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James D. Spinhirne and William D. Hart

Abstract

Remote sensing lidar and imaging spectral radiometer observations were obtained from the ER-2 high-altitude research aircraft during the 1986 FIRE cirrus missions. The dual polarization lidar measurements were nadir directed with 7.5 m vertical and 40 m horizontal resolution, and clearly depicted structure at the top and within the cirrus. Simultaneous radiometric cloud top images were acquired with 5 mrd resolution at ten visible channels, three infrared window channels, and four near-infrared channels. The combined lidar and radiometer data were analyzed for the cirrus structure, radiative parameters, and inferred microphysical properties. On 28 October 1986 a cirrus formation crossed Wisconsin. The results indicate that for the eastern edge of the formation there was a cirrus layer at 9 to 11 km altitude, and a separate lower cloud at 7 to 8 km. The lidar depolarization indicated the upper layer was ice crystals, the lower layer was ice in some areas, and water or possibly mixed phase in others. Split window thermal brightness measurements indicated the upper layer was principally particles of an effective radius less than 25 μm. To the west, the cirrus formation was a denser layer extending between 6 to 11 km altitude. An equivalent height for the thermal IR emission of cirrus was defined. The equivalent height was found to be as much as 4 km below the true cloud top height. The average vertical structure of radiation parameters was derived. For the upward infrared radiance the strongest contribution was from 7 to 8 km altitude but higher cirrus were significant. Cloud visible reflectance approached 0.6 and the 10.84 μm emittance ranged to 0.9. Distinct local vacations in the relation between reflectance and emittance were found, while a significant dispersion of the emittance to reflectance relation for the entire dataset was present. The dispersion was principally due to variations in surface albedo. An overall parameterization for the average measured relation between emittance and visible albedo is given.

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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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Gerald M. Heymsfield, Richard Fulton, and James D. Spinhirne

Abstract

The instrumented NASA ER-2 aircraft overflew severe convection with infrared (IR) V features for the first time in the Midwest United States during May 1984. Measurements taken by the ER-2 were: visible and IR imagery, high-frequency passive microwave (92, 183 GHz) imagery, nadir lidar backscattered return, and flight altitude information. The 7 May and 13 May 1984 cases are analyzed in detail and the various data sources are combined and compared with GOES imagery. Topics addressed in the paper are 1) relation of thermal couplets and V features in aircraft IR measurements to previous findings from GOES data, 2) examination of the cloud radiative hypothesis for the V feature, and 3) stratospheric perturbations above severe thunderstorms and mesoscale convective systems.

The high resolution aircraft IR imagery shows that thermal couplets are considerably more pronounced than in GOES imagery. In one of the cases (7 May 1984) the minimum cloud-top IR temperature was located upshear of the overshooting cloud top in the lidar height field. This was suggested in previous papers to result from cloud top mixing with the stratospheric environment and subsidence. The IR temperatures in the downshear anvils were as much as 5°C warmer than the ambient air temperatures, implying that the upwelling IR radiance comes from about 0.5–1.0 km below the cloud top. Finally, the in situ ER-2 measurements of temperature and air velocity 3–4 km above the overshooting tops showed very intense temperature and vertical velocity perturbations. These perturbations are suggestive of 1) lee waves generated by the overshooting tops, or 2) a cold dome above the squall line possibly due to tropopause lifting by the storms.

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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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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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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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