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Nikulin G.,CNRS Dynamic Meteorology Laboratory | Nikulin G.,Swedish Meteorological and Hydrological Institute | Lott F.,CNRS Dynamic Meteorology Laboratory
Annales Geophysicae | Year: 2010

Three datasets (the NCEP-NCAR reanalysis, the ERA-40 reanalysis and the LMDz-GCM), are used to analyze the relationships between large-scale dynamics of the stratosphere and the tropospheric planetary waves during the Northern Hemisphere (NH) winter. First, a cross-spectral analysis clarifies the time scales at which downward propagation of stratospheric anomalies occurs in the low-frequency band (that is at periods longer than 50 days). At these periods the strength of the polar vortex, measured by the 20-hPa Northern Annular Mode (NAM) index and the wave activity flux, measured by the vertical component of the Eliassen-Palm flux (EPz) from both the troposphere and the stratosphere, are significantly related with each other and in lead-lag quadrature. While, in the low-frequency band of the downward propagation, the EPz anomalies of the opposite sign around NAM extremes drive the onset and decay of NAM events, we found that the EPz anomalies in the troposphere, are significantly larger after stratospheric vortex anomalies than at any time before. This marked difference in the troposphere is related to planetary waves with zonal wavenumbers 1-3, showing that there is a tropospheric planetary wave response to the earlier state of the stratosphere at low frequencies. We also find that this effect is due to anomalies in the EPz issued from the northern midlatitudes and polar regions.

Gouttevin I.,Agro ParisTech | Gouttevin I.,CNRS Laboratory for Glaciology and Environmental Geophysics | Krinner G.,CNRS Laboratory for Glaciology and Environmental Geophysics | Ciais P.,French Climate and Environment Sciences Laboratory | And 2 more authors.
Cryosphere | Year: 2012

Soil freezing is a major feature of boreal regions with substantial impact on climate. The present paper describes the implementation of the thermal and hydrological effects of soil freezing in the land surface model ORCHIDEE, which includes a physical description of continental hydrology. The new soil freezing scheme is evaluated against analytical solutions and in-situ observations at a variety of scales in order to test its numerical robustness, explore its sensitivity to parameterization choices and confront its performance to field measurements at typical application scales.

Our soil freezing model exhibits a low sensitivity to the vertical discretization for spatial steps in the range of a few millimetres to a few centimetres. It is however sensitive to the temperature interval around the freezing point where phase change occurs, which should be 1 °C to 2 °C wide. Furthermore, linear and thermodynamical parameterizations of the liquid water content lead to similar results in terms of water redistribution within the soil and thermal evolution under freezing. Our approach does not allow firm discrimination of the performance of one approach over the other.

The new soil freezing scheme considerably improves the representation of runoff and river discharge in regions underlain by permafrost or subject to seasonal freezing. A thermodynamical parameterization of the liquid water content appears more appropriate for an integrated description of the hydrological processes at the scale of the vast Siberian basins. The use of a subgrid variability approach and the representation of wetlands could help capture the features of the Arctic hydrological regime with more accuracy.

The modeling of the soil thermal regime is generally improved by the representation of soil freezing processes. In particular, the dynamics of the active layer is captured with more accuracy, which is of crucial importance in the prospect of simulations involving the response of frozen carbon stocks to future warming. A realistic simulation of the snow cover and its thermal properties, as well as the representation of an organic horizon with specific thermal and hydrological characteristics, are confirmed to be a pre-requisite for a realistic modeling of the soil thermal dynamics in the Arctic. © Author(s) 2012. CC Attribution 3.0 License.

Boichu M.,CNRS Dynamic Meteorology Laboratory | Clarisse L.,Free University of Colombia | Khvorostyanov D.,CNRS Dynamic Meteorology Laboratory | Clerbaux C.,Free University of Colombia | Clerbaux C.,University Pierre and Marie Curie
Geophysical Research Letters | Year: 2014

Forecasting the dispersal of volcanic clouds during an eruption is of primary importance, especially for ensuring aviation safety. As volcanic emissions are characterized by rapid variations of emission rate and height, the (generally) high level of uncertainty in the emission parameters represents a critical issue that limits the robustness of volcanic cloud dispersal forecasts. An inverse modeling scheme, combining satellite observations of the volcanic cloud with a regional chemistry-transport model, allows reconstructing this source term at high temporal resolution. We demonstrate here how a progressive assimilation of freshly acquired satellite observations, via such an inverse modeling procedure, allows for delivering robust sulfur dioxide (SO2) cloud dispersal forecasts during the eruption. This approach provides a computationally cheap estimate of the expected location and mass loading of volcanic clouds, including the identification of SO2-rich parts. Key Points Refined SO2 cloud dispersal forecasts by assimilation of satellite observations Refined estimation of source emissions using an inverse modeling approach Compared to standard methods, cloud SO2-rich parts are robustly forecasted © 2014. American Geophysical Union. All Rights Reserved.

