Hiroshi Ishimoto

Assessment of the nonsphericity of mineral dust from geostationary satellite measurements

Optical thickness of Asian dust aerosols over the Yellow Sea and the Sea of Japan were retrieved from the visible data of the Geostationary Meteorological Satellite (GMS-5) observed on April 7 and 8, 2000, through look-up tables generated by the doubling and adding method. The retrieved optical thickness, by using conventional spherical particle models, showed unnatural decrease in the middle of day time. The decrease disappeared by the use of the semiempirical theory for nonspherical particles by Pollack and Cuzzi [J. Atmos. Sci. 37 (1980) 868.]. These results are explained by the difference of the phase functions between spherical and nonspherical particles and also the variation of the scattering angle for the target dust area during the satellite measurements. Our results confirm that the use of nonspherical particle model improves the retrieval of dust optical thickness from space.

Retrieval of cirrus optical thickness and ice-shape information using total and polarized reflectance from satellite measurements

Abstract The effect of ice crystal shape on satellite retrieval of cirrus optical thickness is investigated. The cirrus cloud is assumed to be composed of hexagonal ice crystals randomly oriented in three-dimensional space. Theoretical studies followed by an analysis of ADEOS/POLDER measurements show that the use of total and polarized radiance at a wavelength of 865 nm with multi-directional measurements is useful for improving cirrus optical thickness retrieval from space. The analyses of the POLDER data are performed using the columnar (plate like) hexagonal ice crystal models with aspect ratios ranging from 1.0 to 8.0 (from 0.125 to 1.0). The data observed in the scattering angle range between 60° and 120° are used for retrieval because the polarized reflectance measured in this scattering angle range is very sensitive to the aspect ratio of ice crystals; the total reflectance to cirrus optical thickness. The uncertainty of the retrieved cirrus optical thickness using the total reflectance alone, without considering the effect of aspect ratio, is about 1 for the cirrus cloud with optical thickness of 3–4. This uncertainty is eliminated by taking advantage of the polarized reflectance in retrieval.

Fast radiative-transfer model based on the correlated k-distribution method for a high-resolution satellite sounder

A fast radiative-transfer model for the Atmospheric Infrared Sounder (AIRS) was developed by use of a correlated k-distribution method. Transmittances produced by the correlated k-distribution method are systematically displaced from those produced by a line-by-line method, and empirical correction is possible. A fast radiative model that includes this empirical correction has exhibited practical performance in tests of transmittance and brightness temperature that used an independent set of atmospheric profiles.

Influence of particle orientation on retrieving cirrus cloud properties by use of total and polarized reflectances from satellite measurements

Abstract The influence of ice crystal orientation was investigated on retrieving cirrus optical thickness ( τ ) and aspect ratio of ice crystals ( Q ) from satellite measurements using the total and polarized reflectances at a wavelength of 865 nm (r 865 and pr 865 ) . We considered columnar and plate like hexagonal ice crystals whose long axes are randomly oriented in the horizontal plane (2D model) with some amplitude of oscillation expressed by a Gaussian distribution function with the standard deviation of σ . The retrieved τ and Q values significantly depend on the assumption of σ , in particular for the plate type. Furthermore, the relationship between σ and the retrieved values depends on the solar, satellite, and target geometries. In our case study, for one target area, τ value retrieved using the 2D model with σ =5° was approximately twice larger than that using the 2D model with σ =20°, while the retrieved Q value was not significantly influenced by σ . For another target area, the τ (Q) retrieved using the 2D model with σ =5° was about 1.5 (1.8) times larger than that retrieved using the 2D model with σ =20°.

Numerical simulation of spectral albedos of glacier surfaces covered with glacial microbes in Northwestern Greenland

To clarify the effect of light absorbing impurities including glacial microbes spectral albedo measurements using a spectrometer for spectral domains of the ultraviolet, visible and near-infrared have been carried out on ablation area in Qaanaaq Glacier in northwestern Greenland in July 2011. The almost glacier surfaces in the ablation area were covered with cryoconite (biogenic dust) on thin ice grain layer above bare ice. There were also snow-covered surfaces including red snow (snow algae). The measured spectral albedos had a remarkable contrast between red snow surface and cryoconite-covered ice surface in the spectral domain from the ultraviolet to the visible, where red snow albedo increased rapidly with the wavelength, while the cryoconite albedo was relatively flat to the wavelength. We simulated the spectral albedos of these surfaces with a radiative transfer model for the atmosphere-snow system. The single scattering properties are calculated with Mie theory by assuming red snow gains to be sphe...

