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Szymon P. Malinowski and Isztar Zawadzki

Abstract

The cloud–environment mixing process is considered in terms of fractal properties of the cloud-clear air interface. The fractal dimension of the cloud surface is estimated from high-resolution airborne data. The value obtained is D = 2.55 in a range of scales from at least 10 m to over 1000 m with the possibility of even greater extension. This differs significantly from values obtained in shear-generated, well-developed, and homogeneous turbulence. The distribution of filament sizes of cloudy and clear air and estimates of the cloud surface and characteristic time of mixing process are given.

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Szymon P. Malinowski and Hanna Pawlowska-Mankiewicz

Abstract

The existence small-scale inhomogeneities in cumulus clouds leads to a reinterpretation of the experimental data on total water mixing ratio Q and wet equivalent potential temperature θq. This reinterpretation indicates that the height of the level of entrained air may be sometimes overestimated.

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Szymon P. Malinowski, Isztar Zawadzki, and Piotr Banat

Abstract

Cloud–clear air mixing at scales from 1 mm to 1 m is observed in a laboratory chamber. Cross sections through the volume in which the mixing takes place are obtained by illuminating a planar sheet of cloud with laser light (λ = 0.488 μm, 1.2-mm thickness); the light is scattered by cloud droplets and photographed. Images indicate that complicated filament-like structures are created during mixing. Due to the properties of Mie scattering, this technique is in principle more sensitive to the larger cloud drops, and volumes with the small droplets may be underrepresented in the images. After digitization of the images, an interface between cloudy and clear-air filaments is investigated. Preliminary results indicate that at the scale of 2 cm the nature of the interface changes: at larger scales it exhibits self-similar properties, whereas at smaller scales it has a simple geometrical structure.

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H. Gerber, Szymon P. Malinowski, and Haflidi Jonsson

Abstract

Buoyancy reversal by evaporative cooling in entrainment holes has a minimal influence on stratocumulus (Sc) observed during the Physics of Stratocumulus Top (POST) aircraft field study held off the California coast in 2008. High-resolution temperature and microphysics measurements show only small differences for Sc with and without buoyancy reversal predicted by mixing fraction analysis that relates mixtures of cloudy air and free-atmospheric air to buoyancies of the mixtures. The reduction of LWC due to evaporation in the holes is a small percentage (average ~12%) of liquid water diluted in the Sc by entrainment from the entrainment interface layer (EIL) located above unbroken cloud top where most mixing, evaporation, and reduction of the large buoyancy jump between the cloud and free atmosphere occur. Entrainment is dominated by radiative cooling at cloud top.

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Szymon P. Malinowski, Monique Y. Leclerc, and Darrel G. Baumgardner

Abstract

Fractal analyses of individual cloud droplet distributions using aircraft measurements along one-dimensional horizontal cross sections through clouds are performed. Box counting and cluster analyses are used to determine spatial scales of inhomogeneity of cloud droplet spacing. These analyses reveal that droplet spatial distributions do not exhibit a fractal behavior. A high variability in local droplet concentration in cloud volumes undergoing mixing was found. In these regions, thin filaments of cloudy air with droplet concentration close to those observed in cloud cores were found. Results suggest that these filaments may be anisotropic. Additional box counting analyses performed for various classes of cloud droplet diameters indicate that large and small droplets are similarly distributed, except for the larger characteristic spacing of large droplets.

A cloud-clear air interface defined by a certain threshold of total droplet count (TDC) was investigated. There are indications that this interface is a convoluted surface of a fractal nature, at least in actively developing cumuliform clouds. In contrast, TDC in the cloud interior does not have fractal or multifractal properties. Finally a random Cantor set (RCS) was introduced as a model of a fractal process with an ill-defined internal scale. A uniform measure associated with the RCS after several generations was introduced to simulate the TDC records. Comparison of the model with real TDC records indicates similar properties of both types of data series.

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Krzysztof E. Haman, Andrzej Makulski, Szymon P. Malinowski, and Reinhold Busen

Abstract

A new aircraft device for measuring temperature in clouds is described. Its sensor is a resistance thermometer made of platinum-coated tungsten wire 5 mm long and 2.5 μm in diameter. The sensor is located on a rotatable vane behind a thin rod aimed at protecting it against the impact of cloud droplets, which according to limited experience gathered until now seems to be sufficiently effective as an antiwetting protection for the speeds of motorgliders. Contrary to the massive housings usually adopted in other constructions, the rod creates only negligible disturbances in the thermodynamic properties of the ambient air. The time constant of the sensor is of the order 10−4 s, which permits measurements of temperature in clouds with a resolution of a few centimeters, depending on aircraft velocity. The thermometer was tested in a wind tunnel, and on an Ogar motorglider and a Do-228 aircraft. Its present version performs fairly well at low airspeeds of up to about 40 m s−1. For faster aircraft further improvements seem necessary. The paper presents a detailed description of the instrument, discussion of test results, and examples of centimeter-scale features of temperature fields in clouds measured with the thermometer.

