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David W J Thompson - One of the best experts on this subject based on the ideXlab platform.
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a critical comparison of stratosphere troposphere coupling indices
Quarterly Journal of the Royal Meteorological Society, 2009Co-Authors: Mark P Baldwin, David W J ThompsonAbstract:Assessing stratosphere-troposphere coupling in observational data or model output requires a multi-level index with high time resolution. Ideally, such an index would (1) represent spatial patterns in the troposphere that are most strongly coupled with stratospheric variability and (2) be robust and computationally feasible in both observations and standard model output. Several of the indices used to diagnose extratropical stratosphere-troposphere coupling are based on the Northern and Southern Hemisphere annular modes. The annular mode indices are commonly defined as the leading empirical orthogonal functions (EOFs) of monthly-mean, hemispheric Geopotential height. In the lowermost troposphere, the structure of the annular modes is defined as the leading EOF of the near-surface Geopotential height field, and these patterns correspond well to the patterns of variability induced by stratospheric circulation changes. At pressure levels above the surface, the structure of the annular modes is typically found by either calculating the local EOF or regressing Geopotential height data onto the leading principal component time series of near-surface Geopotential height. Here we make a critical comparison of the existing methodologies used to diagnose stratosphere-troposphere coupling, including EOF-based indices as well as measures based on zonal-mean wind at a fixed latitude and Geopotential height over the polar cap. We argue in favour of an alternative methodology based on EOFs of daily zonally-averaged Geopotential. We find that (1) the daily evolution of stratosphere-troposphere coupling events is seen most clearly with this methodology, and (2) the methodology is robust and requires few subjective choices, making it readily applicable to climate model output available only in zonal-mean form. Copyright c ! 2009 Royal Meteorological Society
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A critical comparison of stratosphere–troposphere coupling indices
Quarterly Journal of the Royal Meteorological Society, 2009Co-Authors: Mark P Baldwin, David W J ThompsonAbstract:Assessing stratosphere-troposphere coupling in observational data or model output requires a multi-level index with high time resolution. Ideally, such an index would (1) represent spatial patterns in the troposphere that are most strongly coupled with stratospheric variability and (2) be robust and computationally feasible in both observations and standard model output. Several of the indices used to diagnose extratropical stratosphere-troposphere coupling are based on the Northern and Southern Hemisphere annular modes. The annular mode indices are commonly defined as the leading empirical orthogonal functions (EOFs) of monthly-mean, hemispheric Geopotential height. In the lowermost troposphere, the structure of the annular modes is defined as the leading EOF of the near-surface Geopotential height field, and these patterns correspond well to the patterns of variability induced by stratospheric circulation changes. At pressure levels above the surface, the structure of the annular modes is typically found by either calculating the local EOF or regressing Geopotential height data onto the leading principal component time series of near-surface Geopotential height. Here we make a critical comparison of the existing methodologies used to diagnose stratosphere-troposphere coupling, including EOF-based indices as well as measures based on zonal-mean wind at a fixed latitude and Geopotential height over the polar cap. We argue in favour of an alternative methodology based on EOFs of daily zonally-averaged Geopotential. We find that (1) the daily evolution of stratosphere-troposphere coupling events is seen most clearly with this methodology, and (2) the methodology is robust and requires few subjective choices, making it readily applicable to climate model output available only in zonal-mean form. Copyright c ! 2009 Royal Meteorological Society
Mark P Baldwin - One of the best experts on this subject based on the ideXlab platform.
