Jean-François Gayet

On the Transition of Contrails into Cirrus Clouds

In situ observations of the microphysical properties of upper-tropospheric contrails and cirrus clouds have been performed during more than 15 airborne missions over central Europe. Experimental and technical aspects concerning in situ characterization of ice clouds with the help of optical and nonoptical detection methods (preferably FSSP-300 and Hallet-type replicator) are addressed. The development of contrails into cirrus clouds on the timescale of 1 h is discussed in terms of a representative set of number densities, and size distributions and surface area distributions of aerosols and cloud elements, with special emphasis on small ice crystals (diameter ,20 mm). Contrails are dominated by high concentrations (.100 cm23) of nearly spherical ice crystals with mean diameters in the range 1‐10 mm. Young cirrus clouds, which mostly contain small regularly shaped ice crystals in the range 10‐20-mm diameter and typical concentrations 2‐5 cm23, have been observed. Measurement results are compared to simple parcel model calculations to identify parameters relevant for the contrail‐cirrus transition. Observations and model estimates suggest that contrail growth is only weakly, if at all, affected by preexisting cirrus clouds.

Water vapour measurements inside cirrus clouds in Northern and Southern hemispheres during INCA

Water vapour data inside cirrus clouds from in-situ measurements with an aircraft-borne frost-point hygrometer are analysed. These data have been obtained during two field campaigns, performed in the Southern and Northern hemisphere mid latitudes. There were many occurrences of ice supersaturation inside the investigated cirrus, with a higher frequency of occurrences in the Southern Hemisphere. The source of the differences in the humidity data from the two hemispheres is not clear, and it is speculated that these differences may be related to different levels of pollution. A distribution law for the relative humidity inside cirrus clouds is inferred.

The Reliability of the PMS FSSP in the Presence of Small Ice Crystals

Abstract The performance of measurement systems for small ice crystals is an important issue because of the need to characterize cirrus cloud microphysics for climate research. In a cloud formed of small quasi-spherical ice particles, as found in recently formed aircraft contrails, the PMS FSSP probe gives reliable measurements consistent with other probes. This is in contrast to the expectation, based on earlier work by others, that the FSSP measurements are unreliable when ice is present in the cloud. In natural cirrus on the other hand, where there are irregular large ice particles, the FSSP indeed gives the exaggerated response reported earlier. Here simple criteria by which the presence of spherically shaped small ice particles can be recognized are proposed. These criteria provide a useful indication of the occasions in ice clouds when FSSP measurements can be relied on.

Quantitative measurement of the microphysical and optical properties of cirrus clouds with four different in situ probes: Evidence of small ice crystals

Original microphysical and optical measurements were obtained in cirrus clouds on the Southern and Northern hemispheres during the INCA experiments using four independent techniques: (1) the Counterflow Virtual Impactor, (2) the PMS FSSP-300, (3) the PMS 2D-C and (4) the Polar Nephelometer probes. The combination of these four techniques provides a description of particles within a diameter range varying from a few micrometers (typically 3 μm) to 800 μm. Because of the presence of small ice crystals in cirrus clouds, it is particularly important to overcome the limited accuracy of the sensors used in the experiments for the cloud microphysical measurements. Representative examples of combined results suggest that the available measurements are reliable and can be used for the ongoing comparison between the results from the SH and NH campaigns. The results give the definite picture that the observations of numerous (5 to 10 cm -3 ) small ice crystals in cirrus clouds are a relatively common microphysical feature.

Polarized light scattering by inhomogeneous hexagonal monocrystals: Validation with ADEOS‐POLDER measurements

Various in situ measurements of the light-scattering diagram in ice clouds were performed with a new nephelometer during several airborne campaigns. These measurements were favorably compared with a theoretical scattering model called Inhomogeneous Hexagonal Monocrystal (IHM) model. This model consists in computing the scattering of light by an ensemble of randomly oriented hexagonal ice crystals containing spherical impurities of soot and air bubbles. It is achieved by using a combination of ray tracing, Mie theory, and Monte Carlo techniques and enables to retrieve the six independent elements of the scattering matrix. This good agreement between nephelometer measurements and IHM model provides an opportunity to use this model in order to analyze ADEOS-POLDER total and polarized reflectance measurements over ice clouds. POLDER uses an original concept to observe ice cloud properties, enabling to measure reflectances and polarized reflectances, for a given scene, under several (up to 14) viewing directions. A first analysis of ice cloud spherical albedoes over the terrestrial globe for November 10, 1996, and April 23, 1997, shows a rather good agreement between measurements and modeling. Moreover, polarized reflectances are also calculated and show a satisfactory agreement with measurements.

