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C. M. R. Platt, N. L. Abshire, and G. T. McNice

Abstract

Lidar observations of a winter ice cloud in the zenith gave a very high reflection but a very small depolarization. When the lidar was tilted more than 0.5° away from the zenith, the reflection amplitude fell to 3% of it's zenith value, but the depolarization increased. The above properties proved unambiguously that reflection was occurring from the specular surfaces of horizontal crystals. These properties were used to estimate some cloud microphysical properties. At a selected time, the estimates gave a mean “diameter” of 74 μm for the horizontal faces, a crystal number density of 0.78 −1, and a maximum departure of the crystal axis from the horizontal of 0.5°. The fraction of the total crystal cross section which was specularly reflecting was estimated as unity.

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C. M. R. Platt, A. C. Dilley, J. C. Scott, I. J. Barton, and G. L. Stephens

Abstract

The infrared properties and structures of some anvils emanating from local thunderstorms were studied by lidar and infrared radiometry at Darwin, tropical Northern Australia. The anvils were typically from 1 to 2 km deep, at altitudes from 7 to 16 km and at temperatures from −15 to −70°C. There was a rough dependence of infrared emittance on temperature, but there was also a dependence on the age of the anvil. The average altitude and calculated wide-band greybody flux emittance were 11 km and 0.65 respectively.

One dense cloud appeared “superblack” when observed from below, due to reflection of upwelling warm radiation from the surface. The magnitude of the effect agreed within experimental error with that predicted from computations on a model cloud of ice cylinders, but was about twice that computed for a model of ice spheres.

Calculated rates of heating in the very cold clouds were very high, reaching 4°C h−1 near cloud base. The survival of these clouds for several hours suggests that the absorbed radiant heat was converted largely into sensible heat in the atmosphere rather than causing evaporation of the crystals.

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S. A. Young, C. M. R. Platt, R. T. Austin, and G. R. Patterson

Abstract

Several cloud optical quantities were measured for the first time in midlevel, mixed-phase clouds. These included cloud infrared emittance and absorption coefficient (10–12 μm), effective backscatter-to-extinction ratio, and lidar depolarization ratio. Contrary to expectations, the supercooled water clouds were not always optically thick and therefore had measurable infrared absorption coefficients. At times, the water clouds had quite low emittances, whereas ice clouds had emittances that sometimes approached unity. On average, the cloud emittances were greater than those measured previously at lower temperatures in cirrus, but with considerable variability. At higher temperatures, the emittance values were skewed toward unity. The infrared absorption coefficients, for the semitransparent cases, showed a similar trend. The effective isotropic backscatter-to-extinction ratio was also measured. When separated into temperature intervals, the ratio was surprisingly constant, with mean values lying between 0.42 and 0.43, but with considerable variation. These ratios were most variable (0.15–0.8) in the −20° to −10°C temperature range where various ice crystal habits can occur. When multiple scattering effects were allowed for, values of backscatter-to-extinction ratio in the supercooled water clouds agreed well with theory. Multiple scattering factors based on previously obtained theoretical values were used and, thus, validated.

Characteristic and well-known patterns of lidar backscatter coefficient and depolarization ratio were used to separate out the incidence of supercooled water and ice layers and to identify layers of horizontal planar hexagonal crystals. This approach allowed the most detailed examination yet of such incidence by ground-based remote sensing. Water was detected for 92% of the time for the temperature interval of −5° to 0°C. Between −20° and −5°C, percentages varied between 33% and 56%, dropping to 21% between −25° and −20°C and to zero below −25°C. Oriented hexagonal plate crystals were present for 20% of the total time in ice layers between −20° and −10°C, the region of their maximum growth. The depolarization ratio varied significantly among different ice fall streaks, indicating considerable variation in ice crystal habit. Although the dependence of depolarization ratio on optical depth had been predicted theoretically, the first experimental validation in terms of IR emittance was obtained in this study.

