The Experts below are selected from a list of 5364 Experts worldwide ranked by ideXlab platform
Bryan R Kerman - One of the best experts on this subject based on the ideXlab platform.
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a multifractal equivalent of the beaufort scale for sea state
Geophysical Research Letters, 1993Co-Authors: Bryan R KermanAbstract:It is reported here that the ocean surface under a sufficiently high wind is a multi-fractal process, consisting of breaking wave singularities. It has been argued elsewhere that the singularity strength associated with individual breaking waves implies a distinct energy state within a continuum of such states whose entropy is associated with a fractal dimension. When the multi-fractal process is modelled in the simplest, non-trivial multiplicative energy flux cascade – as a Besicovitch-Cantor process – 3 independent variables are required for a full description. It is shown that when a Closure Assumption is invoked which relates the 2 sub-process energy fluxes as a power-law in their respective receiving areas (the process' support), the 2 exponents involved are remarkably constant within the experimental variation arising from different aircraft imaging sorties over different sea states. The result is a reduction of complexity from 3 to just 1 independent variable to describe any realization; this parameter is referred to as the multi-fractal equivalent of the Beaufort scale.
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A multifractal equivalent of the Beaufort scale for sea‐state
Geophysical Research Letters, 1993Co-Authors: Bryan R KermanAbstract:It is reported here that the ocean surface under a sufficiently high wind is a multi-fractal process, consisting of breaking wave singularities. It has been argued elsewhere that the singularity strength associated with individual breaking waves implies a distinct energy state within a continuum of such states whose entropy is associated with a fractal dimension. When the multi-fractal process is modelled in the simplest, non-trivial multiplicative energy flux cascade – as a Besicovitch-Cantor process – 3 independent variables are required for a full description. It is shown that when a Closure Assumption is invoked which relates the 2 sub-process energy fluxes as a power-law in their respective receiving areas (the process' support), the 2 exponents involved are remarkably constant within the experimental variation arising from different aircraft imaging sorties over different sea states. The result is a reduction of complexity from 3 to just 1 independent variable to describe any realization; this parameter is referred to as the multi-fractal equivalent of the Beaufort scale.
Marion Benetti - One of the best experts on this subject based on the ideXlab platform.
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Importance of boundary layer mixing for the isotopic composition of surface vapor over the subtropical North Atlantic Ocean
Journal of Geophysical Research: Atmospheres, 2015Co-Authors: Marion Benetti, Gilles Reverdin, Camille Risi, Giovanni Aloisi, Geneviève SèzeAbstract:During the summer 2012, we carried out continuous measurements of the isotopic composition (δ) of water vapor over the near-surface subtropical North Atlantic Ocean (STRASSE cruise). In this region of excess evaporation, we investigate the control of evaporation and mixing with a lower troposphere-derived, isotopically depleted air mass on the near-surface δ. We use a simple model to simulate the near-surface δ as the result of a two end-member mixing of the evaporative flux with free tropospheric air. The evaporative flux δ was estimated by the Craig and Gordon equation while the δ of the lower troposphere was taken from the LMDZ-iso global atmospheric circulation model. This simulation considers instantaneous mixing of lower tropospheric air with the evaporated flux and neglects lateral advection. Despite these simplifications, the simulations allow to identify the controls on the near-surface δ. The d-excess variability is largely a consequence of varying kinetic effects during evaporation, even during a convection event when the input of tropospheric vapor was strong. Kinetic effects and mixing processes affect simultaneously the near-surface δ and result in the vapor occupying distinct domains in the δ 18 O-δD space. The relative humidity-d-excess relationship shows that the Closure Assumption overestimates the d-excess variability at short time scales (less than a day). We interpret this as due to an effect of the residence time of the near-surface water vapor on the d-excess. Finally, we highlight the importance of reproducing mixing processes in models simulating isotopes over the subtropical North Atlantic Ocean and propose an extension of the Closure Assumption for use in initial conditions of distillation calculations.
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deuterium excess in marine water vapor dependency on relative humidity and surface wind speed during evaporation
Journal of Geophysical Research, 2014Co-Authors: Marion Benetti, Gilles Reverdin, Catherine Pierre, Liliane Merlivat, Camille Risi, Hanschristian Steenlarsen, Francoise VimeuxAbstract:We provide the first continuous measurements of isotopic composition (δD and δ18O) of water vapor over the subtropical Eastern North Atlantic Ocean from mid-August to mid-September 2012. The ship was located mostly around 26°N, 35°W where evaporation exceeded by far precipitation and water vapor at 20 m largely originated from surface evaporation. The only large deviations from that occurred during a 2 day period in the vicinity of a weak low-pressure system. The continuous measurements were used to investigate deuterium excess (d-excess) relation to evaporation. During 25 days d-excess was negatively correlated with relative humidity (r2 = 0.89). Moreover, d-excess estimated in an evaporative model with a Closure Assumption reproduced most of the observed variability. From these observations, the d-excess parameter seems to be a good indicator of evaporative conditions. We also conclude that in this region, d-excess into the marine boundary layer is less affected by mixing with the free troposphere than the isotopic composition. From our data, the transition from smooth to rough regime at the ocean surface is associated with a d-excess decrease of 5‰, which suggests the importance of the ocean surface roughness in controlling d-excess in this region.
