Guido Di Donfrancesco

Comparison of various linear depolarization parameters measured by lidar

Different definitions for estimating the degree of changes in signal polarization measured by lidar measurements are used both to detect the presence of nonspherical aerosol particles and to estimate their shape and density. Our aim is to provide a tool for calculation and interpretation of changes in polarization that are due to aerosol backscatter measured by the lidar technique. An overview of several techniques used to calculate linear depolarization from two-channel lidar measurements is given. Advantages and disadvantages of each method are analyzed when we apply them on a lidar vertical profile. Systematic errors are also discussed. First, an overview of different estimations of polarizability of atmospheric molecules is given. The presence of signal with orthogonal polarization in each channel (cross talk) is a source of error in depolarization estimation. It is calculated at various degrees of contamination, and the total uncertainty on depolarization definition is retrieved.

Climatology of polar stratospheric clouds based on lidar observations from 1993 to 2001 over McMurdo Station, Antarctica

[1]xa0A climatology of polar stratospheric clouds (PSCs) based on lidar measurements performed at McMurdo Station, Antarctica (78°S, 167°E) from 1993 to 2001 is presented here. The observations are discussed in terms of occurrence and temporal and spatial variability of PSC types. The climatological analysis reveals that in McMurdo PSCs mainly occur between July and mid-August. During this time their altitude changes from 22 to 14 km, following the stratospheric temperature minimum trend. At the beginning of the accounted period (1993–1994), volcanic aerosols from the Mt. Pinatubo eruption in 1991 were still present in the southern polar vortex. Therefore these 2 years have been corrected for the direct contribution of the volcanic aerosol to the backscatter signal. A close examination of the data set evidences that most PSCs appear either as rather thin layers (<1 km) or as layers with a considerably higher thickness. Therefore all observed PSCs have been divided into two classes, depending on the variation of the backscattering ratio with respect to the altitude (e.g., small-scale variations, or SSV, and large-scale variations, or LSV). The seasonal behavior and the occurrence of PSC types under each class have been studied, keeping 1993 and 1994 separated to better highlighting the effect of the volcanic aerosol load on cloud properties. Finally, in order to shed some light on PSC formation, back-trajectory analysis has been performed for retrieving thermal histories of opportunely selected PSCs.

Large nitric acid particles at the top of an Arctic stratospheric cloud

measurements approximately 200 km upwind of the in situ measurements indicate a similar vertical structure for the cloud. These in situ measurements represent, to our knowledge, the most comprehensive in situ observations of all phases of polar stratospheric cloud particles, while the large particles at cloud top have not been previously observed and may have implications for producing particles large enough to remove reactive nitrogen from the polar stratosphere. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0340 Atmospheric Composition and Structure: Middle atmosphere— composition and chemistry; KEYWORDS: polar stratospheric clouds, in situ stratospheric cloud measurements, Arctic stratospheric clouds, polar stratospheric cloud composition, balloon-borne aerosol measurements, large polar stratospheric cloud particles

Physical properties of stratospheric clouds during the Antarctic winter of 1995

Lidar observations collected during winter 1995 at McMurdo Station, Antarctica (78°S-l67°E), are analyzed to determine polar stratospheric cloud (PSC) physical properties. A scheme to infer PSC phase from lidar depolarization and backscatter profiles is presented. Interpretation is supported by collocated temperature soundings and by isentropic back trajectories. The analysis shows that first appearance of PSC is consistent with frozen sulfates, mixing with liquid ternary solutions (H 2 SO 4 -HNO 3 -H 2 O) when temperature lowers. Finally, solids consistent with HNO 3 mixing ratios form as mixed phases first, then followed by full solid phases. Mixed phases (i.e., coexisting solid and liquid aerosols) are detected during the whole winter. While mixed phase PSCs form particularly in the altitude range 15-20 km and are the last to disappear, full solid phases are mainly observed above 20 km and last until the end of August. Mixed phases possess the largest PSC surface areas and, as a result of selective growth, can reach large, fast settling sizes. The considerable denitrification and halogen activation observed in the Antarctic lower stratosphere, where the ozone hole takes place, appears to be well correlated with the action of this kind of PSC.

