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Steven A. Ackerman, W. L. Smith, H. E. Revercomb, and J. D. Spinhirne

Abstract

Lidar and high spectral resolution infrared radiance observations taken on board the ER-2 on 28 October 1986 are used to study the radiative properties of cirrus cloud in the 8–12 μm window region. Measurements from the High-spectral resolution Interferometer Sounder (HIS) indicate that the spectral variation of the equivalent blackbody temperature across the window can be greater than 5°C for a given cirrus cloud. This difference is attributed to the presence of small particles.

A method for detecting cirrus clouds using 8 μm, 11 μm, and 12 μm bands is presented. The 8 μm band is centered on a weak water-vapor absorption line while the 11 μm and 12 μm bands are between absorption lines. The brightness temperature difference between the 8 and 11 μm bands is negative for clear regions, while for ice clouds it is positive. Differences in the 11 and 12 μm channels are positive, whether viewing a cirrus cloud or a clear region. Inclusion of the 8 μm channel therefore removes the ambiguity associated with the use of 11 and 12 μm channels alone. The method is based on the comparison of brightness temperatures observed in these three channels.

The HIS and lidar observations were combined to derive the spectral effective beam emissivity (ε) of the cirrus clouds. Fifty percent of clouds on this day displayed a spectral variation of ε from 2–10%. These differences, in conjunction with large differences in the HIS observed brightness temperatures, indicate that cirrus clouds cannot be considered gray in the 8–12 μm window region.

The derived spectral transmittance of the cloud is used to infer the effective radii of the particle size distribution, assuming ice spheres. For 28 October 1986 the effective radius of cirrus cloud particle size distribution (r eff) was generally within the 30–40 μm range with 8% of the cases where 10 < r eff < 30 μm and 12% of the cases corresponding to r ref > 40 μm.

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Stefan Kinne, Thomas P. Ackerman, Andrew J. Heymsfield, Francisco P. J. Valero, Kenneth Sassen, and James D. Spinhirne

Abstract

Cloud data acquired during the cirrus intensive field operation of FIRE 86 are analyzed for a 75 × 50-km2 cirrus cloud field that passed over Wausau, Wisconsin, during the morning of 28 October 1986. Remote-sensing measurements from the stratosphere and the ground detect an inhomogeneous cloud structure between 6 and 11 km in altitude. The measurements differentiate between an optically thicker (τ > 3) cirrus deck characterized by sheared precipitation trails and an optically thinner (τ < 2) cirrus cloud field in which individual cells of liquid water are imbedded. Simultaneous measurements of particle-size spectra and broadband radiative fluxes at multiple altitudes in the lower half of the cloud provide the basis for a comparison between measured and calculated fluxes. The calculated fluxes are derived from observations of cloud-particle-size distributions, cloud structure, and atmospheric conditions. Comparison of the modeled fluxes with the measurements shows that the model results underestimate the solar reflectivity and attenuation, as well as the downward infrared fluxes. Some of this discrepancy may be due to cloud inhomogeneities or to uncertainties in cloud microphysics, since there were no measurements of small ice crystals available, nor any microphysical measurements in the upper portion of the cirrus. Reconciling the model results with the measurements can be achieved either by adding large concentrations of small ice crystals or by altering the backscattering properties of the ice crystals. These results suggest that additional theoretical and experimental studies on small compact shapes, hollow ice crystals, and shapes with branches are needed. Also, new aircraft instrumentation is needed that can detect ice crystals with maximum dimensions between 5 and 50 μm.

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E. E. Clothiaux, G. G. Mace, T. P. Ackerman, T. J. Kane, J. D. Spinhirne, and V. S. Scott

Abstract

A cloud detection algorithm for a low power micropulse lidar is presented that attempts to identify all of the significant power returns from the vertical column above the lidar at all times. The main feature of the algorithm is construction of lidar power return profiles during periods of clear sky against which cloudy-sky power returns are compared. This algorithm supplements algorithms designed to detect cloud-base height in that the tops of optically thin clouds are identified and it provides an alternative approach to algorithms that identify significant power returns by analysis of changes in the slope of the backscattered powers with height. The cloud-base heights produced by the current algorithm during nonprecipitating periods are comparable with the results of a cloud-base height algorithm applied to the same data. Although an objective validation of algorithm performance on high, thin cirrus is lacking because of no truth data, the current algorithm produces few false positive and false negative classifications as determined through manual comparison of the original photoelectron count data to the final cloud mask image.

