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: 5.51). 01/2003; 3(2003):1807-1816.
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.

0 Bookmarks
 · 
61 Views
  • Source
    Monthly Weather Review 01/2005; 133(12). · 2.76 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Factors controlling the microphysical link between distributions of relative humidity above ice saturation in the upper troposphere and lowermost stratosphere and cirrus clouds are examined with the help of microphysical trajectory simulations. Our findings are related to results from aircraft measurements and global model studies. We suggest that the relative humidities at which ice crystals form in the atmosphere can be inferred from in situ measurements of water vapor and temperature close to, but outside of, cirrus clouds. The comparison with concomitant measurements performed inside cirrus clouds provides a clue to freezing mechanisms active in cirrus. The analysis of field data taken at northern and southern midlatitudes in fall 2000 reveals distinct differences in cirrus cloud freezing thresholds. Homogeneous freezing is found to be the most likely mechanism by which cirrus form at southern hemisphere midlatitudes. The results provide evidence for the existence of heterogeneous freezing in cirrus in parts of the polluted northern hemisphere, but do not suggest that cirrus clouds in this region form exclusively on heterogeneous ice nuclei, thereby emphasizing the crucial importance of homogeneous freezing. The key features of distributions of upper tropospheric relative humidity simulated by a global climate model are shown to be in general agreement with both, microphysical simulations and field observations, delineating a feasible method to include and validate ice supersaturation in other large-scale atmospheric models, in particular chemistry-transport and weather forecast models.
    ATMOSPHERIC CHEMISTRY AND PHYSICS 10/2003; 3(2003):1791-1806. · 5.51 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)

View
27 Downloads
Available from
May 23, 2014