Abstract
Abstract. Using 2B-CLDCLASS-LIDAR (radar–lidar) cloud classification and 2B-FLXHR-LIDAR radiation products from CloudSat over 4 years, this study evaluates the co-occurrence frequencies of different cloud types, analyzes their along-track horizontal scales and cloud radiative effects (CREs), and utilizes the vertical distributions of cloud types to evaluate cloud-overlap assumptions. The statistical results show that high clouds, altostratus (As), altocumulus (Ac) and cumulus (Cu) tend to coexist with other cloud types. However, stratus (St) (or stratocumulus, Sc), nimbostratus (Ns) and convective clouds are much more likely to exhibit individual features than other cloud types. On average, altostratus-over-stratus/stratocumulus cloud systems have a maximum horizontal scale of 17.4 km, with a standard deviation of 23.5 km. Altocumulus-over-cumulus cloud types have a minimum scale of 2.8 km, with a standard deviation of 3.1 km. By considering the weight of each multilayered cloud type, we find that the global mean instantaneous net CREs of multilayered cloud systems during the daytime are approximately −41.3 and −50.2 W m−2, which account for 40.1 and 42.3% of the global mean total net CREs at the top of the atmosphere (TOA) and at the surface, respectively. The radiative contributions of high-over-altocumulus and high-over-stratus/stratocumulus (or cumulus) in the all multilayered cloud systems are dominant due to their frequency. Considering the overlap of cloud types, the cloud fraction based on the random overlap assumption is underestimated over vast oceans, except in the west-central Pacific Ocean warm pool. Obvious overestimations mainly occur over tropical and subtropical land masses. In view of a lower degree of overlap than that predicted by the random overlap assumption to occur over the vast ocean, particularly poleward of 40° S, the study therefore suggests that a linear combination of minimum and random overlap assumptions may further improve the predictions of actual cloud fractions for multilayered cloud types (e.g., As + St/Sc and Ac + St/Sc) over the Southern Ocean. The establishment of a statistical relationship between multilayered cloud types and the environmental conditions (e.g., atmospheric vertical motion, convective stability and wind shear) would be useful for parameterization design of cloud overlap in numerical models.
Highlights
As the most important regulators of Earth’s climate system, clouds significantly affect the radiation budget, the hydrological cycle and the large-scale circulation on Earth (Hartmann et al, 1992; Stephens, 2005)
As + St/Sc has a maximum scale of 17.4 km and standard deviation (SD) of 23.5 km, while Ac + Cu has a minimum scale of 2.8 km and SD of 3.1 km
Because random cloud overlap is considered a better characterization of cloud overlap behavior than minimum overlap and maximum overlap when two cloud layers separated by clear layers, we only provide the difference in the cloud fractions between random overlap and observed overlap
Summary
As the most important regulators of Earth’s climate system, clouds significantly affect the radiation budget, the hydrological cycle and the large-scale circulation on Earth (Hartmann et al, 1992; Stephens, 2005). Because the passive sensors largely fail to resolve the cloud overlap in the vertical, the 2B-FLXHR-LIDAR product derived from these two active sensors is considered a vital data set for examining the radiative heating features in the atmosphere and for studying the variations in fluxes and heating rate caused by vertically overlapping clouds (L’Ecuyer et al, 2008; Haynes et al, 2013) In this investigation, we only provide the results of the net radiative effect of different multilayered cloud types at the TOA (top of the atmosphere) and at the surface during the daytime by using the 2B-FLXHRLIDAR. The calculated annual mean cloud fractions for different cloud types in this investigation are reliable
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