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C. M. R. Platt, D. M. Winker, M. A. Vaughan, and S. D. Miller

Abstract

Cloud-integrated attenuated backscatter from observations with the Lidar In-Space Technology Experiment (LITE) was studied over a range of cirrus clouds capping some extensive mesoscale convective systems (MCSs) in the Tropical West Pacific. The integrated backscatter when the cloud is completely attenuating, and when corrected for multiple scattering, is a measure of the cloud particle backscatter phase function.

Four different cases of MCS were studied. The first was very large, very intense, and fully attenuating, with cloud tops extending to 17 km and a maximum lidar pulse penetration of about 3 km. It also exhibited the highest integrated attenuated isotropic backscatter, with values in the 532-nm channel of up to 2.5 near the center of the system, falling to 0.6 near the edges. The second MCS had cloud tops that extended to 14.8 km. Although MCS2 was almost fully attenuating, the pulse penetration into the cloud was up to 7 km and the MCS2 had a more diffuse appearance than MCS1. The integrated backscatter values were much lower in this system but with some systematic variations between 0.44 and 0.75. The third MCS was Typhoon Melissa. Values of integrated backscatter in this case varied from 1.64 near the eye of the typhoon to between 0.44 and 1.0 in the areas of typhoon outflow and in the 532-nm channel. Mean pulse penetration through the cloud top was 2–3 km, the lowest penetration of any of the systems. The fourth MCS consisted of a region of outflow from Typhoon Melissa. The cloud was semitransparent for more than half of the image time. During that time, maximum cloud depth was about 7 km. The integrated backscatter varied from about 0.38 to 0.63 in the 532-nm channel when the cloud was fully attenuating.

In some isolated cirrus between the main systems, a plot of integrated backscatter against one minus the two-way transmittance gave a linear dependence with a maximum value of 0.35 when the clouds were fully attenuating. The effective backscatter-to-extinction ratios, when allowing for different multiple-scattering factors from space, were often within the range of those observed with ground-based lidar. Exceptions occurred near the centers of the most intense convection, where values were measured that were considerably higher than those in cirrus observed from the surface. In this case, the values were more compatible with theoretical values for perfectly formed hexagonal columns or plates. The large range in theoretically calculated backscatter-to-extinction ratio and integrated multiple-scattering factor precluded a closer interpretation in terms of cloud microphysics.

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Robert S. Stone, Graeme L. Stephens, C. M. R. Platt, and S. Banks

Abstract

The problem of retrieving cirrus cloud optical depth from radiance measurements made by instruments aboard operational meteorological satellites is addressed. A method is proposed that exploits the relationship between observed differences in the near infrared (NIR) and infrared (IR) window radiances (expressed in terms of brightness temperature differences ΔT) and the optical depth of the cloud. The approach designed to test this method relies on the simultaneous collection of ground-based lidar and infrared radiometric (LIRAD) data, radiosonde data and bispectral satellite images.

Two case studies are described for which independent estimates of satellite pixel and coincident time-averaged LIRAD optical depths are compared with radiative transfer calculations made for hypothetical clouds characterized by distributions of spherical ice particles. Such comparative analyses yield information about cloud microphysics and enable the selection of representative theoretical relationships between estimates of cloud optical depth and observed spectral differences. A third case demonstrates the potential use of this split window technique to estimate cirrus cloud optical depth when only operational data is available.

In the first two cases, it was found that the LIRAD-derived optical depths agree to within 70% of the satellite estimates for optical depths greater than about 0.3, and that the differences tend to be systematic. Larger discrepancies are noted for thinner clouds, however, indicating inaccuracies in one or the other, or possibly both of these methods when applied to very thin layers. Another possible cause for these large discrepancies is the potential ambiguity in comparing the spatially averaged satellite data with time-averaged LIRAD data if physical changes in cloud structure occur during the course of the experiment.

We also found that, in all cases, the observed spectral differences (NIR-IR) agree reasonably well with model simulations if the clouds are assumed to be composed of distributions of large spherical ice particles having effective radii in the 32–64 μm range.

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