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a critical comparison of stratosphere troposphere coupling indices
Quarterly Journal of the Royal Meteorological Society, 2009Co-Authors: Mark P Baldwin, David W J ThompsonAbstract:Assessing stratosphere-troposphere coupling in observational data or model output requires a multi-level index with high time resolution. Ideally, such an index would (1) represent spatial patterns in the troposphere that are most strongly coupled with stratospheric variability and (2) be robust and computationally feasible in both observations and standard model output. Several of the indices used to diagnose extratropical stratosphere-troposphere coupling are based on the Northern and Southern Hemisphere annular modes. The annular mode indices are commonly defined as the leading empirical orthogonal functions (EOFs) of monthly-mean, hemispheric Geopotential height. In the lowermost troposphere, the structure of the annular modes is defined as the leading EOF of the near-surface Geopotential height field, and these patterns correspond well to the patterns of variability induced by stratospheric circulation changes. At pressure levels above the surface, the structure of the annular modes is typically found by either calculating the local EOF or regressing Geopotential height data onto the leading principal component time series of near-surface Geopotential height. Here we make a critical comparison of the existing methodologies used to diagnose stratosphere-troposphere coupling, including EOF-based indices as well as measures based on zonal-mean wind at a fixed latitude and Geopotential height over the polar cap. We argue in favour of an alternative methodology based on EOFs of daily zonally-averaged Geopotential. We find that (1) the daily evolution of stratosphere-troposphere coupling events is seen most clearly with this methodology, and (2) the methodology is robust and requires few subjective choices, making it readily applicable to climate model output available only in zonal-mean form. Copyright c ! 2009 Royal Meteorological Society
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A critical comparison of stratosphere–troposphere coupling indices
Quarterly Journal of the Royal Meteorological Society, 2009Co-Authors: Mark P Baldwin, David W J ThompsonAbstract:Assessing stratosphere-troposphere coupling in observational data or model output requires a multi-level index with high time resolution. Ideally, such an index would (1) represent spatial patterns in the troposphere that are most strongly coupled with stratospheric variability and (2) be robust and computationally feasible in both observations and standard model output. Several of the indices used to diagnose extratropical stratosphere-troposphere coupling are based on the Northern and Southern Hemisphere annular modes. The annular mode indices are commonly defined as the leading empirical orthogonal functions (EOFs) of monthly-mean, hemispheric Geopotential height. In the lowermost troposphere, the structure of the annular modes is defined as the leading EOF of the near-surface Geopotential height field, and these patterns correspond well to the patterns of variability induced by stratospheric circulation changes. At pressure levels above the surface, the structure of the annular modes is typically found by either calculating the local EOF or regressing Geopotential height data onto the leading principal component time series of near-surface Geopotential height. Here we make a critical comparison of the existing methodologies used to diagnose stratosphere-troposphere coupling, including EOF-based indices as well as measures based on zonal-mean wind at a fixed latitude and Geopotential height over the polar cap. We argue in favour of an alternative methodology based on EOFs of daily zonally-averaged Geopotential. We find that (1) the daily evolution of stratosphere-troposphere coupling events is seen most clearly with this methodology, and (2) the methodology is robust and requires few subjective choices, making it readily applicable to climate model output available only in zonal-mean form. Copyright c ! 2009 Royal Meteorological Society
P. Bénard - One of the best experts on this subject based on the ideXlab platform.
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an oblate spheroid Geopotential approximation for global meteorology
Quarterly Journal of the Royal Meteorological Society, 2014Co-Authors: P. BénardAbstract:The spherical Geopotential approximation used in most meteorological global models assumes a spherical shape for the Earth and its Geopotential field. Consequently, the deviation of the geoid surface from a sphere, and the observed meridional variations of the apparent gravity are not represented. These two errors are small but their effect on medium or long-term forecasts is debated because they are systematic and might have cumulative effects. Various formulations with spheroidal iso-Geopotential surfaces have been proposed recently, but none of them combines the advantages of an accurate description for the Geopotential field and of horizontal/vertical orthogonal coordinate surfaces. This article proposes a spheroidal coordinate system which meets these two requirements. The transformation and metric factors are defined analytically. The coordinate system is defined as an approximation of orthogonal horizontal/vertical coordinates in which the vertical lines are not exactly orthogonal to true horizontal surfaces. The consequences of this deviation from orthogonality are quantified and upper bounds for the resulting errors are obtained mathematically, i.e. independently of any arbitrary numerical process. The precision of the coordinate can be made as large as desired by raising the truncation of the Taylor series used to approach its exact value, and it is shown that, in practice, truncation values in the range 5 to 8 are appropriate for global numerical weather prediction.
