Pub Date : 2020-11-25DOI: 10.1080/03091929.2020.1842391
Koushik Kanti Barman, S. Bora
ABSTRACT The present work is concerned with the interaction of oblique surface gravity waves by a simple and composite porous block of finite width placed on a multi-step bottom in a two-layer fluid. The ocean depth is taken to be finite and its bed impermeable. The problem is studied by employing linearised water wave theory and eigenfunction expansion. The dispersion relations and their roots are analysed which give a clear understanding of the phenomenon. The cases of simple and composite interface-piercing structures are taken up separately to investigate the impact of porosity in wave attenuation for surface and interface modes. Waves propagate through the porous structure with distinct eigenvalues. The appropriateness of structures of various configurations on the scattering of surface waves is investigated by examining the reflection coefficients for waves in surface and interface modes as well as their effects on the free surface and interface elevations, the wave-loads on the structure and the rigid wall supporting the structure at one end. Further, as a special case, the sea-bed preceding the step bottom is considered to be porous and its effect on reflection is examined. The investigation establishes that for a suitable configuration of the porous structure, an optimum width can be ascertained to design a breakwater of reasonable efficiency possessing characteristics of both reflection and dissipation processes. The problems are solved analytically and the results are presented in graphical form. This kind of study is likely to have immense significance for designing of different types of coastal structures with respect to reflection and dissipation of wave energy at continental shelves which is influenced by a stratified fluid, which is modelled in this work as a two-layer fluid for convenience. Comparison of present results with available results show good agreement and this points towards the effectiveness of the model described in this work.
{"title":"Linear water wave interaction with a composite porous structure in a two-layer fluid flowing over a step-like sea-bed","authors":"Koushik Kanti Barman, S. Bora","doi":"10.1080/03091929.2020.1842391","DOIUrl":"https://doi.org/10.1080/03091929.2020.1842391","url":null,"abstract":"ABSTRACT The present work is concerned with the interaction of oblique surface gravity waves by a simple and composite porous block of finite width placed on a multi-step bottom in a two-layer fluid. The ocean depth is taken to be finite and its bed impermeable. The problem is studied by employing linearised water wave theory and eigenfunction expansion. The dispersion relations and their roots are analysed which give a clear understanding of the phenomenon. The cases of simple and composite interface-piercing structures are taken up separately to investigate the impact of porosity in wave attenuation for surface and interface modes. Waves propagate through the porous structure with distinct eigenvalues. The appropriateness of structures of various configurations on the scattering of surface waves is investigated by examining the reflection coefficients for waves in surface and interface modes as well as their effects on the free surface and interface elevations, the wave-loads on the structure and the rigid wall supporting the structure at one end. Further, as a special case, the sea-bed preceding the step bottom is considered to be porous and its effect on reflection is examined. The investigation establishes that for a suitable configuration of the porous structure, an optimum width can be ascertained to design a breakwater of reasonable efficiency possessing characteristics of both reflection and dissipation processes. The problems are solved analytically and the results are presented in graphical form. This kind of study is likely to have immense significance for designing of different types of coastal structures with respect to reflection and dissipation of wave energy at continental shelves which is influenced by a stratified fluid, which is modelled in this work as a two-layer fluid for convenience. Comparison of present results with available results show good agreement and this points towards the effectiveness of the model described in this work.","PeriodicalId":56132,"journal":{"name":"Geophysical and Astrophysical Fluid Dynamics","volume":"28 1","pages":"577 - 611"},"PeriodicalIF":1.3,"publicationDate":"2020-11-25","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81665778","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-11-01DOI: 10.1080/03091929.2019.1692205
T. K. Edwards, L. Smith, S. Stechmann
Atmospheric water has a complex behaviour partly due to the influence of precipitation. Consequently, it is challenging to explain properties of water such as the scale-dependence of its variance, for which a range of spectral exponents has been identified in observational data. Here, a precipitating quasi-geostrophic (PQG) model is explored as a possible prototype for contributing to understanding of water spectra, in an idealised setting. Geostrophic turbulence is examined in numerical simulations, where precipitation is included to explore its effect on the water spectrum, but where phase changes are neglected to allow corresponding theoretical analysis. The water spectral exponent is seen to range from approximately −1.4 to approximately −5 depending on the rainfall speed parameter, , which indicates a significant influence of precipitation on the water spectrum. The limiting values of this range are explained through asymptotic analyses for large and small values of . To obtain this theoretical understanding of the model, a key observation is that water can be written as a linear combination of two other tracers (equivalent potential temperature and a moist variable M), which themselves have theoretically tractable spectra. These two other tracers are linked to distinct modes of the PQG equations–the vortical mode and a moist mode – and the analysis here highlights the usefulness of wave or mode decompositions for understanding water in a saturated domain.
