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Masataka Shiobara, James D. Spinhirne, Akihiro Uchiyama, and Shoji Asano

Abstract

Optical depths in the visible to infrared spectral region were obtained from solar extinction measurements with two sun photometers during the First ISCCP Regional Experiment Phase II Cirrus Intensive Field Observation in Kansas.

A method is described to correct sun photometry for gaseous absorption and is extended to estimate the water vapor amount. The approach uses a prior computation of gaseous absorption for the narrowband-pass sun photometry, parameterized with the slant-path absorber amount. These produce correction coefficients for gaseous absorption, as determined by LOWTRAN 7 models. Near-infrared channels were calibrated by modified Langley plots taking account of gaseous absorption.

After the correction and calibration, the aerosol optical depths at the wavelength of 0.4–4 µm were obtained for clear sky conditions. The aerosol optical depth at the wavelength λ = 0.5 µm was 0.1–0.2 during the campaign. The cloud optical depth at λ = 0.5 µm was obtained for cirrus events on 26 November and 5 December 1991 correction of multiple scattering effects involved in sun photometry. The column amount of water vapor was estimated from the 0.94-µm-channel measurement and compared with results from radiosonde measurements. The comparison has shown a good agreement within a 10% difference during the campaign when the equivalent water vapor amount ranges from 0.3 to 1.2 g cm−2.

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Reinout Boers, James D. Spinhirne, and William D. Hart

Abstract

A Nd:YAG lidar system was flown aboard NASA's ER-2 high altitude aircraft. Observations of cloud top height were made with 70 m along-track and 7.5 m vertical-height resolution. The lidar data observed from an East Pacific stratocumulus cloud height deck revealed large cloud variability on 1–5 km scales. The cloud deck sloped upward from 700 to 1000 m in a northeast-southwest direction over a distance of 120 km. Vertical cloud top distributions were negatively skewed indicating flat-topped clouds. The dominant spectral peak of the cloud top variations was found at 4.5 km, which is 5 to 7 times the depth of the local boundary layer. No other peaks were significant in the average spectrum, The cloud layer was stable with respect to cloud top entrainment instability. The southwestern region of the study area was more prone to shear instability at cloud top than the northeastern region. The results of this study show that a lidar system is ideal to provide the topography of clouds and local boundary layer depth. This information is useful in the study of cloud top radiation and parameterization of clouds in numerical models.

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Hélène Chepfer, Philippe Goloub, James Spinhirne, Pierre H. Flamant, Mario Lavorato, Laurent Sauvage, Gérard Brogniez, and Jacques Pelon

Abstract

Bidirectional polarized reflectances measured with the POLDER-1 instrument on board Advanced Earth Observing Satellite-1 have been used to infer cloud altitude and thermodynamical phase (ice/liquid) at a global scale. This paper presents a validation of these properties for cirrus clouds. The validation presented here is based on comparisons between POLDER-1 retrievals and measurements collected with a ground-based lidar network. The scale differences between POLDER measurements and lidar data are treated by selecting homogeneous and stable cloud layers.

These comparisons show that the cloud altitude retrieval with POLDER is valid for optically thick cloud, and nonvalid for semitransparent and thin cirrus clouds. The limitations of the cloud altitude retrieval method are analyzed by using both comparisons between POLDER and lidar and simulations of the bidirectional polarized reflectances performed with a radiative transfer code to assess a threshold of validity of the POLDER retrieval method. The comparisons of lidar and POLDER data show that the cloud thermodynamical phase (ice/liquid) retrieval is satisfactory, and examples of cloud thermodynamical phase retrieval are presented as a function of cloud temperatures.

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