Abstract
Abstract A methodology of employing statistical procedures, specifically the principal component analysis (PCA) technique, to assess cirrus cloud data reliability is described. The approach is demonstrated by an example of a study of optical and microphysical characteristics measured during two campaigns performed at midlatitudes in the pristine Southern (SH) and polluted Northern (NH) Hemispheres within the international INCA project (Interhemispheric Differences in Cirrus Cloud Properties from Anthropogenic Emissions). The datasets were obtained by using state-of-the-art airborne instruments including the polar nephelometer and PMS particle size spectrometers for the ice-particle characterization. The approach is applied to both the measured angular scattering intensities and the ice-particle size distributions. It is shown that the PCA technique allows for impartial elimination of nonreliable channels of instruments. Furthermore, this technique is efficient in a study if the dataset is statistically homogeneous, and provides the possibility of removing specific records corresponding to distinguishing statistical ensembles. The results, expressed in terms of significant components and corresponding eigenvalues, show that the Southern and Northern Hemisphere datasets are in good agreement and they can be considered as statistically representative of the sampled cirrus. Furthermore, the frequency distributions of the cirrus cloud microphysical and optical properties can be regarded as arbitrary positive quantities, which are lognormally distributed. The validation of the measurements is provided by intercomparison of parameters estimated from different and independent techniques. The statistical relationships between quantities derived from angular scattering intensities and from ice-particle distributions as well as the similarity of the results obtained for the Southern and Northern Hemisphere cases serve as proof of the reliability of the measured cloud properties.
Highlights
It is generally recognized that inadequate parameterization of ice-phase clouds is one of the greatest sources of uncertainty in both the modeling of climate processes and the interpretation of satellite remote sensing data
Another objective is to report on statistical parameters of cirrus cloud optical and microphysical characteristics measured during two campaigns of the European INCA experiment (Interhemispheric Differences in Cirrus Properties from Anthropogenic Emissions; Ström et al 2001)
This paper describes the methodology of applying statistical procedures and, in particular, the principal component analysis technique to assess cirrus cloud data reliability
Summary
It is generally recognized that inadequate parameterization of ice-phase clouds is one of the greatest sources of uncertainty in both the modeling of climate processes and the interpretation of satellite remote sensing data (see, among others, Gonzalez et al 2002). Several intensive field campaigns have been carried out over the last two decades in order to improve our knowledge of icephase cloud systems—for example, the First International Satellite Cloud Climatology Project (ISCCP) Regional Experiment (FIRE) experiments (Starr and Wylie 1990; Randall et al 1995), the International Cirrus Experiment (ICE’89; Raschke et al 1990), the European Cloud and Radiation Experiment (EUCREX’94; Sauvage et al 1999), Formation Processes and Radiative Properties of Particles in Aircraft Wakes (AEROCONTRAIL; Schröder et al 2000), and the Subsonic Aircraft: Contrail and Cloud Effects Special Study (SUCCESS; Lawson et al 1998) These experiments involve airborne instruments that simultaneously measure microphysical, optical, and other characteristics of clouds as well as meteorological parameters. The main goal of this paper is to describe a methodology of applying statistical procedures, the PCA technique, to assess data reliability Another objective is to report on statistical parameters of cirrus cloud optical and microphysical characteristics measured during two campaigns of the European INCA experiment (Interhemispheric Differences in Cirrus Properties from Anthropogenic Emissions; Ström et al 2001). The differences in cirrus microphysical and optical properties between the two hemispheres, together with interpretation of the results are presented in detail by Gayet et al (2004)
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