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J. D. Spinhirne, R. Boers, and W. D. Hart

Abstract

Maine stratus clouds were simultaneously observed by nadir Nd:YAG lidar measurements and in situ cloud physics measurements. A procedure was applied to derive the two-dimensional vertical cross section of the liquid water from within the cloud top lidar observations. A comparison to direct in-cloud liquid water observations gave good results. The liquid water retrieval was limited to an effective optical of 1.5. The true cloud optical thickness was also obtained from the retrieval procedure to a corresponding limit of 3.8. The optical thickness of the observed marine stratus clouds was predominantly below 3.0.

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J. D. Spinhirne, J. A. Reagan, and B. M. Herman

Abstract

Vertical profiles of aerosol extinction and backscatter in the troposphere are obtained from multizenith angle lidar measurements. A direct slant path solution was found to be not possible due to horizontal inhomogeneity of the atmosphere. Regression analysis with respect to zenith angle for a layer integration of the angle-dependent lidar equation was thus employed to determine the optical thickness and aerosol extinction-to-backscatter ratio for defined atmospheric layers, and subsequently, cross-section profiles could be evaluated. Measurements were made with an elastic backscatter ruby lidar system with calibration by a standard target procedure. The results from 20 measurement cases are presented. For layer-aerosol optical thicknesses >0.04, useful results were obtained, and corroboration by solar radiometer aerosol optical depth data was found. The mean mixed-layer aerosol extinction-to-backscatter ratio for the measurements was 19.5 sr with a standard deviation of 8.3 sr. With the use of an aerosol size distribution inverted from wavelength-dependent solar aerosol optical depth data, the measured extinction-to-backscatter ratio was compared to Mie theory calculations, and the imaginary index giving best agreement was determined. A maximum upper limit of 0.015 was indicated for the aerosol imaginary index. but the mean result was 0.003 for a real index of 1.52.

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J. D. Spinhirne, M. Z. Hansen, and J. Simpson

Abstract

High-resolution observations of the structure of cloud tops have been obtained with polarization lidar operated from a high altitude aircraft. Case studies of measurements acquired from cumuliform cloud systems are presented, two from September 1979 observations in the area of Florida and adjacent waters and a third during the May 1981 CCOPE experiment in southeast Montana. Accurate cloud top height structure and relative density of hydrometers are obtained from the lidar return signal intensity. Correlation between the signal return intensity and active updrafts was noted. Thin cirrus overlying developing turrets was observed in some cases. Typical values of the observed backscatter cross section were 0.1–0.5 (km sr−1) for cumulonimbus tops.

The depolarization ratio of the lidar signals was a function of the thermodynamic phase of cloud top areas. An increase of the cloud top depolarization with decreasing temperature was found for temperatures above and below −40°C. The observed values of depolarization from water clouds were greater than reported by previous studies. Increased multiple scattering due to a larger range from the receiver to scattering medium is thought to have given rise to the greater water cloud depolarization for the cloud top measurements.

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Andrew J. Heymsfield, Karen M. Miller, and James D. Spinhirne

Abstract

The temperature and windfield structure and hydrometeor composition of cirrus clouds sampled by the NCAR King Air and Sabreliner aircraft on 28 October 1986 near Madison, Wisconsin are described as part of a case study that examines cirrus cloud radiative and microphysical properties. Two cloud layers were sampled from top to base. The upper layer was found at altitudes between 8.5 and 11.5 km and the lower between 6.0 and 8.5 km. Vertical velocities calculated from the increase in ice mass flux with height were typical of synoptic scale lifting. Stronger vertical velocities were measured in convective cells at the top of the lower layer.

The total ice particle concentration was dominated by particles <200 μm. Mean particle size and ice water content increased with decreasing altitude. The largest particles, exceeding 1000 μm in the upper layer and 1500 μm in the lower layer, probably resulted from aggregation, even at cold temperatures. Cloud emissivity and optical depth were calculated from the ice particle size spectra.

The distribution of ice mass was narrow at cloud top and broadened with decreasing altitude. At the highest levels of the upper cloud, half the mass was in particles <150 μm. In this region, we underestimate the mass by a significant fraction presumably contained in particles too small to detect. In the lower levels, half the mass was in particles <200–400 μm. In the cloud sampled by the King Air, half the mass was in particles <400–600 μm. Up to 10% of the mass in the higher cloud and up to 30% in the lower cloud was contained in particles >500 μm.

We relate the microstructure of a shallow liquid water layer associated with an altocumulus to lidar observations. Thirteen separate episodes of liquid water were sampled at about −30°C. Mean droplet dimensions were <9 μm, and the liquid water contents were low. Virtually no ice particles were detected within and below the layer. We surmised that under such conditions these liquid water clouds remained colloidally stable. Kelvin-Helmholz waves may have produced the undulations observed at cloud top.

