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Hironobu Iwabuchi and Tadahiro Hayasaka

Abstract

Cloud remote sensing techniques are conventionally based on the independent pixel approximation (IPA). Here, three-dimensional (3D) radiative effects on IPA-based retrieved optical thickness from a visible-wavelength moderate-resolution (about 1 km) sensor are investigated. A Monte Carlo 3D radiative transfer model and a lognormal spectral cloud model are used to simulate monochromatic radiance reflected from overcast boundary layer cloud. A characterization of statistical properties of the optical thickness by the mean (M) and variance (S 2) of the logarithm of the optical thickness is proposed, where S represents a degree of cloud inhomogeneity. Biases in retrieved values of the two parameters with the IPA are defined as ΔM and ΔS 2 and attributed to neglect of net horizontal radiative transport in the IPA. Sensitivities of ΔM and ΔS 2 are tested with respect to geometrical roughness, M, S, mean geometrical thickness, spectral exponent of optical thickness fluctuation, ground surface reflectance, and bidirectional angles. The 3D radiative effects are sensitive to the geometrical roughness of cloud top rather than internal inhomogeneity of the extinction coefficient. The bias ΔM is negative in forward scattering viewing geometry due to cloud-side shadowing, while positive in backscattering viewing geometry due to side illumination. It is found that ΔM is proportional to S 2 and large for a dense cloud. On the other hand, ΔS 2 largely depends on the solar zenith angle; smoothing is exhibited for high solar elevation and roughening for low solar elevation. The smoothing and roughening phenomena are found to be almost independent of the inhomogeneity parameter. An optically thick cloud exhibits more roughening, while for a geometrically thick cloud both smoothing and roughening are enhanced. It is suggested that for the bias removal some empirical assumptions are required in geometrical and microphysical properties of cloud, which should be studied with in situ observation data.

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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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Maki Hirakata, Hajime Okamoto, Yuichiro Hagihara, Tadahiro Hayasaka, and Riko Oki

Abstract

This study analyzed the global and seasonal characteristics of cloud phase and ice crystal orientation (CTYPE-lidar) by using the Cloud–Aerosol Lidar with Orthogonal Polarization (CALIOP) on board the Cloud–Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). A dataset from September 2006 to August 2007 was used to derive the seasonal characteristics. The discrimination scheme was originally developed by Yoshida et al., who classified clouds mainly into warm water, supercooled water, and randomly oriented ice crystals or horizontally oriented ice plates. This study used the following products for the comparison with CTYPE-lidar: (i) the vertical feature mask (VFM) of the National Aeronautics and Space Administration (NASA), (ii) the Moderate Resolution Imaging Spectroradiometer (MODIS), and (iii) European Centre for Medium-Range Weather Forecasts (ECMWF). Overall, the results showed that the CTYPE-lidar discrimination scheme was consistent with the outputs from VFM, MODIS, and ECMWF. The zonal mean water cloud cover in daytime from this study showed good agreement with that derived from MODIS; the slope of the linear regression was 1.06 and the offset was 0.002. The CTYPE-lidar ice cloud occurrence frequency and the ECMWF ice supersaturation occurrence frequency were also in good agreement; the slope of the linear regression of the two products was 1.02 in the temperature range −60°C ≤ T ≤ −30°C. The maximum occurrence frequencies in this study and ECMWF were recognized around −60°C of the equator, with their peak shifted from several degrees north (~9°N) in September–November (SON) to south (~9°S) in December–February (DJF) and back to north (~7°N) in March–May (MAM) and June–August (JJA).

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Tadahiro Hayasaka, Teruyuki Nakajima, Yasushi Fujiyoshi, Yutaka Ishizaka, Takao Takeda, and Masayuki Tanaka

Abstract

An algorithm was developed for retrieving cloud geometrical thickness from a measured liquid water path and equivalent width of 0.94-µm water vapor absorption band. The algorithm was applied to aircraft observations obtained by a microwave radiometer and a spectrometer in the winter of 1991 over the western North Pacific Ocean. Retrieved values of the cloud geometrical thickness are apt to be smaller than those observed by eye, especially for horizontally inhomogeneous clouds. Measured cloud albedos in the visible and near-infrared spectral region were also compared with calculated values. For homogeneous clouds there exists a single droplet size distribution that satisfies both spectral regions. However, for inhomogeneous clouds no single size distribution exists that satisfies the albedo observed in both spectral regions.

