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Journal ArticleDOI

High‐frequency gravity waves and homogeneous ice nucleation in tropical tropopause layer cirrus

TL;DR: In this article, the impact of high-frequency gravity waves on homogeneous-freezing ice nucleation in cold cirrus clouds is examined using parcel model simulations driven by superpressure balloon measurements of temperature variability experienced by air parcels in the tropical tropopause region.
Abstract: The impact of high-frequency gravity waves on homogeneous-freezing ice nucleation in cold cirrus clouds is examined using parcel model simulations driven by superpressure balloon measurements of temperature variability experienced by air parcels in the tropical tropopause region. We find that the primary influence of high-frequency waves is to generate rapid cooling events that drive production of numerous ice crystals. Quenching of ice nucleation events by temperature tendency reversal in the highest-frequency waves does occasionally produce low ice concentrations, but the overall impact of high-frequency waves is to increase the occurrence of high ice concentrations. The simulated ice concentrations are considerably higher than indicated by in situ measurements of cirrus in the tropical tropopause region. One-dimensional simulations suggest that although sedimentation reduces mean ice concentrations, a discrepancy of about a factor of 3 with observed ice concentrations remains. Reconciliation of numerical simulations with the observed ice concentrations will require inclusion of physical processes such as heterogeneous nucleation and entrainment.
Citations
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Journal ArticleDOI
01 Feb 2017
TL;DR: The authors assesses recent observational and modeling evidence of how anthropogenic activities might affect the formation of cirrus clouds and conclude that changes in physical properties and chemical composition of liquid aerosol particles will not affect cirrus significantly, but anthropogenic influences may occur through changes in heterogeneous ice nuclei.
Abstract: Cirrus clouds are ubiquitous, long-lived, high-level ice clouds that exert a considerable global radiative effect on the climate system. This review assesses recent observational and modeling evidence of how anthropogenic activities might affect cirrus. Changes in physical properties and chemical composition of liquid aerosol particles will unlikely affect cirrus significantly, but anthropogenic influences may occur through changes in heterogeneous ice nuclei. Two main uncertain factors contribute to the current inability to constrain background cirrus formation: small-scale variability in dynamical forcings that drive ice nucleation parameterizations and the ability of airborne particles to act as efficient heterogeneous ice nuclei. These uncertainties keep us from drawing robust conclusions about anthropogenic influences on cirrus. Microphysical and macrophysical representation of cirrus in global models must first be advanced before we can predict changes in climate with fewer uncertainties. Some climate intervention studies suggest a potential cooling effect of deliberately perturbed cirrus, but at the risk of modifying precipitation inadvertently.

65 citations

Journal ArticleDOI
TL;DR: In this paper, the results from 22 airborne field campaigns, including more than 10 million high-resolution particle images collected in cirrus formed in situ and in convective anvils, are interpreted in terms of particle shapes and their potential impact on radiative transfer.
Abstract: Results from 22 airborne field campaigns, including more than 10 million high‐resolution particle images collected in cirrus formed in situ and in convective anvils, are interpreted in terms of particle shapes and their potential impact on radiative transfer. Emphasis is placed on characterizing ice particle shapes in tropical maritime and midlatitude continental anvil cirrus, as well as in cirrus formed in situ in the upper troposphere, and subvisible cirrus in the upper tropical troposphere layer. There is a distinctive difference in cirrus ice particle shapes formed in situ compared to those in anvils that are generated in close proximity to convection. More than half the mass in cirrus formed in situ are rosette shapes (polycrystals and bullet rosettes). Cirrus formed from fresh convective anvils is mostly devoid of rosette‐shaped particles. However, small frozen drops may experience regrowth downwind of an aged anvil in a regime with RHice > ~120% and then grow into rosette shapes. Identifiable particle shapes in tropical maritime anvils that have not been impacted by continental influences typically contain mostly single plate‐like and columnar crystals and aggregates. Midlatitude continental anvils contain single‐rimed particles, more and larger aggregates with riming, and chains of small ice particles when in a highly electrified environment. The particles in subvisible cirrus are < ~100 μm and quasi‐spherical with some plates and rare trigonal shapes. Percentages of particle shapes and power laws relating mean particle area and mass to dimension are provided to improve parameterization of remote retrievals and numerical simulations.

