J. Argain - Academia.edu (original) (raw)

Papers by J. Argain

Research paper thumbnail of The momentum flux profiles produced by trapped lee waves

<p>Orographic gravity waves (also known as mountain waves) cause the atmosp... more <p>Orographic gravity waves (also known as mountain waves) cause the atmosphere to exert a drag force on mountains. By Newton’s 3<sup>rd</sup> law, the mountains exert an equal and opposite force on the atmosphere. It is clear from linear wave theory how to develop a framework for representing this reaction force in parametrizations for vertically propagating waves in climate and weather prediction models: the waves break and dissipate either due to critical levels (where the wind speed is perpendicular to the horizontal wavenumber vector, or zero), or due to the progressive decrease of density with height. But the situation is more complicated for trapped lee waves, which propagate horizontally near the surface, and where the wave energy is alternately reflected at the ground and at an elevated layer where the waves become evanescent. It is clear that boundary layer friction should be responsible for most of the dissipation of trapped lee waves, but it is not clear, even in the inviscid approximation, what form the wave momentum flux profiles that force the large-scale mean flow will take. This is due to the complications associated with the fact that trapped lee waves have both horizontal and vertical momentum (and pseudo-momentum) fluxes, which oscillate indefinitely with the wave phase downstream of the orography. No mechanism equivalent to critical levels, or density decay with height, acting on vertically propagating mountain waves, is available for trapped lee waves. In this study, this limitation is overcome by accounting for the effects of weak friction. While for an inviscid trapped lee wave train, the horizontally integrated momentum flux is ill-defined (except at the surface), in a dissipative problem where friction exists, no matter how small, the wave train necessarily decays downstream, and so is spatially bounded. This allows the areally integrated effect of the trapped lee wave to be expressed in terms of the divergence of the vertical flux of horizontal wave momentum (as for vertically-propagating waves). On the other hand, the form of the momentum flux profile (which defines this divergence) is different from any form that could be inferred from inviscid theory, although it is independent of the magnitude of friction, as long as this is small. These results from linear theory are compared with high-resolution numerical simulations of trapped lee waves for the two-layer atmosphere of Scorer, which confirm the form of the momentum flux profiles, and suggest that these may be independent of the adopted form of friction, at least to some extent. The results therefore facilitate the formulation of parametrizations for trapped lee waves with a much more solid physical  basis, and are likely to be generalizable to other atmospheric profiles.</p>

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Research paper thumbnail of A Review on the Current Status of Numerical Weather Prediction in Portugal 2021: Surface–Atmosphere Interactions

Atmosphere

Earth system modelling is currently playing an increasing role in weather forecasting and underst... more Earth system modelling is currently playing an increasing role in weather forecasting and understanding climate change, however, the operation, deployment and development of numerical Earth system models are extremely demanding in terms of computational resources and human effort. Merging synergies has become a natural process by which national meteorological services assess and contribute to the development of such systems. With the advent of joining synergies at the national level, the second edition of the workshop on Numerical Weather Prediction in Portugal was promoted by the Portuguese Institute for the Sea and Atmosphere, I.P. (IPMA), in cooperation with several Portuguese Universities. The event was hosted by the University of Évora, during the period of 11–12 of November 2021. It was dedicated to surface–atmosphere interactions and allowed the exchange of experiences between experts, students and newcomers. The workshop provided a refreshed overview of ongoing research and ...

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Research paper thumbnail of Estimating the friction velocity in neutral and unstable boundary layers

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Research paper thumbnail of Trapped lee waves in layered atmospheres: an important source of low-level drag?

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Research paper thumbnail of The drag exerted by weakly dissipative trapped lee waves on the atmosphere: application to Scorer's two‐layer model

Quarterly Journal of the Royal Meteorological Society

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Research paper thumbnail of The dependence of mountain wave reflection on the abruptness of atmospheric profile variations

Quarterly Journal of the Royal Meteorological Society, 2020

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Research paper thumbnail of Estimation of Surface-Layer Scaling Parameters in the Unstable Boundary Layer: Implications for Orographic Flow Speed-Up

Boundary-Layer Meteorology, 2017

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Research paper thumbnail of Drag produced by trapped lee waves and propagating mountain waves in a two-layer atmosphere