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An oblate‐spheroid Geopotential approximation for global meteorology
Quarterly Journal of the Royal Meteorological Society, 2013Co-Authors: P. BénardAbstract:The spherical Geopotential approximation used in most meteorological global models assumes a spherical shape for the Earth and its Geopotential field. Consequently, the deviation of the geoid surface from a sphere, and the observed meridional variations of the apparent gravity are not represented. These two errors are small but their effect on medium or long-term forecasts is debated because they are systematic and might have cumulative effects. Various formulations with spheroidal iso-Geopotential surfaces have been proposed recently, but none of them combines the advantages of an accurate description for the Geopotential field and of horizontal/vertical orthogonal coordinate surfaces. This article proposes a spheroidal coordinate system which meets these two requirements. The transformation and metric factors are defined analytically. The coordinate system is defined as an approximation of orthogonal horizontal/vertical coordinates in which the vertical lines are not exactly orthogonal to true horizontal surfaces. The consequences of this deviation from orthogonality are quantified and upper bounds for the resulting errors are obtained mathematically, i.e. independently of any arbitrary numerical process. The precision of the coordinate can be made as large as desired by raising the truncation of the Taylor series used to approach its exact value, and it is shown that, in practice, truncation values in the range 5 to 8 are appropriate for global numerical weather prediction.
Andrew Staniforth - One of the best experts on this subject based on the ideXlab platform.
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Spheroidal and spherical Geopotential approximations
Quarterly Journal of the Royal Meteorological Society, 2014Co-Authors: Andrew StaniforthAbstract:It is customary in atmospheric modelling to approximate the equi-Geopotential surfaces of apparent gravity as spheres, and to use spherical polar coordinates to represent the global atmosphere. However Earth's mean surface is more accurately approximated by a spheroid of revolution than by a sphere, and therefore the Geopotential surfaces are better represented as spheroidal surfaces than spherical ones. Several authors have considered how to develop a spheroidal coordinate system. The keystone for this is a sufficiently accurate, yet simple and flexible, mathematical approximation of the Geopotential for a spheroidal Earth. Geopotential approximation is a compromise between the extremes of being either too simple, with deficient representation of the essentials of the underlying physics, or too complicated, leading to overly complicated coordinate systems. A new spheroidal Geopotential approximation is proposed herein. It is relatively simple, and analytically tractable, yet properly represents the underlying physics. Using this new approximation, a new, relatively simple, quasi-orthogonal spheroidal coordinate system is developed. It is then straightforward to obtain the customary spherical Geopotential approximation, with its associated use of spherical polar coordinates, as an asymptotic limit of this new formulation. This confirms a previous finding, obtained using a different quasi-orthogonal coordinate system. The spheroidal Geopotential approximation, and quasi-orthogonal coordinate system, proposed herein are however much simpler, yet no less accurate. They thereby lead to a much simpler, more direct, yet equally rigorous, justification for the spherical Geopotential approximation. Copyright © 2010 Royal Meteorological Society
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Deriving significant-level Geopotentials from radiosonde reports
Monthly Weather Review, 1995Co-Authors: Clément Chouinard, Andrew StaniforthAbstract:Abstract An algorithm is developed to derive hydrostatically balanced Geopotentials at significant levels from radiosonde reports of significant-level temperatures and mandatory-level Geopotentials and temperatures. It minimizes the square of the nonhydrostatic differences in a layer where at least one significant-level datum is reported and can be viewed as being a ID analysis step that returns an estimate of the departures from hydrostatic balance within the layer. The piecewise-polynomial interpolation of the minimization procedure is used to produce an expanded Geopotential profile in any layer where significant-level data are reported, and the integrated minimization error can be used as a quality-control measure. The algorithm's performance has been evaluated using the global radiosonde dataset for a given synoptic time, and it is found that it produces equivalent layer-mean temperature errors that are generally smaller than radiosonde observational errors.
Viliam Vatrt - One of the best experts on this subject based on the ideXlab platform.