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Krzysztof E. Haman, Szymon P. Malinowski, Bożena D. Struś, Reinhold Busen, and Andrzej Stefko

Abstract

A new version of an ultrafast aircraft resistance thermometer (UFT-F) with a time constant of the order 10−4 s,for use in both cloudy and cloudless air, is described. It evolved from an earlier version (UFT-S). Its sensing element is similar to that in UFT-S and consists of a 5-mm-long and 2.5-μm-thick platinum-coated tungsten wire, located on a rotatable vane behind a thin vertical rod that protects the sensor against direct impact of cloud droplets and other objects. Such construction introduces much smaller thermal disturbances than do more massive housings of other types of immersion thermometers and permits taking full advantage of low thermal inertia of the sensing wire. However, aerodynamic disturbances created by vortex shedding from the protective rod induce adiabatic fluctuations of temperature, which appear on the temperature records as “noise.” In the case of the UFT-S the level of this noise has become intolerable at airspeeds of about 40 m s−1, limiting applicability of this instrument to slow aircraft or gliders. For UFT-F the shape of the protective rod has been redesigned and endowed with a special system of reducing aerodynamic disturbances behind it, which made it usable at airspeeds up to 100 m s−1 in cloudless air or warm clouds. For use in supercooled clouds, a special variety of UFT-F (denoted here UFT-D) has been designed. As in its predecessor, its sensing element is a 5-mm-long, 2.5-μm-thick, platinum-coated tungsten resistive wire protected against impact of cloud droplets by an airfoil-shaped rod, but all its icing-sensitive parts are electrically heated to prevent buildup of ice. This modification required a total change of mechanical structure of the instrument. Tests during the Third Canadian Freezing Drizzle Experiment showed that UFT-D can perform fairly well in water clouds supercooled down to at least −8°C and that its heating system introduces no intolerable disturbances into the record. Use of UFT-D in ice or mixed clouds is limited by the fact that the protective rod is not effective enough against ice crystals bigger than about 200 μm, which can quickly destroy the delicate sensing element.

The paper gives details of construction as well as results of wind tunnel and in-flight tests of these instruments.

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Miroslaw Andrejczuk, Wojciech W. Grabowski, Szymon P. Malinowski, and Piotr K. Smolarkiewicz

Abstract

This note presents an analysis of several dozens of direct numerical simulations of the cloud–clear air mixing in a setup of decaying moist turbulence with bin microphysics. The goal is to assess the instantaneous relationship between the homogeneity of mixing and the ratio of the time scales of droplet evaporation and turbulent homogenization. Such a relationship is important for developing improved microphysical parameterizations for large-eddy simulation of clouds. The analysis suggests a robust relationship for the range of time scale ratios between 0.5 and 10. Outside this range, the scatter of numerical data is significant, with smaller and larger time scale ratios corresponding to mixing scenarios that approach the extremely inhomogeneous and homogeneous limits, respectively. This is consistent with the heuristic argument relating the homogeneity of mixing to the time scale ratio.

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Emmanuel O. Akinlabi, Marta Wacławczyk, Juan Pedro Mellado, and Szymon P. Malinowski

Abstract

In this work, direct numerical simulation (DNS) of the stratocumulus cloud-top mixing layer is used to test various approaches to estimate the turbulence kinetic energy (TKE) dissipation rate ε from one-dimensional (1D) intersections that resemble experimental series. Results of these estimates are compared with “true” (DNS) values of ε in buoyant and inhomogeneous atmospheric flows. We focus on recently proposed methods of the TKE dissipation-rate retrievals based on zero crossings and recovering the missing part of the spectrum. These methods are tested on fully resolved turbulence fields and compared to standard retrievals from power spectra and structure functions. Anisotropy of turbulence due to buoyancy is shown to influence retrievals based on the vertical velocity component. TKE dissipation-rate estimates from the number of crossings correspond well to spectral estimates. The method based on the recovery of the missing part of the spectrum works best for Pope’s model of the dissipation spectrum and is sensitive to external intermittency. This allows for characterization of external intermittency by the Taylor-to-Liepmann scale ratio. Further improvements of this method are possible when the variance of the velocity derivative is used instead of the number of zero crossings per unit length. In conclusion, the new methods of TKE dissipation-rate retrieval from 1D series provide a valuable complement to standard approaches.

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Miroslaw Andrejczuk, Wojciech W. Grabowski, Szymon P. Malinowski, and Piotr K. Smolarkiewicz

Abstract

This paper extends the previously published numerical study of Andrejczuk et al. on microscale cloud–clear air mixing. Herein, the primary interest is on microphysical transformations. First, a convergence study is performed—with well-resolved direct numerical simulation of the interfacial mixing in the limit—to optimize the design of a large series of simulations with varying physical parameters. The principal result is that all conclusions drawn from earlier low-resolution (Δx = 10−2 m) simulations are corroborated by the high-resolution (Δx = 0.25 × 10−2 m) calculations, including the development of turbulent kinetic energy (TKE) and the evolution of microphysical properties. This justifies the use of low resolution in a large set of sensitivity simulations, where microphysical transformations are investigated in response to variations of the initial volume fraction of cloudy air, TKE input, liquid water mixing ratio in cloudy filaments, relative humidity (RH) of clear air, and size of cloud droplets. The simulations demonstrate that regardless of the initial conditions the evolutions of the number of cloud droplets and the mean volume radius follow a universal path dictated by the TKE input, RH of clear air filaments, and the mean size of cloud droplets. The resulting evolution path only weakly depends on the progress of the homogenization. This is an important conclusion because it implies that a relatively simple rule can be developed for representing the droplet-spectrum evolution in cloud models that apply parameterized microphysics. For the low-TKE input, when most of the TKE is generated by droplet evaporation during mixing and homogenization, an inhomogeneous scenario is observed with approximately equal changes in the dimensionless droplet number and mean volume radius cubed. Consistent with elementary scale analysis, higher-TKE inputs, higher RH of cloud-free filaments, and larger cloud droplets enhance the homogeneity of mixing. These results are discussed in the context of observations of entrainment and mixing in natural clouds.

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