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Tadahiro Hayasaka, Nobuyuki Kikuchi, and Masayuki Tanaka

Abstract

Aircraft observations of shortwave radiative properties of stratocumulus clouds were carried out over the western North Pacific Ocean during January 1991. Two aircraft were equipped with a pair of pyranometers and near-infrared pyranometers. Downward and upward shortwave fluxes above and below the cloud were synchronously measured by two aircraft. The cloud radiative properties, especially the absorptance obtained from measurements, were compared with those calculated. Aircraft measurements and Monte Carlo calculations showed that spatial inhomogeneities of clouds cause horizontal radiative convergence and divergence, and that vertical radiative convergence-that is, absorptance with a usual definition-apparently becomes extremely large or negative. The apparent absorptance could be corrected by a method that evaluates the true absorption from the difference between the apparent visible and near-infrared absorptions. The corrected absorptance agreed well with the theoretical absorptance calculated with plane-parallel cloud models. It is also inferred that the anomalous absorption pointed out by aircraft observations in previous studies does not exist.

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Makoto Kuji, Tadahiro Hayasaka, Nobuyuki Kikuchi, Teruyuki Nakajima, and Masayuki Tanaka

Abstract

An algorithm was developed to retrieve both the optical thickness and the effective particle radius of low-level marine clouds simultaneously from National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (AVHRR) data. The algorithm uses the combination of the visible (channel 1) and the middle-infrared (channel 3) reflected radiation. The thermal component in the middle infrared was corrected with the thermal-infrared (channel 4) radiance by a statistical technique using a regressive formula. The liquid water path (i.e., vertically integrated liquid water content) was also estimated as a by-product. The algorithm was applied to AVHRR datasets for which almost-synchronized airborne observations were conducted around the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) and the Western North Pacific Experiment (WENPEX) regions. The two regions are different in the characteristics of cloud fields:summer stratus and stratiform clouds that result from outbreaks of cold air mass over the warm sea in winter seasons, respectively.

In the FIRE region, the retrieved parameters are almost consistent with those of in situ airborne observations, even when using a more practical approach than the algorithms adopted in previous studies, but there is still a discrepancy between the satellite-derived results and those of in situ airborne observations around the drizzle-dominated portion. In the WENPEX region, it is suggested that cloud fractional coverage in a pixel may cause error in the retrieval, particularly for horizontally inhomogeneous cloud field analyses with an assumption of a plane-parallel atmospheric model. It is found also that the thermal-infrared information has a potential to estimate the inhomogeneity of cloud fields as a result of the comparison between stratus and broken cloud cases.

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Haruma Ishida, Takashi Y. Nakjima, Tatsuya Yokota, Nobuyuki Kikuchi, and Hiroshi Watanabe

Abstract

In this work, the Greenhouse Gases Observing Satellite (GOSAT) Thermal and Near-infrared Sensor for Carbon Observation–Cloud and Aerosol Imager (TANSO-CAI) cloud screening results, which are necessary for the retrieval of carbon dioxide (CO2) and methane (CH4) gas amounts from GOSAT TANSO–Fourier Transform Spectrometer (FTS) observations, are compared with results from Aqua/Moderate Resolution Imaging Spectroradiometer (MODIS) in four seasons. A large number of pixels, acquired from both satellites with nearly coincident locations and times, are extracted for statistical comparisons. The same cloud screening algorithm was applied to both satellite datasets to focus on the performance of the individual satellite sensors, without concern for differences in algorithms. The comparisons suggest that CAI is capable of discriminating between clear and cloudy areas over water without sun glint and also may be capable of identifying thin cirrus clouds, which are generally difficult to detect without thermal infrared or near-infrared bands. On the other hand, cloud screening over land by CAI resulted in greater cloudy discrimination than that by MODIS, whereas detection of thin cirrus clouds tended to be more difficult over land than water, resulting in incorrect identification of thin cirrus as clear. The amount of missed thin cirrus had a seasonal variation, with the maximum occurring in summer. The cloudy tendency of CAI over half vegetation is caused by lack of an effective threshold test that can be applied to MODIS. The statistical results of the comparison clarified the important points to consider when using the results of CAI cloud screening.

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