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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. Source


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. Source


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. Source


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. Source


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. Source

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