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J. R. Wang, J. D. Spinhirne, P. Racette, L. A. Chang, and W. Hart

Abstract

Simultaneous measurements with the millimeter-wave imaging radiometer (MIR), cloud lidar system (CLS), and the MODIS airborne simulator (MAS) were made aboard the NASA ER-2 aircraft over the western Pacific Ocean on 17–18 January 1993. These measurements were used to study the effects of clouds on water vapor profile retrievals based on millimeter-wave radiometer measurements. The CLS backscatter measurements (at 0.532 and 1.064 μm) provided information on the heights and a detailed structure of cloud layers; the types of clouds could be positively identified. All 12 MAS channels (0.6–13 μm) essentially respond to all types of clouds, while the six MIR channels (89–220 GHz) show little sensitivity to cirrus clouds. The radiances from the 12-μm and 0.875-μm channels of the MAS and the 89-GHz channel of the MIR were used to gauge the performance of the retrieval of water vapor profiles from the MIR observations under cloudy conditions. It was found that, for cirrus and absorptive (liquid) clouds, better than 80% of the retrieval was convergent when one of the three criteria was satisfied; that is, the radiance at 0.875 μm is less than 100 W cm−3 sr−1, or the brightness at 12 μm is greater than 260 K, or brightness at 89 GHz is less than 270 K (equivalent to cloud liquid water of less than 0.04 g cm−2). The range of these radiances for convergent retrieval increases markedly when the condition for convergent retrieval was somewhat relaxed. The algorithm of water vapor profiling from the MIR measurements could not perform adequately over the areas of storm-related clouds that scatter radiation at millimeter wavelengths.

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J. A. Reagan, J. D. Spinhirne, D. M. Byrne, D. W. Thomson, R. G. De Pena, and Y. Mamane

Abstract

Particulate size and height distributions, complex refractive index and mass loading have been measured and inferred from direct aircraft and indirect lidar-solar radiometer observations made during a unique joint experiment conducted the week of 18 November 1974 in Tucson, Ariz. The aircraft and lidar-solar radiometer measurements were first analyzed independently and the results were then intercompared. Vertical profiles of particulate extinction obtained from the lidar (monostatic) and aircraft measurements were found to be in excellent agreement on both a relative and absolute basis. Lidar (bistatic and monostatic) inferences of particulate mass loading agreed favorably with the aircraft mass monitor measurements. The aircraft and lidar (bistatic) size distribution determinations were found to be similar in shape and agreed in absolute value within an order of magnitude. The mean particle refractive index inferred from the lidar (bistatic) measurements (n = 1.40 − i0.000) agreed with the index of a significant fraction of the particles identified by electron microscope analysis of impactor samples collected with the aircraft.

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Kenneth Sassen, Christian J. Grund, James D. Spinhirne, Michael M. Hardesty, and Jose M. Alvarez

Abstract

Optical remote sensing measurements of cirrus cloud properties were collected by one airborne and four ground-based lidar systems over a 32-h period during this cue study from the First ISCCP (International Satellite Cloud Climatology Program) Regional Experiment (FIRE) Intensive Field Observation (IFO) program. The lidar systems were variously equipped to collect linear depolarization, intrinsically calibrated backscatter, and Doppler velocity information. Data presented here describe the temporal evolution and spatial distribution of cirrus clouds over an area encompassing southern and central Wisconsin. The cirrus cloud types include: (a) dissipating subvisual and “thin” fibrous cirrus cloud bands, (b) an isolated mesoscale uncinus complex (MUC), (c) a large-scale, deep cloud that developed into an organized cirrus structure within the lidar array, and (d) a series of intensifying mesoscale cirrus cloud masses. Although the cirrus frequently developed in the vertical from particle fallstreaks emanating from generating regions at or near cloud tops, glaciating supercooled (−30° to −35°C) altocumulus clouds contributed to the production of ice mass at the base of the deep cirrus cloud, apparently even through riming, and other mechanisms involving evaporation, wave motions, and radiative effects are indicated. The generating regions ranged in scale from ∼1.0-km cirrus uncinus cells, to organized MUC structures up to ∼120 km across.

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S. H. Melfi, J. D. Spinhirne, S-H. Chou, and S. P. Palm

Abstract

Observations of a convective planetary boundary layer (PBL) were made with an airborne, downward-looking lidar system over the Atlantic Ocean during a cold air outbreak. The lidar data revealed well-organized, regularly spaced cellular convection with dominant spacial scales between two and four times the height of the boundary layer. It is demonstrated that the lidar can accurately measure the structure of the PBL with high vertical and horizontal resolution. Parameters important for PBL modeling such as entrainment zone thickness, entrainment rate, PBL height and relative heat flux can be inferred from the lidar data. It is suggested that wind shear at the PBL top may influence both entrainment and convective cell size.

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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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