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Teruyuki Nakajima, Tadahiro Hayasaka, Akiko Higurashi, Gen Hashida, Naser Moharram-Nejad, Yahya Najafi, and Hamzeh Valavi

Abstract

Solar radiation measurements were made using sun photometers and pyranometers during 31 May-7 June 1991 at several places in Iran and during 12 June-17 September 1991 at a fixed place, Bushehr, Iran. In the first period the aerosol optical thickness had values about 0.4 at the wavelength of 0.5 μm in the coastal area and about 0.2 in the plateau area. The Ångström's exponent, which is the slope of optical thickness spectrum, had values around 1 for large city areas and less than 0.5 for inland arid areas. Chemical analyses of sampled air indicate an effect of fossil fuel burning from local sources. Such optical and chemical characteristics of atmospheres suggest that soil-derived coarse particles contributed considerably to the atmospheric turbidity in arid areas, whereas an active generation of aerosols was dominant near large cities.

Significant rises in atmospheric turbidity were observed in the earlier part of the second period at Bushehr about once a week with a duration of about one day, which may have been caused by smoke from oil-well fires in Kuwait. The aerosol optical thickness in these events had values of about 1.5, which is equivalent to a columnar aerosol volume of 4.4 × 10−4 cm3 cm−2. The absorption index ranged from 0.005 to 0.02 with several peaks reaching 0.1 in the second period. These peaks can be attributed to prevailing smoke particles. In spite of the large variety of optical thicknesses and absorption indices, there existed stable power-law size distributions with an exponent about 3.7.

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Pingping Xie, Mingyue Chen, Song Yang, Akiyo Yatagai, Tadahiro Hayasaka, Yoshihiro Fukushima, and Changming Liu

Abstract

A new gauge-based analysis of daily precipitation has been constructed on a 0.5° latitude–longitude grid over East Asia (5°–60°N, 65°–155°E) for a 26-yr period from 1978 to 2003 using gauge observations at over 2200 stations collected from several individual sources. First, analyzed fields of daily climatology are computed by interpolating station climatology defined as the summation of the first six harmonics of the 365-calendar-day time series of the mean daily values averaged over a 20-yr period from 1978 to 1997. These fields of daily climatology are then adjusted by the Parameter-Elevation Regressions on Independent Slopes Model (PRISM) monthly precipitation climatology to correct the bias caused by orographic effects. Gridded fields of the ratio of daily precipitation to the daily climatology are created by interpolating the corresponding station values using the optimal interpolation method. Analyses of total daily precipitation are finally calculated by multiplying the daily climatology by the daily ratio.

Cross-validation tests indicated that this gauge-based analysis has high quantitative quality with a negligible bias and a correlation coefficient of ∼0.6 for comparisons between withdrawn station data and the analysis at a 0.05° latitude–longitude grid box. The quality of the analysis increases with the gauge network density. The mean distribution and annual cycle of this new gauge analysis present similar patterns but with more detailed structures and slightly larger magnitude compared to other published monthly gauge analyses over the region.

The East Asia gauge analysis is applied to verify the performance of five satellite-based precipitation estimates. This examination reveals the regionally and seasonally dependent performance of the satellite products with the best statistics observed for relatively wet regions. Further improvements of the daily gauge analysis are underway to increase the gauge network density and to refine the algorithm to better deal with the orographic effects especially over South and Southeast Asia.

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Guang-Yu Shi, Tadahiro Hayasaka, Atsumu Ohmura, Zhi-Hua Chen, Biao Wang, Jian-Qi Zhao, Hui-Zheng Che, and Li Xu

Abstract

Solar radiation is one of the most important factors affecting climate and the environment. Routine measurements of irradiance are valuable for climate change research because of long time series and areal coverage. In this study, a set of quality assessment (QA) algorithms is used to test the quality of daily solar global, direct, and diffuse radiation measurements taken at 122 observatories in China during 1957–2000. The QA algorithms include a physical threshold test (QA1), a global radiation sunshine duration test (QA2), and a standard deviation test applied to time series of annually averaged solar global radiation (QA3). The results show that the percentages of global, direct, and diffuse solar radiation data that fail to pass QA1 are 3.07%, 0.01%, and 2.52%, respectively; the percentages of global solar radiation data that fail to pass the QA2 and QA3 are 0.77% and 0.49%, respectively. The method implemented by the Global Energy Balance Archive is also applied to check the data quality of solar radiation in China. Of the 84 stations with a time series longer that 20 yr, suspect data at 35 of the sites were found. Based on data that passed the QA tests, trends in ground solar radiation and the effect of the data quality assessment on the trends are analyzed. There is a decrease in ground solar global and direct radiation in China over the years under study. Although the quality assessment process has significant effects on the data from individual stations and/or time periods, it does not affect the long-term trends in the data.

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