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J. M. Austin

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J. M. Austin

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J. M. Austin and R. Shapiro

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The hypothesis is investigated that there is a physical difference between the development and motion components of a surface pressure change. Temperature changes indicate that deepening and filling are accompanied by high-level heating and cooling, respectively, while the motion part of pressure changes is associated with low-level temperature variations.

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H. G. Houghton and J. M. Austin

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H. G. Houghton and J. M. Austin

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Surface pressure changes can occur only when an accelerational field exists. The regularity of occurrence, the distribution, and the magnitudes of the accelerational fields found in the atmosphere have been determined from the available data. The most direct method used was to plot maps of the deviation of the observed wind from the geostrophic wind. Charts of the horizontal divergence, as determined from the observed winds, were prepared for several levels. Charts were also drawn of the non-geostrophic temperature changes, which are defined as the difference between the actual 12-hour temperature changes and the temperature changes which would result from geostrophic advection of the temperature field. It is shown that the magnitudes of the divergence and the non-geostrophic temperature changes are consistent with the observed deviations from the geostrophic wind. The errors of each method are investigated and it is concluded that they are not sufficient to affect the order of magnitude of the results. All of the charts exhibit definite patterns which show a considerable degree of correspondence with the weather conditions. It is concluded that accelerational fields regularly occur in the atmosphere which are one order of magnitude greater than cyclostrophic accelerations and accelerations due to the variation of the Coriolis parameter.

The equation for the pressure tendency is discussed with reference to the observational data. Since the total divergence in a vertical column is the relatively small difference between large divergences of opposite sign, the divergence integral in the tendency equation apparently cannot be evaluated from the data. Furthermore the sum of the divergence and advective integrals yield only the surface pressure tendency, which is already available. It does not appear that the divergence can be prognosticated as accurately as the pressure field. It is pointed out that the vertical velocities associated with a field of divergence may cause large pressure and temperature changes aloft with no surface pressure change. This shows that it is not possible to determine the regions responsible for surface pressure changes by considering the changes in the several layers. The influence of vertical stability on surface pressure changes was investigated statistically with indeterminate results.

A model of a cyclonic development based on the latent heat of condensation is discussed. It appears that this mechanism is incapable of explaining pressure changes of the magnitude commonly observed. A mechanism by which additional accelerations and pressure changes might result from the deformation of the field of mass by an initial accelerational field is presented. Sufficient evidence has not been accumulated to determine whether this mechanism operates in the atmosphere.

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J. B. Jensen, P. H. Austin, M. B. Baker, and A. M. Blyth

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The analysis of Paluch suggests that some cumuli contain cloudy air from only two sources: cloud base and cloud top. A framework is presented for the investigation of droplet spectral evolution in clouds composed of air from only these two sources. The key is the investigation of the dependence of droplet concentration N on the fraction of cloud base air F in a sample of cloudy air. This N-vs-F analysis is coupled with an investigation of droplet spectral parameters to infer the types and scales of entrainment and mixing events.

The technique is used in a case study of a small, nonprecipitating continental cumulus cloud which was sampled during the 1981 CCOPE project in eastern Montana. The mixing between cloudy and entrained air in this cloud often appears to occur without total removal of droplets, although there is evidence that total evaporation occurs in some regions with low liquid water content. The observed droplet spectra are compared with those calculated from an adiabatic parcel model. The spectral comparison and the results of the N-vs-F analysis support the hypothesis that cloudy and environmental air interact on fairly large scales with subsequent homogenization of the large-scale regions. This description is consistent with recent models of mixing in turbulent flows.

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P. H. Austin, M. B. Baker, A. M. Blyth, and J. B. Jensen

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We have analyzed small-scale fluctuations in microphysical, dynamical and thermodynamical parameters measured in two warm cumulus clouds during the Cooperative Convective Precipitation Experiment (CCOPE) project (1981) in light of predictions of several recent models. The measurements show the existence at all levels throughout the sampling period of two statistically distinct kinds of cloudy regions, termed “variable” and “steady,” often separated by transition zones of less than ten meters. There is some evidence for microphysical variability induced by local fluctuations in thermodynamic and dynamic parameters; however, the predominant variations are of a nature consistent with laboratory evidence suggesting that mixing is dominated by large structures. Entrainment appears to occur largely near cloud top but the data presented here do not permit identification of a mechanism for transport of the entrained air throughout the cloud.

