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K. M. Gierens
and
J. Ström

Abstract

Numerical simulations of ice cloud formation in the wake of an aircraft flying at cruise altitude have been performed. The engine exhaust has been excluded from the simulations in order to study cloud formation due solely to aerodynamic effects. The ice is formed via homogeneous freezing nucleation of ambient haze droplets in the upwelling limbs of the vortex pair behind the aircraft. Properties of wake ice clouds are compared with properties of contrails obtained with in situ measurements and recent simulations. In particular, the authors find that aerodynamically induced ice clouds are similar in microphysical and radiative respects to contrails that are formed from the nucleation of exhaust particles. This means that under cold and moist conditions contrails as young as 2–5 min may consist of similar amounts of aerodynamically produced and exhaust triggered ice crystals.

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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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J. Ström
,
B. Strauss
,
T. Anderson
,
F. Schröder
,
J. Heintzenberg
, and
P. Wendling

Abstract

In situ measurements made in cold (−35° to −60°C) cirrus clouds over southern Germany in March 1994 are presented. The clouds appeared to be in an early stage of their life cycle and their properties in many ways resemble those reported for ice fogs. Crystal concentrations were high (median 2.5 cm−3, STP) and sizes small with a diameter of mean mass of typically 16 μm. The cloud on 18 March presents an interesting case for modeling studies of cirrus formation. On that particular day, the bulk properties of the cloud appeared to be connected to wave structures in the vertical wind field consistent with the Brunt–Väisälä frequency, which gave a corresponding wavelength of 40–50 km. Furthermore, analyses of potential temperature and vertical wind suggested that the vertical displacement producing these clouds was less than 100 m. Size distribution measurements of interstitial particles and crystal residues (particles remaining after evaporation of the crystals) show that small aerosol particles (diameters <0.5 μm) participate in the nucleation of cirrus crystals at low temperatures. Because the aerosol in this small size range is readily perturbed by anthropogenic activity, it is important to study the link between upper tropospheric aerosol properties and cirrus cloud microphysics. While the observations presented here are not adequate to quantify this link, they pave the way for modeling studies and would be interesting to compare to cirrus observations from cleaner regions.

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F. Schröder
,
B. Kärcher
,
C. Duroure
,
J. Ström
,
A. Petzold
,
J.-F. Gayet
,
B. Strauss
,
P. Wendling
, and
S. Borrmann

Abstract

In situ observations of the microphysical properties of upper-tropospheric contrails and cirrus clouds have been performed during more than 15 airborne missions over central Europe. Experimental and technical aspects concerning in situ characterization of ice clouds with the help of optical and nonoptical detection methods (preferably FSSP-300 and Hallet-type replicator) are addressed. The development of contrails into cirrus clouds on the timescale of 1 h is discussed in terms of a representative set of number densities, and size distributions and surface area distributions of aerosols and cloud elements, with special emphasis on small ice crystals (diameter <20 μm). Contrails are dominated by high concentrations (>100 cm−3) of nearly spherical ice crystals with mean diameters in the range 1–10 μm. Young cirrus clouds, which mostly contain small regularly shaped ice crystals in the range 10–20-μm diameter and typical concentrations 2–5 cm−3, have been observed. Measurement results are compared to simple parcel model calculations to identify parameters relevant for the contrail–cirrus transition. Observations and model estimates suggest that contrail growth is only weakly, if at all, affected by preexisting cirrus clouds.

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