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Hagen M.,German Aerospace Center | Van Baelen J.,CNRS Laboratory of Physics and Meteorology | Richard E.,CNRS Laboratory for Aerology
Quarterly Journal of the Royal Meteorological Society | Year: 2011

A number of days with small precipitating convective cells were investigated using weather radars during the COPS (Convective and Orographically-induced Precipitation Study) field campaign in the region of the Vosges and the Rhine Valley in Central Europe. Depending on the weather situation, two distinct mechanisms could be identified for the initiation of convection. On some days, cells were initiated over the ridge of the Vosges, whereas on other days cells were initiated in the lee of the Vosges. The initiation of convection appeared to be concentrated in a few favourable locations. Using the Froude number, it was possible to describe the two distinct mechanisms. When the Froude number was low, the flow was diverted around the Vosges and thermally driven convergence at the ridge initiated convection, whereas when the Froude number was high, the flow passed through mountain gaps and then converged on the lee side with the flow in the Rhine Valley. The convergence on the lee side was enhanced at locations where the outflows through valleys converged. Low Froude numbers were accompanied by weak winds varying with height, whereas high Froude numbers were observed during situations with stronger southwesterly winds increasing with height. Copyright © 2011 Royal Meteorological Society Copyright © 2011 Royal Meteorological Society.

Dalou C.,CNRS Magmas and Volcanoes Laboratory | Koga K.T.,CNRS Magmas and Volcanoes Laboratory | Shimizu N.,Woods Hole Oceanographic Institution | Boulon J.,CNRS Laboratory of Physics and Meteorology | Devidal J.-L.,CNRS Magmas and Volcanoes Laboratory
Contributions to Mineralogy and Petrology | Year: 2012

We experimentally determined F and Cl partition coefficients together with that of 19 trace elements (including REE, U-Th, HFSE and LILE) between basaltic melt and lherzolite minerals: olivine, orthopyroxene, clinopyroxene, plagioclase and garnet. Under conditions from 8 to 25 kbars and from 1,265 to 1,430°C, compatibilities of F and Cl are globally ordered as D Cpx/melt > D Opx/melt > D Grt/melt > D Ol/melt > D Plag/melt, and D F mineral/melt is larger than D Cl mineral/melt. Four other major results were brought to light. (1) Chlorine partition coefficients positively correlate with the jadeite component in orthopyroxene, and increase of the CaTs component promotes Cl incorporation in clinopyroxene. (2) Variations of fluorine partition coefficients correlate strongly with melt viscosity. (3) F and Cl partition coefficients correlate with the Young's modulus (E 0) of pyroxene octahedral sites (M sites) and with Raman vibrational modes of pyroxenes. This demonstrates a fundamental interaction between the M site of pyroxenes and the incorporation of F and Cl. (4) We also determined the parameters of the lattice-strain model applied to 3+ cation trace elements for the two M sites in orthopyroxene and clinopyroxene: D 0 M1, D 0 M2, r 0 M1, r 0 M2, E 0 M1 and E 0 M2. © 2011 Springer-Verlag.

Langmann B.,University of Hamburg | Sellegri K.,CNRS Laboratory of Physics and Meteorology | Freney E.,CNRS Laboratory of Physics and Meteorology
Atmospheric Chemistry and Physics | Year: 2014

