Possibility of the Visible-Channel Calibration Using Deep Convective Clouds Overshooting the TTL

The authors examined the possible use of deep convective clouds (DCCs), defined as clouds that overshoot the tropical tropopause layer (TTL), for the calibration of satellite measurements at solar channels. DCCs are identified in terms of the Moderate Resolution Imaging Spectroradiometer (MODIS) 10.8-mu m brightness temperature (TB11) on the basis of a criterion specified by TB11 <= 190 K. To determine the characteristics of these clouds, the MODIS-based cloud optical thickness (COT) and effective radius (r(e)) for a number of identified DCCs are analyzed. It is found that COT values for most of the 4249 DCC pixels observed in January 2006 are close to 100. Based on the MODIS quality-assurance information, 90% and 70.2% of the 4249 pixels have COT larger than 100 and 150, respectively. On the other hand, the r(e) values distributed between 15 and 25 mu m show a sharp peak centered approximately at 20 mu m. Radiances are simulated at the MODIS 0.646-mu m channel by using a radiative transfer model under homogeneous overcast ice cloudy conditions for COT = 200 and r(e) = 20 mu m. These COT and r(e) values are assumed to be typical for DCCs. A comparison between the simulated radiances and the corresponding Terra/Aqua MODIS measurements for 6 months in 2006 demonstrates that, on a daily basis, visible-channel measurements can be calibrated within an uncertainty range of +/-5%. Because DCCs are abundant over the tropics and can be identified from infrared measurements, the present method can be applied to the calibration of a visible-channel sensor aboard a geostationary or low-orbiting satellite platform.