Elfatih A. B. Eltahir - One of the best experts on this subject based on the ideXlab platform.
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Modeling the hydroclimatology of the midwestern United States. Part 1: current climate
Climate Dynamics, 2011Co-Authors: Jonathan M. Winter, Elfatih A. B. EltahirAbstract:An ensemble of six 22-year numerical experiments was conducted to evaluate the ability of Regional Climate Model version 3 (RegCM3) to simulate the energy and water budgets of the midwestern United States. RegCM3 was run using two surface physics schemes: Integrated Biosphere Simulator (IBIS) and Biosphere-Atmosphere Transfer Scheme 1e (BATS1e), and two convective Closure Assumptions: Fritsch & Chappell (FC80) and Arakawa & Schubert (AS74). Boundary conditions were provided by the National Centers for Environmental Prediction-Department of Energy Reanalysis 2 dataset and the ECHAM5 general circulation model. A companion paper examines the American Midwest under future climate scenarios. Overall, the model that reproduces the observed seasonal cycles of the midwestern United States climate system best is RegCM3 using IBIS and the AS74 convective Closure Assumption. IBIS simulates shortwave radiation more accurately, while BATS1e simulates longwave radiation more accurately. Summer two-meter air temperature is overestimated by the combination of IBIS and the FC80 convective Closure Assumption. All models contain a wet bias and overestimate evapotranspiration during the spring. Total runoff, surface runoff, groundwater runoff, and root zone soil moisture are best simulated by RegCM3 using IBIS and the AS74 convective Closure Assumption. While BATS1e does capture the seasonal cycle of total runoff, gross errors in the partitioning of total runoff between surface runoff and groundwater runoff exist. The seasonal cycle of root zone soil moisture simulated by RegCM3 using IBIS and the AS74 convective Closure Assumption is dry, but agrees with observations during the summer. The rest of the models underestimate root zone soil moisture.National Science Foundation (U.S.) (Award EAR-04500341)Martin Family Society of Fellows for Sustainabilit
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Modeling the hydroclimatology of the midwestern United States. Part 1: current climate
Climate Dynamics, 2011Co-Authors: Jonathan M. Winter, Elfatih A. B. EltahirAbstract:An ensemble of six 22-year numerical experiments was conducted to evaluate the ability of Regional Climate Model version 3 (RegCM3) to simulate the energy and water budgets of the midwestern United States. RegCM3 was run using two surface physics schemes: Integrated Biosphere Simulator (IBIS) and Biosphere-Atmosphere Transfer Scheme 1e (BATS1e), and two convective Closure Assumptions: Fritsch & Chappell (FC80) and Arakawa & Schubert (AS74). Boundary conditions were provided by the National Centers for Environmental Prediction-Department of Energy Reanalysis 2 dataset and the ECHAM5 general circulation model. A companion paper examines the American Midwest under future climate scenarios. Overall, the model that reproduces the observed seasonal cycles of the midwestern United States climate system best is RegCM3 using IBIS and the AS74 convective Closure Assumption. IBIS simulates shortwave radiation more accurately, while BATS1e simulates longwave radiation more accurately. Summer two-meter air temperature is overestimated by the combination of IBIS and the FC80 convective Closure Assumption. All models contain a wet bias and overestimate evapotranspiration during the spring. Total runoff, surface runoff, groundwater runoff, and root zone soil moisture are best simulated by RegCM3 using IBIS and the AS74 convective Closure Assumption. While BATS1e does capture the seasonal cycle of total runoff, gross errors in the partitioning of total runoff between surface runoff and groundwater runoff exist. The seasonal cycle of root zone soil moisture simulated by RegCM3 using IBIS and the AS74 convective Closure Assumption is dry, but agrees with observations during the summer. The rest of the models underestimate root zone soil moisture.
Christine Lac - One of the best experts on this subject based on the ideXlab platform.
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A New Framework to Compare Mass-Flux Schemes Within the AROME Numerical Weather Prediction Model
Boundary-Layer Meteorology, 2016Co-Authors: Sébastien Riette, Christine LacAbstract:In the Application of Research to Operations at Mesoscale (AROME) numerical weather forecast model used in operations at Météo-France, five mass-flux schemes are available to parametrize shallow convection at kilometre resolution. All but one are based on the eddy-diffusivity–mass-flux approach, and differ in entrainment/detrainment, the updraft vertical velocity equation and the Closure Assumption. The fifth is based on a more classical mass-flux approach. Screen-level scores obtained with these schemes show few discrepancies and are not sufficient to highlight behaviour differences. Here, we describe and use a new experimental framework, able to compare and discriminate among different schemes. For a year, daily forecast experiments were conducted over small domains centred on the five French metropolitan radio-sounding locations. Cloud base, planetary boundary-layer height and normalized vertical profiles of specific humidity, potential temperature, wind speed and cloud condensate were compared with observations, and with each other. The framework allowed the behaviour of the different schemes in and above the boundary layer to be characterized. In particular, the impact of the entrainment/detrainment formulation, Closure Assumption and cloud scheme were clearly visible. Differences mainly concerned the transport intensity thus allowing schemes to be separated into two groups, with stronger or weaker updrafts. In the AROME model (with all interactions and the possible existence of compensating errors), evaluation diagnostics gave the advantage to the first group.
