A continuum method for modeling surface tension

The major source of cloud-condensation nuclei (CCN) over the oceans appears to be dimethylsulphide, which is produced by planktonic algae in sea water and oxidizes in the atmosphere to form a sulphate aerosol Because the reflectance (albedo) of clouds (and thus the Earth's radiation budget) is sensitive to CCN density, biological regulation of the climate is possible through the effects of temperature and sunlight on phytoplankton population and dimethylsulphide production. To counteract the warming due to doubling of atmospheric CO2, an approximate doubling of CCN would be needed.

Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate

The present paper reviews existing knowledge with regard to Organic Aerosol (OA) of importance for global climate modelling and defines critical gaps needed to reduce the involved uncertainties. All pieces required for the representation of OA in a global climate model are sketched out with special attention to Secondary Organic Aerosol (SOA): The emission estimates of primary carbonaceous particles and SOA precursor gases are summarized. The up-to-date understanding of the chemical formation and transformation of condensable organic material is outlined. Knowledge on the hygroscopicity of OA and measurements of optical properties of the organic aerosol constituents are summarized. The mechanisms of interactions of OA with clouds and dry and wet removal processes parameterisations in global models are outlined. This information is synthesized to provide a continuous analysis of the flow from the emitted material to the atmosphere up to the point of the climate impact of the produced organic aerosol. The sources of uncertainties at each step of this process are highlighted as areas that require further studies.

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Organic aerosol and global climate modelling: a review

The software package OPAC (Optical Properties of Aerosols and Clouds) is described. It easily provides optical properties in the solar and terrestrial spectral range of atmospheric particulate matter. Microphysical and optical properties of six water clouds, three ice clouds, and 10 aerosol components, which are considered as typical cases, are stored as ASCII files. The optical properties are the extinction, scattering, and absorption coefficients, the single scattering albedo, the asymmetry parameter, and the phase function. They are calculated on the basis of the microphysical data (size distribution and spectral refractive index) under the assumption of spherical particles in case of aerosols and cloud droplets and assuming hexagonal columns in case of cirrus clouds. Data are given for up to 61 wavelengths between 0.25 and 40 μm and up to eight values of the relative humidity. The software package also allows calculation of derived optical properties like mass extinction coefficients and Angstrom coefficients. Real aerosol in the atmosphere always is a mixture of different components. Thus, in OPAC it is made possible to get optical properties of any mixtures of the basic components and to calculate optical depths on the base of exponential aerosol height profiles. Typical mixtures of aerosol components as well as typical height profiles are proposed as default values, but mixtures and profiles for the description of individual cases may also be achieved simply.

Optical Properties of Aerosols and Clouds: The Software Package OPAC

. Aerosols affect the climate system by changing cloud characteristics in many ways. They act as cloud condensation and ice nuclei, they may inhibit freezing and they could have an influence on the hydrological cycle. While the cloud albedo enhancement (Twomey effect) of warm clouds received most attention so far and traditionally is the only indirect aerosol forcing considered in transient climate simulations, here we discuss the multitude of effects. Different approaches how the climatic implications of these aerosol effects can be estimated globally as well as improvements that are needed in global climate models in order to better represent indirect aerosol effects are discussed in this paper.

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Global indirect aerosol effects: a review

Cloud physics has achieved such a voluminous literature over the past few decades that a significant quantitative study of the entire field would prove unwieldy. This book concentrates on one major aspect: cloud microphysics, which involves the processes that lead to the formation of individual cloud and precipitation particles. Common practice has shown that one may distinguish among the following additional major aspects: cloud dynamics, which is concerned with the physics responsible for the macroscopic features of clouds; cloud electricity, which deals with the electrical structure of clouds and the electrification processes of cloud and precipitation particles; and cloud optics and radar meteorology, which describe the effects of electromagnetic waves interacting with clouds and precipitation. Another field intimately related to cloud physics is atmospheric chemistry, which involves the chemical composition of the atmosphere and the life cycle and characteristics of its gaseous and particulate constituents. In view of the natural interdependence of the various aspects of cloud physics, the subject of microphysics cannot be discussed very meaningfully out of context. Therefore, we have found it necessary to touch briefly upon a few simple and basic concepts of cloud dynamics and thermodynamics, and to provide an account of the major characteristics of atmospheric aerosol particles. We have also included a separate chapter on some of the effects of electric fields and charges on the precipitation-forming processes.

