Guyancourt, France
Guyancourt, France

Time filter

Source Type

Protat A.,Center for Australian Weather and Climate Research | Young S.A.,Center for Australian Weather and Climate Research | McFarlane S.A.,Pacific Northwest National Laboratory | L'Ecuyer T.,University of Wisconsin - Madison | And 5 more authors.
Journal of Applied Meteorology and Climatology | Year: 2014

The objective of this paper is to investigate whether estimates of the cloud frequency of occurrence and associated cloud radiative forcing as derived from ground-based and satellite active remote sensing and radiative transfer calculations can be reconciled over a well-instrumented active remote sensing site located in Darwin, Australia, despite the very different viewing geometry and instrument characteristics. It is found that the ground-based radar-lidar combination at Darwin does not detect most of the cirrus clouds above 10km (because of limited lidar detection capability and signal obscuration by low-level clouds) and that the CloudSat radar-Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) combination underreports the hydrometeor frequency of occurrence below 2-km height because of instrument limitations at these heights. The radiative impact associated with these differences in cloud frequency of occurrence is large on the surface downwelling shortwave fluxes (ground and satellite) and the top-of-atmosphere upwelling shortwave and longwave fluxes (ground). Good agreement is found for other radiative fluxes. Large differences in radiative heating rate as derived from ground and satellite radar-lidar instruments and radiative transfer calculations are also found above 10km (up to 0.35 K day-1 for the shortwave and 0.8 K day-1 for the longwave). Given that the ground-based and satellite estimates of cloud frequency of occurrence and radiative impact cannot be fully reconciled over Darwin, caution should be exercised when evaluating the representation of clouds and cloud-radiation interactions in large-scale models, and limitations of each set of instrumentation should be considered when interpreting model-observation differences. © 2014 American Meteorological Society.

Protat A.,Center for Australian and Weather and Climate Research | Protat A.,Laboratoire Atmosphere | McFarquhar G.M.,University of Illinois at Urbana - Champaign | Um J.,University of Illinois at Urbana - Champaign | Delanoe J.,University of Reading
Journal of Applied Meteorology and Climatology | Year: 2011

Best estimates of the bulk microphysical and radiative properties (ice water content, visible extinction, effective radius, and total concentration) are derived for three case studies of tropical ice clouds sampled during the Tropical Warm Pool International Cloud Experiment (TWP-ICE). Two case studies are aged cirrus clouds produced by deep convection (the so-called 27/01 and 29/01 cases), and the third ("02/02") is a fresh anvil produced by deep convective activity over the Tiwi Islands. Using crystal images obtained by a Cloud Particle Imager (CPI), it is observed that small ice particles (with maximum dimension D < 50-100 μm) were predominantly quasi spherical, with the degree of nonsphericity increasing rapidly in the 50 < D < 100-μm range. For D > 100 μm, the aged cirrus clouds were predominantly characterized by bullet rosettes and aggregates of bullet rosettes, plates, and columns. In contrast, the fresh anvil had more frequent occurrences of plates, columns, aggregates of plates, and occasionally capped columns. The impact of shattering of large ice crystals on probe tips and the overall quality of the TWP-ICE in situ microphysical measurements are assessed. It is suggested that shattering has a relatively small impact on the CPI and cloud droplet probe (CDP) TWP-ICE data and a large impact on the Cloud Aerosol Spectrometer data, as already documented by others. It is also shown that the CPI size distributions must be multiplied by a factor of 4 to match those of the cloud imaging probe (CIP) for maximum dimension larger than 100 μm (taken as a reference). A technique [named Best Estimate of Area and Density (BEAD)] to minimize errors associated with the density (ρ)-D and projected area (A)-D assumptions in bulk microphysics calculation is introduced and applied to the TWP-ICE data. The method makes direct use of the frequency of occurrence of each particle habit as classified from the CPI data and prescribed ρ-D and A-D relationships from the literature. This approach produces ice water content (IWC) estimates that are virtually unbiased relative to bulk measures obtained from a counterflow spectrometer and impactor (CSI) probe. In contrast, the use of ρ-D and A-D relationships for single habits does produce large biases relative to the CSI observations: from 250% for bullet rosettes to 170%-80% for aggregates. The so-called width, length, area, and perimeter (WLAP) technique, which also makes use of individual CPI images, is found to produce positively biased IWCs (by 40% or so), and has a standard deviation of the errors similar to the BEAD technique. The impact of the large variability of the size distributions measured by different probe combinations on the bulk microphysical properties is characterized. The mean fractional differences with respect to the CSI measurements are small for the CPI 1 CIP, CPI, and CDP 1 CIP combinations (2.2%, 20.8%, and 21.1%, respectively), with standard deviations of the fractional differences ranging from 7% to 9%. This result provides an independent validation of the CPI scaling factor. The fractional differences produced between the CPI 1 CIP, CPI, and CDP 1 CIP combinations for extinction, effective radius, and total concentration are 33%, 10%-20%, and 90%, respectively, with relatively small standard deviations of 5%-8%. The fractional difference on total concentration varies greatly over the concentration range though, with values being larger than a factor of 2 for total concentrations smaller than 40 L-1, but reducing to 10%-20% for concentrations larger than 100 L-1. Therefore, caution should be exercised when using total concentrations smaller than 60-80 L-1 as references for radar-lidar retrieval evaluation. © 2011 American Meteorological Society.

