Influence of surface roughness of ice crystal particles on bulk scattering properties of cirrus clouds

2019 ◽  
Author(s):  
Shenhe Ren ◽  
Ming Gao ◽  
Yanxiang Liu ◽  
Yan Li
Keyword(s):  
Surface Roughness ◽  
Cirrus Clouds ◽  
Ice Crystal ◽  
Bulk Scattering

scholarly journals Cirrus clouds in a global climate model with a statistical cirrus cloud scheme

2010 ◽  
Vol 10 (12) ◽  
pp. 5449-5474 ◽  
Author(s):  
M. Wang ◽  
J. E. Penner
Keyword(s):  
Climate Model ◽  
Global Climate ◽  
Global Climate Model ◽  
Lower Troposphere ◽  
Circulation Model ◽  
Cirrus Clouds ◽  
Cloud Formation ◽  
Cirrus Cloud ◽  
Ice Crystal ◽  
Homogeneous Freezing

Abstract. A statistical cirrus cloud scheme that accounts for mesoscale temperature perturbations is implemented in a coupled aerosol and atmospheric circulation model to better represent both subgrid-scale supersaturation and cloud formation. This new scheme treats the effects of aerosol on cloud formation and ice freezing in an improved manner, and both homogeneous freezing and heterogeneous freezing are included. The scheme is able to better simulate the observed probability distribution of relative humidity compared to the scheme that was implemented in an older version of the model. Heterogeneous ice nuclei (IN) are shown to decrease the frequency of occurrence of supersaturation, and improve the comparison with observations at 192 hPa. Homogeneous freezing alone can not reproduce observed ice crystal number concentrations at low temperatures (<205 K), but the addition of heterogeneous IN improves the comparison somewhat. Increases in heterogeneous IN affect both high level cirrus clouds and low level liquid clouds. Increases in cirrus clouds lead to a more cloudy and moist lower troposphere with less precipitation, effects which we associate with the decreased convective activity. The change in the net cloud forcing is not very sensitive to the change in ice crystal concentrations, but the change in the net radiative flux at the top of the atmosphere is still large because of changes in water vapor. Changes in the magnitude of the assumed mesoscale temperature perturbations by 25% alter the ice crystal number concentrations and the net radiative fluxes by an amount that is comparable to that from a factor of 10 change in the heterogeneous IN number concentrations. Further improvements on the representation of mesoscale temperature perturbations, heterogeneous IN and the competition between homogeneous freezing and heterogeneous freezing are needed.

scholarly journals The roles of dynamical variability and aerosols in cirrus cloud formation

2003 ◽  
Vol 3 (2) ◽  
pp. 1415-1451 ◽  
Author(s):  
B. Kärcher ◽  
J. Ström
Keyword(s):  
Probability Distributions ◽  
Cirrus Clouds ◽  
Cloud Formation ◽  
Cirrus Cloud ◽  
Ice Crystal ◽  
Mesoscale Variability ◽  
Aerosol Composition ◽  
Airborne Measurements ◽  
Cloud Properties

Abstract. The probability of occurrence of ice crystal number densities in young cirrus clouds is examined based on airborne measurements. The observations have been carried out at midlatitudes in both hemispheres at equivalent latitudes (~52–55° N/S) during the same season (local autumn in 2000). The in situ measurements considered in the present study include temperatures, vertical velocities, and ice crystal concentrations, the latter determined with high precision and accuracy using a counterflow virtual impactor. Most young cirrus clouds typically contain high number densities (1–10 cm−3) of small (diameter <20 μm) ice crystals. This mode dominates the probability distributions in both hemispheres and is shown to be caused by rapid cooling rates associated with updraft speeds in the range 10–100 cm s-1. A second mode containing larger crystals extends from ~1 cm−3 to low concentrations close to the detection threshold (~3×104cm−3) and is associated with lower updraft speeds. Results of a statistical analysis provide compelling evidence that the dynamical variability of vertical air motions on the mesoscale is the key factor determining the observed probability distributions of pristine ice crystal concentrations in cirrus. Other factors considered are variations of temperature as well as size, number, and ice nucleation thresholds of the freezing aerosol particles. The variability in vertical velocities is likely caused by atmospheric waves. Inasmuch as gravity waves are widespread, mesoscale variability in vertical velocities can be viewed as a universa  feature of young cirrus clouds. Large-scale models that do not account for this subgrid-scale variability yield erroneous predictions of the variability of basic cirrus cloud properties. Climate change may bring about changes in the global distribution of updraft speeds, mean air temperatures, and aerosol properties. As shown in this work, these changes could significantly modify the probability distribution of cirrus ice crystal concentrations. This study emphasizes the key role of vertical velocities and mesoscale variability in vertical velocities in controlling cirrus properties. The results suggest that, in any effort to ascribe cause to trends of cirrus cloud properties, a careful evaluation of dynamical changes in cloud formation should be done before conclusions regarding the role of other anthropogenic factors, such as changes in aerosol composition, are made.

