An intercomparison of aerosol backscatter lidar algorithms was performed in 2001 within the framework of the European Aerosol Research Lidar Network to Establish an Aerosol Climatology (EARLINET). The objective of this research was to test the correctness of the algorithms and the influence of the lidar ratio used by the various lidar teams involved in the EARLINET for calculation of backscatter-coefficient profiles from the lidar signals. The exercise consisted of processing synthetic lidar signals of various degrees of difficulty. One of these profiles contained height-dependent lidar ratios to test the vertical influence of those profiles on the various retrieval algorithms. Furthermore, a realistic incomplete overlap of laser beam and receiver field of view was introduced to remind the teams to take great care in the nearest range to the lidar. The intercomparison was performed in three stages with increasing knowledge on the input parameters. First, only the lidar signals were distributed; this is the most realistic stage. Afterward the lidar ratio profiles and the reference values at calibration height were provided. The unknown height-dependent lidar ratio had the largest influence on the retrieval, whereas the unknown reference value was of minor importance. These results show the necessity of making additional independent measurements, which can provide us with a suitable approximation of the lidar ratio. The final stage proves in general, that the data evaluation schemes of the different groups of lidar systems work well.

[1] In situ measurements of light scattering and microphysical characteristics of young and 20-min-aged persistent contrails and of frontal cirrus clouds were carried out with the airborne polar nephelometer and microphysical Particle Measuring Systems instruments. Optical and microphysical properties of contrails at different stages of evolution and of cirrus clouds sampled in ice-supersaturated air masses at ambient temperatures near À60°C are examined. The results show that quasi-spherical ice particles with diameters smaller than 5 mm control the optical properties of the plume shortly after formation. In slightly aged contrails the optical properties are governed by larger nonspherical ice crystals. Very similar optical properties are observed in frontal cirrus but with a different ice particle size distribution. The contrail optical and microphysical ice particle data obtained in this study are representative of the visible persistent contrail created by a midsized airliner after the wake vortices have decayed.

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 Ͼ50 m 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 between the two instruments showed that 22°and 46°peaks in the phase function were present when diamond dust was recorded by the CPI, but not when rosette shapes were present. Visual observations confirmed the presence of 22°and 46°a tmospheric halos in some, but not all, of the diamond dust events. No visual halos were observed when rosette shapes were precipitating. Average PN phase functions are presented for diamond dust and rosette shapes. The diamond dust and rosette-shaped ice crystals appear to be very similar in shape to those observed by CPIs in cirrus clouds. Cloud conditions at the South Pole that were associated with various crystal types are discussed, as are some effects of blowing snow.

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.

Abstract. This study reports on the origin of small-scale ice crystal complexity and its influence on the angular light scattering properties of cirrus clouds. Cloud simulation experiments were conducted at the AIDA (Aerosol Interactions and Dynamics in the Atmosphere) cloud chamber of the Karlsruhe Institute of Technology (KIT). A new experimental procedure was applied to grow and sublimate ice particles at defined super-and subsaturated ice conditions and for temperatures in the −40 to −60 • C range. The experiments were performed for ice clouds generated via homogeneous and heterogeneous initial nucleation. Small-scale ice crystal complexity was deduced from measurements of spatially resolved single particle light scattering patterns by the latest version of the Small Ice Detector (SID-3). It was found that a high crystal complexity dominates the microphysics of the simulated clouds and the degree of this complexity is dependent on the available water vapor during the crystal growth. Indications were found that the small-scale crystal complexity is influenced by unfrozen H 2 SO 4 /H 2 O residuals in the case of homogeneous initial ice nucleation. Angular light scattering functions of the simulated ice clouds were measured by the two currently available airborne polar nephelometers: the polar nephelometer (PN) probe of Laboratoire de Métérologie et Physique (LaMP) and the Particle Habit Imaging and Polar Scattering (PHIPS-HALO) probe of KIT. The measured scattering functions are featureless and flat in the side and backward scattering directions. It was found that these functions have a rather low sensitivity to the small-scale crystal complexity for ice clouds that were grown under typical atmospheric conditions. These results have implications for the microphysical properties of cirrus clouds and for the radiative transfer through these clouds.

