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

ATMOSPHERIC CHEMISTRY AND PHYSICS (Impact Factor: 5.51). 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.

  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: This study presents the sensitivity of global ice crystal number concentration, Nc, to the parameterization of heterogeneous ice nuclei (IN). Simulations are carried out with the NASA Global Modeling Initiative chemical and transport model coupled to an analytical ice microphysics parameterization. Heterogeneous freezing is described using nucleation spectra derived from theoretical considerations and empirical data for dust, black carbon, ammonium sulfate, and glassy aerosol as IN precursors. When competition between homogeneous and heterogeneous freezing is considered, global mean Nc vary by up to a factor of twenty depending on the heterogeneous freezing spectrum used. IN effects on Nc strongly depend on dust and black carbon concentrations and are strongest under conditions of weak updraft and high temperature. Regardless of the heterogeneous spectrum used, dust is an important contributor of IN over large regions of the Northern Hemisphere. Black carbon however exhibits appreciable effects on Nc when the freezing fraction is greater than 1%. Compared to in situ observations, Nc is overpredicted at temperatures below 205 K, even if a fraction of liquid aerosol is allowed to act as glassy IN. Assuming that cirrus formation is forced by weak updraft addressed this overprediction but promoted heterogeneous freezing effects to the point where homogeneous freezing is inhibited for IN concentrations as low as 1 L−1. Chemistry and dynamics must be considered to explain cirrus characteristics at low temperature. Only cloud formation scenarios where competition between homogeneous and heterogeneous freezing is the dominant feature would result in maximum supersaturation levels consistent with observations.
    Journal of Geophysical Research Atmospheres 01/2010; 115. · 3.44 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Atmospheric aerosol particles serve as nuclei for ice-crystal formation. As such, these particles are critical to the generation of cirrus clouds, which form from gas and liquid water. Atmospheric aerosols also initiate ice formation in warmer, mixed-phase clouds, where ice crystals coexist with aqueous droplets. Biogenic aerosol particles of terrestrial origin, including bacteria and pollen, can act as ice nuclei. Whether biogenic particles of marine origin also act as ice nuclei has remained uncertain. We exposed the cosmopolitan planktonic diatom species Thalassiosira pseudonana to water vapour and supercooled aqueous sodium chloride under typical tropospheric conditions conducive to cirrus-cloud formation. Ice nucleation was determined using a controlled vapour cooling-stage microscope system. Under all conditions, diatoms initiated ice formation. The presence of diatoms in water increased the temperature for ice formation up to 13K, and in aqueous sodium chloride, ice formed at temperatures up to 30K higher than when diatoms were not present. In addition, diatoms initiated ice formation from water vapour at relative humidities as low as 65%. The rate of ice nucleation was rapid and independent of surface area. We suggest that marine biogenic particles such as diatoms help explain high values and seasonal variations in ice-nuclei concentrations in subpolar regions.
    Nature Geoscience 01/2010; 4(2):88-90. · 11.67 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: A methodology to identify and characterize cirrus clouds has been developed and applied to the multichannel-multiwavelength Rayleigh-Mie-Raman (RMR) lidar in Rome-Tor Vergata (RTV). A set of 167 cirrus cases, defined on the basis of quasi-stationary temporal period conditions, has been selected in a dataset consisting of about 500 h of nighttime lidar sessions acquired between February 2007 and April 2010. The derived lidar parameters (effective height, geometrical and optical thickness and mean back-scattering ratio) and the cirrus mid-height temperature (estimated from the radiosoundings of Pratica di Mare, WMO site #16245) of this sample have been analyzed by the means of a clustering multivariate analysis. This approach identified four cirrus classes above the RTV site: two thin cirrus clusters in mid and upper troposphere and two thick cirrus clusters in mid-upper troposphere. These results, which are very similar to those derived through the same approach in the lidar site of the Observatoire of Haute Provence (OHP), allows characterizing cirrus clouds over RTV site and attests the robustness of such classification. To have some indications about the cirrus generation methods for the different classes, the analyses of the extinction-to-backscatter ratio (lidar ratio, LReff), in terms of the frequency distribution functions and depending on the mid-height cirrus temperature have been performed. This study suggests that smaller (larger) ice crystals compose thin (thick) cirrus classes. This information, together with the value of relative humidity over ice (110 ± 30%), calculated through the simultaneous WV Raman measurements for the mid-tropospheric thin class, indicates that this class could be formed by an heterogeneous nucleation mechanism. The RTV cirrus results, re-computed through the cirrus classification by Sassen and Cho (1992), shows good agreement to other mid-latitude lidar cirrus observation for the relative occurrence of subvisible (SVC), thin and opaque cirrus classes (10%, 49% and 41%, respectively). The overall mean value of cirrus optical depth is 0.37 ± 0.18 , while most retrieved LReff values ranges between 10-60 sr and the estimated mean value is 31 ± 15 sr, similar to LR values of lower latitude cirrus measurements. The obtained results are consistent with previous studies conducted with different systems and confirm that cirrus classification based on a statistical approach seems to be a good tool both to validate the height-resolved cirrus fields, calculated by models, and to investigate the key processes governing cirrus formation and evolution. These are fundamental elements to improve the characterization of the cirrus optical properties and, thus, the determination of their radiative impact.
    Atmospheric Chemistry and Physics 04/2013; 13(4):9615-9652. · 4.88 Impact Factor

Full-text (4 Sources)

Available from
May 23, 2014