Comparisons of the mineral dust optical thickness retrieved from Geostationary Meteorological Satellite with ground-based radiometer measurements

Abstract Optical thickness of the Asian dust aerosols over the ocean near Japan was retrieved from the visible data of the Geostationary Meteorological Satellite (GMS-5) from April 7 to 22, 2002 using the method by Masuda et al. [Remote Sens. Environ. 82 (2002) 238] in which the nonsphericity of dust particles was taken into account. The retrieved optical thicknesses were then compared with those derived from the sunphotometer and the skyradiometer measurements. The mean and the standard deviation of the optical thickness difference, GMS minus radiometers, were −0.03 and 0.17, respectively, for 85 samples where the optical thickness ranges from 0.25 to 1.07.

One‐dimensional variational (1D‐Var) retrieval of middle to upper tropospheric humidity using AIRS radiance data

A one-dimensional variational analysis (1D-Var retrieval) of tropospheric humidity was conducted using hyper-spectral radiance data from the Atmospheric Infrared Sounder (AIRS). For the vertical range of the atmosphere between 200 and 600 hPa, the same high-resolution retrieval of humidity profiles as for clear-sky conditions was possible over low clouds if the cloud height was lower than 800 hPa. Global analyses from a global data assimilation system were used for initial profiles, and clear conditions over 800 hPa height were determined from AIRS radiance data. Results of analyses for 50 days of global radiosonde matchup data from 21 December 2008 to 8 February 2009 revealed that our 1D-Var calculations derived humidity profiles were closer to the sonde profiles than those of a global analysis at a height over 600 hPa. Furthermore, the results of 1D-Var retrieval often represented high and supersaturated relative humidity around the supposed ice clouds. The altitudes of the high humidity region agreed with the height of ice clouds that had been detected by CloudSat/CALIPSO. As well as possibly improving the humidity profiles in a numerical model by data assimilation, it is expected that these humidity analyses using AIRS radiance data will provide additional information for the study of ice clouds in the middle to upper troposphere.

Physical model for multiple scattered spaceborne lidar returns from clouds

A practical model for determining the time-dependent lidar attenuated backscattering coefficient β was developed for application to global lidar data. An analytical expression for the high-order phase function was introduced to reduce computational cost for simulating the angular distribution of the multiple scattering irradiance. The decay rate of the multiple scattering backscattered irradiance was expressed by incorporating the dependence on the scattering angle and the scattering order based on the path integral approach. The estimated β over time and the actual range showed good agreement with Monte Carlo simulations for vertically homogeneous and inhomogeneous cloud profiles, resulting in about 15% mean relative error corresponding to 4 times improved accuracy against the Ornstein-Fürth Gaussian approximation method.

Relationship between ice supersaturation and ice microphysics inferred from CloudSat, CALIPSO and AIRS

Super saturation of ice clouds was investigated by using CloudSat, CALIPSO and AIRS on Aqua. Global distributions of relative humidity and temperature were first retrieved by using Ishimoto 2009 AIRS algorithm that used 110 channels and 46 channels for temperature and humidity retrievals, respectively. Retrievals have been conducted between 600hPa and 200hPa and also above clouds when cloud top height was below 800hPa. We relied on the information of clouds, i.e., cloud location and ice water content, from CloudSat radar and CALIOP lidar. The water vapor density from AIRS had been sampled along CloudSat and CALIPSO tracks in order to retrieve relative humidity in the vicinity of ice clouds. Relative humidity when ice formation occurred was estimated by using ice water content and the water vapor density near ice clouds.

Ground-based atmospheric water vapor monitoring system with spectroscopy of radiation in 20-30 GHz and 50-60 GHz bands

We propose a novel ground-based meteorological monitoring system. In the 20{30 GHz band, our system simultaneously measures a broad absorption peak of water vapor and cloud liquid water. Additional observation in the 50{60 GHz band obtains the radiation of oxygen. Spectral results contain vertical profiles of the physical temperature of atmospheric molecules. We designed a simple method for placing the system atop high buildings and mountains and on decks of ships. There is a simple optical system in front of horn antennas for each frequency band. A focused signal from a reflector is separated into two polarized optical paths by a wire grid. Each signal received by the horn antenna is amplified by low-noise amplifiers. Spectra of each signal are measured as a function of frequency using two analyzers. A blackbody calibration source is maintained at 50 K in a cryostat. The calibration signal is led to each receiver via the wire grid. The input path of the signal is selected by rotation of the wire grid by 90°, because the polarization axis of the reflected path and axis of the transparent path are orthogonal. We developed a prototype receiver and demonstrated its performance using monitoring at the zenith.