Microphysics of Clouds and Precipitation

A physical model which predicts the shape of water drops falling at terminal velocity in air is presented. The model is based on a balance of the forces which act on a drop falling under gravity in a viscous medium. The model was evaluated by numerical techniques and the shape of water drops of radii between 170 and 4000 μ (equivalent to Reynolds numbers between 30 and 4900) was determined. The results of our investigation show that the drop shapes predicted by the model agree well with those experimentally observed in our wind tunnel. Both theory and experiment demonstrate that: 1) drops with radii ≲170 μ are very slightly deformed and can be considered spherical, 2) the shape of drops between about 170 and 500 μ can be closely approximated by an oblate spheroid, 3) drops between about 500 and 2000 μ are deformed into an asymmetric oblate spheroid with an increasingly pronounced flat base, and 4) drops ≳2000 μ develop a concave depression in the base which is more pronounced for larger drop size...

A Semi-Empirical Determination of the Shape of Cloud and Rain Drops

A theoretical model is formulated which allows the processes that control the wet deposition of atmospheric pollutants to be included in cloud dynamic models. The model considers the condensation process and the collision-coalescence process which, coupled together, control the fate of atmospheric aerosol particles removed by clouds and precipitation through nucleation scavenging and impaction scavenging. The model was tested by substituting a simple parcel model for the dynamic framework. In this form the model was used to determine the time evolution of the aerosol particle mass scavenged by drops as well as the aerosol particle mass left unactivated in air as “drop-interstitial” aerosol. In the present computation all aerosol particles are assumed to have the same composition. Our study shows for inside cloud scavenging: 1) collision and coalescencence causes among the various drop size categories a redistribution of the scavenged aerosol particles in such a manner that the main aerosol partic...

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A Theoretical Study of the Wet Removal of Atmospheric Pollutants. Part I: The Redistribution of Aerosol Particles Captured through Nucleation and Impaction Scavenging by Growing Cloud Drops

An experimental study of the effect of ventilation on the rate of evaporation of millimeter sized water drops failing at terminal velocity in air has been carried out in a wind tunnel where drops were suspended freely in the tunnel air stream. It was found that for drops in the size range 1150 µm≤a0≤2500 µm, the mean ventilation coefficient fvfh could be expressed as f=(0.78±0.02)+(0.308±0.010)X, where X=N&frac13Sc,v N½Re. Previously, we showed that this relation holds for drops in the size range 60 µm≤a0≤400 µm. Taken together, our present and previous data suggest that with reasonable accuracy f=0.78+0.308X, for 1.4≤X≤51.4 (60 µm≤a0≤2500 µm). For 0≤X≤1.4 (0≤a≤60 µm), one may use our previous result f=1.00+0.108 X2. To illustrate how the present data may be applied, we computed the distance which is required for a water drop to travel from cloud base through a NACA Standard Atmosphere of various relative humidities, in order to reach the earth's surface with a given size.

Hans R. Pruppacher

Papers

Microphysics of Clouds and Precipitation

Microphysics of Clouds and Precipitation

A Semi-Empirical Determination of the Shape of Cloud and Rain Drops

A Theoretical Study of the Wet Removal of Atmospheric Pollutants. Part I: The Redistribution of Aerosol Particles Captured through Nucleation and Impaction Scavenging by Growing Cloud Drops

A Wind Tunnel Investigation of the Rate of Evaporation of Small Water Drops Falling at Terminal Velocity in Air