Quarterly Journal of the Royal Meteorological Society, 2012

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Research paper thumbnail of The importance of friction in mountain wave drag amplification by Scorer parameter resonance

Quarterly Journal of the Royal Meteorological Society, 2012

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Research paper thumbnail of Mountain Waves in Two-Layer Sheared Flows: Critical-Level Effects, Wave Reflection, and Drag Enhancement

Journal of the Atmospheric Sciences, 2008

Internal gravity waves generated in two-layer stratified shear flows over mountains are investiga... more Internal gravity waves generated in two-layer stratified shear flows over mountains are investigated here using linear theory and numerical simulations. The impact on the gravity wave drag of wind profiles with constant unidirectional or directional shear up to a certain height and zero shear above, with and without critical levels, is evaluated. This kind of wind profile, which is more realistic than the constant shear extending indefinitely assumed in many analytical studies, leads to important modifications in the drag behavior due to wave reflection at the shear discontinuity and wave filtering by critical levels. In inviscid, nonrotating, and hydrostatic conditions, linear theory predicts that the drag behaves asymmetrically for backward and forward shear flows. These differences primarily depend on the fraction of wavenumbers that pass through their critical level before they are reflected by the shear discontinuity. If this fraction is large, the drag variation is not too dif...

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Research paper thumbnail of Orographic drag associated with lee waves trapped at an inversion

Journal of the Atmospheric Sciences, 2013

The drag produced by 2D orographic gravity waves trapped at a temperature inversion and waves pro... more The drag produced by 2D orographic gravity waves trapped at a temperature inversion and waves propagating in the stably stratified layer existing above are explicitly calculated using linear theory, for a two-layer atmosphere with neutral static stability near the surface, mimicking a well-mixed boundary layer. For realistic values of the flow parameters, trapped-lee-wave drag, which is given by a closed analytical expression, is comparable to propagating-wave drag, especially in moderately to strongly nonhydrostatic conditions. In resonant flow, both drag components substantially exceed the single-layer hydrostatic drag estimate used in most parameterization schemes. Both drag components are optimally amplified for a relatively low-level inversion and Froude numbers Fr ≈ 1. While propagating-wave drag is maximized for approximately hydrostatic flow, trapped-lee-wave drag is maximized for l2a = O(1) (where l2 is the Scorer parameter in the stable layer and a is the mountain width). ...

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Research paper thumbnail of Estimation of the Friction Velocity in Stably Stratified Boundary-Layer Flows Over Hills

Boundary-Layer Meteorology, 2008

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Research paper thumbnail of A linear study of high-drag states and flow stagnation produced by mountain waves

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Research paper thumbnail of Orographic gravity wave drag amplification by parametric resonance

ABSTRACT One of the mechanisms identified by Wells and Vosper (2010) for the amplification of gra... more ABSTRACT One of the mechanisms identified by Wells and Vosper (2010) for the amplification of gravity wave drag produced in flow over mountains, which makes this force substantially exceed its linear estimate even for very low mountain heights, requires a Scorer parameter profile that oscillates with height with a wavelength half that of the primary waves generated by the vertically-averaged Scorer parameter. This mechanism, first mentioned by Phillips (1968) in an oceanic context, relies on a resonance process, and Wells and Vosper claim that this resonance is intrinsically nonlinear. In the present study (see Teixeira et al. 2012), we examine this mechanism in more detail, considering flow over an isolated mountain with a Scorer parameter that is constant in the vertical apart from a superposed small-amplitude sinusoidal oscillation. This problem is treated both using an analytical perturbation method and numerical simulations with a mesoscale model. It is shown that the processes leading to drag enhancement (or reduction) are linear, since they result from the interaction between the primary wave and the Scorer parameter oscillation. These processes may therefore be captured in a linear framework, by expanding the solution to the Taylor-Goldstein equation in powers of a small parameter proportional to the amplitude of the Scorer parameter oscillation. A semi-analytical model based on this approach produces results that are qualitatively similar to those obtained in the numerical simulations. The drag normalized by its value for a constant Scorer parameter is studied as a function of the wavelength and phase of the Scorer parameter oscillation. Only higher-order effects may be attributed to nonlinearity. However, it is shown that the results are quite sensitive to friction, in particular no drag enhancement is produced for some values of the phase of the Scorer parameter oscillation unless friction is included in some form. This implies that, in numerical simulations, the drag behaviour should be sensitive to the numerical diffusion inherent to the discretization schemes and to the type of turbulence closure employed in the numerical model. In our study, the semi-analytical model uses a simple Rayleigh damping coefficient to represent friction, and the value of this parameter is estimated from the numerical simulations. While most of the numerical simulations are nominally inviscid, a first assessment of the effect of physical friction, using simulations with turbulence closures, is also carried out. Additionally, non-hydrostatic effects are assessed, and found to have an important impact on the drag amplification, weakening the resonance process due to wave dispersion.