The Origin of High Ice Crystal Number Densities in Cirrus Clouds

2005 ◽  
Vol 62 (7) ◽  
pp. 2568-2579 ◽  
Author(s):  
C. R. Hoyle ◽  
B. P. Luo ◽  
T. Peter
Keyword(s):  
Cirrus Clouds ◽  
Small Scale ◽  
Ice Crystal ◽  
Cooling Rates ◽  
Density Distributions ◽  
In Situ Forming ◽  
Independent Particle ◽  
Scale Temperature ◽  
High Cooling Rates ◽  
Homogeneous Freezing

Abstract Recent measurements with four independent particle instruments in cirrus clouds, which formed without convective or orographic influence, report high number densities of ice particles (as high as nice = 50 cm−3) embedded in broad density distributions (nice = 0.1–50 cm−3). It is shown here that small-scale temperature fluctuations related to gravity waves, mechanical turbulence, or other small-scale air motions are required to explain these observations. These waves have typical peak-to-peak amplitudes of 1–2 K and frequencies of up to 10 h−1, corresponding to instantaneous cooling rates of up to 60 K h−1. Such waves remain unresolved in even the most advanced state-of-the-art global atmospheric models. Given the ubiquitous nature of these fluctuations, it is suggested that the character of young in situ forming cirrus clouds is mostly determined by homogeneous freezing of ice in solution droplets, driven by a broad range of small-scale fluctuations (period ∼a few minutes) with moderate to high cooling rates (1–100 K h−1).

402 Investigation on influence of surface roughness of cooling surface on ice structure formed by control of initial ice crystal orientation

2014 ◽  
Vol 2014.24 (0) ◽  
pp. 121-122
Author(s):  
Takahiro IKEYA ◽  
Koji MATSUMOTO ◽  
Yoshikazu TERAOKA ◽  
Kazuyoshi MATSUNAGA ◽  
Masato HONDA
Keyword(s):  
Surface Roughness ◽  
Crystal Orientation ◽  
Ice Crystal ◽  
Cooling Surface

scholarly journals Mesoscopic surface roughness of ice crystals pervasive across a wide range of ice crystal conditions

2014 ◽  
Vol 14 (22) ◽  
pp. 12357-12371 ◽  
Author(s):  
N. B. Magee ◽  
A. Miller ◽  
M. Amaral ◽  
A. Cumiskey
Keyword(s):  
Surface Roughness ◽  
Environmental Scanning Electron Microscopy ◽  
Diffusion Chamber ◽  
Ice Crystals ◽  
Environmental Scanning ◽  
Ice Crystal ◽  
Hexagonal Ice ◽  
Wide Range ◽  
Ice Surfaces ◽  
Cloud Modeling

Abstract. Here we show high-magnification images of hexagonal ice crystals acquired by environmental scanning electron microscopy (ESEM). Most ice crystals were grown and sublimated in the water vapor environment of an FEI-Quanta-200 ESEM, but crystals grown in a laboratory diffusion chamber were also transferred intact and imaged via ESEM. All of these images display prominent mesoscopic topography including linear striations, ridges, islands, steps, peaks, pits, and crevasses; the roughness is not observed to be confined to prism facets. The observations represent the most highly magnified images of ice surfaces yet reported and expand the range of conditions in which rough surface features are known to be conspicuous. Microscale surface topography is seen to be ubiquitously present at temperatures ranging from −10 °C to −40 °C, in supersaturated and subsaturated conditions, on all crystal facets, and irrespective of substrate. Despite the constant presence of surface roughness, the patterns of roughness are observed to be dramatically different between growing and sublimating crystals, and transferred crystals also display qualitatively different patterns of roughness. Crystals are also demonstrated to sometimes exhibit inhibited growth in moderately supersaturated conditions following exposure to near-equilibrium conditions, a phenomenon interpreted as evidence of 2-D nucleation. New knowledge about the characteristics of these features could affect the fundamental understanding of ice surfaces and their physical parameterization in the context of satellite retrievals and cloud modeling. Links to supplemental videos of ice growth and sublimation are provided.

Elemental composition and morphology of ice-crystal residual particles in cirrus clouds and contrails

1998 ◽  
Vol 49 (1) ◽  
pp. 21-34 ◽  
Author(s):  
A Petzold ◽  
J Ström ◽  
S Ohlsson ◽  
F.P Schröder
Keyword(s):  
Elemental Composition ◽  
Cirrus Clouds ◽  
Ice Crystal

scholarly journals Observations and modelling of microphysical variability, aggregation and sedimentation in tropical storm cirrus outflow regions

2011 ◽  
Vol 11 (8) ◽  
pp. 23761-23800
Author(s):  
M. W. Gallagher ◽  
P. J. Connolly ◽  
A. Heymsfield ◽  
K. N. Bower ◽  
T. W. Choularton ◽  
...  
Keyword(s):  
Collection Efficiency ◽  
Aerosol Particles ◽  
Tropical Storm ◽  
Cirrus Clouds ◽  
Ice Crystals ◽  
Aircraft Measurements ◽  
Ice Crystal ◽  
Crystal Aggregation ◽  
Small Crystals ◽  
Bulk Model

Abstract. Aircraft measurements of the microphysics of a tropical convective anvil (at temperatures ~−60 °C) forming above the HECTOR storm have been performed. The observed microphysics has been compared to a bulk and explicit microphysical model of the anvil region including crystal aggregation and sedimentation. It has been found that in flights made using straight and level runs perpendicular to the storm that the number of ice crystals initially decreased with distance from the storm as aggregation took place resulting in larger crystals followed by their loss due to sedimentation. At still greater distances from the storm the number of very small crystals increased. This is attributed to the formation of new ice crystals on aerosol particles as the ice super saturation rose following the depletion of the larger ice particles following aggregation and sedimentation. Comparison with the explicit microphysics model showed that the changes in the shapes of the ice crystal spectra as a function of distance from the storm could be explained by the explicit microphysical model if the aggregation efficiency was set to E~0.02. It is noteworthy that this aggregation efficiency is much larger than values normally used in cloud resolving models at these temperatures (typically E~0.0016). Furthermore if the bulk model is used then optimum agreement was reached with a collection efficiency for aggregation of E~0.05. These results are important for the treatment of the evolution and lifetime of tropical cirrus clouds.

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