The fall speeds of sub‐100 µm ice crystals

Westbrook, C. D.

doi:10.1002/qj.290

Cited by 51 publications

(95 citation statements)

References 33 publications

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“…The time interval t was defined with regard to excluding reductions of the observed ice crystal concentration by sedimentation, assuming that the largest ice crystals grow to approximately 100 µm. These large crystals determine the sedimentation timescale and sediment with terminal velocities between 0.1 and 10 cm s −1 (Westbrook, 2008). This corresponds to sedimentation times between 35 and 3500 s for an average fall distance of 3.5 m (half of the cloud chamber height).…”

Section: Ice Nucleation Active Surface Site Densitiesmentioning

confidence: 99%

A new temperature- and humidity-dependent surface site density approach for deposition ice nucleation

et al. 2015

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Abstract. Deposition nucleation experiments with ArizonaTest Dust (ATD) as a surrogate for mineral dusts were conducted at the AIDA cloud chamber at temperatures between 220 and 250 K. The influence of the aerosol size distribution and the cooling rate on the ice nucleation efficiencies was investigated. Ice nucleation active surface site (INAS) densities were calculated to quantify the ice nucleation efficiency as a function of temperature, humidity and the aerosol surface area concentration. Additionally, a contact angle parameterization according to classical nucleation theory was fitted to the experimental data in order to relate the ice nucleation efficiencies to contact angle distributions. From this study it can be concluded that the INAS density formulation is a very useful tool to describe the temperature-and humidity-dependent ice nucleation efficiency of ATD particles.Deposition nucleation on ATD particles can be described by a temperature-and relative-humidity-dependent INAS density function n s (T , S ice ) withwhere the temperature-and saturation-dependent function x therm is defined aswith the saturation ratio with respect to ice S ice > 1 and within a temperature range between 226 and 250 K. For lower temperatures, x therm deviates from a linear behavior with temperature and relative humidity over ice.Also, two different approaches for describing the time dependence of deposition nucleation initiated by ATD particles are proposed. Box model estimates suggest that the timedependent contribution is only relevant for small cooling rates and low number fractions of ice-active particles.

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Section: Ice Nucleation Active Surface Site Densitiesmentioning

confidence: 99%

A new temperature- and humidity-dependent surface site density approach for deposition ice nucleation

et al. 2015

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“…It also affects instruments [Lavigne et al, 2008], leading to important bias when retrieving cloud optical properties [Masuda and Ishimoto, 2004]. Moreover, crystal orientation impacts sedimentation speed [Heymsfield and Iaquinta, 2000;Westbrook, 2008], which affects cloud lifetime and persistence in atmospheric models [Spichtinger and Gierens, 2009]. We thus need to progress in our understanding of this phenomena: quantify the global spatiotemporal distribution of horizontally oriented crystals and the amount of crystal that orient themselves preferentially, understand what thermodynamic conditions drive orientation and explore possible links between oriented crystals and large-scale dynamics.…”

Section: Introductionmentioning

confidence: 99%

A global view of horizontally oriented crystals in ice clouds from Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)

Noël¹,

Chepfer²

2010

J. Geophys. Res.

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[1] We analyze optical signatures in 18 months of CALIOP layer-integrated backscatter and depolarization ratio to investigate the geographical and seasonal distribution of oriented crystals in ice clouds on a global scale. Oriented crystals are found to be rare: they appear in ∼6% of all ice cloud layers, and inside these layers the proportion of oriented crystals is estimated below 5%, even though they have a significant effect on the cloud optical properties. The geographical pattern of crystal orientation is very stable over the year, without any noticeable cycle. We investigate the atmospheric conditions which might lead to crystal orientation, including synoptic-scale dynamics and thermodynamic profiles. In the tropics, detections of crystal orientation are more numerous in areas dominated by convection on a monthly basis, and at midlatitudes less numerous in areas dominated by strong horizontal winds. Synoptic effects, however, appear secondary; orientation is primarily driven by temperature. Oriented crystals are mostly nonexistent in ice clouds colder than −30°C, and very frequent in warmer ice clouds, appearing in 30% of such clouds in the tropics and up to 50% at higher latitudes. The temperatures where oriented crystals are found (−30°C to −10°C) are conducive to the formation of planar crystals. Results suggest oriented crystals are more frequent just above cloud base in slightly thicker cloud layers, which might provide clues to how and why orientation takes place.

