The Experts below are selected from a list of 249 Experts worldwide ranked by ideXlab platform
Pascal Marquet - One of the best experts on this subject based on the ideXlab platform.
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The first and second order approximations of the third-law moist-air entropy Potential Temperature
arXiv: Atmospheric and Oceanic Physics, 2019Co-Authors: Pascal MarquetAbstract:It is important to be able to calculate the moist-air entropy of the atmosphere with precision. A Potential Temperature has already been defined from the third law of thermodynamics for this purpose. However, a doubt remains as to whether this entropy Potential Temperature can be represented with simple but accurate first- or second-order approximate formulas. These approximations are rigorously defined in this paper using mathematical arguments and numerical adjustments to some datasets. The differentials of these approximations lead to simple but accurate formulations for tendencies, gradients and turbulent fluxes of the moist-air entropy. Several physical consequences based on these approximations are described and can serve to better understand moist-air processes (like turbulence or diabatic forcing) or properties of certain moist-air quantities (like the static energies).
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On a general definition of the squared Brunt–Väisälä frequency associated with the specific moist entropy Potential Temperature
Quarterly Journal of the Royal Meteorological Society, 2012Co-Authors: Pascal Marquet, Jean-françois GeleynAbstract:The squared Brunt–Vaisala frequency (BVF) is computed in terms of the moist entropy Potential Temperature recently defined by Marquet (2011. Q. J. R. Meteorol. Soc. 137: 768–791). Both homogeneously saturated and non-saturated versions of N2 (the squared BVF) are derived. The method employed for computing these special homogeneous cases relies on the expression of density written as a function of pressure, total water content and specific moist entropy only. The associated conservative variable diagrams are discussed and compared with existing ones. Despite being obtained without any simplification, the formulations for N2 remain nicely compact and are clearly linked with the squared BVF expressed in terms of the adiabatic non-saturated and saturated lapse rates. As in previous similar expressions, the extreme homogeneous solutions for N2 are of course different, but they are not analytically discontinuous. This allows us to define a simple bridging expression for a single general shape of N2, depending only on the basic mean atmospheric quantities and on a transition parameter, to be defined (or parametrized) in connection with the type of application sought. This integrated result remains a linear combination (with complex but purely local weights) of two terms only, namely the environmental gradient of the moist entropy Potential Temperature and the environmental gradient of the total water content. Simplified versions of the various equations are also proposed for the case in which the moist entropy Potential Temperature is approximated by a function of both so-called moist conservative variables of Betts. Copyright © 2012 Royal Meteorological Society
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definition of a moist entropy Potential Temperature application to fire i data flights
Quarterly Journal of the Royal Meteorological Society, 2011Co-Authors: Pascal MarquetAbstract:A moist entropy Potential Temperature–denoted by θs–is defined analytically in terms of the specific entropy for moist air. The expression for θs is valid for a general mixing of dry air, water vapour and possible condensed water species. It displays the same conservative properties as the moist entropy, even for varying dry air or total water content. The moist formulation for θs is equal to the dry formulation θ if dry air is considered, and it displays new properties valid for the moist air cases, both saturated or unsaturated ones. Exact and approximate versions of θs are evaluated for several stratocumulus cases, in particular by using the aircraft observation datasets from the FIRE-I experiment. It appears that there is no (or only a small) jump in θs at the top of the planetary boundary layer (PBL). The mixing in moist entropy is almost complete in the PBL, with the same values observed in the clear air and the cloudy regions, including the very top of the entrainment region. The Randall–Deardorff Cloud-Top Entrainment Instability analysis may be interpreted as a mixing in moist entropy criterion. The iso-θs lines are plotted on skew T–log p and conserved variable diagrams. All these properties could suggest some hints on the use of moist entropy (or θs) in cloud modelling or in mixing processes, with the marine stratocumulus considered as a paradigm of moist turbulence. Copyright © 2011 Royal Meteorological Society
George H. Bryan - One of the best experts on this subject based on the ideXlab platform.
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on the computation of pseudoadiabatic entropy and equivalent Potential Temperature
Monthly Weather Review, 2008Co-Authors: George H. BryanAbstract:A set of approximate equations for pseudoadiabatic thermodynamics is developed. The equations are derived by neglecting the entropy of water vapor and then compensating for this error by using a constant (but relatively large) value for the latent heat of vaporization. The subsequent formulations for entropy and equivalent Potential Temperature have errors that are comparable to those of previous formulations, but their simple form makes them attractive for use in theoretical studies. It is also shown that, if the latent heat of vaporization is replaced with a constant value, an optimal value should be chosen to minimize error; a value of 2.555 10 6 Jk g 1 is found in tests herein.
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A reevaluation of ice-liquid water Potential Temperature
Monthly Weather Review, 2004Co-Authors: George H. Bryan, J. Michael FritschAbstract:Abstract A synthesis of previous studies suggests the need for new, more accurate approximations for ice–liquid water Potential Temperature (θil), a thermodynamic variable utilized in some numerical models. Starting from equations presented in a previous study, two new approximate formulations of θil are derived, along with their governing equations. The new formulations are significant improvements over previous ones because no terms are dropped during their derivation and no Taylor series approximations are utilized. The governing equations for the new formulations reveal conditions under which θil can be considered a conserved variable. Potential Temperature lapse rates determined from a reference thermodynamic equation are compared numerically against lapse rates determined from several approximations of θil. Many of the findings agree with previous studies. However, the results show that a commonly used formulation does not account for the specific heats of water, and thus has an inherent cold bias. ...