Mailler S.,CNRS Dynamic Meteorology Laboratory | Lott F.,Ecole Nationale des Ponts et Chaussees
Monthly Weather Review | Year: 2015

The dynamical relations between equatorial atmospheric angular momentum (EAAM), equatorialmountain torques, and cold surges are analyzed in a general circulation model (GCM). First, the authors show that the global EAAM budget is well closed in the GCM, much better than in the NCEP-NCAR reanalysis. They then confirm that the equatorial torques due to the Tibetan Plateau, the Rockies, and the Andes are well related to the cold surges developing over East Asia, North America, and South America, respectively. For all these mountains, a peak in the equatorial mountain torque components precedes by few days the development of a cold surge, confirming that the cold surge's "preconditioning" is dynamically driven by large-scale mountains. The authors also analyze the contribution of the subgrid-scale orography (SSO) parameterizations and find that they contribute substantially to the torques. In experiments where these parameterizations are almost entirely reduced over a given massif, the authors find that the explicit pressure torques produced by that massif largely compensate the reduction in the parameterized torques. On the one hand, this proves that the explicitly resolved equatorial mountain torques are effective dynamical drivers of the flow dynamics, since they are enhanced when a parameterized torque is reduced. On the other hand, this shows that the cold surges can be captured in GCMs, provided that the synoptic conditions prior to their onset are realistic. The compensation between torques is nevertheless not complete and some weakening of the cold surges is found when the parameterized mountain torques are reduced. © 2015 American Meteorological Society.

Jouve L.,French National Center for Scientific Research | Brun A.S.,University Paris Diderot | Talagrand O.,CNRS Dynamic Meteorology Laboratory
Astrophysical Journal | Year: 2011

We have developed a variational data assimilation technique for the Sun using a toy αΩ dynamo model. The purpose of this work is to apply modern data assimilation techniques to solar data using a physically based model. This work represents the first step toward a complete variational model of solar magnetism. We derive the adjoint αΩ dynamo code and use a minimization procedure to invert the spatial dependence of key physical ingredients of the model. We find that the variational technique is very powerful and leads to encouraging results that will be applied to a more realistic model of the solar dynamo. © 2011. The American Astronomical Society. All rights reserved..

Lemaire V.E.P.,INERIS | Colette A.,INERIS | Menut L.,CNRS Dynamic Meteorology Laboratory
Atmospheric Chemistry and Physics | Year: 2016

Because of its sensitivity to unfavorable weather patterns, air pollution is sensitive to climate change so that, in the future, a climate penalty could jeopardize the expected efficiency of air pollution mitigation measures. A common method to assess the impact of climate on air quality consists in implementing chemistry-transport models forced by climate projections. However, the computing cost of such methods requires optimizing ensemble exploration techniques. By using a training data set from a deterministic projection of climate and air quality over Europe, we identified the main meteorological drivers of air quality for eight regions in Europe and developed statistical models that could be used to predict air pollutant concentrations. The evolution of the key climate variables driving either particulate or gaseous pollution allows selecting the members of the EuroCordex ensemble of regional climate projections that should be used in priority for future air quality projections (CanESM2/RCA4; CNRM-CM5-LR/RCA4 and CSIRO-Mk3-6-0/RCA4 and MPI-ESM-LR/CCLM following the EuroCordex terminology). After having tested the validity of the statistical model in predictive mode, we can provide ranges of uncertainty attributed to the spread of the regional climate projection ensemble by the end of the century (2071-2100) for the RCP8.5. In the three regions where the statistical model of the impact of climate change on PM2.5 offers satisfactory performances, we find a climate benefit (a decrease of PM2.5 concentrations under future climate) of -1.08 (±0.21), -1.03 (±0.32), -0.83 (±0.14) μg m-3, for respectively Eastern Europe, Mid-Europe and Northern Italy. In the British-Irish Isles, Scandinavia, France, the Iberian Peninsula and the Mediterranean, the statistical model is not considered skillful enough to draw any conclusion for PM2.5. In Eastern Europe, France, the Iberian Peninsula, Mid-Europe and Northern Italy, the statistical model of the impact of climate change on ozone was considered satisfactory and it confirms the climate penalty bearing upon ozone of 10.51 (±3.06), 11.70 (±3.63), 11.53 (±1.55), 9.86 (±4.41), 4.82 (±1.79) μg m-3, respectively. In the British-Irish Isles, Scandinavia and the Mediterranean, the skill of the statistical model was not considered robust enough to draw any conclusion for ozone pollution. © Author(s) 2016.

