Marcel Snels - Academia.edu (original) (raw)

Papers by Marcel Snels

Springer atmospheric sciences, Dec 31, 2022

Scattering codes are used to study the optical properties of Polar Stratospheric Clouds (PSC). Pa... more Scattering codes are used to study the optical properties of Polar Stratospheric Clouds (PSC). Particle backscattering and depolarization coefficients can be computed with available scattering codes once the particle size distribution (PSD) is known and a suitable refractive index is assumed. However, PSCs often appear as external mixtures of Supercooled Ternary Solution (STS) droplets, solid Nitric Acid Trihydrate (NAT) and possibly ice particles, making questionable the assumption of a single refractive index and a single morphology to model the scatterers.Here we consider a set of fifteen coincident measurements of PSCs above McMurdo Station, Antarctica, by ground-based lidar and balloon-borne Optical Particle Counter (OPC), and in situ observations taken by a laser backscattersonde and OPC during four balloon stratospheric flights from Kiruna, Sweden. This unique dataset of microphysical and optical observations allows to test the performances of optical scattering models when both spherical and aspherical scatterers of different composition and, possibly, shapes are present.We consider particles as STS if their radius is below a certain threshold value Rth and NAT or possibly ice if above it. The refractive indices are assumed known from the literature. Mie scattering is used for the STS, assumed spherical, while scattering from NAT particles, considered as spheroids of different Aspect Ratio (AR), is treated with T-Matrix results where applicable, and of geometric-optics-integral-equation approach where the particle size parameter is too large to allow for a convergence of the T-matrix method.The parameters Rth and AR of our model have been varied between 0.1 and 2 micrometers and between 0.3 and 3, respectively, and the calculated backscattering coefficient and depolarization were compared with the observed ones. The best agreement was found for Rth between 0.5 and 0.8 micrometers, and for AR less than 0.55 and greater than 1.5.To further constrain the variability of AR within the identified intervals we have sought an agreement with the experimental data by varying AR on a case-by-case basis, and further optimizing the agreement by a proper choice of AR smaller than 0.55 and greater than 1.5, and Rth within the interval 0.5 and 0.8 micrometers. The ARs identified in this way cluster around the values 0.5 and 2.5.The comparison of the calculations with the measurements is presented and discussed. The results of this work help to set limits to the variability of the dimensions and asphericity of PSC solid particles, within the limits of applicability of our model based on the T-matrix theory of scattering and on assumptions on a common particle shape in a PSD and a common threshold radius for all the PSDs.

Atmospheric Measurement Techniques, Jan 26, 2023

Journal of Chemical Physics, Jun 12, 2006

Two rotationally fully resolved vibrational bands have been assigned unambiguously to the linear ... more Two rotationally fully resolved vibrational bands have been assigned unambiguously to the linear deuteron bound Ar⋯DN2+ complex by using ground state combination differences. The ionic complex is formed in a supersonic planar plasma expansion optimized and controlled by a mass spectrometer and is detected in direct absorption using tunable diode lasers and applying production modulation spectroscopy. The band origins are located at 2436.272 cm−1 and at 2435.932cm−1 and correspond to the ν1 band (NN stretch) and to the ν2+4νs combination band (DN and intermolecular stretch), respectively. The two bands overlap strongly and the large intensity of the combination band is explained in terms of a Fermi interaction. This interaction perturbs the observed transitions, particularly for low J values. Least-squares fitting yields values for the Fermi interaction parameters of F0=0.332cm−1 and FJ=−0.00146cm−1 and results in accurate rotational constants. These are discussed both from an experimental and a theoretical point of view.