Protat A.,Center for Australian Weather and Climate Research | Protat A.,Laboratoire Atmosphere | Bouniol D.,Meteo - France | O'Connor E.J.,University of Reading | And 4 more authors.
Journal of Atmospheric and Oceanic Technology | Year: 2011

The calibration of the CloudSat spaceborne cloud radar has been thoroughly assessed using very accurate internal link budgets before launch, comparisons with predicted ocean surface backscatter at 94 GHz, direct comparisons with airborne cloud radars, and statistical comparisons with ground-based cloud radars at different locations of the world. It is believed that the calibration of CloudSat is accurate to within 0.5-1 dB. In the present paper it is shown that an approach similar to that used for the statistical comparisons with groundbased radars can now be adopted the other way around to calibrate other ground-based or airborne radars against CloudSat and/or to detect anomalies in long time series of ground-based radar measurements, provided that the calibration of CloudSat is followed up closely (which is the case). The power of using CloudSat as a global radar calibrator is demonstrated using the Atmospheric Radiation Measurement cloud radar data taken at Barrow, Alaska, the cloud radar data from the Cabauw site, Netherlands, and airborne Doppler cloud radar measurements taken along the CloudSat track in the Arctic by the Radar System Airborne (RASTA) cloud radar installed in the French ATR-42 aircraft for the first time. It is found that the Barrow radar data in 2008 are calibrated too high by 9.8 dB, while the Cabauw radar data in 2008 are calibrated too low by 8.0 dB. The calibration of the RASTA airborne cloud radar using direct comparisons with CloudSat agrees well with the expected gains and losses resulting from the change in configuration that required verification of the RASTA calibration. © 2011 American Meteorological Society.

Franklin C.N.,CSIRO | Jakob C.,Monash University | Dix M.,CSIRO | Protat A.,CSIRO | And 2 more authors.
Quarterly Journal of the Royal Meteorological Society | Year: 2012

Single column model simulations using the UK Met Office Unified Model, as used in the Australian Community Climate Earth System Simulator, are presented for the Tropical Warm Pool-International Cloud Experiment (TWP-ICE) field study. Two formulations for the representation of clouds are compared with the extensive observations taken during the campaign, giving insight into the ability of the model to simulate tropical cloud systems. During the active monsoon phase the modelled cloud cover has a stronger dependence on relative humidity than the observations. Observed ice cloud properties during the suppressed monsoon period show that the ice water content is significantly underestimated in the simulations. The profiles of modelled ice fall speeds are faster than those observed in the levels above 12 km, implying that the observations have smaller sized particles in larger concentrations than the models. Both simulations show similar errors in the diurnal cycle of relative humidity during the active monsoon phase, suggesting that the error is less sensitive to the choice of cloud scheme and rather is driven by the convection scheme. However, during the times of suppressed convection the relative humidity error is different between the simulations, with congestus convection drying the environment too much, particularly in the prognostic cloud-scheme simulation. This result shows that the choice of cloud scheme and the way that the cloud and convection schemes interact plays a role in the temperature and moisture errors during the suppressed monsoon phase, which will impact the three-dimensional model simulations of tropical variability. © 2011 Royal Meteorological Society.

