The effect of multilayered clouds on cloud pressure retrievals in near-global MODIS data

In this study we investigate the errors in CO2-slicing cloud top pressure retrievals due to the presence of multilayered clouds. The CO2-slicing technique assumes a single cloud layer. When two cloud layers are present, the CO2 slicing retrievals result in cloud top pressures located somewhere between the upper and lower cloud layers. The errors decrease as the optical thickness of the uppermost cloud layer increases. To detect multilayered clouds, specifically for the case of thin cirrus overlying lower level water clouds, we use a technique which identifies MODIS pixels that potentially contain thin cirrus overlying lower-level water clouds. Our approach is suitable for daytime imagery and uses the 1.63 µm or 2.13 µm band reflectance, and the 8.5 and 11 µm band brightness temperatures. No existing satellite-based cloud climatologies have accounted for multilayered clouds. For this study we processed eights days, 1-8 April 2003, of global MODIS data from both Aqua and Terra for cloud top pressure, effective cloud amount, cloud thermodynamic phase, and for the presence of multilayered clouds. Near global daytime CO2-slicing results for all cloud observations are compared with results in which multilayered observations are removed. Geographic trends in multilayered cloud occurrence and CO2-slicing errors due to multilayered clouds are investigated.