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- Author or Editor: Claude Basdevant x
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Abstract
A lateral diffusion scheme designed to efficiently parameterize the subgrid scale lures associated with barotropic and baroclinic transients is presented and tested on a quasi-geostrophic, two layer model, with law-scale thermal forcing. The scheme is based on formal energy conservation and potential enstrophy dissipation. At very coarse resolutions, where the cutoff scale is of the order of the internal radius of deformation, the diffusion scheme is shown to produce a realistic amount of potential-to-kinetic energy conversions, and realistic amplitudes of the large-scale barotropic modes.
Abstract
A lateral diffusion scheme designed to efficiently parameterize the subgrid scale lures associated with barotropic and baroclinic transients is presented and tested on a quasi-geostrophic, two layer model, with law-scale thermal forcing. The scheme is based on formal energy conservation and potential enstrophy dissipation. At very coarse resolutions, where the cutoff scale is of the order of the internal radius of deformation, the diffusion scheme is shown to produce a realistic amount of potential-to-kinetic energy conversions, and realistic amplitudes of the large-scale barotropic modes.
Abstract
This article estimates the biases and standard deviations of the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) and the 50-yr National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) Reanalysis (NN50) in the upper troposphere and lower stratosphere in 1971–72. These estimates are obtained by comparing the reanalyzed temperatures and winds with EOLE observations, a dataset collected during 480 superpressure-ballon flights in the Southern Hemisphere (SH). Dedicated algorithms have been developped to control the quality of this dataset and a stringent selection has been performed on the observations. None of the atmospheric centers has assimilated the EOLE dataset, which is therefore fully independent from the reanalyses. It is furthermore argued that the statistics obtained in this study at the end of the presatellite era may be representative of the reanalysis accuracy since 1957. The results of these comparisons indicate that NN50 tends to be a few degrees colder than the observations in the SH subpolar latitudes, while ERA-40 is less hit by this cold-pole issue. Both reanalyses, on the other hand, are found to be warmer than the observations by about 1 K in the subtropics. In contrast, the wind comparisons only exhibit nonsignificant or small reanalysis biases, even though the reanalyzed subtropical jet is slightly displaced equatorward with respect to the observations. The ability of reanalyses to capture the atmospheric synoptic-scale variability in the upper troposphere is assessed by computing the standard deviations of the reanalysis minus observation differences. The ERA-40 and NN50 standard deviations show a maximum (i.e., a poorer reanalysis accuracy) in the SH storm track. However, ERA-40 standard deviations are found to be much larger than NN50 standard deviations. The standard deviations also exhibit a marked decrease above the continents, stressing the heterogeneity of the atmospheric observation network during the presatellite era. Finally, in contrast with previous studies, the reanalysis accuracy does not appear to be better during summer than during winter.
Abstract
This article estimates the biases and standard deviations of the 40-yr European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-40) and the 50-yr National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) Reanalysis (NN50) in the upper troposphere and lower stratosphere in 1971–72. These estimates are obtained by comparing the reanalyzed temperatures and winds with EOLE observations, a dataset collected during 480 superpressure-ballon flights in the Southern Hemisphere (SH). Dedicated algorithms have been developped to control the quality of this dataset and a stringent selection has been performed on the observations. None of the atmospheric centers has assimilated the EOLE dataset, which is therefore fully independent from the reanalyses. It is furthermore argued that the statistics obtained in this study at the end of the presatellite era may be representative of the reanalysis accuracy since 1957. The results of these comparisons indicate that NN50 tends to be a few degrees colder than the observations in the SH subpolar latitudes, while ERA-40 is less hit by this cold-pole issue. Both reanalyses, on the other hand, are found to be warmer than the observations by about 1 K in the subtropics. In contrast, the wind comparisons only exhibit nonsignificant or small reanalysis biases, even though the reanalyzed subtropical jet is slightly displaced equatorward with respect to the observations. The ability of reanalyses to capture the atmospheric synoptic-scale variability in the upper troposphere is assessed by computing the standard deviations of the reanalysis minus observation differences. The ERA-40 and NN50 standard deviations show a maximum (i.e., a poorer reanalysis accuracy) in the SH storm track. However, ERA-40 standard deviations are found to be much larger than NN50 standard deviations. The standard deviations also exhibit a marked decrease above the continents, stressing the heterogeneity of the atmospheric observation network during the presatellite era. Finally, in contrast with previous studies, the reanalysis accuracy does not appear to be better during summer than during winter.
Abstract
The classical phenomenological relations between dispersion laws, second-order structure functions and energy spectra are reexamined from a more quantitative standpoint. It is shown that when a nonlocal energy spectrum (steeper than k −3) is substituted into the relation giving structure functions or dispersion laws, an infrared divergence occurs so that the structure functions or dispersion laws at inertial-range separation are not dominated by contributions from the inertial range spectrum; they saturate and become independent of spectral steepness. It follows that the spectral steepness of real flows in the enstrophy inertial range must be extremely difficult to estimate from correlation or dispersion measurements alone. This might explain why the existence of steep spectra, speculated on the basis of numerical modeling, has not been confirmed by real flow measurements.
