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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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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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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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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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