Legras B.,CNRS Dynamic Meteorology Laboratory | Mestre O.,National School of Meteorological Studies | Bard E.,Collège de France | Yiou P.,CEA Saclay Nuclear Research Center
Climate of the Past | Year: 2010

A key issue of climate change is to identify the forcings and their relative contributions. The solar-climate relationship is currently the matter of a fierce debate. We address here the need for high quality observations and an adequate statistical approach. A recent work by Le Mouël et al. (2010) and its companion paper by Kossobokov et al. (2010) show spectacular correlations between solar activity and temperature series from three European weather stations over the last two centuries. We question both the data and the method used in these works. We stress (1) that correlation with solar forcing alone is meaningless unless other forcings are properly accounted for and that sunspot counting is a poor indicator of solar irradiance, (2) that long temperature series require homogenization to remove historical artefacts that affect long term variability, (3) that incorrect application of statistical tests leads to interpret as significant a signal which arises from pure random fluctuations. As a consequence, we reject the results and the conclusions of Le Mouël et al. (2010) and Kossobokov et al. (2010). We believe that our contribution bears some general interest in removing confusion from the scientific debate.© Author(s) 2010.

Angot G.,University of Versailles | Keckhut P.,University of Versailles | Hauchecorne A.,University of Versailles | Claud C.,CNRS Dynamic Meteorology Laboratory
Journal of Geophysical Research: Atmospheres | Year: 2012

This study describes a method to calculate long-term temperature trends, as an alternative to the ones based on monthly mean temperatures, which are highly impacted by the high winter variability partially due to wave-mean flow interactions like Sudden Stratospheric Warmings (SSW). This method avoids the strong influence of SSWs and provides "background" temperature trend estimates which are in better agreement with expected direct radiative effects. The data set used results from lidar measurements - performed above southern France continuously since late 1978 - combined with radiosonde profiles. With this new methodology, the long-term trends during winter at 40 km shows a larger cooling per decade (-2 0.4 K) than when the mean temperature is used (-0.4 0.4 K). The background temperature trend is closer to the summer trend estimates which are similar whatever the temperature proxy used, due to the absence of SSWs (-2.9 0.3 K per decade with the mean-based method and -3.4 0.3 K per decade with the background-based calculation). Based on this background temperature, composite evolutions of winter anomalies for both vortex-displacement and vortex-splitting major SSWs have been displayed: in both cases the largest warming occurs at the time of the SSW in the upper stratosphere, with mean amplitudes of more than 10 K. A warm signal in the upper mesosphere could suggest a potential precursory role of gravity waves. Displacement-type events present an 18-day periodicity, which is a clear sign of the wave number one Rossby wave. Colder tropospheric temperatures are noticed before and during the SSW, and warmer ones after the event, with a stronger signal for split-type events. © 2012. American Geophysical Union. All Rights Reserved.

Lebonnois S.,CNRS Dynamic Meteorology Laboratory | Hourdin F.,CNRS Dynamic Meteorology Laboratory | Eymet V.,CNRS LAPLACE Lab | Crespin A.,CNRS Dynamic Meteorology Laboratory | And 2 more authors.
Journal of Geophysical Research E: Planets | Year: 2010

A general circulation model (GCM) has been developed for the Venus atmosphere, from the surface up to 100 km altitude, based on the GCM developed for Earth at our laboratory. Key features of this new GCM include topography, diurnal cycle, dependence of the specific heat on temperature, and a consistent radiative transfer module based on net exchange rate matrices. This allows a consistent computation of the temperature field, in contrast to previous GCMs of Venus atmosphere that used simplified temperature forcing. The circulation is analyzed after 350 Venus days (111 Earth years). Superrotation is obtained above roughly 40 km altitude. Below, the zonal wind remains very small compared to observed values, which is a major pending question. The meridional circulation consists of equator-to-pole cells, the dominant one being located within the cloud layers. The modeled temperature structure is globally consistent with observations, though discrepancies persist in the stability of the lowest layers and equator-pole temperature contrast within the clouds (10 K in the model compared to the observed 40 K). In agreement with observational data, a convective layer is found between the base of the clouds (around 47 km) and the middle of the clouds (55-60 km altitude). The transport of angular momentum is analyzed, and comparison between the reference simulation and a simulation without diurnal cycle illustrates the role played by thermal tides in the equatorial region. Without diurnal cycle, the Gierasch-Rossow-Williams mechanism controls angular momentum transport. The diurnal tides add a significant downward transport of momentum in the equatorial region, causing low latitude momentum accumulation. © 2010 by the American Geophysical Union.

Mailler S.,CNRS Dynamic Meteorology Laboratory | Khvorostyanov D.,CNRS Dynamic Meteorology Laboratory | Menut L.,CNRS Dynamic Meteorology Laboratory
Atmospheric Chemistry and Physics | Year: 2013

Five one-year air quality simulations over a domain covering Europe have been performed using the CHIMERE chemistry transport model and the EMEP emission dataset for Europe. These five simulations differ only by the representation of the effective emission heights for anthropogenic emissions: one has been run using the EMEP standard recommendations, three others with vertical injection profiles derived from the EMEP recommendations but multiplying the injection height by 0.75, 0.50 and 0.25, respectively, while the last one uses vertical profiles derived from the recent literature. It is shown that using injection heights lower than the EMEP recommendations leads to significantly improved simulation of background SO2, NO2 and O3 concentrations when compared to the Airbase station measurements. © 2013 Author(s).

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