{"title":"Spectra of atmospheric water in precipitating quasi-geostrophic turbulence","authors":"T. K. Edwards, L. Smith, S. Stechmann","doi":"10.1080/03091929.2019.1692205","DOIUrl":"https://doi.org/10.1080/03091929.2019.1692205","url":null,"abstract":"Atmospheric water has a complex behaviour partly due to the influence of precipitation. Consequently, it is challenging to explain properties of water such as the scale-dependence of its variance, for which a range of spectral exponents has been identified in observational data. Here, a precipitating quasi-geostrophic (PQG) model is explored as a possible prototype for contributing to understanding of water spectra, in an idealised setting. Geostrophic turbulence is examined in numerical simulations, where precipitation is included to explore its effect on the water spectrum, but where phase changes are neglected to allow corresponding theoretical analysis. The water spectral exponent is seen to range from approximately −1.4 to approximately −5 depending on the rainfall speed parameter, , which indicates a significant influence of precipitation on the water spectrum. The limiting values of this range are explained through asymptotic analyses for large and small values of . To obtain this theoretical understanding of the model, a key observation is that water can be written as a linear combination of two other tracers (equivalent potential temperature and a moist variable M), which themselves have theoretically tractable spectra. These two other tracers are linked to distinct modes of the PQG equations–the vortical mode and a moist mode – and the analysis here highlights the usefulness of wave or mode decompositions for understanding water in a saturated domain.","PeriodicalId":56132,"journal":{"name":"Geophysical and Astrophysical Fluid Dynamics","volume":"70 1","pages":"715 - 741"},"PeriodicalIF":1.3,"publicationDate":"2020-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83806166","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-11-01DOI: 10.1080/03091929.2019.1694676
Y. Afanasyev, Y. Huang
Atmospheres of gas-giant planets are driven by thermal convection and often exhibit cyclonic circulation at the poles. Here we present the results of the numerical simulations of individual cold and warm blobs in a polar area of a rotating deep spherical layer. The simulations show that the cyclones created at the top of the atmosphere by sinking cold blobs translate northward. The cyclones are the surface signatures of the Taylor columns formed above the descending cold blobs. The Taylor columns are aligned with the planetary axis of rotation and are created by inertial (gyroscopic) waves emitted by the blobs. In contrast, the cyclones created at the bottom of the shell by rising warm blobs move southwards. The numerical results exclude beta-drift from possible reasons of the observed translation.
{"title":"Poleward translation of vortices due to deep thermal convection on a rotating planet","authors":"Y. Afanasyev, Y. Huang","doi":"10.1080/03091929.2019.1694676","DOIUrl":"https://doi.org/10.1080/03091929.2019.1694676","url":null,"abstract":"Atmospheres of gas-giant planets are driven by thermal convection and often exhibit cyclonic circulation at the poles. Here we present the results of the numerical simulations of individual cold and warm blobs in a polar area of a rotating deep spherical layer. The simulations show that the cyclones created at the top of the atmosphere by sinking cold blobs translate northward. The cyclones are the surface signatures of the Taylor columns formed above the descending cold blobs. The Taylor columns are aligned with the planetary axis of rotation and are created by inertial (gyroscopic) waves emitted by the blobs. In contrast, the cyclones created at the bottom of the shell by rising warm blobs move southwards. The numerical results exclude beta-drift from possible reasons of the observed translation.","PeriodicalId":56132,"journal":{"name":"Geophysical and Astrophysical Fluid Dynamics","volume":"120 1","pages":"821 - 834"},"PeriodicalIF":1.3,"publicationDate":"2020-11-01","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"74937396","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-10-29DOI: 10.1080/03091929.2020.1831483
M. Kalashnik
ABSTRACT Surface quasi-geostrophic (SQG) flows with a much larger horizontal scale than the Rossby radius of deformation are considered. A new version of the SQG model with two boundaries, which is reduced to a nonlinear system of partial differential equations, is proposed to describe the dynamics of such flows. This system describes the interaction between the barotropic and baroclinic components of the stream function and generalises the two-dimensional Euler equations for flows with a vertical velocity shear. The laws of conservation of both total and surface potential energies, which follow from this system, have been formulated. The solutions of a number of problems in the theory of baroclinic instability, which are in agreement with already known solutions, have been obtained within the framework of this system. It is shown that vertical shear flows are absolutely unstable, i.e. their instability is independent of the horizontal velocity profile structure. A generalised system of the SQG model equations, which additionally takes into account the β-effect and the Ekman bottom friction, has also been proposed. The transformation of jet flows due to the bottom friction and the influence of the β-effect on the stability of shear flows have been studied based on this system.