S S Zilitinkevich - One of the best experts on this subject based on the ideXlab platform.
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similarity theory and calculation of turbulent fluxes at the surface for the stably stratified atmospheric boundary layer
Boundary-Layer Meteorology, 2007Co-Authors: S S Zilitinkevich, Igor EsauAbstract:In this paper we revise the similarity theory for the stably stratified atmospheric boundary layer (ABL), formulate analytical approximations for the wind velocity and Potential Temperature profiles over the entire ABL, validate them against large-eddy simulation and observational data, and develop an improved surface flux calculation technique for use in operational models.
Axel Seifert - One of the best experts on this subject based on the ideXlab platform.
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A Budget Analysis of the Variances of Temperature and Moisture in Precipitating Shallow Cumulus Convection
Boundary-Layer Meteorology, 2017Co-Authors: Vera Schemann, Axel SeifertAbstract:Large-eddy simulations of an evolving cloud field are used to investigate the contribution of microphysical processes to the evolution of the variance of total water and liquid water Potential Temperature in the boundary layer. While the first hours of such simulations show a transient behaviour and have to be analyzed with caution, the final portion of the simulation provides a quasi-equilibrium situation. This allows investigation of the budgets of the variances of total water and liquid water Potential Temperature and quantification of the contribution of several source and sink terms. Accretion is found to act as a strong sink for the variances, while the contributions from the processes of evaporation and autoconversion are small. A simple parametrization for the sink term connected to accretion is suggested and tested with a different set of simulations.
Trevor J. Mcdougall - One of the best experts on this subject based on the ideXlab platform.
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Quantifying the Nonconservative Production of Conservative Temperature, Potential Temperature, and Entropy
Journal of Physical Oceanography, 2013Co-Authors: F Graham, Trevor J. McdougallAbstract:AbstractThe evolution equation of Potential Temperature has to date been treated as an approximation to the oceanic version of the first law of thermodynamics. That is, oceanographers have regarded the advection and diffusion of Potential Temperature as the advection and diffusion of “heat.” However, the nonconservative source terms that arise in the evolution equation for Potential Temperature are estimated to be two orders of magnitude larger than the corresponding source terms for Conservative Temperature. In this paper the nonconservative source terms of Potential Temperature, Conservative Temperature, and entropy are derived for a stratified turbulent fluid, then quantified using the output of a coarse-resolution ocean model and compared to the rate of dissipation of mechanical energy, epsilon. It is shown that the error incurred in ocean models by assuming that Conservative Temperature is 100% conservative is approximately 120 times smaller than the corresponding error for Potential Temperature and ...
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Algorithms for Density, Potential Temperature, Conservative Temperature, and the Freezing Temperature of Seawater
Journal of Atmospheric and Oceanic Technology, 2006Co-Authors: David R. Jackett, Trevor J. Mcdougall, Rainer Feistel, Daniel G. Wright, Stephen M. GriffiesAbstract:Algorithms are presented for density, Potential Temperature, conservative Temperature, and the freezing Temperature of seawater. The algorithms for Potential Temperature and density (in terms of Potential Temperature) are updates to routines recently published by McDougall et al., while the algorithms involving conservative Temperature and the freezing Temperatures of seawater are new. The McDougall et al. algorithms were based on the thermodynamic Potential of Feistel and Hagen; the algorithms in this study are all based on the “new extended Gibbs thermodynamic Potential of seawater” of Feistel. The algorithm for the computation of density in terms of salinity, pressure, and conservative Temperature produces errors in density and in the corresponding thermal expansion coefficient of the same order as errors for the density equation using Potential Temperature, both being twice as accurate as the International Equation of State when compared with Feistel’s new equation of state. An inverse function relating Potential Temperature to conservative Temperature is also provided. The difference between practical salinity and absolute salinity is discussed, and it is shown that the present practice of essentially ignoring the difference between these two different salinities is unlikely to cause significant errors in ocean models.
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accurate and computationally efficient algorithms for Potential Temperature and density of seawater
Journal of Atmospheric and Oceanic Technology, 2003Co-Authors: Trevor J. Mcdougall, David R. Jackett, Daniel G. Wright, Rainer FeistelAbstract:Abstract An equation of state for seawater is presented that contains 25 terms and is an excellent fit to the Feistel and Hagen equation of state. It is written in terms of Potential Temperature (rather than in situ Temperature), as required for efficient ocean model integrations. The maximum density error of the fit is 3 × 10–3 kg m–3 in the oceanographic ranges of Temperature, salinity, and pressure. The corresponding maximum error in the thermal expansion coefficient is 4 × 10–7 °C–1, which is a factor of 12 less than the corresponding maximum difference between the Feistel and Hagen equation of state and the widely used but less accurate international equation of state. A method is presented to convert between Potential Temperature and in situ Temperature using specific entropy based on the Gibbs function of Feistel and Hagen. The resulting values of Potential Temperature are substantially more accurate than those based on the lapse rate derived from the international equation of state.