SUMMARYAirborne microphysical and optical properties of cirrus clouds, jet-stream and wave clouds were measured at temperatures ranging from −25 • C to −62 • C in the southern hemisphere from Punta Arenas (53 • S) in March and April 2000 during the INCA experiment (INterhemispheric differences in Cirrus properties from Anthropogenic emissions). The observations related to cirrus clouds show general decreases of the ice water content (18 mg m −3 to 0.05 mg m −3 ), extinction coefficient (0.70 km −1 to 0.08 km −1 ), ice particle concentration (2.2 cm −3 to 0.5 cm −3 ), and the effective diameter (80 μm to 17 μm) linked to the variation of ambient temperature (−25 • C to −60 • C). The lowest temperature at which supercooled water droplets were detected was −33 • C. The asymmetry parameter shows relatively small variations, with the smallest values (0.758) observed at the lowest temperatures. High-altitude clouds which form with a rapid vertical transport, i.e. jet-stream cirrus and orographic-wave ice-cloud, are characterized by very high values of ice particle concentrations (up to 100 cm −3 ) compared to mean values as a whole (1.45 cm −3 ). These two kinds of high clouds are the subjects of detailed case-studies. Although formed at a similar range of temperatures and with similar aerosol properties, the strong differences in cloud properties and humidity fields highlight the key role of the dynamical structure in controlling the formation, evolution and subsequent radiative properties of such high-altitude clouds.

Abstract. During the CIRCLE-2 experiment carried out over Western Europe in May 2007, combined in situ and remote sensing observations allowed to describe microphysical and optical properties near-top of an overshooting convective cloud (11 080 m/−58 • C). The airborne measurements were performed with the DLR Falcon aircraft specially equipped with a unique set of instruments for the extensive in situ cloud measurements of microphysical and optical properties (Polar Nephelometer, FSSP-300, Cloud Particle Imager and PMS 2-D-C) and nadir looking remote sensing observations (DLR WALES Lidar). Quasi-simultaneous space observations from MSG/SEVIRI, CALIPSO/CALIOP-WFC-IIR and CloudSat/CPR combined with airborne RASTA radar reflectivity from the French Falcon aircraft flying above the DLR Falcon depict very well convective cells which overshoot by up to 600 m the tropopause level. Unusual high values of the concentration of small ice particles, extinction, ice water content (up to 70 cm −3 , 30 km −1 and 0.5 g m −3 , respectively) are experienced. The mean effective diameter and the maximum particle size are 43 µm and about 300 µm, respectively. This very dense cloud causes a strong attenuation of the WALES and CALIOP lidar returns. The SE-VIRI retrieved parameters confirm the occurrence of small ice crystals at the top of the convective cell. Smooth and featureless phase functions with asymmetry factors of 0.776 indicate fairly uniform optical properties. Due to small ice crystals the power-law relationship between ice water content (IWC) and radar reflectivity appears to be very different from those usually found in cirrus and anvil clouds. For a given equivalent reflectivity factor, IWCs are significantly larger for the overshooting cell than for the cirrus. Assuming the same prevalent microphysical properties over the depth of the overshooting cell, RASTA reflectivity profiles scaled into ice water content show that retrieved IWC up to 1 g m −3 may be observed near the cloud top. Extrapolating the relationship for stronger convective clouds with similar ice particles, IWC up to 5 g m −3 could be experienced with reflectivity factors no larger than about 20 dBZ. This means that for similar situations, indication of rather weak radar echo does not necessarily warn the occurrence of high ice water content carried by small ice crystals. All along the cloud penetration the shape of the ice crystals is dominated by chainlike aggregates of frozen droplets. Our results confirm previous observations that the chains of ice crystals are found in a continental deep convective systems which are known generally to generate intense electric fields causing efficient ice particle aggregation processes. Vigorous updrafts could lift supercooled droplets which are frozen extremely rapidly by homogeneous nucleation near the −37 • C level, producing therefore high concentrations of very small ice particles at upper altitudes. They are sufficient to deplete the water vapour and suppress further nucleation as confirmed by humidity measure...

Homogeneous freezing of supercooled droplets occurs in convective systems in low-and in mid-latitudes. This droplet freezing process leads to the formation of a large amount of small ice particles, so called frozen droplets, that are transported to the upper parts of anvil outflows, where they can influence the cloud radiative properties. However, the detailed microphysics and, thus, the scattering properties of these small ice particles are highly uncertain.Here, we investigate the link between the microphysical and optical properties of frozen droplets in cloud chamber experiments, where the frozen droplets were formed, grown and sublimated under controlled conditions. It was found that frozen droplets developed a high degree of small-scale complexity after their initial formation and subsequent growth. During sublimation the smallscale complexity disappeared releasing a smooth and near-spherical ice particle. Angular light scattering and depolarization measurements confirmed that these sublimating frozen droplets scattered light similar to spherical particles, i.e. they had angular light scattering properties similar to water droplets. The knowledge gained from this laboratory study was applied to two case studies of aircraft measurements in a mid-latitude and in a tropical convective systems. The in-situ aircraft measurements confirmed that the microphysics of frozen droplets is dependent on the humidity conditions they are exposed to (growth or sublimation). The existence of optically spherical frozen droplets can be important for the radiative properties of detraining convective outflows.