Article

Cirrus Cloud Occurrence as Function of Ambient Relative Humidity: A Comparison of Observations Obtained during the INCA Experiment

ATMOSPHERIC CHEMISTRY AND PHYSICS (Impact Factor: 5.3). 01/2003; 3(2003):1807-1816. DOI: 10.5194/acp-3-1807-2003
Source: DLR

ABSTRACT Based on in-situ observations performed during
the Interhemispheric differences in cirrus properties from anthropogenic
emissions (INCA) experiment, we introduce and
discuss the cloud presence fraction (CPF) defined as the ratio
between the number of data points determined to represent
cloud at a given ambient relative humidity over ice
(RHI) divided by the total number of data points at that value
of RHI. The CPFs are measured with four different cloud
probes. Within similar ranges of detected particle sizes and
concentrations, it is shown that different cloud probes yield
results that are in good agreement with each other. The CPFs
taken at Southern Hemisphere (SH) and Northern Hemisphere
(NH) midlatitudes differ from each other. Above ice
saturation, clouds occurred more frequently during the NH
campaign. Local minima in the CPF as a function of RHI
are interpreted as a systematic underestimation of cloud presence
when cloud particles become invisible to cloud probes.
Based on this interpretation, we find that clouds during the
SH campaign formed preferentially at RHIs between 140
and 155%, whereas clouds in the NH campaign formed at
RHIs somewhat below 130%. The data show that interstitial
aerosol and ice particles coexist down to RHIs of 70–90%,
demonstrating that the ability to distinguish between different
particle types in cirrus conditions depends on the sensors
used to probe the aerosol/cirrus system. Observed distributions
of cloud water content differ only slightly between the
NH and SH campaigns and seem to be only weakly, if at all,
affected by the freezing aerosols.

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    • "Their maximum dimensions decreased to values of 100 µm or less. It is well known from in situ observations that cirrus ice particles with low ice water content may be present in interstitial, subsaturated regions (Ström et al., 2003). "
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    ABSTRACT: We apply a cloud-resolving model with explicit aerosol and ice microphysics and Lagrangian ice particle tracking to simulate the evolution of a cirrus cloud field observed during the US Atmospheric Radiation Measurement Program Intensive Operational Period in March 2000. This comprehensive data set includes remote sensing, radiosonde, and aircraft measurements of a midlatitude cirrus cloud system, supported by estimates of the dynamical cloud forcing. The dataset allows us to evaluate and study in great detail the process-oriented representation of the microphysical processes relevant to the formation and evolution of deep, stratiform cirrus (in particular ice crystal sedimentation and aggregation). The suite of explicitly resolved physical processes in our model enables us to better understand the sensitivity of the simulated cirrus properties on a large number ofmicrophysical and environmental parameters. The evolution of the domain-integrated cloud optical depth is largely dominated by homogeneous freezing processes.Wefind that the evolution of the observed cirrus cloud system is most dependent on updraught speed and ice supersaturation and that homogeneous freezing leads to a total, cloud-averaged ice crystal concentration of 0.1 cm−3 of air. It is not necessary to invoke heterogeneous ice nuclei to explain most of the data, but we cannot rule out that a small concentration (up to 0.002 cm−3) of such particles may have affected the cirrus cloud field in nature. Cloud-averaged ice particle size distributions are bimodal, separating two distinct growth regimes in the developed cloud. The small mode (ice particle sizes below a few 100μm) forms by homogeneous freezing of supercooled aerosol droplets and grows by deposition of water molecules from the gas phase. The large mode (sizes up to several 1000μm) forms and grows by aggregation. We demonstrate that the formation of the largest crystals by aggregation in deep cirrus is controlled in part by the nucleation of new ice crystals in dynamically active, highly supersaturated upper cloud regions. Furthermore, a pronounced increase in the number of aggregation events is predicted in sublimation zones. The combined effect of sublimation and sedimentation leads to the formation of a very thin (vertical extension ∼ 100 m) sublimation microlayer mainly composed of aggregated ice crystals, containing relatively high total ice crystal number concentrations (∼ 0.02 cm−3) comparable to those generated locally by homogeneous freezing in the upper cloud layers.
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    • "To derive these distributions , we have used the Forward Scattering Spectrometer Probe (FSSP-300) and 2D-C cloud probe data with 1 s time resolution. Together, these cloud probes cover the ice crystal size range 3–800 µm; details of their characteristics are summarised by Ström et al. (2003) and Gayet et al. (2006). For the solid stepped line, we have used measurements for which T lay within ±5 K around the mean temperature 225 K, in which most of the data were taken. "
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    ABSTRACT: A statistical cloud scheme for non-convective cirrus formed by homogeneous freezing of supercooled aerosols is presented. As large-scale cirrus clouds exhibit metastable thermodynamic states and have long lifetimes, a fully prognostic approach is developed. The scheme is based on separate probability distribution functions of total water representing clear-sky and in-cloud conditions. These distributions are based on in situ observations and have few degrees of freedom so that they can be used in forecast and climate models. Predictive equations are derived for horizontal cloud fraction and mixing ratios of grid-mean water vapour and ice water, in-cloud water vapour and ice crystal number. The scheme allows sub- and supersaturations with respect to ice to occur in cloud-free air and inside cirrus, and produces nucleated ice crystal concentrations and sizes in good agreement with observations, an important prerequisite for an improved treatment of cirrus cloud radiative properties. Implementation in general circulation models, open issues and future research avenues are discussed.
  • Source
    • "To derive these distributions , we have used the Forward Scattering Spectrometer Probe (FSSP-300) and 2D-C cloud probe data with 1 s time resolution. Together, these cloud probes cover the ice crystal size range 3–800 µm; details of their characteristics are summarised by Ström et al. (2003) and Gayet et al. (2006). For the solid stepped line, we have used measurements for which T lay within ±5 K around the mean temperature 225 K, in which most of the data were taken. "
    [Show abstract] [Hide abstract]
    ABSTRACT: A statistical cloud scheme for non-convective cirrus formed by homogeneous freezing of supercooled aerosols is presented. As large-scale cirrus clouds exhibit metastable thermodynamic states and have long lifetimes, a fully prognostic approach is developed. The scheme is based on separate probability distribution functions of total water representing clear-sky and in-cloud conditions. These distributions are based on in situ observations and have few degrees of freedom so that they can be used in forecast and climate models. Predictive equations are derived for horizontal cloud fraction and mixing ratios of grid-mean water vapour and ice water, in-cloud water vapour and ice crystal number. The scheme allows sub- and supersaturations with respect to ice to occur in cloud-free air and inside cirrus, and produces nucleated ice crystal concentrations and sizes in good agreement with observations, an important prerequisite for an improved treatment of cirrus cloud radiative properties. Implementation in general circulation models, open issues and future research avenues are discussed. Copyright c � 2008 Royal Meteorological Society
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