Frédérique Auriol

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.

Cirrus cloud microphysical and optical properties at southern and northern midlatitudes during the INCA experiment

[1]xa0Microphysical and optical measurements were performed in midlatitude cirrus clouds at temperatures between −33°C and −60°C during southern and northern Interhemispheric Differences in Cirrus Properties From Anthropogenic Emissions (INCA) field experiments carried out at equivalent latitudes (53°S and 55°N) from Punta Arenas in the Southern Hemisphere (SH, Chile) and Prestwick in the Northern Hemisphere (NH, Scotland). The aim of this paper is to analyze the microphysical and optical properties of cirrus clouds and to compare the results of the two campaigns. Compared with the mean properties of cirrus clouds in SH, the cirrus sampled in NH were characterized by a greater concentration of ice crystals (2.2 cm−3 versus 1.4 cm−3, respectively) with a lower effective diameter (36 μm versus 42 μm, respectively). A significant contrast in extinction coefficient was also evidenced with larger values in NH than in SH (0.61 km−1 versus 0.49 km1, respectively), whereas no significant differences in the ice water content were observed (8 mg m−3). The first measurements of the asymmetry parameter obtained in midlatitude cirrus clouds revealed rather uniform particle scattering properties (median g values between 0.76 and 0.78) with small differences between SH and NH (0.770 versus 0.767, respectively). No significant differences in crystal shape were evidenced between the two data sets. For similar environmental conditions (i.e., over a similar range of temperature and vertical velocity) and for given values of the relative humidity, the comparisons clearly show distinct differences between the microphysical and optical properties of cirrus clouds sampled in the SH and NH field experiments. These differences may be related to the contrasts in cirrus freezing thresholds in terms of relative humidity over ice for onset of clouds, which is about 20% lower in NH than in SH, as reported in previous INCA data analyses. Nevertheless, definite conclusions are hampered by the large natural variability of cirrus cloud properties and the limited number of flights performed, not forgetting both the actual freezing mechanisms and potential differences in small-scale dynamical variability, which are still insufficiently understood.

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.

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.

Two case studies of winter continental‐type water and mixed‐phase stratocumuli over the sea 1. Microphysical and optical properties

[1]xa0Collocated aircraft observations of microstructure and radiative properties of winter boundary layer clouds over the East China Sea and the Japan Sea have been carried out in January 1999 within the Japanese Cloud and Climate Study (JACCS) program. The first part of the paper describes the in situ measured microphysical and optical properties for two cases of boundary layer winter stratocumulus clouds, which concern, first, a rather uniform, supercooled water cloud contaminated by aerosols and, second, a highly heterogeneous, mixed-phase stratiform cloud. Using the Polar Nephelometer, a new instrument for measuring, in situ, the scattering phase function of cloud droplets and ice particles, the polluted, continental-type stratocumulus cloud can be optically regarded as a liquid water cloud because the measured scattering phase functions fitted very well with those calculated from Mie theory for the directly measured FSSP size distributions. In mixed-phase cloud, the measured scattering phase function shows that ice particles strongly affect optical properties of the cloud, where large number of liquid water droplets with higher extinction coefficient and asymmetry factor values were converted into a much smaller number of large ice crystals with lower extinction coefficient and asymmetry factor. Furthermore, a quasi-stable liquid-topped cloud layer with precipitating ice particles was noticed; the layer may, first, affect the cloud radiative properties and, second, seriously restrict the interpretation of satellite cloud composition retrievals.

Uptake of reactive nitrogen on cirrus cloud particles during INCA

[1]xa0The uptake of reactive nitrogen species on ice crystals in cirrus clouds was investigated by simultaneous aircraft-based measurements of gas- and condensed-phase NOy, ice particle size distribution and total aerosol surface area. The measurements were performed in 2000 during the INCA campaign at northern and southern midlatitudes at local autumn. Cirrus ice particles were frequently encountered during these flights in the upper troposphere. Concurrently with the occurrence of cirrus ice particles particulate NOy was observed. It was found that the coverage of the cirrus ice particles with particulate NOy strongly depends on the temperature and the gas-phase NOy concentration. A substantial coverage of the ice particles with values larger than one percent of a monolayer was observed only for temperatures below about 217 K. On average only about one percent of the available gas-phase NOy was found as particulate NOy.

Aerosol retrieval over land using a multiband polarimeter and comparison with path radiance method

[1]xa0This study focuses on the development of a new approach to retrieve aerosol properties over land, based on the use of multispectral polarized measurements (0.67–2.2 μm). We use the measurements of the airborne MICROPOL polarimeter during regional aircraft field experiments located in France and dedicated to the study of aerosol (pollutant and mineral dust particles) and surface properties. We have developed a multiband polarization algorithm (MBP) and compared the retrievals with both a path radiance algorithm and Sun photometer data. It is shown that surface polarized reflectance exhibits only a small spectral variation for forward scattering geometries (3% on average, 15–20% for a single view and scattering angle < 110°). The atmospheric contribution at 2.2 μm is small for aerosol optical thicknesses (AOTs) up to 0.15 at 0.67 μm and can be accounted for in the retrieval of surface properties. The 2.2 μm channel therefore enables us to accurately derive the surface polarization in the shorter MICROPOL bands. For the observations we have made, the surface model developed for the analysis of the Polarization and Directionality of the Earth Reflectance (POLDER) measurements overestimates surface polarization from a few to fifty percents. This leads to AOT underestimation by a factor two. For the pollutant aerosol cases (0.065 < AOT < 0.20), the MBP approach retrieves AOT with an accuracy of 0.03, over both natural and urban surfaces. However, this method remains only weakly sensitive to coarse mode particles and fails when dust particles associated with large AOTs (0.5) are considered.

In situ observations of a reduction in effective crystal diameter in cirrus clouds near flight corridors

Measurements of the crystal size distribution performed over Southern Germany show that the effective diameter decrease 10-30% in cirrus clouds perturbed by aircraft. The reduction is more pronounced at higher cloud water content and is not observed at temperatures warmer than -30°C.

Microphysical properties of mixed-phase & Ice clouds retrieved from In Situ airborne “polar nephelometer” measurements

In this paper we present microphysical properties of natural clouds retrieved from an airborne “Polar Nephelometer” measurements. The retrieval was done by means of an iterative inversion method, which is based on the bi-component (water droplets and ice crystals) representation of ice and mixed phase cloud composition. The present study shows that experimental scattering phase functions of ice crystals are characterized by high information content with respect to the aspect ratio of ice crystals which can be estimated in addition to their effective size distributions.

In Situ Observation of Cirrus Scattering Phase Functions with 22° and 46° Halos: Cloud Field Study on 19 February 1998

Abstract Observations of halos and related phenomena due to ice crystals are commonly reported from ground observations and presented in the literature. Nevertheless, ice crystal characteristics have only been poorly documented from in situ measurements performed in halo-producing cirrus with simultaneous observations of optical phenomena. Using the Polar Nephelometer, a new instrument for in situ measuring of the scattering phase function of cloud droplets and ice particles, 22° and 46° halo features have been evidenced during a cirrus uncinus cloud case study between −30°C and −38°C. Simultaneous microphysical measurements were made with a 2D-C probe manufactured by Particle Measuring Systems Inc. (PMS). The results show that ice crystal properties derived from 2D-C measurements do not present substantial differences when comparing cirrus cloud samples with and without halos. Consequently, the cloud scattering properties appear to be dominated by small ice particles (smaller than about 100 μm), which ar...