surface albedo, and 20% uncertainty in overlying water vapor used in atmospheric correction, will be incorporated into collection 5 of the MODIS cloud optical properties retrieval for both Terra and Aqua. The figure above demonstrates the cloud optical thickness and effective radius retrieved on March 22, 2001 over Greenland and the nearby Atlantic Ocean. This figure shows (a) the true color image that is often difficult to distinguish clouds from snow and sea ice, (b) the thermodynamic phase of all clouds identified for this scene, (c) the cloud optical thickness of liquid water and ice clouds, and (d) cloud effective radius of all liquid water and ice clouds. Panels (e) and (f) show the new algorithm applied to this scene of Terra data, and shows the percent uncertainty in retrieved cloud optical thickness and effective radius (log scale).

The new algorithm shows that much larger uncertainties in cloud optical thickness occur in the Greenland sea to the east of the mainland, which are due largely to the fact that this part of the scene contains extensive sea ice with high surface albedo below the cloud. The effective radius is still retrieved with high accuracy in these regions. (Submitted by M. D. King and S. Platnick.)