Chaufray J.-Y.,Laboratoire Of Meteorologie Dynamique | Gonzalez-Galindo F.,Institute Astrofisica Of Andalucia | Forget F.,Laboratoire Of Meteorologie Dynamique | Lopez-Valverde M.,Institute Astrofisica Of Andalucia | And 7 more authors.
Journal of Geophysical Research: Planets | Year: 2014

To study the transport of the ionospheric plasma on Mars, we have included a 3-D multifluid dynamical core in a Martian general circulation model. Vertical transport modifies the ion density above ~160 km on the dayside, especially the ions produced at high altitudes like O+, N+, and C +. Near the exobase, the dayside to nightside flow velocity reaches few hundreds of m/s, due to a large horizontal pressure gradient. Comparison with Mars Express/Analyzer of Space Plasmas and Energetic Atoms-3 measurements between 290 and 500 km suggests that this flow could account for at least 20% of the flow produced by the solar wind. This flow is not sufficient to populate substantially the nightside ionosphere at high altitudes, in agreement with recent observations, because of a strong nightside downward flow produced by vertical pressure gradient. The O2+ and NO+ ion densities on the nightside at low altitudes (~130 km) are modified by this downward flow, compared to simulated densities without ion dynamics, while other ions are lost by chemical reactions. Variability at different time scales (diurnal, seasonal, and solar cycles) are studied. We simulate diurnal and seasonal variations of the ionospheric composition due to the variability of the neutral atmosphere and solar flux at the top of the atmosphere. The ionospheric dynamics are not strongly affected by seasons and solar cycles, and the retroaction of the ionosphere on the neutral atmosphere temperature and velocity is negligible compared to other physical processes below the exobase. Key Points Three-dimensional GCM-Ionosphere model Effects of pressure gradients on the vertical and horizontal ions density Diurnal, seasonal, and solar activity variations of the Martian ionosphere © 2014. American Geophysical Union. All Rights Reserved.

Jouan C.,University of Quebec at Montréal | Jouan C.,University Pierre and Marie Curie | Pelon J.,University Pierre and Marie Curie | Girard E.,University of Quebec at Montréal | And 3 more authors.
Atmospheric Chemistry and Physics | Year: 2014

Recently, two types of ice clouds (TICs) properties have been characterized using the Indirect and Semi-Direct Aerosol Campaign (ISDAC) airborne measurements (Alaska, April 2008). TIC-2B were characterized by fewer (< 10 L−1) and larger (> 110 μm) ice crystals, and a larger ice supersaturation (> 15%) compared to TIC-1/2A. It has been hypothesized that emissions of SO2 may reduce the ice nucleating properties of ice nuclei (IN) through acidification, resulting in a smaller concentration of larger ice crystals and leading to precipitation (e.g., cloud regime TIC-2B). Here, the origin of air masses forming the ISDAC TIC-1/2A (1 April 2008) and TIC-2B (15 April 2008) is investigated using trajectory tools and satellite data. Results show that the synoptic conditions favor air masses transport from three potential SO2 emission sources into Alaska: eastern China and Siberia where anthropogenic and biomass burning emissions, respectively, are produced, and the volcanic region of the Kamchatka/Aleutians. Weather conditions allow the accumulation of pollutants from eastern China and Siberia over Alaska, most probably with the contribution of acidic volcanic aerosol during the TIC-2B period. Observation Monitoring Instrument (OMI) satellite observations reveal that SO2 concentrations in air masses forming the TIC-2B were larger than in air masses forming the TIC-1/2A. Airborne measurements show high acidity near the TIC-2B flight where humidity was low. These results support the hypothesis that acidic coating on IN could be at the origin of the formation of TIC-2B. © 2014 Author(s).