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Research paper thumbnail of Resonant gravity-wave drag enhancement in linear stratified flow over mountains

Quarterly Journal of the Royal Meteorological Society, 2005

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Research paper thumbnail of Neutral and stably-stratified turbulent boundary layer flows past topography

In this work, we study neutral and stably-stratified turbulent boundary layer flows over orograph... more In this work, we study neutral and stably-stratified turbulent boundary layer flows over orography using a non-hydrostatic microscale-mesoscale numerical model, which has been validated previously with observations. To allow for a simulation of flows over complex topography, the ...

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Research paper thumbnail of Trapped lee wave drag in two-layer atmospheres

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Research paper thumbnail of Estimating the friction velocity in stable boundary layers

... JL Argain, University of Algarve, Faro, Portugal; and PMA Miranda and MAC Teixeira. A new met... more ... JL Argain, University of Algarve, Faro, Portugal; and PMA Miranda and MAC Teixeira. A new method is proposed for the calculation of friction velocity in stable turbulent boundary layer flows, where gravity waves propagation is possible. ...

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Research paper thumbnail of The dependence of mountain wave reflection on the abruptness of atmospheric profile variations

Quarterly Journal of the Royal Meteorological Society, 2020

It is known from geometric optics that a change in refractive index is potentially reflective if ... more It is known from geometric optics that a change in refractive index is potentially reflective if it occurs over scales much smaller than the wavelength of the incident waves. The limitations of this assumption for hydrostatic orographic gravity waves are tested here using linear theory and a method recently developed by the authors to evaluate the reflection coefficient, based on the wave drag. Two atmospheric profiles optimally suited to this method are adopted, the first with piecewise constant static stability (representative of a tropopause), and the second with constant wind speed near the surface, and a linearly decreasing wind aloft below a critical level (relevant to downslope windstorms). Both profiles consist of two atmospheric layers separated by a transition layer with controllable thickness, where the parameters vary continuously. The variation of the reflection coefficient between its maximum (for a zero‐thickness transition layer) and zero, as the ratio of the thickne...

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Research paper thumbnail of Reflection of nonlinear mountain waves by critical levels: behaviour of the reflection coefficient

Quarterly Journal of the Royal Meteorological Society

Critical levels, where the wind vanishes in the atmosphere, are of key importance for gravity wav... more Critical levels, where the wind vanishes in the atmosphere, are of key importance for gravity wave drag parametrization. The reflectivity of these levels to mountain waves is investigated here using a combination of high-resolution numerical simulations and insights from linear theory. A methodology is developed for relating the reflection coefficient R of 2D hydrostatic orographic gravity waves to the extrema of the associated drag as a function of an independent flow parameter. This method is then used to infer the variation of the reflection coefficient with flow nonlinearity. To isolate the effect of critical levels, a wind profile with negative shear is adopted, which is characterized by its Richardson number Ri and the dimensionless mountain height Nh0/U0, based on the mountain height h0, Brunt-Vaisala frequency N and surface incoming wind speed U0. Subject to the assumptions of linear theory, the drag is shown to be modified by wave refraction and reflection. The modulation of the drag by wave reflection is used to derive the reflection coefficient from the drag diagnosed from the numerical simulations. Despite considerable uncertainty, the critical level is found to have an R that first increases with Nh0/U0 for low values of this parameter, and for stronger nonlinearity saturates to a value of about 0.6. The flow configuration in this saturated regime is characterized in the case of high-drag states by constructive wave interference, resembling downslope windstorms. Wave reflection by critical levels enhances the flow nonlinearity and the associated drag amplification, more than doubling it for values of Nh0/U0 as low as 0.12. These results emphasize the need to represent this process in orographic gravity wave drag parametrizations, and suggest a possible way of doing it using a prescribed critical level reflection coefficient, derived using the present methodology.