55 citations

01 May 2013
TL;DR: In this paper, the composition of the residual particles within cirrus crystals after the ice was sublimated was determined in situ, showing that mineral dust and metallic particles are the dominant sources of residual particles, whereas sulfate and organic particles are underrepresented, and elemental carbon and biological materials are essentially absent.
Abstract: Formation of cirrus clouds depends on the availability of ice nuclei to begin condensation of atmospheric water vapor. Although it is known that only a small fraction of atmospheric aerosols are efficient ice nuclei, the critical ingredients that make those aerosols so effective have not been established. We have determined in situ the composition of the residual particles within cirrus crystals after the ice was sublimated. Our results demonstrate that mineral dust and metallic particles are the dominant source of residual particles, whereas sulfate and organic particles are underrepresented, and elemental carbon and biological materials are essentially absent. Further, composition analysis combined with relative humidity measurements suggests that heterogeneous freezing was the dominant formation mechanism of these clouds.

44 citations

Journal ArticleDOI
Blaž Gasparini1, Steffen Münch1, Laure Poncet1, Monika Feldmann1, Ulrike Lohmann1 
TL;DR: In this paper, the authors compare simulations where the ice crystal sedimentation velocity is increased at temperatures colder than −35°C with simulations of cirrus seeding with ice nucleating particles using the ECHAM-HAM general circulation model.
Abstract: . The complex microphysical details of cirrus seeding with ice nucleating particles (INPs) in numerical simulations are often mimicked by increasing ice crystal sedimentation velocities. So far it has not been tested whether these results are comparable to geoengineering simulations in which cirrus clouds are seeded with INPs. We compare simulations where the ice crystal sedimentation velocity is increased at temperatures colder than −35 °C with simulations of cirrus seeding with INPs using the ECHAM-HAM general circulation model. The radiative flux response of the two methods shows a similar behaviour in terms of annual and seasonal averages. Both methods decrease surface temperature but increase precipitation in response to a decreased atmospheric stability. Moreover, simulations of seeding with INPs lead to a decrease in liquid clouds, which counteracts part of the cooling due to changes in cirrus clouds. The liquid cloud response is largely avoided in a simulation where seeding occurs during night only. Simulations with increased ice crystal sedimentation velocity, however, lead to counteracting mixed-phase cloud responses. The increased sedimentation velocity simulations can counteract up to 60 % of the radiative effect of CO2 doubling with a maximum net top-of-the-atmosphere forcing of −2. 2 W m−2. They induce a 30 % larger surface temperature response, due to their lower altitude of maximum diabatic forcing compared with simulations of seeding with INPs.

26 citations

References
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Journal ArticleDOI
10 Aug 2000-Nature
TL;DR: This work shows from experimental data that the homogeneous nucleation of ice from supercooled aqueous solutions is independent of the nature of the solute, but depends only on the water activity of the solution, and presents a thermodynamic theory for homogeneous ice nucleation, which expresses the nucleation rate coefficient as a function of water activity and pressure.
Abstract: The unique properties of water in the supercooled (metastable) state are not fully understood. In particular, the effects of solutes and mechanical pressure on the kinetics of the liquid-to-solid phase transition of supercooled water and aqueous solutions to ice have remained unresolved. Here we show from experimental data that the homogeneous nucleation of ice from supercooled aqueous solutions is independent of the nature of the solute, but depends only on the water activity of the solution--that is, the ratio between the water vapour pressures of the solution and of pure water under the same conditions. In addition, we show that the presence of solutes and the application of pressure have a very similar effect on ice nucleation. We present a thermodynamic theory for homogeneous ice nucleation, which expresses the nucleation rate coefficient as a function of water activity and pressure. Recent observations from clouds containing ice are in good agreement with our theory and our results should help to overcome one of the main weaknesses of numerical models of the atmosphere, the formulation of cloud processes.

1,093 citations

Journal ArticleDOI
14 Jun 2013-Science
TL;DR: Results demonstrate that mineral dust and metallic particles are the dominant source of residual particles, whereas sulfate and organic particles are underrepresented, and elemental carbon and biological materials are essentially absent.
Abstract: Formation of cirrus clouds depends on the availability of ice nuclei to begin condensation of atmospheric water vapor. Although it is known that only a small fraction of atmospheric aerosols are efficient ice nuclei, the critical ingredients that make those aerosols so effective have not been established. We have determined in situ the composition of the residual particles within cirrus crystals after the ice was sublimated. Our results demonstrate that mineral dust and metallic particles are the dominant source of residual particles, whereas sulfate and organic particles are underrepresented, and elemental carbon and biological materials are essentially absent. Further, composition analysis combined with relative humidity measurements suggests that heterogeneous freezing was the dominant formation mechanism of these clouds.