Until recently secondary organic carbon aerosol (SOA) mass concentrations have been systematically underestimated by three-dimensional atmospheric-chemistry-aerosol models. With a newly proposed concept of aging of organic vapours, more realistic model results for organic carbon aerosol mass concentrations can be achieved. Applying a mixed thermodynamic-kinetic approach for SOA formation shifted the aerosol size distribution towards particles in the cloud condensation nuclei size range, thereby emphasising the importance of SOA formation schemes for modelling realistic cloud and precipitation formation. The additional importance of hetero-molecular nucleation between H2SO4 and organic vapours remains to be evaluated in three-dimensional atmospheric-chemistry-aerosol models. Here a case study is presented focusing on Puy-de-Dôme, France in June 2010. The measurements indicate a considerable increase in SOA mass concentration during the measurement campaign, which could be reproduced by modelling using a simplified thermodynamic-kinetic approach for SOA formation and increased biogenic volatile organic compound (VOC) precursor emissions. Comparison with a thermodynamic SOA formation approach shows a huge improvement in modelled SOA mass concentration with the thermodynamic-kinetic approach for SOA formation. SOA mass concentration increases by a factor of up to 6 accompanied by a slight improvement of modelled particle size distribution. Even though nucleation events at Puy-de-Dôme were rare during the chosen period of investigation, a weak event in the boundary layer could be reproduced by the model in a sensitivity study when nucleation of low-volatile secondary organic vapour is included. Differences in the model results with and without nucleation of organic vapour are visible in the lower free troposphere over several days. Taking into account the nucleation of organic vapour leads to an increase in accumulation mode particles due to coagulation and condensational growth of nucleation and Aitken mode particles. © Author(s) 2014. CC Attribution 3.0 License.

Cornet C.,CNRS Optical Atmosphere Laboratory | C-Labonnote L.,CNRS Optical Atmosphere Laboratory | Szczap F.,CNRS Laboratory of Physics and Meteorology
Journal of Quantitative Spectroscopy and Radiative Transfer | Year: 2010

A polarized atmospheric radiative transfer model for the computation of radiative transfer inside three-dimensional inhomogeneous mediums is described. This code is based on Monte Carlo methods and takes into account the polarization state of the light. Specificities introduced by such consideration are presented. After validation of the model by comparisons with adding-doubling computations, examples of reflectances simulated from a synthetic inhomogeneous cirrus cloud are analyzed and compared with reflectances obtained with the classical assumption of a plane parallel homogeneous cloud (1D approximation). As polarized reflectance is known to saturate for optical thickness of about 3, one could think that they should be less sensitive to 3D effects than total reflectances. However, at high spatial resolution (80 m), values of polarized reflectances much higher than the ones predicted by the 1D theory can be reached. The study of the reflectances of a step cloud shows that these large values are the results of illumination and shadowing effects similar to those often observed on total reflectances. In addition, we show that for larger spatial resolution (10 km), the so-called plane-parallel bias leads to a non-negligible overestimation of the polarized reflectances of about 7-8%. © 2009 Elsevier Ltd. All rights reserved.

Shcherbakov V.,CNRS Laboratory of Physics and Meteorology | Shcherbakov V.,Institut Universitaire de France
Journal of Quantitative Spectroscopy and Radiative Transfer | Year: 2013

The goal of this study is to explore some effects of surface roughness on phase functions of hexagonal ice crystals. The simulations were performed using a Monte-Carlo ray-tracing method. The peculiarity of our statistical model of surface roughness is the following: (a) the surface roughness of the prismatic facets can be anisotropic, and (b) the roughness degree of basal and prismatic facets can be different.The main result is the existence of a sufficiently large range of the roughness parameters when a phase function of hexagonal ice crystals has a pronounced 22° halo whereas the 46° halo is totally smoothed. It is speculated that the anisotropic roughness is the unique model that is able to explain the only-22°-halo feature of phase functions of thin plates. It is also proposed to use the model of rough basal-facets as a proxy for ensembles of hollow-ended columns and/or bullets. © 2013 Elsevier Ltd.

Fauchez T.,Lille University of Science and Technology | Cornet C.,Lille University of Science and Technology | Szczap F.,CNRS Laboratory of Physics and Meteorology | Dubuisson P.,Lille University of Science and Technology | Rosambert T.,CNRS Laboratory of Physics and Meteorology
Atmospheric Chemistry and Physics | Year: 2014