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“…Furthermore, compact (i.e. mass density of about 1.62 g cm −3 ) needle-or diskshaped particles would also have reduced settling velocities compared to mass-equivalent compact spherical particles (Westbrook, 2008). More complex combinations of particle shapes and mass densities are possible, but in the following we will focus on only these cases.…”

Section: Discussionmentioning

confidence: 99%

“…column or disk). For estimating the sizes of the indicated aspheric particle types associated with the discussed relative settling velocity conditions the respective hydrodynamic radii were approximated by the corresponding capacitances according to Westbrook (2008). The relation between particle size and capacitance for cylindrical particles was taken from Smythe (1962).…”

Section: Discussionmentioning

confidence: 99%

“…Depending on the crystallisation conditions, plates, needles and more complex crystals are found, with many of the particles found being considerably aspheric. For aspheric ice particles, Westbrook (2008) estimated that the settling velocities are significantly underestimated in simulations when the settling characteristics of spherical particles are assumed.…”

Section: Introductionmentioning

confidence: 99%

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Denitrification by large NAT particles: the impact of reduced settling velocities and hints on particle characteristics

Woiwode

Grooß²,

Oelhaf

et al. 2014

Atmos. Chem. Phys.

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Abstract. Vertical redistribution of HNO 3 through large HNO 3 -containing particles associated with polar stratospheric clouds (PSCs) plays an important role in the chemistry of the Arctic winter stratosphere. During the RECON-CILE (Reconciliation of essential process parameters for an enhanced predictability of Arctic stratospheric ozone loss and its climate interactions) campaign, apparently very large NAT (nitric acid trihydrate) particles were observed by the airborne in situ probe FSSP-100 . Our analysis shows that the FSSP-100 observations associated with the flight on 25 January 2010 cannot easily be explained assuming compact spherical NAT particles due to much too short growing time at temperatures below the existence temperature of NAT (T NAT ). State-of-the-art simulations using CLaMS (Chemical Lagrangian Model of the Stratosphere; Grooß et al., 2014) suggest considerably smaller particles. We consider the hypothesis that the simulation reproduces the NAT particle masses in a realistic way, but that real NAT particles may have larger apparent sizes compared to compact spherical particles, e.g. due to non-compact morphology or aspheric shape. Our study focuses on the consequence that such particles would have reduced settling velocities compared to compact spheres, altering the vertical redistribution of HNO 3 . Utilising CLaMS simulations, we investigate the impact of reduced settling velocities of NAT particles on vertical HNO 3 redistribution and compare the results with observations of gas-phase HNO 3 by the airborne Fourier transform spectrometer MIPAS-STR associated with two REC-ONCILE flights. The MIPAS-STR observations confirm conditions consistent with denitrification by NAT particles for the flight on 25 January 2010 and show good agreement with the simulations within the limitations of the comparison. Best agreement is found if settling velocities between 100 and 50 % relative to compact spherical particles are considered (slight preference for the 70 % scenario). In contrast, relative settling velocities of 30 % result in too weak vertical HNO 3 redistribution. Sensitivity simulations considering temperature biases of ±1 K and multiplying the simulated nucleation rates by factors of 0.5 and 2.0 affect the comparisons to a similar extent, but result in no effective improvement compared to the reference scenario. Our results show that an accurate knowledge of the settling velocities of NAT particles is important for quantitative simulations of vertical HNO 3 redistribution.

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The fall speeds of sub‐100 µm ice crystals

Cited by 51 publications

References 33 publications

A new temperature- and humidity-dependent surface site density approach for deposition ice nucleation

A new temperature- and humidity-dependent surface site density approach for deposition ice nucleation

A global view of horizontally oriented crystals in ice clouds from Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO)

Denitrification by large NAT particles: the impact of reduced settling velocities and hints on particle characteristics

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