In situ mountain-wave polar stratospheric cloud measurements: Implications for nitric acid trihydrate formation

0.2 cm 3 , median radii of 1 to 2 mm and volumes up to 1 mm 3 cm 3 . A comparison between optical PSC data and optical simulations based on the measured particle size distribution indicates that the NAT particles were aspherical with an aspect ratio of 0.5. The NAT particle properties have been compared to another PSC observation on 25 January 2000, where NAT particle number densities were about an order of magnitude higher. In both cases, microphysical modeling indicates that the NAT particles have formed on ice particles in the mountain-wave events. Differences in the NAT particle number density can be explained by the meteorological conditions. We suggest that the higher NAT number density on 25 January can be caused by stronger wave activity observed on that day, larger cooling rates and therefore higher NAT supersaturation. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0340 Atmospheric Composition and Structure: Middle atmosphere— composition and chemistry; KEYWORDS: polar stratospheric cloud (PSC), nitric acid trihydrate (NAT), ozone, gravity wave, PSC formation

Correction scheme for close-range lidar returns

Because of the effect of defocusing and incomplete overlap between the laser beam and the receiver field of view, elastic lidar systems are unable to fully capture the close-range backscatter signal. Here we propose a method to empirically estimate and correct such effects, allowing to retrieve the lidar signal in the region of incomplete overlap. The technique is straightforward to implement. It produces an optimized numerical correction by the use of a simple geometrical model of the optical apparatus and the analysis of two lidar acquisitions taken at different elevation angles. Examples of synthetic and experimental data are shown to demonstrate the validity of the technique.

Multiwavelength Aerosol Scatterometer for Airborne Experiments to Study the Optical Properties of Stratospheric Aerosol

In the recent past the role of polar stratospheric clouds (PSCs) and of stratospheric aerosol in polar ozone depletion raised the attention of the scientific community (Solomon 1990; Fiocco et al. 1997). The understanding of PSCs in terms of concentrations of particles, sizes, optical parameters, formation processes, and microphysics is relevant for evaluating their contribution to chlorine activation, dehydration, and denitrification of the lower polar stratosphere. In view of a better understanding of PSC properties, during the last few years joint lidar and optical particle counters (OPC) measurements were carried out in Antarctica (Deshler et al. 1991; Adriani et al. 1992). While the OPC was able to assess particle concentrations and sizing, the lidar could give a contemporary measurement of the aerosol volume cross section for backscattered signals and information on the depolarizing properties of the particles, hence their thermodynamical phase. In the approximation of the Mie theory, the two measurement techniques were compared to estimate the refractive index of the particles (Adriani et al. 1995). However, the comparison between lidar and balloon-borne measurements needs some care, due to differences in the sampling technique. The lidar is a ground-based instrument and the OPC is carried by balloons and moves with respect to the ground, so that the two measurements are not taken on the same air mass. Ancillary information about the wind speed profile have to be used to choose, among the lidar temporal sequence, the best lidar profile to be compared—at a given altitude—with data from the balloon-borne OPC, drifting with the

Polar stratospheric clouds observed during the Airborne Polar Experiment–Geophysica Aircraft in Antarctica (APE‐GAIA) campaign

[1]xa0In this work, observations of polar stratospheric clouds (PSC) carried out during one flight of the stratospheric research aircraft Geophysica deployed in the Airborne Polar Experiment–Geophysica Aircraft in Antarctica (APE-GAIA) campaign in September–October 1999 are presented. An analysis of data from the two lidars and a backscatter sonde on board the Geophysica aircraft is presented, coupled with temperature measurements. The observations are analyzed with the aid of a mesoscale model to provide air mass thermal histories and a microphysical box model to simulate PSC formation. The results obtained from this comparison are discussed in view of the theories for PSC formation processes.

The AMMA mulid network for aerosol characterization in West Africa

Three ground-based portable low power consumption micro lidars (MULID) have been built and deployed at three remote sites in Banizoumbou (Niger), Cinzana (Mali) and MBour (Senegal) in the framework of the African Monsoon Multidisciplinary Analyses (AMMA) project for the characterization of aerosols optical properties. A description of the instrument and a discussion of the data inversion method, including a careful analysis of measurement uncertainties (systematic and statistical errors), are presented. Some case studies of typical lidar profiles observed over the Banizoumbou site during 2006 are shown and discussed with respect to the Aerosol Robotic Network (AERONET) 7-day back-trajectories and the biomass burning emissions from the Combustion Emission database for the AMMA campaign.

Balloonborne lidar for cloud physics studies

An innovative balloonborne microjoule lidar (MULID) has been developed within the framework of the HIBISCUS project to provide nighttime measurements of visible and subvisible cirrus and aerosols. MULID has been designed to be a low-cost and an ultralow consumption instrument, due to the remote possibilities of payload recovery and the necessity of a low-weight battery power supply. Ground tests have been performed at the Observatory of Haute Provence (France), and the first technical flight has been made from Trapani, Italy, on a stratospheric balloon; finally, the instrument has been scientifically deployed during the pre-HIBISCUS and HIBISCUS tropical campaigns in Bauru, Brazil, in February 2003 and February 2004, respectively. A description of the instrument is provided together with the results of the ground-based and flight tests as well as an overview and discussion of the first results.