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C. M. R. Platt, R. T. Austin, S. A. Young, and A. J. Heymsfield

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During the Maritime Continent Thunderstorm Experiment (MCTEX), several decaying storm anvils were observed. The anvil clouds exhibited typical patterns of fallout and decay over a number of hours of observation. The anvil bases were initially very attenuating to lidar pulses, and continued that way until anvil breakup commenced. During that time, the anvil base reached some characteristic altitude (∼7 km) below which the cloud particles had evaporated fully. Some typical “tongues” of fallout below such levels also occurred. Millimeter radar showed the storm anvil cloud tops to be much higher than detected by lidar until the anvil was well dissipated.

The infrared properties of the anvils were calculated. In three of the four anvils studied, the calculated emittance never exceeded 0.8–0.85. In the remaining case, the cloud emittance approached unity only in the period before the anvil had descended appreciably. Radiative transfer calculations showed that the infrared emission originated mostly from the layer between cloud base and the height at which complete attenuation of the lidar pulse occurred. However, the correct blackbody emission at cloud base could only be obtained by assuming the existence of an additional layer, situated above the first, 1.8 km deep and with a specific backscatter coefficient. The depressed values of emittance were interpreted as a cooling (below those temperatures measured by radiosonde) for some distance above anvil cloud base due to evaporation of the cloud. Typically, this cooling amounted to about 10°C, depending on the layer thickness above cloud base at which cooling was occurring. A reexamination of data taken in 1981 at Darwin, Northern Territory, Australia, indicated a similar depression in emittance in all cases of attenuating storm anvils. A simple model of ice-mass evaporation saturating the ambient air was used to approximate the observed cooling in one anvil. Millimeter radar reflectivity measurements, which also yielded ice water content at cloud base, were also used to find equivalent cooling rates. By varying the mean volume diameter in the calculation, cooling rates similar to those found from the radiometric method could be obtained. The values of mean volume diameter agreed, within uncertainties, with those obtained by the lidar–radar method. Estimated cooling to over 1 km above cloud base confirms earlier work on anvil mammata. Values of backscatter-to-extinction ratio at the base of the anvils showed some consistent variations, indicating a change of ice-crystal habit, or size, with time.

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C. M. R. Platt, S. A. Young, P. J. Manson, G. R. Patterson, S. C. Marsden, R. T. Austin, and J. H. Churnside

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The optical properties of equatorial cirrus were studied during a three-week period of the ARM Pilot Radiation and Observation Experiment at Kavieng, Papua New Guinea, in January and February 1993. The experiment consisted of vertical lidar (532 nm) and passive infrared filter radiometer (10.84 μm) observations of cirrus clouds. The observations gave values of cloud height, depth, structure, infrared emittance, infrared absorption, and visible optical depth and linear depolarization ratio. A standard lidar–radiometer analysis, with some improvements, was used to calculate these quantities. The cirrus was found to vary in altitude from a maximum cloud top of 17.6 km to a minimum cloud base of 6 km with equivalent temperatures of −82°C to −7°C respectively. The cirrus also varied widely in depth (0.7 to 7.5 km). The mean emittance (for each temperature interval) of the cooler clouds was found to be higher than that observed previously at tropical and midlatitude sites and at equivalent temperatures. The mean infrared absorption coefficients were similar to those of midlatitude clouds, except at the extreme temperature ranges, but were higher than those observed in tropical synoptic clouds over Darwin. Infrared optical depths varied from 0.01 to 2.4 and visible optical depths from 0.01 to 8.6.

Plots of integrated attenuated backscatter versus infrared emittance, for various ranges of cloud temperature, showed characteristic behavior. Values of the measured quantity k/2η, where k is the visible backscatter to extinction ratio and η a multiple scattering factor, were found to increase with temperature from 0.14 at −70°C to 0.30 at −20°C.

Values of the quantity 2αη, where α is the ratio of visible extinction to infrared absorption coefficient, varied from about 1.7 to 3.8, depending somewhat on the cloud temperature. Deduced values of α were as high as 5.3 at the lower temperature ranges, indicating smaller particles.

The lidar integrated attenuated depolarization ratio Δ decreased with temperature, as found previously in midlatitude cirrus. Values of Δ varied from 0.42 at −70°C to 0.18 at −10°C. Data obtained from the NOAA/ETL microwave radiometer gave values of water path, varying from 4 to 6 cm precipitable water. A value of the water vapor continuum absorption coefficient at 10.84 μm equal to 9.0 ± 0.5 g−1 cm2 atm−1 was obtained in agreement with previous observations.

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