, during nighttime. Each lidar profile extends from the ground to 30 km height. Here the dataset ... more , during nighttime. Each lidar profile extends from the ground to 30 km height. Here the dataset is presented and discussed in terms of the temperature structure of the UTLS, obtained from co-located radiosondings. During the campaign, several high altitude clouds were observed, peaking approximately 3 km below the Cold 5 Point Tropopause (CPT) located above 17 km. Their occurrence was associated with cold anomalies in the Upper Troposphere (UT). Conversely, when warm UT anomalies occurred, the presence of cirrus was restricted to a 5 km thick layer centered 5 km below the CPT. Thin and subvisible cirrus were frequently detected close to the CPT. The particle depolarization ratios of these cirrus was generally lower than the values detected in the UT clouds. CPT cirrus occurrence showed a correlation with cold anomalies likely triggered by stratospheric wave activity penetrating the UT. The back trajectories study revealed a thermal and 10 convective history compatible with the convective outflow formation for most of the cirrus clouds, suggesting that the majority of air masses related to the clouds had encountered convection in the past and had reached the minimum temperature during its transport in less than 48 hours before the observation. A subset of SVC, with low depolarization and high lidar ratio and with no sign of significative recent uplifting, may be originated in situ. 1 Introduction Cirrus are clouds composed of ice particles which form in the upper troposphere, covering about 20-25 % of the Earth (Rossow and Schiffer, 1999). They play an important role in climate: cirrus control the amount of solar radiative energy reaching the 1

HAL (Le Centre pour la Communication Scientifique Directe), Sep 17, 2017

Temperature estimations at an altitude of about 95 km on the night side of Venus are provided. Th... more Temperature estimations at an altitude of about 95 km on the night side of Venus are provided. They are derived from hydroxyl nightglow emissions, observed in the infrared spectral range at 2.7-3.5 micron, using the Visible and Infrared Thermal Imaging Spectrometer on board Venus Express

A statistical comparison between PSC observations observed by ground- and satellite based lidars ... more A statistical comparison between PSC observations observed by ground- and satellite based lidars around McMurdo is presented. PSC lidar observations are compared to climate model predictions and balloon borne optical particle counter measurements at McMurdo.

An FSSP-100 Optical Particle Counter designed to count and size particles in the micron range and... more An FSSP-100 Optical Particle Counter designed to count and size particles in the micron range and a backscattersonde that measures in-situ particle optical properties such as backscatter and depolarization ratio, are part of the payload of the high altitude research aircraft M55 Geophysica. This aircraft was deployed in tropical field

Molecular Physics, Oct 10, 1989

The 2~ band of CFrCl and several hot bands of the most abundant isotope, CF335CI, have been inves... more The 2~ band of CFrCl and several hot bands of the most abundant isotope, CF335CI, have been investigated using a tunable diode laser spectrometer. The K fine structure of the 2v, fundamental of the CF335CI isotopic species has been measured for about 50 multiplets from P(70) up to R(101). Accurate molecular constants for the 2v, band were obtained from a least-squares fit of more than 1000 P(J, K) and R(J, K) transitions (K Q 48). Assuming additivity for aA and (1' constants, values of (Y: = 28.6(3) X 1O-6 cm-' and (I? = 70.34(4) X 10e6 cm-' were calculated. Many unresolved P(J) and R(J) multiplets were observed and assigned to the 2~3 fundamental of the CFa3'CI isotopic variety and to the hot bands 2vj + vg-+,, 2v, + V~-y5, and 3~-v, of the most abundant CF3"CI. 8 1989 Academx PW.. IIIC.

Molecular Physics, Apr 10, 2002

We report results from measurements of the high resolution FTIR spectrum for the fully deuterated... more We report results from measurements of the high resolution FTIR spectrum for the fully deuterated benzene molecule C 6 D 6 in the range 450-3500 cm -1 . Accurate spectroscopic constants have been obtained for the fundamental vibration nu11 at 496.208 cm -1 and improved ground state constants have been deduced from a fit of ground state combination differences. The J structure of the combination parallel bands nu2 + nu11 (at 2798.1 cm -1 ), nu5 + nu12 (1802.5 cm -1 ) and nu7 + nu16 (2619.3 cm -1 ) of C6D6 has been analysed as well, from which improved values of the band origin and of the B and D J constants of the excited states have been obtained. The strongest hot bands accompanying these parallel transitions have been assigned by means of the anharmonic force field calculated by MASLEN et al . [1992, J. chem. Phys. , 97 , 4233]. In particular ( nu11 + nu16 ) - nu16 is assigned to the band at 492.4 cm -1 even though its shape is typical of a perpendicular transition (PAPE). New values for the nu5 , nu12 and nu16 band origins are determined from the band origins of combination bands and from calculated anharmonic constants. Numerous anharmonic constants are derived from the assignment of hot band and combination transitions.