{"title":"Long-wave instabilities in the SQG model with two boundaries","authors":"M. Kalashnik","doi":"10.1080/03091929.2020.1831483","DOIUrl":"https://doi.org/10.1080/03091929.2020.1831483","url":null,"abstract":"ABSTRACT Surface quasi-geostrophic (SQG) flows with a much larger horizontal scale than the Rossby radius of deformation are considered. A new version of the SQG model with two boundaries, which is reduced to a nonlinear system of partial differential equations, is proposed to describe the dynamics of such flows. This system describes the interaction between the barotropic and baroclinic components of the stream function and generalises the two-dimensional Euler equations for flows with a vertical velocity shear. The laws of conservation of both total and surface potential energies, which follow from this system, have been formulated. The solutions of a number of problems in the theory of baroclinic instability, which are in agreement with already known solutions, have been obtained within the framework of this system. It is shown that vertical shear flows are absolutely unstable, i.e. their instability is independent of the horizontal velocity profile structure. A generalised system of the SQG model equations, which additionally takes into account the β-effect and the Ekman bottom friction, has also been proposed. The transformation of jet flows due to the bottom friction and the influence of the β-effect on the stability of shear flows have been studied based on this system.","PeriodicalId":56132,"journal":{"name":"Geophysical and Astrophysical Fluid Dynamics","volume":"4 1","pages":"393 - 411"},"PeriodicalIF":1.3,"publicationDate":"2020-10-29","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85663684","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-10-12DOI: 10.1080/03091929.2020.1828402
J. Reinaud
The self-similar collapse of three vortices is the motion of three vortices colliding at a single point at finite time. Such a motion has first been shown to exist for two-dimensional, planar, point vortices. In this paper, we show that the concept generalises naturally to three-dimensional quasi-geostrophic vortices as well as to surface quasi-geostrophic vortices. We first determine the conditions that lead to the collapse for these singular vortices. We then show how these conditions precipitate the merger of finite core vortices both in a three-dimensional quasi-geostrophic flow and in a surface quasi-geostrophic flow.
{"title":"Self-similar collapse of three geophysical vortices","authors":"J. Reinaud","doi":"10.1080/03091929.2020.1828402","DOIUrl":"https://doi.org/10.1080/03091929.2020.1828402","url":null,"abstract":"The self-similar collapse of three vortices is the motion of three vortices colliding at a single point at finite time. Such a motion has first been shown to exist for two-dimensional, planar, point vortices. In this paper, we show that the concept generalises naturally to three-dimensional quasi-geostrophic vortices as well as to surface quasi-geostrophic vortices. We first determine the conditions that lead to the collapse for these singular vortices. We then show how these conditions precipitate the merger of finite core vortices both in a three-dimensional quasi-geostrophic flow and in a surface quasi-geostrophic flow.","PeriodicalId":56132,"journal":{"name":"Geophysical and Astrophysical Fluid Dynamics","volume":"4 1","pages":"369 - 392"},"PeriodicalIF":1.3,"publicationDate":"2020-10-12","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"83681050","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-09-24DOI: 10.1080/03091929.2020.1817913
D. Grandi, A. Passerini
ABSTRACT We consider a model for convection in compressible fluids in two dimensions. A constitutive limit is studied in which both the mechanical compressibility and thermal expansion affect the buoyancy force. The motion is no longer isochoric as in the classical Boussinesq approximation but has a uniform expansion rate associated to the upward motion: . By using a perturbative approach, we study a Boussinesq-like approximation with pressure-dependent buoyancy force. The existence of weak solutions for the approximated system is proved and their stability is investigated.