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James R. Campbell, Dennis L. Hlavka, Ellsworth J. Welton, Connor J. Flynn, David D. Turner, James D. Spinhirne, V. Stanley Scott III, and I. H. Hwang

Abstract

Atmospheric radiative forcing, surface radiation budget, and top-of-the-atmosphere radiance interpretation involve knowledge of the vertical height structure of overlying cloud and aerosol layers. During the last decade, the U.S. Department of Energy, through the Atmospheric Radiation Measurement (ARM) program, has constructed four long-term atmospheric observing sites in strategic climate regimes (north-central Oklahoma; Barrow, Alaska; and Nauru and Manus Islands in the tropical western Pacific). Micropulse lidar (MPL) systems provide continuous, autonomous observation of nearly all significant atmospheric clouds and aerosols at each of the central ARM facilities. These systems are compact, and transmitted pulses are eye safe. Eye safety is achieved by expanding relatively low-powered outgoing pulse energy through a shared, coaxial transmit/receive telescope. ARM MPL system specifications and specific unit optical designs are discussed. Data normalization and calibration techniques are presented. These techniques, in tandem, represent an operational value-added processing package used to produce normalized data products for ARM cloud and aerosol research.

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Bruce A. Wielicki, J.T. Suttles, Andrew J. Heymsfield, Ronald M. Welch, James D. Spinhirne, Man-Li C. Wu, David O'C. Starr, Lindsay Parker, and Robert F. Arduini

Abstract

Observations of cirrus and altocumulus clouds during the First International Satellite Cloud Climatology Project Regional Experiment (FIRE) are compared to theoretical models of cloud radiative properties. Three tests are performed. First, Landsat radiances are used to compare the relationship between nadir reflectance at 0.83 μm and beam emittance at 11.5 μm with that predicted by model calculations using spherical and nonspherical phase functions. Good agreement is found between observations and theory when water droplets dominate. Poor agreement is found when ice particles dominate, especially if scattering phase functions for spherical particles am used. Even when compared to a laboratory measured ice particle phase function (Volkovitskiy et al. 1980), the observations show increased side scattered radiation relative to the theoretical calculations. Second, the anisotropy of conservatively scattered radiation is examined using simultaneous multiple-angle views of the cirrus from Landsat and ER-2 aircraft radiometers. Observed anisotropy gives good agreement with theoretical calculations using the laboratory measured ice-particle phase function and poor agreement with a spherical-particle phase function. Third, Landsat radiances at 0.83 μm, 1.65 μm, and 2.21 μm are used to infer particle phase and particle size. For water droplets, good agreement is found with King Air FSSP particle probe measurements in the cloud. For ice particles, the Landsat radiance observations predict an effective radius of 60 μm versus aircraft observations of about 200 μm. It is suggested that this discrepancy may be explained by uncertainty in the imaginary index of ice and by inadequate measurements of small ice particles by microphysical probes.

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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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R. A. Peppler, C. P. Bahrmann, J. C. Barnard, J. R. Campbell, M.-D. Cheng, R. A. Ferrare, R. N. Halthore, L. A. HeiIman, D. L. Hlavka, N. S. Laulainen, C.-J. Lin, J. A. Ogren, M. R. Poellot, L. A. Remer, K. Sassen, J. D. Spinhirne, M. E. Splitt, and D. D. Turner

Drought-stricken areas of Central America and Mexico were victimized in 1998 by forest and brush fires that burned out of control during much of the first half of the year. Wind currents at various times during the episode helped transport smoke from these fires over the Gulf of Mexico and into portions of the United States. Visibilities were greatly reduced during favorable flow periods from New Mexico to south Florida and northward to Wisconsin as a result of this smoke and haze. In response to the reduced visibilities and increased pollutants, public health advisories and information statements were issued by various agencies in Gulf Coast states and in Oklahoma.

This event was also detected by a unique array of instrumentation deployed at the U.S. Department of Energy's Atmospheric Radiation Measurement (ARM) program Southern Great Plains Cloud and Radiation Testbed and by sensors of the Oklahoma Department of Environmental Quality/Air Quality Division. Observations from these measurement devices suggest elevated levels of aerosol loading and ozone concentrations during May 1998 when prevailing winds were favorable for the transport of the Central American smoke pall into Oklahoma and Kansas. In particular, aerosol extinction profiles derived from the ARM Raman lidar measurements revealed large variations in the vertical distribution of the smoke.

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