This paper presents a study of the impact of cirrus cloud heterogeneities on the thermal infrared brightness temperatures at the top of the atmosphere (TOA). Realistic 3-D cirri are generated by a cloud generator based on simplified thermodynamic and dynamic equations and on the control of invariant scale properties. The 3-D thermal infrared radiative transfer is simulated with a Monte Carlo model for three typical spectral bands in the infrared atmospheric window. Comparisons of TOA brightness temperatures resulting from 1-D and 3-D radiative transfer show significant differences for optically thick cirrus (Ï., > 0.3 at 532 nm) and are mainly due to the plane-parallel approximation (PPA). At the spatial resolution of 1 km × 1 km, two principal parameters control the heterogeneity effects on brightness temperatures: i) the optical thickness standard deviation inside the observation pixel, ii) the brightness temperature contrast between the top of the cirrus∼and the clear-sky atmosphere. Furthermore, we show that the difference between 1-D and 3-D brightness temperatures increases with the zenith view angle from two to ten times between 0° and 60° due to the tilted independent pixel approximation (TIPA). © Author(s) 2014. CC Attribution 3.0 License.

Adachi K.,Arizona State University | Freney E.J.,Arizona State University | Freney E.J.,CNRS Laboratory of Physics and Meteorology | Buseck P.R.,Arizona State University
Geophysical Research Letters | Year: 2011

Hygroscopic aerosol particles change the magnitude of light scattering through condensation and evaporation of water vapor. We collected aerosol particles from two megacities and observed the particle shapes at various values of relative humidity (RH) using an environmental cell within a transmission electron microscope. Many Mexico City samples had sulfate particles that were embedded within weakly hygroscopic organic aerosol, whereas the Los Angeles samples mainly consisted of externally mixed sulfate particles. For the Mexico City samples, when the RH was increased in the microscope, only the sulfate parts deliquesced, but the entire particle did not become spherical, i.e., particles containing deliquescent phases do not necessarily become spherical upon deliquescence. This result conflicts with the assumption used in many models, i.e., that deliquesced particles become spherical. Using a discrete-dipole approximation to calculate light scattering of simulated particles that resemble the observed ones, we show that, for particles >1.0 m, the spherical-shape assumption used in Mie theory underestimates the light scattering by ∼50%, with the exact value depending on the sizes and relative volumes of the constituent phases. © 2011 by the American Geophysical Union.

Flossmann A.I.,CNRS Laboratory of Physics and Meteorology | Wobrock W.,CNRS Laboratory of Physics and Meteorology
Atmospheric Research | Year: 2010

This review compiles the main results obtained using a mesoscale cloud model with bin resolved cloud micophysics and aerosol particle scavenging, as developed by our group over the years and applied to the simulation of shallow and deep convective clouds. The main features of the model are reviewed in different dynamical frameworks covering parcel model dynamics, as well as 1.5D, 2D and 3D dynamics. The main findings are summarized to yield a digested presentation which completes the general understanding of cloud-aerosol interaction, as currently available from textbook knowledge. Furthermore, it should provide support for general cloud model development, as it will suggest potentially minor processes that might be neglected with respect to more important ones and can support development of parameterizations for air quality, chemical transport and climate models.Our work has shown that in order to analyse dedicated campaign results, the supersaturation field and the complex dynamics of the specific clouds needs to be reproduced. Only 3D dynamics represents the variation of the supersaturation over the entire cloud, the continuous nucleation and deactivation of hydrometeors, and the dependence upon initial particle size distribution and solubility.However, general statements on certain processes can be obtained also by simpler dynamics. In particular, we found:. Nucleation incorporates about 90% of the initial aerosol particle mass inside the cloud drops. Collision and coalescence redistributes the scavenged aerosol particle mass in such a way that the particle mass follows the main water mass. Small drops are more polluted than larger ones, as pollutant mass mixing ratio decreases with drops size. Collision and coalescence mixes the chemical composition of the generated drops. Their complete evaporation will release processed particles that are mostly larger and more hygroscopic than the initial particles. An interstitial aerosol is left unactivated between the cloud drops which is reduced in number and almost devoid of large particles. Consequently, impaction scavenging can probably be neglected inside clouds. Below clouds, impaction scavenging contributes around 30% to the particle mass reaching the ground by a rainfall event. The exact amount depends on the precise case studied. Nucleation and impaction scavenging directly by the ice phase in mixed phase clouds seems to play a minor role with respect to the particle mass that enters the ice particles via freezing of the liquid phase.The aerosol scavenging efficiency generally follows rather closely the precipitation scavenging value. The nucleation scavenging efficiency is around 90% for the liquid phase clouds and impaction scavenging generally contributed to about 30% of the particle mass in the rain. Clouds are very efficient in pumping up the boundary layer aerosol which essentially determines the cloud properties. For a marine case studied the net pumping depleted about 70% of the aerosol from the section of the boundary layer considered. The larger particles (and thus 70% of the mass vented up) got activated inside the cloud. A weak net import through cloud top and the upwind side was found, as well as a larger net export at the downwind side. The outside cloud subsidence can add to the replenishment of the boundary layer and eventually cause a recycling of the particles into the cloud.The results of the parcel model studies seem to indicate that increasing particulate pollution and decreasing solubility suppresses rain formation. In individual and short time cloud simulations this behaviour was even confirmed in our 3D model studies. However, taking into account entire cloud fields over longer periods of time yields the strong spatial and temporal variability of the results with isolated regions of inverse correlation of the effects. Even though in general initially the expected behaviour was found, after several hours of simulation, the overall precipitation amounts of the more polluted cases caught up. This suggests that a changing pollution will affect the spatial and temporal pattern of precipitation, but will probably not reduce the overall long term precipitation amount which might be entirely governed by the moisture state of the atmosphere. Our results regarding mixed phase precipitation with respect to "all liquid" cases seem to confirm this idea, as with increasing modelling time the precipitation mass of both cases also become similar. © 2010 Elsevier B.V.