{"title":"Approximation à la Oberbeck-Boussinesq for fluids with pressure-induced stratified density","authors":"D. Grandi, A. Passerini","doi":"10.1080/03091929.2020.1817913","DOIUrl":"https://doi.org/10.1080/03091929.2020.1817913","url":null,"abstract":"ABSTRACT We consider a model for convection in compressible fluids in two dimensions. A constitutive limit is studied in which both the mechanical compressibility and thermal expansion affect the buoyancy force. The motion is no longer isochoric as in the classical Boussinesq approximation but has a uniform expansion rate associated to the upward motion: . By using a perturbative approach, we study a Boussinesq-like approximation with pressure-dependent buoyancy force. The existence of weak solutions for the approximated system is proved and their stability is investigated.","PeriodicalId":56132,"journal":{"name":"Geophysical and Astrophysical Fluid Dynamics","volume":"22 1","pages":"412 - 435"},"PeriodicalIF":1.3,"publicationDate":"2020-09-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"74173154","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-09-02DOI: 10.1080/03091929.2020.1814275
Adam Ayouche, X. Carton, G. Charria, Sebastien Theettens, N. Ayoub
In the Bay of Biscay (north-east Atlantic), long-living eddies and the frontal activity that they induce substantially contribute to mesoscale and submesoscale dynamics. Tides and river plumes also contribute to frontal activity. Biological productivity is sensitive to river plume fronts and to external forcings (tides and wind). Considering the importance of river plumes, we study here the structure, stability and vertical mixing processes in such river plumes (similar to those generated by the Gironde river). Restratification budget is considered here for evaluating stirring (frontogenetic/frontolytic) or vertical mixing (parametrised here from Ertel potential vorticity mixing) processes. Using high-resolution idealised numerical simulations, we analyse the evolution of the bulge and of the coastal part of this plume and we conduct sensitivity experiments to the river discharge, to southwesterly winds and to M2 tides. The bulge and the coastal current are stable (unstable) in case of moderate (high) river discharge, due to mixed barotropic/baroclinic instabilities. In the unstable case, near surface symmetric and vertical shear instabilities develop in the coastal current and in the core of the bulge where the Rossby number is large. When southwesterly winds blow, the river plume is squeezed near the coast by Ekman transport. The river plume is then subject to frontal symmetric, baroclinic, barotropic and vertical shear instabilities in the coastal part, north of the estuary (its far field). Conversely, in the presence of M2 tides, the river plume is barotropically, baroclinically and symmetrically unstable in its near field. Interior vertical mixing is induced by advective (stirring) and frontogenetic processes. Frontogenesis is dominant in the far-field (in the presence of southwesterlies) or in the near-field (when M2 tide is active). Frontogenesis is important in the far-field region in unforced river plumes (both with moderate and high river discharges). Potential vorticity is eroded in the far-field when southwesterlies blow. This is primarily due to the frictional processes which are dominant at the surface. This study has identified the instabilities which affect a river plume in different cases, and the local turbulent processes which alter the stratification.
