In this study, we examine the sensitivity of surface irradiances to cloud properties derived from satellite sensors. We computed surface irradiances in two different ways, one using cloud properties solely from MODerate resolution Imaging Spectroradiometer (MODIS), and the other using MODIS data supplemented with CALIPSO and CloudSat (hereafter CLCS) cloud vertical structure information [Kato et. al, 2010].
Incorporating cloud properties from CLCS into radiative transfer calculations changes cloud radiative effects (CRE) at the surface depending on cloud types. For example, when multi-layer clouds are present, the active sensors are able to detect lower clouds beneath higher layers that are not detected by MODIS. CLCS can also dertermine the cloud base height directly, whereas the MODIS estimates rely on an empirical relationship between cloud physical thickness and other cloud properties that can be derived from MODIS. From all the multilayered clouds more than 50% are composed of two cloud layers. For all the overcast CERES footprints, we separate two-layer overlapping clouds into 6 different combinations of cloud types that determined by cloud top height: High-high (HH), high-middle (HM), high-low (HL), middle-middle (MM), middle-low (ML) and low-low (LL). Among these combinations, three most frequent 2-layer cloud systems are: HL (56.1%), HM (22.3%) and HH (12.1%) over all the overcast CERES footprints with only 2 overlapped layers. We found that the MODIS-derived cloud base height is in average 2.9±2.4 km higher than the lower cloud from CLCS. Cloud base height difference (MODIS-CLCS) for three most frequent multi-layer cloud systems are, 3.3±2.3 km for HL, 3.4±2.3 km for HM and 2.8±1.6 km for HH.
When clouds are overcast over a CERES footprint, the mean difference in CRE is 1.0±24.2 Wm-2 for downward shortwave, -8.0±13.2 Wm-2 for downward longwave, and -9.0±28.3 Wm-2 for net irradiance at the surface. The mean diffefence of CRE for the most frequent two-layer overlapping cloud system (HL) is 4.6±40.0 Wm-2 for downward shortwave -7.9±57.0 Wm-2 for downward longwave.
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