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Greg M. McFarquhar
,
Andrew J. Heymsfield
,
James Spinhirne
, and
Bill Hart

Abstract

In situ microphysical, remote sensing, and satellite observations of thin and subvisible cirrus have been used to establish their frequency of occurrence, determine their mean optical depths and radiative forcings, and to analyze their association with deep convection. A spatially thin layer of cirrus, with both base and top above 15 km, was observed in the central Pacific Tropics 29% of the time, with a mean thickness of 0.47 km, using a nadir-pointing Nd:YAG lidar operating at 1.064 μm during the Central Equatorial Pacific Experiment (CEPEX). In situ microphysical data collected in the mid-1970s and mid-1980s by a WB-57 and Learjet near Kwajalein, Marshall Islands, are revisited to determine typical ice crystal sizes and shapes that occur in this cloud type. Three observed vertical profiles, obtained from ascents/descents through cloud, are used with a δ-four-stream radiative transfer model to calculate observed heating rates of up to 1.0 K day−1, principally in the infrared, and cloud radiative forcings of up to 1.2 W m−2. These calculations are extended using remotely sensed optical depths acquired with the airborne lidar on four days during CEPEX; the average τ estimated was 0.01, and the corresponding heating rates and cloud radiative forcings were 1.66 K day−1 and 1.6 W m−2, respectively. Using a visibility threshold for τ of 0.03, this suggests that the majority of the thin cirrus observed are subvisible. The calculated radiative effects depend principally on the observed optical thickness of the cloud layers.

Altitude-dependent average extinction coefficients between 0.001 and 0.004 km−1 were calculated from the limb-viewing Stratospheric Aerosol and Gas Experiment (SAGE) II satellite, confirming that the lidar estimated τ is representative for the Tropics. A comparison of the SAGE II observed occurrence times with cloud properties estimated from collocated International Satellite Cloud and Climatology Project retrievals for the closest time shows that 28% of the subvisible cirrus occurred within the same 2.5° by 2.5° grid box as deep convection.

Although the effect of subvisible cirrus on the radiative budget of the Tropics is not as large as for other ice clouds, their effects are not negligible and their other impacts, such as enhancing upper-tropospheric vertical motions and the lower-stratospheric water vapor, should not be ignored.

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James D. Spinhirne
,
William D. Hart
, and
Dennis L. Hlavka

Abstract

A summary of experimental observations and analysis of cirrus from high-altitude aircraft remote sensing is presented. The vertical distribution of cirrus optical and infrared cross-section parameters and the relative effective emittance and visible reflectance are derived from nadir-viewing lidar and multispectral radiometer data for observations during the 1986 and 1991 FIRE cirrus experiments. Statistics on scattering and absorption cross sections in relation to altitude and temperature are given. The emittance and reflectance results are considered as a function of solar zenith angle. Comparative radiative transfer calculations based on the discrete-ordinate method were carried out for three representative cloud phase function models: a spherical water droplet, an ice column crystal cloud, and a Henyey-Greenstein function. The agreements between observations of the effective emittance and shortwave reflectance and the model calculations were a function of the solar zenith angle. At angles between 54° and 60° a Henyey-Greenstein (HG) function with an asymmetry factor of 0.6–0.7 produced the best comparison. At 66°–72° the ice column model was equally comparable to observations. Comparisons to the water cloud model wore poor in all cases. The effects of ice crystal microphysical variations on the observed results were not generally apparent, but one dramatic example of difference was found. In order to explain the variations noted for solar zenith angle, an instrument–the Tilt Scan CCD Camera radiometer–was developed to directly observe the shortwave bidirectional reflectance function for 1991 measurements. The results indicate a characteristic angular function of the visible reflectance of cirrus that is flatter than predicted by the ice column scattering model, but the overall asymmetry factor is comparable. The good agreement with values from an HG function at some angles is not generally applicable. The characteristics of the observed cirrus angular reflectance pattern correlate well with, and are explained by, the results that were found for the solar zenith angle dependence of the eminence and reflectance.