{"title":"Instabilities and vertical mixing in river plumes: application to the Bay of Biscay","authors":"Adam Ayouche, X. Carton, G. Charria, Sebastien Theettens, N. Ayoub","doi":"10.1080/03091929.2020.1814275","DOIUrl":"https://doi.org/10.1080/03091929.2020.1814275","url":null,"abstract":"In the Bay of Biscay (north-east Atlantic), long-living eddies and the frontal activity that they induce substantially contribute to mesoscale and submesoscale dynamics. Tides and river plumes also contribute to frontal activity. Biological productivity is sensitive to river plume fronts and to external forcings (tides and wind). Considering the importance of river plumes, we study here the structure, stability and vertical mixing processes in such river plumes (similar to those generated by the Gironde river). Restratification budget is considered here for evaluating stirring (frontogenetic/frontolytic) or vertical mixing (parametrised here from Ertel potential vorticity mixing) processes. Using high-resolution idealised numerical simulations, we analyse the evolution of the bulge and of the coastal part of this plume and we conduct sensitivity experiments to the river discharge, to southwesterly winds and to M2 tides. The bulge and the coastal current are stable (unstable) in case of moderate (high) river discharge, due to mixed barotropic/baroclinic instabilities. In the unstable case, near surface symmetric and vertical shear instabilities develop in the coastal current and in the core of the bulge where the Rossby number is large. When southwesterly winds blow, the river plume is squeezed near the coast by Ekman transport. The river plume is then subject to frontal symmetric, baroclinic, barotropic and vertical shear instabilities in the coastal part, north of the estuary (its far field). Conversely, in the presence of M2 tides, the river plume is barotropically, baroclinically and symmetrically unstable in its near field. Interior vertical mixing is induced by advective (stirring) and frontogenetic processes. Frontogenesis is dominant in the far-field (in the presence of southwesterlies) or in the near-field (when M2 tide is active). Frontogenesis is important in the far-field region in unforced river plumes (both with moderate and high river discharges). Potential vorticity is eroded in the far-field when southwesterlies blow. This is primarily due to the frictional processes which are dominant at the surface. This study has identified the instabilities which affect a river plume in different cases, and the local turbulent processes which alter the stratification.","PeriodicalId":56132,"journal":{"name":"Geophysical and Astrophysical Fluid Dynamics","volume":"26 1","pages":"650 - 689"},"PeriodicalIF":1.3,"publicationDate":"2020-09-02","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"87475319","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-08-20DOI: 10.1080/03091929.2020.1804900
J. Šimkanin, J. Kyselica
We carry out numerical dynamo simulations in a spherical shell to assess the influence of varying Prandtl number on the morphology of the induced magnetic field in both cases when the convection is laminar and turbulent. We work in a regime of large magnetic Prandtl number ( ) due to computational constraints of the full three-dimensional numerical model. We consider dynamos driven by thermal convection in the regimes of small and unit Prandtl numbers. Unlike the laminar convection, in the turbulent regime the morphology of the induced magnetic fields depends only weakly on the Prandtl number. For both values of considered the magnetic field is non-dipolar and small-scale. Together with the previous results of other authors, who found that the morphology of the magnetic field does not change substantially with when is small, the results of our study suggest that the induced magnetic field does not vary substantially with regardless of the strength of the magnetic diffusion.
{"title":"Transition from laminar to turbulent dynamo: the effect of varying Prandtl number","authors":"J. Šimkanin, J. Kyselica","doi":"10.1080/03091929.2020.1804900","DOIUrl":"https://doi.org/10.1080/03091929.2020.1804900","url":null,"abstract":"We carry out numerical dynamo simulations in a spherical shell to assess the influence of varying Prandtl number on the morphology of the induced magnetic field in both cases when the convection is laminar and turbulent. We work in a regime of large magnetic Prandtl number ( ) due to computational constraints of the full three-dimensional numerical model. We consider dynamos driven by thermal convection in the regimes of small and unit Prandtl numbers. Unlike the laminar convection, in the turbulent regime the morphology of the induced magnetic fields depends only weakly on the Prandtl number. For both values of considered the magnetic field is non-dipolar and small-scale. Together with the previous results of other authors, who found that the morphology of the magnetic field does not change substantially with when is small, the results of our study suggest that the induced magnetic field does not vary substantially with regardless of the strength of the magnetic diffusion.","PeriodicalId":56132,"journal":{"name":"Geophysical and Astrophysical Fluid Dynamics","volume":"16 1","pages":"192 - 206"},"PeriodicalIF":1.3,"publicationDate":"2020-08-20","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"74605505","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-08-19DOI: 10.1080/03091929.2020.1805448
M. Rostami, V. Zeitlin
It is shown that steady large-scale slowly eastward-moving twin-cyclone coherent structures, the equatorial modons, exist in both one- and two layer versions of the rotating shallow water model on the equatorial beta plane. They arise via the process of “ageostrophic adjustment” from the analytic asymptotic modon solutions of the vorticity equation obtained in the limit of small pressure perturbations. Evolution of these structures in adiabatic and moist-convective environments, and also in the presence of topography is analysed, showing their robustness in the one-layer model. It is demonstrated that moist convection enhances and helps maintain the modons. In the two-layer model the barotropic and quasi-barotropic modons display similar to one-layer modon features, while increasing baroclinicity leads to eventual loss of coherence and arrest of the eastward propagation. Some features of equatorial modons resemble those observed in the Madden-Julian Oscillation events in tropical atmosphere, which hints at their possible relevance to the dynamics of this phenomenon.