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Assessment of the LVD offsets for the normal-orthometric heights and different permanent tide systems—a case study of New Zealand
Applied Geomatics, 2011Co-Authors: Robert Tenzer, Viliam Vatrt, Ahmed Abdalla, Nadim DayoubAbstract:The Geopotential-value approach is utilized in this study to estimate the average offsets of local vertical datums (LVDs) in New Zealand realized in the system of normal-orthometric heights. The LVD offsets are taken relative to the World Height System (WHS). We adopt the geoidal Geopotential value W _0 = 62,636,856 m^2 s^−2 for a definition of WHS. The conversion of heights between different permanent tide systems is taken into consideration. The Geopotential-value approach utilizes Molodensky’s theory of the normal heights. The normal-orthometric heights at global positioning system (GPS)-leveling points are thus first converted to the normal heights. The normal to normal-orthometric height correction is computed and applied along the leveling lines using the leveling data, and the gravity disturbances are computed approximately from the EGM08 global Geopotential model. The numerical study is conducted for 18 LVDs in the North and South Islands of New Zealand. The LVD offsets are estimated from EGM08 to GPS-leveling data. The estimated average LVD offsets vary between 1 cm (Wellington 1953 LVD) and 37 cm (One Tree Point 1964 LVD).
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Dimension of the Earth's General Ellipsoid
Earth Moon and Planets, 2002Co-Authors: Milan Burša, S. Kenyon, Jan Kouba, Karel Raděj, Zdislav Šíma, Viliam Vatrt, Marie VojtíškováAbstract:The problem of specifying the Earth's mean (general)ellipsoid is discussed. This problem has been greatly simplified in the era of satellite altimetry, especially thanks to the adopted geoidal Geopotential value, W0 = (62 636 856.0 ± 0.5) m2 s-2.Consequently, the semimajor axis a of the Earth's mean ellipsoid can be easily derived. However, an a priori condition must be posed first. Two such a priori conditions have been examined, namely an ellipsoid with the corresponding Geopotential that fits best W0 in the least squares sense and an ellipsoid that has the global Geopotential average equal to W0. It has been demonstrated that both a priori conditions yield ellipsoids of the same dimension, with a–values that are practically identical to the value corresponding to the Pizzetti theory of the level ellipsoid: a = (6 378 136.68 ± 0.06) m.
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Determination of Geopotential Differences between Local Vertical Datums and Realization of a World Height System
Studia Geophysica et Geodaetica, 2001Co-Authors: Milan Burša, Jan Kouba, Karel Raděj, Viliam Vatrt, Achim Müller, Scott A. True, Marie VojtíškováAbstract:The methodology developed for connecting Local Vertical Datums (LVD) was applied to the Australian Height Datum (AHD) and the North American Vertical Datum (NAVD88). The Geopotential values at AHD and NAVD88 were computed and the corresponding vertical offset of 974 mm with rms 51 mm was obtained between the zero reference surfaces defined by AHD and NAVD88. The solution is based on the four primary geodetic parameters, the GPS/levelling sites and the Geopotential model EGM96. The Global Height System (or the Major Vertical Datum) can be defined by a geoidal Geopotential value used in the solution as the reference value, or by the Geopotential value of the LVD, e.g. NAVD88.
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Methodology of Testing Geopotential Models Specified in Different Tide Systems
Studia Geophysica et Geodaetica, 1999Co-Authors: Viliam VatrtAbstract:It is proved that the Testing Geopotential Model (TGM) results in identical model distortions when TGM is performed in the mean, zero, and tide- free systems. The Molodensky quasigeoid height is invariant in relation to different tide systems, however, the Molodensky normal height, the ellipsoidal height, as well as, the actual Geopotential, expressed in the above different tide systems, differ.
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Estimation of the accuracy of Geopotential models.
Studia Geophysica et Geodætica, 1995Co-Authors: Milan Burša, Karel Raděj, B. Bystrzycká, Viliam VatrtAbstract:The new Geopotential Model Testing (GMT) method has been theoretically developed and practically applied. It is free of any hypothesis, the limiting factors are the accuracy of the geocentric position of the GMT sites and of their normal heights, as well as the accuracy of the Geopotential value W0 on the geoid used as the testing value given a-priori. The GMT procedure occurs on the physical Earth's surface, no reductions are applied. No limits as regards the magnitude of the heights above sea level of the GMT sites are required. The rms error at discrete points of the most recent Geopotential model JGM-3 comes out at about ± 1·5 m.