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Teruyuki Nakajima
,
Michael D. King
,
James D. Spinhirne
, and
Lawrence F. Radke

Abstract

A multispectral scanning radiometer has been used to obtain measurements of the reflection function of marine stratocumulus clouds at 0.75, 1.65 and 2.16 μm. These observations were obtained from the NASA ER-2 aircraft as part of the First ISCCP [International Satellite Cloud Climatology Project] Regional Experiment (FIRE), conducted off the coast of southern California during July 1987. Multispectral images of the reflection function were used to derived the optical thickness and effective particle radius of stratiform cloud layers on four days. In addition to the radiation measurements, in situ microphysical measurements were obtained from the University of Washington Convair C-131A aircraft. In this paper we compare remote sensing results with in situ observations, which show a good spatial correlation for both optical thicknesses and effective radius. These comparisons further show systematic differences between remote sensing and in situ values, with a tendency for remote sensing to overestimate the effective radius by ∼2–3 μm, independent of particle radius. The optical thickness, in contrast, is somewhat overestimated for small optical thickness and underestimated for large optical thicknesses. An introduction of enhanced gaseous absorption at a wavelength of 2.16 μm successfully explains some of these observed discrepancies.

Marginal probability density functions of optical thickness, liquid water path and effective radius have been derived from our remote sensing results. The joint probability density function of liquid water path and effective radius shows that the effective radius increases as the liquid water path increases for optically thin clouds, in contrast to optically thick clouds for which the effective radius decreases with increasing liquid water path.

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Yuekui Yang
,
Alexander Marshak
,
J. Christine Chiu
,
Warren J. Wiscombe
,
Stephen P. Palm
,
Anthony B. Davis
,
Douglas A. Spangenberg
,
Louis Nguyen
,
James D. Spinhirne
, and
Patrick Minnis

Abstract

Laser beams emitted from the Geoscience Laser Altimeter System (GLAS), as well as other spaceborne laser instruments, can only penetrate clouds to a limit of a few optical depths. As a result, only optical depths of thinner clouds (< about 3 for GLAS) are retrieved from the reflected lidar signal. This paper presents a comprehensive study of possible retrievals of optical depth of thick clouds using solar background light and treating GLAS as a solar radiometer. To do so one must first calibrate the reflected solar radiation received by the photon-counting detectors of the GLAS 532-nm channel, the primary channel for atmospheric products. Solar background radiation is regarded as a noise to be subtracted in the retrieval process of the lidar products. However, once calibrated, it becomes a signal that can be used in studying the properties of optically thick clouds. In this paper, three calibration methods are presented: (i) calibration with coincident airborne and GLAS observations, (ii) calibration with coincident Geostationary Operational Environmental Satellite (GOES) and GLAS observations of deep convective clouds, and (iii) calibration from first principles using optical depth of thin water clouds over ocean retrieved by GLAS active remote sensing. Results from the three methods agree well with each other. Cloud optical depth (COD) is retrieved from the calibrated solar background signal using a one-channel retrieval. Comparison with COD retrieved from GOES during GLAS overpasses shows that the average difference between the two retrievals is 24%. As an example, the COD values retrieved from GLAS solar background are illustrated for a marine stratocumulus cloud field that is too thick to be penetrated by the GLAS laser. Based on this study, optical depths for thick clouds will be provided as a supplementary product to the existing operational GLAS cloud products in future GLAS data releases.

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Kenneth Sassen
,
David O'C. Starr
,
Gerald G. Mace
,
Michael R. Poellot
,
S.H. Melfi
,
Wynn L. Eberhard
,
James D. Spinhirne
,
E.W. Eloranta
,
Donald E. Hagen
, and
John Hallett

Abstract

In presenting an overview of the cirrus clouds comprehensively studied by ground-based and airborne sensors from Coffeyville, Kansas, during the 5–6 December 1992 Project FIRE IFO II case study period, evidence is provided that volcanic aerosols from the June 1991 Pinatubo eruptions may have significantly influenced the formation and maintenance of the cirrus. Following the local appearance of a spur of stratospheric volcanic debris from the subtropics, a series of jet streaks subsequently conditioned the troposphere through tropopause foldings with sulfur-based particles that became effective cloud-forming nuclei in cirrus clouds. Aerosol and ozone measurements suggest a complicated history of stratospheric-tropospheric exchanges embedded within the upper-level flow, and cirrus cloud formation was noted to occur locally at the boundaries of stratospheric aerosol-enriched layers that became humidified through diffusion, precipitation, or advective processes. Apparent cirrus cloud alterations include abnormally high ice crystal concentrations (up to ∼600 L−1), complex radial ice crystal types, and relatively large haze particles in cirrus uncinus cell heads at temperatures between −40° and −50°C. Implications for volcanic-cirrus cloud climate effects and usual (nonvolcanic aerosol) jet stream cirrus cloud formation are discussed.

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