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Johan Ström and Jost Heintzenberg

Abstract

Results are presented from measurements made with a counterflow virtual impactor (CVI) in cirriform clouds containing crystals with dimensions typically less than 30 μm. Independent measurements of crystal number concentration and cloud water content are presented, and for the first time consistent instrumentation is used to measure both the cloud condensate and water vapor. The experiment was conducted over the Alps where the vertical wind field was very persistent in the stable air, which suggests the presence of standing gravity waves. For the two flights presented in this paper, the relative humidity over ice within the cloud ranged from about 50% to 110%, and the cloud water content ranged from about 1 to 10 mg m−3. The number concentration was typically a few hundred crystals per liter, reaching peak concentrations of several thousand per liter. In general, the cloud water comprised less than 10% of the total water content. Often, the crystals were not detected at all by concurrent PMS-230X optical array probe measurements due to the small size of the crystals. These results add to the understanding of the properties of cirrus, particularly for the constituents of less than 30 μm in dimension.

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Hong Lin, Kevin J. Noone, Johan Ström, and Andrew J. Heymsfield

Abstract

An air parcel model has been used to study dynamic influences on cirrus cloud microphysical processes. Representative data selected from a measurement campaign carried out over southern Germany during March 1994 were used for a base-case model run where a modeled air parcel moved in a wave trajectory with a period similar to the measured Brunt–Väisälä frequency and an amplitude of about 30 m. Six case studies were performed for this paper. In each case, ice crystal nucleation processes were examined as an air parcel moved with trajectories having different wave forms. A random walk trajectory simulating turbulence with turbulent structure was also considered. The relationships between the parameters in the air parcel trajectories and crystal microphysical properties are discussed. Simulation results show that after two wave cycles, the model-produced crystal spectra are usually narrower than typical measurement data;however, broader spectra can be produced for certain types of trajectories. The broadness of crystal spectra is closely related to the air parcel’s initial position in the wave trajectory. It is not necessary to invoke entrainment to produce a broad crystal spectrum.

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Hong Lin, Kevin J. Noone, Johan Ström, and Andrew J. Heymsfield

Abstract

An air parcel model including homogeneous freezing nucleation of ice crystals has been used to study the formation and development of cirrus clouds. In situ measurements taken during March 1994 over southern Germany were used for comparison with model predictions. Typical experimental data were chosen for a base-case model run. Using measured aerosol properties as input values, the model predicts the measured ice crystal size distribution. In particular, both measurements and model results show the presence of numerous small ice crystals (diameter between 1 and 20 μm). Both measurements and model results also show that small aerosol particles (below 0.1 μm diameter) are active in forming cirrus cloud particles. The modeled microphysical properties including ice crystal size distribution, number concentration, and the residual particle size distribution are in good agreement with the experimental data. Based on the measured parameter values, a model sensitivity study considering air parcel updraft velocity, initial temperature, relative humidity, aerosol size distribution, number concentration, and air parcel vertical displacement is presented.

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Annica M. L. Ekman, Chien Wang, Johan Ström, and Radovan Krejci

Abstract

Large concentrations of small aerosols have been previously observed in the vicinity of anvils of convective clouds. A 3D cloud-resolving model (CRM) including an explicit size-resolving aerosol module has been used to examine the origin of these aerosols. Five different types of aerosols are considered: nucleation mode sulfate aerosols (here defined by 0 ≤ d ≤5.84 nm), Aitken mode sulfate aerosols (here defined by 5.84 nm ≤ d ≤ 31.0 nm), accumulation mode sulfate aerosols (here defined by d ≥ 31.0 nm), mixed aerosols, and black carbon aerosols.

The model results suggest that approximately 10% of the initial boundary layer number concentration of Aitken mode aerosols and black carbon aerosols are present at the top of the convective cloud as the cloud reaches its decaying state. The simulated average number concentration of Aitken mode aerosols in the cloud anvil (∼1.6 × 104 cm−3) is in the same order of magnitude as observations. Thus, the model results strongly suggest that vertical convective transport, particularly during the active period of the convection, is responsible for a major part of the appearance of high concentrations of small aerosols (corresponding to the Aitken mode in the model) observed in the vicinity of cloud anvils.

There is some formation of new aerosols within the cloud, but the formation is small. Nucleation mode aerosols are also efficiently scavenged through impaction scavenging by precipitation. Accumulation mode and mixed mode aerosols are efficiently scavenged through nucleation scavenging and their concentrations in the cloud anvil are either very low (mixed mode) or practically zero (accumulation mode).

In addition to the 3D CRM, a box model, including important features of the aerosol module of the 3D model, has been used to study the formation of new aerosols after the cloud has evaporated. The possibility of these aerosols to grow to suitable cloud condensation or ice nuclei size is also examined. Concentrations of nucleation mode aerosols up to 3 × 104 cm−3 are obtained. The box model simulations thus suggest that new particle formation is a substantial source of small aerosols in the upper troposphere during and after the dissipation of the convective cloud. Nucleation mode and Aitken mode aerosols grow due to coagulation and condensation of H2SO4 on the aerosols, but the growth rate is low. Provided that there is enough OH available to oxidize SO2, parts of the aerosol population (∼400 cm−3) can reach the accumulation mode size bin of the box model after 46 h of simulation.

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Valery Shcherbakov, Jean-François Gayet, Olivier Jourdan, Andreas Minikin, Johan Ström, and Andreas Petzold

Abstract

A methodology of employing statistical procedures, specifically the principal component analysis (PCA) technique, to assess cirrus cloud data reliability is described. The approach is demonstrated by an example of a study of optical and microphysical characteristics measured during two campaigns performed at midlatitudes in the pristine Southern (SH) and polluted Northern (NH) Hemispheres within the international INCA project (Interhemispheric Differences in Cirrus Cloud Properties from Anthropogenic Emissions). The datasets were obtained by using state-of-the-art airborne instruments including the polar nephelometer and PMS particle size spectrometers for the ice-particle characterization. The approach is applied to both the measured angular scattering intensities and the ice-particle size distributions. It is shown that the PCA technique allows for impartial elimination of nonreliable channels of instruments. Furthermore, this technique is efficient in a study if the dataset is statistically homogeneous, and provides the possibility of removing specific records corresponding to distinguishing statistical ensembles. The results, expressed in terms of significant components and corresponding eigenvalues, show that the Southern and Northern Hemisphere datasets are in good agreement and they can be considered as statistically representative of the sampled cirrus. Furthermore, the frequency distributions of the cirrus cloud microphysical and optical properties can be regarded as arbitrary positive quantities, which are lognormally distributed. The validation of the measurements is provided by intercomparison of parameters estimated from different and independent techniques. The statistical relationships between quantities derived from angular scattering intensities and from ice-particle distributions as well as the similarity of the results obtained for the Southern and Northern Hemisphere cases serve as proof of the reliability of the measured cloud properties.

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