458 citations

Journal ArticleDOI
TL;DR: In this paper, the design, laboratory calibrations, and flight tests of a new optical imaging instrument, the twodimensional stereo (2D-S) probe, are presented.
Abstract: The design, laboratory calibrations, and flight tests of a new optical imaging instrument, the twodimensional stereo (2D-S) probe, are presented. Two orthogonal laser beams cross in the middle of the sample volume. Custom, high-speed, 128-photodiode linear arrays and electronics produce shadowgraph images with true 10-m pixel resolution at aircraft speeds up to 250 m s 1 . An overlap region is defined by the two laser beams, improving the sample volume boundaries and sizing of small (100 m) particles, compared to conventional optical array probes. The stereo views of particles in the overlap region can also improve determination of three-dimensional properties of some particles. Data collected by three research aircraft are examined and discussed. The 2D-S sees fine details of ice crystals and small water drops coexisting in mixed-phase cloud. Measurements in warm cumuli collected by the NCAR C-130 during the Rain in Cumulus over the Ocean (RICO) project provide a test bed to compare the 2D-S with 2D cloud (2D-C) and 260X probes. The 2D-S sees thousands of cloud drops 150 m when the 2D-C and 260X probes see few or none. The data suggest that particle images and size distributions ranging from 25 to 150 m and collected at airspeeds 100 m s 1 by the 2D-C and 260X probes are probably (erroneously) generated from out-of-focus particles. Development of the 2D-S is in its infancy, and much work needs to be done to quantify its performance and generate software to analyze data.

308 citations

Journal ArticleDOI
TL;DR: In this paper, the number of ice crystals formed via homogeneous freezing of aqueous solution droplets is rather insensitive to details of the aerosol size distribution, but increases rapidly with updraft velocity and decreases with temperature.
Abstract: [1] The nucleation and initial growth of ice crystals in cirrus clouds at low (<235 K) temperatures prevailing in the upper troposphere and in the tropopause region is theoretically considered. The analysis explains the dependence of the number density of ice crystals on the vertical velocity and temperature seen in numerical simulations of cirrus formation when the timescale of depositional growth of the pristine ice particles is fast compared to the timescale of the freezing event. In such cases, applicable in many situations, the number of crystals formed via homogeneous freezing of aqueous solution droplets is rather insensitive to details of the aerosol size distribution, but increases rapidly with updraft velocity and decreases with temperature. The derived parameterization is validated with parcel model simulations, and its applicability for use in climate models is discussed. The potential role of aerosol size and heterogeneous freezing processes in altering the predicted cirrus properties is briefly addressed.

277 citations

Journal ArticleDOI
TL;DR: In this paper, it was shown that heterogeneous nucleation of ice on glassy solution droplets is an important mechanism for ice nucleation in the tropical tropopause layer.
Abstract: Ice clouds in the tropical tropopause layer have a key role in dehydrating air that is entering the stratosphere. Cloud-chamber measurements suggest that their high humidity can be explained if heterogeneous ice nucleation on glassy aerosols is a significant nucleation mechanism in this region. Ice clouds in the tropical tropopause layer play a key role in dehydrating air as it enters the stratosphere1,2. However, in situ measurements show that water vapour within these clouds is unexpectedly supersaturated3,4,5; normally the growth of ice crystals rapidly quenches supersaturation3. The high in-cloud humidity may be related to the low number of ice crystals found in these clouds4,6, but low ice number densities are inconsistent with standard models of cirrus cloud formation involving homogeneous freezing of liquid aerosols7. Aqueous aerosols rich in organic matter are ubiquitous in the atmosphere8,9, and under cirrus conditions they are known to become glassy10,11, that is, amorphous, non-crystalline solids. Here we report experiments in a cloud simulation chamber that demonstrate heterogeneous nucleation of ice on glassy solution droplets. Cirrus residues measured in situ showed ice nuclei rich in oxidized organic matter12, consistent with heterogeneous nucleation on glassy aerosols. In addition, using a one-dimensional cirrus model, we show that nucleation on glassy aerosols may explain low ice crystal numbers and high in-cloud humidity in the tropical tropopause layer. We propose that heterogeneous nucleation on glassy aerosols is an important mechanism for ice nucleation in the tropical tropopause layer.

272 citations