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A New Framework to Compare Mass-Flux Schemes Within the AROME Numerical Weather Prediction Model
Boundary-Layer Meteorology, 2016Co-Authors: Sébastien Riette, Christine LacAbstract:In the Application of Research to Operations at Mesoscale (AROME) numerical weather forecast model used in operations at Meteo-France, five mass-flux schemes are available to parametrize shallow convection at kilometre resolution. All but one are based on the eddy-diffusivity–mass-flux approach, and differ in entrainment/detrainment, the updraft vertical velocity equation and the Closure Assumption. The fifth is based on a more classical mass-flux approach. Screen-level scores obtained with these schemes show few discrepancies and are not sufficient to highlight behaviour differences. Here, we describe and use a new experimental framework, able to compare and discriminate among different schemes. For a year, daily forecast experiments were conducted over small domains centred on the five French metropolitan radio-sounding locations. Cloud base, planetary boundary-layer height and normalized vertical profiles of specific humidity, potential temperature, wind speed and cloud condensate were compared with observations, and with each other. The framework allowed the behaviour of the different schemes in and above the boundary layer to be characterized. In particular, the impact of the entrainment/detrainment formulation, Closure Assumption and cloud scheme were clearly visible. Differences mainly concerned the transport intensity thus allowing schemes to be separated into two groups, with stronger or weaker updrafts. In the AROME model (with all interactions and the possible existence of compensating errors), evaluation diagnostics gave the advantage to the first group.
Gilles Reverdin - One of the best experts on this subject based on the ideXlab platform.
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Importance of boundary layer mixing for the isotopic composition of surface vapor over the subtropical North Atlantic Ocean
Journal of Geophysical Research: Atmospheres, 2015Co-Authors: Marion Benetti, Gilles Reverdin, Camille Risi, Giovanni Aloisi, Geneviève SèzeAbstract:During the summer 2012, we carried out continuous measurements of the isotopic composition (δ) of water vapor over the near-surface subtropical North Atlantic Ocean (STRASSE cruise). In this region of excess evaporation, we investigate the control of evaporation and mixing with a lower troposphere-derived, isotopically depleted air mass on the near-surface δ. We use a simple model to simulate the near-surface δ as the result of a two end-member mixing of the evaporative flux with free tropospheric air. The evaporative flux δ was estimated by the Craig and Gordon equation while the δ of the lower troposphere was taken from the LMDZ-iso global atmospheric circulation model. This simulation considers instantaneous mixing of lower tropospheric air with the evaporated flux and neglects lateral advection. Despite these simplifications, the simulations allow to identify the controls on the near-surface δ. The d-excess variability is largely a consequence of varying kinetic effects during evaporation, even during a convection event when the input of tropospheric vapor was strong. Kinetic effects and mixing processes affect simultaneously the near-surface δ and result in the vapor occupying distinct domains in the δ 18 O-δD space. The relative humidity-d-excess relationship shows that the Closure Assumption overestimates the d-excess variability at short time scales (less than a day). We interpret this as due to an effect of the residence time of the near-surface water vapor on the d-excess. Finally, we highlight the importance of reproducing mixing processes in models simulating isotopes over the subtropical North Atlantic Ocean and propose an extension of the Closure Assumption for use in initial conditions of distillation calculations.
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deuterium excess in marine water vapor dependency on relative humidity and surface wind speed during evaporation
Journal of Geophysical Research, 2014Co-Authors: Marion Benetti, Gilles Reverdin, Catherine Pierre, Liliane Merlivat, Camille Risi, Hanschristian Steenlarsen, Francoise VimeuxAbstract:We provide the first continuous measurements of isotopic composition (δD and δ18O) of water vapor over the subtropical Eastern North Atlantic Ocean from mid-August to mid-September 2012. The ship was located mostly around 26°N, 35°W where evaporation exceeded by far precipitation and water vapor at 20 m largely originated from surface evaporation. The only large deviations from that occurred during a 2 day period in the vicinity of a weak low-pressure system. The continuous measurements were used to investigate deuterium excess (d-excess) relation to evaporation. During 25 days d-excess was negatively correlated with relative humidity (r2 = 0.89). Moreover, d-excess estimated in an evaporative model with a Closure Assumption reproduced most of the observed variability. From these observations, the d-excess parameter seems to be a good indicator of evaporative conditions. We also conclude that in this region, d-excess into the marine boundary layer is less affected by mixing with the free troposphere than the isotopic composition. From our data, the transition from smooth to rough regime at the ocean surface is associated with a d-excess decrease of 5‰, which suggests the importance of the ocean surface roughness in controlling d-excess in this region.