{"title":"Eastward-moving equatorial modons in moist-convective shallow-water models","authors":"M. Rostami, V. Zeitlin","doi":"10.1080/03091929.2020.1805448","DOIUrl":"https://doi.org/10.1080/03091929.2020.1805448","url":null,"abstract":"It is shown that steady large-scale slowly eastward-moving twin-cyclone coherent structures, the equatorial modons, exist in both one- and two layer versions of the rotating shallow water model on the equatorial beta plane. They arise via the process of “ageostrophic adjustment” from the analytic asymptotic modon solutions of the vorticity equation obtained in the limit of small pressure perturbations. Evolution of these structures in adiabatic and moist-convective environments, and also in the presence of topography is analysed, showing their robustness in the one-layer model. It is demonstrated that moist convection enhances and helps maintain the modons. In the two-layer model the barotropic and quasi-barotropic modons display similar to one-layer modon features, while increasing baroclinicity leads to eventual loss of coherence and arrest of the eastward propagation. Some features of equatorial modons resemble those observed in the Madden-Julian Oscillation events in tropical atmosphere, which hints at their possible relevance to the dynamics of this phenomenon.","PeriodicalId":56132,"journal":{"name":"Geophysical and Astrophysical Fluid Dynamics","volume":"30 1","pages":"345 - 367"},"PeriodicalIF":1.3,"publicationDate":"2020-08-19","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"81079131","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
Pub Date : 2020-08-05DOI: 10.1080/03091929.2020.1795150
John McHugh, R. Sharman
The vertical profile of the Earth's atmosphere in middle latitudes contains a sharp transition region between two roughly constant stability layers, termed the tropopause and also exhibits jet streams at nearly the same altitude, with the jet stream core possibly above or below the tropopause, depending on time and location. This proximity of the jet to the tropopause would be expected to greatly affect the dynamic stability of the jet, treated here with the jet modelled with the Bickley profile and the tropopause modelled as a smooth transition region with a tanh profile. Stability results are obtained numerically using a Chebyshev collocation spectral method. The results show that the jet becomes more unstable as it is moved further beneath the tropopause. Corresponding two- and three-dimensional nonlinear simulations of the flow confirm the initial growth, and indicate that when a jet is above the tropopause, the configuration is more stable and more likely to produce a strong single unstable mode. The simulations indicate that this instability will grow to form a solitary wave envelope pattern. Conversely, when a jet is below the tropopause, the jet is more likely to form a broad spectrum of motion.
{"title":"Dynamic stability of a jet near a transition in static stability","authors":"John McHugh, R. Sharman","doi":"10.1080/03091929.2020.1795150","DOIUrl":"https://doi.org/10.1080/03091929.2020.1795150","url":null,"abstract":"The vertical profile of the Earth's atmosphere in middle latitudes contains a sharp transition region between two roughly constant stability layers, termed the tropopause and also exhibits jet streams at nearly the same altitude, with the jet stream core possibly above or below the tropopause, depending on time and location. This proximity of the jet to the tropopause would be expected to greatly affect the dynamic stability of the jet, treated here with the jet modelled with the Bickley profile and the tropopause modelled as a smooth transition region with a tanh profile. Stability results are obtained numerically using a Chebyshev collocation spectral method. The results show that the jet becomes more unstable as it is moved further beneath the tropopause. Corresponding two- and three-dimensional nonlinear simulations of the flow confirm the initial growth, and indicate that when a jet is above the tropopause, the configuration is more stable and more likely to produce a strong single unstable mode. The simulations indicate that this instability will grow to form a solitary wave envelope pattern. Conversely, when a jet is below the tropopause, the jet is more likely to form a broad spectrum of motion.","PeriodicalId":56132,"journal":{"name":"Geophysical and Astrophysical Fluid Dynamics","volume":"9 1","pages":"155 - 183"},"PeriodicalIF":1.3,"publicationDate":"2020-08-05","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"85171959","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":4,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
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