de Coetlogon G.,Laboratoire Atmosphere | Leduc-Leballeur M.,Laboratoire Atmosphere | Meynadier R.,Laboratoire Atmosphere | Bastin S.,Laboratoire Atmosphere | And 5 more authors.
Quarterly Journal of the Royal Meteorological Society | Year: 2014

The surface-wind response to sea-surface temperature (SST) and SST meridional gradient is investigated in the Gulf of Guinea by using daily observations and re-analyses in the 2000-2009 decade, with a focus on boreal spring and summer months (May to August), where quasi-biweekly fluctuations in the position of the northern front of the equatorial cold tongue induce quasi-biweekly equatorial SST anomalies. Following a large-scale wind acceleration (deceleration), an equatorial SST cold (warm) anomaly is created within a few days. In order to explain the local atmospheric response to this SST anomaly, the two following mechanisms are invoked: first, a colder (warmer) ocean decreases (increases) the vertical stability in the marine atmospheric boundary layer, which favours a weaker (stronger) surface wind; and second, a negative (positive) anomaly of SST meridional gradient induces a positive (negative) anomaly of the sea-level-pressure meridional gradient, which decelerates (accelerates) the surface wind. The first mechanism has an immediate effect in the equatorial belt between 1°S and 1°N (and to a lesser extent between 3°S and 1°S), whereas the second takes 1 or 2 days to adjust and damps anomalous southeasterlies up to 800 hPa in the low troposphere between 7°S and 1°N, through reversed anomalies of meridional SST and pressure gradient. This negative feedback leads to weaker (stronger) winds in the southeastern tropical Atlantic, which forces the opposite phase of the oscillation within about 1 week. Around the Equator, where the amplitude of the oscillation is found to be maximal, both mechanisms combine to maximize the wind response to the front fluctuations. Between the Equator and the coast, a low-level secondary atmospheric circulation takes control of the surface-wind acceleration or deceleration around 3°N, which reduces the influence of the SST-front fluctuations. © 2013 Royal Meteorological Society.

Protat A.,Center for Australian Weather and Climate Research | Protat A.,Laboratoire Atmosphere | Delanoe J.,University of Reading | O'Connor E.J.,University of Reading | L'Ecuyer T.S.,Colorado State University
Journal of Atmospheric and Oceanic Technology | Year: 2010

In this paper, the statistical properties of tropical ice clouds (ice water content, visible extinction, effective radius, and total number concentration) derived from 3 yr of ground-based radar-lidar retrievals from the U.S. Department of Energy Atmospheric Radiation Measurement Climate Research Facility in Darwin, Australia, are compared with the same properties derived using the official CloudSat microphysical retrieval methods and from a simpler statistical method using radar reflectivity and air temperature. It is shown that the two official CloudSat microphysical products (2B-CWC-RO and 2B-CWC-RVOD) are statistically virtually identical. The comparison with the ground-based radar-lidar retrievals shows that all satellite methods produce ice water contents and extinctions in a much narrower range than the ground-based method and overestimate the mean vertical profiles of microphysical parameters below 10-km height by over a factor of 2. Better agreements are obtained above 10-km height. Ways to improve these estimates are suggested in this study. Effective radii retrievals from the standard CloudSat algorithms are characterized by a large positive bias of 8-12 μm.Asensitivity test shows that in response to such a bias the cloud longwave forcing is increased from 44.6 to 46.9 W m-2 (implying an error of about 5%), whereas the negative cloud shortwave forcing is increased from 281.6 to 282.8 W m-2. Further analysis reveals that these modest effects (although not insignificant) can be much larger for optically thick clouds. The statistical method using CloudSat reflectivities and air temperature was found to produce inaccurate mean vertical profiles and probability distribution functions of effective radius. This study also shows that the retrieval of the total number concentration needs to be improved in the official CloudSat microphysical methods prior to a quantitative use for the characterization of tropical ice clouds. Finally, the statistical relationship used to produce ice water content from extinction and air temperature obtained by the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) satellite is evaluated for tropical ice clouds. It is suggested that the CALIPSOice water content retrieval is robust for tropical ice clouds, but that the temperature dependence of the statistical relationship used should be slightly refined to better reproduce the radar-lidar retrievals. © 2010 American Meteorological Society.