Molecular Physics, 1991

The v3(F2) fundamental band of ruthenium tetroxide (RuO4) vapour is investigated by high-resoluti... more The v3(F2) fundamental band of ruthenium tetroxide (RuO4) vapour is investigated by high-resolution (0·004 cm-1 FWHM) interferometric FTIR spectroscopy in the range 850-1000 cm-1. The rotational fine structure is largely resolved and accurately analysed on the basis of third- and fourth-order (diagonal) effective Hamiltonians in the theory of Moret-Bailly. The analysis of the enriched isotopomer spectra gives ..d83 cm-1, m

Mie scattering codes are used to study the optical properties of Polar Stratospheric Clouds (PSC)... more Mie scattering codes are used to study the optical properties of Polar Stratospheric Clouds (PSC). Backscattering and extinction can be computed once the particle size distribution (PSD) is known and a suitable refractive index is assumed. However, PSCs often appear as external mixtures of Supercooled Ternary Solution (STS) droplets, solid Nitric Acid Trihydrate (NAT) and possibly ice particles, making questionable the use of Mie theory with a single refractive index and with the underlying assumption of spherical scatterers. Here we consider a set of fifteen coincident measurements of PSC above McMurdo Station, Antarctica, by ground-based lidar and balloon-borne Optical Particle Counters (OPC), and in situ observations taken by a laser backscattersonde and an OPC during four balloon stratospheric flights from Kiruna, Sweden. This unique dataset of microphysical and optical observations allows to test the performances of Mie theory under fairly reasonable corrections when aspherical scatterers are present. Here we consider particles as STS if their radius is below a certain threshold value R th and NAT or possibly ice if above it. The refractive indexes are assumed known from literature. Moreover, the Mie result for solid particles are reduced by a factor C <1, which takes into account the backscattering depression expected from the asphericity. Finally, we consider the fraction X of the backscattering from the aspherical part of the PSD as polarized, and the remaining (1-X) as depolarized. The three parameters R th , C and X of our model are chosen to provide the best match with the observed optical backscattering and depolarization. The comparison of the calculations with the measures is satisfactory for the backscattering but not for the depolarization, and possible causes are discussed. The results of this work help to understand the limits of the application of Mie theory in modeling the optical response of particles of different composition and morphology.

Journal of Molecular Spectroscopy, Aug 1, 1988

Journal of Chemical Physics, Jun 28, 2013

Ab initio calculations of the shapes of pure CO 2 infrared and Raman bands under (pressure) condi... more Ab initio calculations of the shapes of pure CO 2 infrared and Raman bands under (pressure) conditions for which line-mixing effects are important have been performed using requantized classical molecular dynamics simulations. This approach provides the autocorrelation functions of the dipole vector and isotropic polarizability whose Fourier-Laplace transforms yield the corresponding spectra. For that, the classical equations of dynamics are solved for each molecule among several millions treated as linear rigid rotors and interacting through an anisotropic intermolecular potential. Two of the approximations used in the previous studies have been corrected, allowing the consideration of line-mixing effects without use of any adjusted parameters. The comparisons between calculated and experimental spectra under various conditions of pressure and temperature demonstrate the quality of the theoretical model. This opens promising perspectives for first principle ab initio predictions of line-mixing effects in absorption and scattering spectra of various systems involving linear molecules.

An analysis of depolarization, optical thickness and LR, and their relationship with the thermal ... more An analysis of depolarization, optical thickness and LR, and their relationship with the thermal and dynamic histories of the air masses is presented.