Guy Febvre

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.

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...

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...

Microphysical characteristics of a marine stratiform cloud obtained from lidar and in situ measurements

Abstract Lidar and in situ aircraft measurements were performed in a marine stratiform layer during the intensive field observation of the International Cirrus Experiment (ICE 89). The analysis of the results shows that this marine boundary layer is mainly characterized by two decoupled stratiform cloud layers. Drizzle formation is evidenced to be efficient on an horizontal scale ranged between 20 and 30 km in the upper stratiform cloud and promotes depletion of the cloud water. The drops are large enough to fall through the lower layer without evaporation. Subsequent differences in microphysical properties between cloud and drizzle regions are clearly evidenced and may have a strong influence on visible optical depth and cloud albedo. The calibration of the Lidar data we propose leads to describe the vertical microstructure of the precipitating drizzle regions.

Lidar data inversion for Cirrus clouds: An approach based on a statistical analysis of in situ microphysical measurements

The problem of the lidar equation inversion lies in the fact that it requires a lidar calibration or else a reference value from the studied medium. This paper presents an approach to calibrate the lidar by calculating the constant Ak (lidar constant A multiplied by the ratio of backscatter coefficient to extinction coefficient k). This approach is based on statistical analysis of in situ measurements. This analysis demonstrates that the extinction coefficient has a typical probablility distribution in cirrus clouds. The property of this distribution, as far as the attenuation of laser beam in the cloud, is used as a constraint to calculate the value of Ak. The validity of this method is discussed and results compared with two other inversion methods.