Delanoe J.,Laboratoire Atmosphere | Protat A.,Center for Australian and Weather and Climate Research | Jourdan O.,CNRS Laboratory of Physics and Meteorology | Pelon J.,Laboratoire Atmosphere | And 4 more authors.
Journal of Atmospheric and Oceanic Technology | Year: 2013

This study illustrates the high potential of RALI, the French airborne radar-lidar instrument, for studying cloud processes and evaluating satellite products when satellite overpasses are available. For an Arctic nimbostratus ice cloud collected on 1 April 2008 during the Polar Study using Aircraft, Remote Sensing, Surface Measurements and Models, of Climate, Chemistry, Aerosols, and Transport (POLARCAT) campaign, the capability of this synergistic instrument to retrieve cloud properties and to characterize the cloud phase at scales smaller than a kilometer, which is crucial for cloud process analysis, is demonstrated. A variational approach, which combines radar and lidar, is used to retrieve the ice-water content (IWC), extinction, and effective radius. The combination of radar and lidar is shown to provide better retrievals than do stand-alone methods and, in general, the radar overestimates and the lidar underestimates IWC. As the sampled ice cloud was simultaneously observed by CloudSat and Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellites, a new way to assess satellite cloud products by combining in situ and active remote sensing measurements is identified. It was then possible to compare RALI to three satellite ice cloud products: CloudSat, CALIPSO, and the Cloud-Aerosol-Water-Radiation Interactions (ICARE) center's radar-lidar project (DARDAR). © 2013 American Meteorological Society.

Diakhate M.,Laboratoire Of Physique Of Latmosphere Et Of Locean Simeon Fongang Lpao Sf | Diakhate M.,Laboratoire dOceanographie et Climat | de Coetlogon G.,Laboratoire Atmosphere | Lazar A.,Laboratoire dOceanographie et Climat | And 2 more authors.
Quarterly Journal of the Royal Meteorological Society | Year: 2016

Tropical Atlantic sea-surface temperatures (SSTs) maximum intraseasonal variability (ISV) and their interaction with local surface winds are investigated, applying statistical analysis to observations and to a recent coupled reanalysis over the 2000-2009 decade. Five cores of strong ISV emerge, with standard deviation reaching about 1 °C in frontal areas of the three main upwelling systems: equatorial, Angola-Benguela and Senegal-Mauritania (the southern side of the Canary upwelling). West of 10 °W along the Equator, a 20-60-day peak caused by tropical instability waves is shown to generate surface wind anomalies through the adjustment of the horizontal surface pressure gradient in addition to the modification of near-surface atmospheric stratification. East of 10°W along the Equator, an intense biweekly oscillation increases the ocean and atmosphere ISV. In the two coastal upwelling fronts, intraseasonal SST anomalies resemble each other. They are shown to be influenced by coastal Kelvin waves in addition to large-scale wind forcing. Over the Angola-Benguela upwelling, coastal wind bursts controlling the SST ISV are associated with anomalously strong pressure patterns related to the Madden-Julian Oscillation, the St Helena anticyclone and the Antarctic Oscillation. In the Senegal-Mauritania upwelling, the wind anomalies mainly linked to the Azores anticyclone in the southern front during November to May appear to be connected to the Saharan heat-low in the northern front from June to September. In all five regions and as expected for such upwelling regimes, vertical oceanic mixing represents the dominant term in the mixed-layer heat budget. In the equatorial band, as found in previous studies, horizontal advection is equally important, while it appears surprisingly weak in coastal fronts. Finally, a striking result is the general lack of surface wind signal related to the SST ISV in the coastal upwellings. © 2016 Royal Meteorological Society.

Loading Laboratoire Atmosphere collaborators
Loading Laboratoire Atmosphere collaborators