Tridon F.,CNRS Laboratory of Physics and Meteorology | Van Baelen J.,CNRS Laboratory of Physics and Meteorology | Pointin Y.,CNRS Laboratory of Physics and Meteorology
Geophysical Research Letters | Year: 2011

Vertically pointing Micro Rain Radars (MRRs) provide profiles of drop size distributions (DSDs) from the measured Doppler reflectivity spectra. However, in the presence of strong vertical winds, the measured spectra can suffer from aliasing errors. These errors can considerably affect the derived DSD, and hence, the retrieved rain parameters. In this work, we show that such aliasing can be automatically detected and that this detection can be used at our benefit to identify strong vertical winds and eliminate incorrect retrievals. Likewise, we show that when this aliasing is adequately corrected, the retrieved DSD is then fit for further parameter retrievals. Copyright © 2011 by the American Geophysical Union.

Baran A.J.,UK Met Office | Gayet J.-F.,CNRS Laboratory of Physics and Meteorology | Shcherbakov V.,CNRS Laboratory of Physics and Meteorology
Atmospheric Chemistry and Physics | Year: 2012

In-situ Polar Nephelometer (PN) measurements of unusual ice crystal scattering phase functions, obtained near the cloud-top of a mid-latitude anvil cloud, at a temperature of about-58 °C, were recently reported by Gayet et al. (2012). The ice crystal habits that produced the phase functions consisted of aggregates of ice crystals and aggregates of quasi-spherical ice particles. The diameters of the individual quasi-spherical ice particles were estimated to be between about 15 μm and 20 μm. The measured-averaged scattering phase functions were featureless, at scattering angles less than about 100°, but an ice bow-like feature was noted between the scattering angles of about 120° to 160°. The estimated asymmetry parameter was 0.78 ± 0.04. In this paper, the averaged scattering phase function is interpreted in terms of a weighted habit mixture model. The model that provides the best overall fit to the measured scattering phase function comprises of highly distorted ten-element hexagonal ice aggregates and quasi-spherical ice particles. The smaller quasi-spherical ice crystals are represented by Chebyshev ice particles of order 3, and were assumed to have equivalent spherical diameters of 24 μm. The asymmetry parameter of the best overall model was found to be 0.79. It is argued that the Chebyshev-like ice particles are responsible for the ice bow-like feature and mostly dominate the scattered intensity measured by the PN. The results from this paper have important implications for climate modelling (energy balance of anvils), cloud physics and the remote sensing of cirrus properties. © 2012 Author(s).

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