Abstract
The classical phenomenological relations between dispersion laws, second-order structure functions and energy spectra are reexamined from a more quantitative standpoint. It is shown that when a nonlocal energy spectrum (steeper than k −3) is substituted into the relation giving structure functions or dispersion laws, an infrared divergence occurs so that the structure functions or dispersion laws at inertial-range separation are not dominated by contributions from the inertial range spectrum; they saturate and become independent of spectral steepness. It follows that the spectral steepness of real flows in the enstrophy inertial range must be extremely difficult to estimate from correlation or dispersion measurements alone. This might explain why the existence of steep spectra, speculated on the basis of numerical modeling, has not been confirmed by real flow measurements.
Abstract
A method of parameterization of the energy and enstropy transfers involving subgrid scales is proposed in order to predict the time evolution of the energy spectrum at larger scales. The method is based on the strong non-localness of enstrophy transfer within the enstrophy inertial range of turbulence in two dimensions. It is applied first to a master equation derived from an eddy-damped quasi-normal Markovian model. Comparisons with reference calculations including all the scales up to the dissipation range show that the method simulates the enstrophy inertial range accurately up to the cutoff wavenumber. It is then generalized to direct simulation of the two-dimensional Navier-Stokes equation: the inertial range is again accurately simulated.
Abstract
A method of parameterization of the energy and enstropy transfers involving subgrid scales is proposed in order to predict the time evolution of the energy spectrum at larger scales. The method is based on the strong non-localness of enstrophy transfer within the enstrophy inertial range of turbulence in two dimensions. It is applied first to a master equation derived from an eddy-damped quasi-normal Markovian model. Comparisons with reference calculations including all the scales up to the dissipation range show that the method simulates the enstrophy inertial range accurately up to the cutoff wavenumber. It is then generalized to direct simulation of the two-dimensional Navier-Stokes equation: the inertial range is again accurately simulated.
Abstract
In September and October 2005, the Stratéole/Vorcore campaign flew 27 superpressure balloons from McMurdo, Antarctica, into the stratospheric polar vortex. Long-duration flights were successfully achieved, 16 of those flights lasting for more than 2 months. Most flights were terminated because they flew out of the authorized flight domain or because of energy shortage in the gondola. The atmospheric pressure (1-Pa precision) was measured every minute during the flights, whereas air temperature observations (0.25-K accuracy) and balloon positions (absolute GPS observations, 10-m accuracy) were obtained every 15 min. Fifteen-minute-averaged horizontal velocities of the wind were deduced from the successive balloon positions with a corresponding accuracy ≲0.1 m s−1. The collected dataset (more than 150 000 independent observations) provides a thorough high-resolution sampling of the polar lower stratosphere in the Southern Hemisphere from its wintertime state up to the establishment of the summer circulation in December–January. Most of the balloons stayed inside the vortex until its final breakdown, although a few were ejected toward the midlatitudes in November during filamention events associated with an increase in planetary wave activity. The balloons behaved as quasi-Lagrangian tracers during the first part of the campaign (quiescent vortex) and after the vortex breakdown in early December. Large-amplitude mountain gravity waves were detected over the Antarctic Peninsula and caused one flight termination associated with the sudden burst in the balloon superpressure.
Abstract
In September and October 2005, the Stratéole/Vorcore campaign flew 27 superpressure balloons from McMurdo, Antarctica, into the stratospheric polar vortex. Long-duration flights were successfully achieved, 16 of those flights lasting for more than 2 months. Most flights were terminated because they flew out of the authorized flight domain or because of energy shortage in the gondola. The atmospheric pressure (1-Pa precision) was measured every minute during the flights, whereas air temperature observations (0.25-K accuracy) and balloon positions (absolute GPS observations, 10-m accuracy) were obtained every 15 min. Fifteen-minute-averaged horizontal velocities of the wind were deduced from the successive balloon positions with a corresponding accuracy ≲0.1 m s−1. The collected dataset (more than 150 000 independent observations) provides a thorough high-resolution sampling of the polar lower stratosphere in the Southern Hemisphere from its wintertime state up to the establishment of the summer circulation in December–January. Most of the balloons stayed inside the vortex until its final breakdown, although a few were ejected toward the midlatitudes in November during filamention events associated with an increase in planetary wave activity. The balloons behaved as quasi-Lagrangian tracers during the first part of the campaign (quiescent vortex) and after the vortex breakdown in early December. Large-amplitude mountain gravity waves were detected over the Antarctic Peninsula and caused one flight termination associated with the sudden burst in the balloon superpressure.