Modeling of light scattering in cirrus clouds with inhomogeneous hexagonal monocrystals. Comparison with in-situ and ADEOS-POLDER measurements

An Inhhomogeneous Hexagonal Monocrystal (IHM) model is used to simulate light scattering by randomly oriented hexagonal ice crystals containing air bubbles. This model based on a combination of ray-tracing, Mie theory and Monte-Carlo techniques, allows to retrieve the scattering phase function. In-situ measurements of the light scattering diagram in natural cirrus clouds with an airborne nephelometer have been performed. The results given by the IHM model have been favorably adjusted with these measurements. This agreement provides an opportunity to use this model in order to analyze ADEOS-POLDER reflectance measurements over cirrus clouds. POLDER uses an original concept to measure, for a given scene, total and polarized reflectances under several viewing directions. A first analysis of cirrus cloud spherical albedoes for the 10th November 1996 shows a rather good agreement between measurements and calculations.

In situ measurements of the scattering phase function of stratocumulus, contrails and cirrus

Original measurements were obtained in stratocumulus, contrails and cirrus clouds by using a new optical airborne probe, the ‘Polar Nephelometer’, which is the first airborne instrument to make direct in situ measurements of the scattering phase function of cloud particles over a broad range of sizes (from a few micrometers to about 500 µm diameter). Preliminary measurements show that in stratocumulus water droplet cloud, the measured phase function fits very well with the phase function derived from direct PMS probes measurements. This definitively confirms the reliability of the Polar Nephelometer for airborne measurements. In contrails and natural cirrus, measured scattering phase function indicates major differences with those used in cloud models which assume ice spheres or simple geometric shape of ice particles. These results highlight new potential insights on both modelling of climate processes and methodologies for cloud remote sensing from satellite measurements.

Microphysical and optical properties of cirrus and contrails : cloud field study on 13 October 1989

Abstract During the intensive International Cirrus Experiment conducted over the North Sea during fall 1989, natural cirrus and contrail-induced cirrus were analyzed from in situ and remote sensing measurements (lidar and infrared radiometer). These two cloud types primarily formed at the same range of altitude (8200 m, −37°C). Analysis of the measurements depicts distinctive microphysical and optical properties in the two types of cirrus. Natural cirrus exhibits sheared fallstreaks of ice crystals up to 750 µm in size near the base level. From the top to the base of this cloud the mean values of ice water content and particle concentration increase from 15 to 50 mg m−3 and from 26 to 60 L−1, respectively. The corresponding visible optical depth is around 2.0. Greatest particle concentration and smallest ice crystals are measured at all levels in contrails leading to an optical depth of 0.8 in the denser cloud despite an ice water content that never exceeds 18 mg m−3. These results are consistent with rem...

Light Scattering by Single Natural Ice Crystals

During the South Pole Ice Crystal Experiment, angular scattering intensities (ASIs) of single ice crystals formed in natural conditions were measured for the first time with the polar nephelometer instrument. The microphysical properties of the ice crystals were simultaneously obtained with a cloud particle imager. The observations of the scattering properties of numerous ice crystals reveal high variability of the ASIs in terms of magnitude and distribution over scattering angles. To interpret observed ASI features, lookup tables were computed with a modified ray tracing code, which takes into account the optical geometry of the polar nephelometer. The numerical simulations consider a wide range of input parameters for the description of the ice crystal properties (particle orientation, aspect ratio, surface roughness, and internal inclusions). A new model of surface roughness, which assumes the Weibull statistics, was proposed. The simulations reproduce the overwhelming majority of the observed ASIs features and trace very well the quasi-specular reflection from crystal facets. The discrepancies observed between the model and the experimental data correspond to the rays, which pass through the ice crystal and are scattered toward the backward angles. This feature may be attributed to the internal structure of the ice crystals that should be considered in modeling refinements.

Microphysical and Optical Properties of Atmospheric Ice Crystals at South Pole Station

Abstract In early February 2001 (during the austral summer), over 900 000 digital images of ice crystals were recorded at the South Pole using two ground-based cloud particle imagers (CPIs). Of these, 721 572 crystals >50 μm were classified into crystal habits. When sorted by number, 30% of the crystals were rosette shaped (mixed-habit rosettes, platelike polycrystals, and rosette shapes with side planes), 45% were diamond dust (columns, thick plates, and plates), and 25% were irregular. When sorted by area, rosette shapes comprised 50%, diamond dust 30% and irregular 20%. By mass, the percentages were 57% rosette shapes, 23% diamond dust, and 20% irregular. Particle size distributions as a function of maximum dimension and equivalent radius are compared with previous studies. Particles are generally found to be slightly larger than previous austral wintertime studies. In 2002, a polar nephelometer (PN) that measures scattering phase function was incorporated with one of the CPIs. Correlated measurements ...