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Research paper thumbnail of The momentum flux profiles produced by trapped lee waves

<p>Orographic gravity waves (also known as mountain waves) cause the atmosp... more <p>Orographic gravity waves (also known as mountain waves) cause the atmosphere to exert a drag force on mountains. By Newton’s 3<sup>rd</sup> law, the mountains exert an equal and opposite force on the atmosphere. It is clear from linear wave theory how to develop a framework for representing this reaction force in parametrizations for vertically propagating waves in climate and weather prediction models: the waves break and dissipate either due to critical levels (where the wind speed is perpendicular to the horizontal wavenumber vector, or zero), or due to the progressive decrease of density with height. But the situation is more complicated for trapped lee waves, which propagate horizontally near the surface, and where the wave energy is alternately reflected at the ground and at an elevated layer where the waves become evanescent. It is clear that boundary layer friction should be responsible for most of the dissipation of trapped lee waves, but it is not clear, even in the inviscid approximation, what form the wave momentum flux profiles that force the large-scale mean flow will take. This is due to the complications associated with the fact that trapped lee waves have both horizontal and vertical momentum (and pseudo-momentum) fluxes, which oscillate indefinitely with the wave phase downstream of the orography. No mechanism equivalent to critical levels, or density decay with height, acting on vertically propagating mountain waves, is available for trapped lee waves. In this study, this limitation is overcome by accounting for the effects of weak friction. While for an inviscid trapped lee wave train, the horizontally integrated momentum flux is ill-defined (except at the surface), in a dissipative problem where friction exists, no matter how small, the wave train necessarily decays downstream, and so is spatially bounded. This allows the areally integrated effect of the trapped lee wave to be expressed in terms of the divergence of the vertical flux of horizontal wave momentum (as for vertically-propagating waves). On the other hand, the form of the momentum flux profile (which defines this divergence) is different from any form that could be inferred from inviscid theory, although it is independent of the magnitude of friction, as long as this is small. These results from linear theory are compared with high-resolution numerical simulations of trapped lee waves for the two-layer atmosphere of Scorer, which confirm the form of the momentum flux profiles, and suggest that these may be independent of the adopted form of friction, at least to some extent. The results therefore facilitate the formulation of parametrizations for trapped lee waves with a much more solid physical  basis, and are likely to be generalizable to other atmospheric profiles.</p>

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Research paper thumbnail of A Review on the Current Status of Numerical Weather Prediction in Portugal 2021: Surface–Atmosphere Interactions

Atmosphere

Earth system modelling is currently playing an increasing role in weather forecasting and underst... more Earth system modelling is currently playing an increasing role in weather forecasting and understanding climate change, however, the operation, deployment and development of numerical Earth system models are extremely demanding in terms of computational resources and human effort. Merging synergies has become a natural process by which national meteorological services assess and contribute to the development of such systems. With the advent of joining synergies at the national level, the second edition of the workshop on Numerical Weather Prediction in Portugal was promoted by the Portuguese Institute for the Sea and Atmosphere, I.P. (IPMA), in cooperation with several Portuguese Universities. The event was hosted by the University of Évora, during the period of 11–12 of November 2021. It was dedicated to surface–atmosphere interactions and allowed the exchange of experiences between experts, students and newcomers. The workshop provided a refreshed overview of ongoing research and ...

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Research paper thumbnail of Estimating the friction velocity in neutral and unstable boundary layers

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Research paper thumbnail of Trapped lee waves in layered atmospheres: an important source of low-level drag?

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Research paper thumbnail of The drag exerted by weakly dissipative trapped lee waves on the atmosphere: application to Scorer's two‐layer model

Quarterly Journal of the Royal Meteorological Society

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Research paper thumbnail of The dependence of mountain wave reflection on the abruptness of atmospheric profile variations

Quarterly Journal of the Royal Meteorological Society, 2020

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Research paper thumbnail of Estimation of Surface-Layer Scaling Parameters in the Unstable Boundary Layer: Implications for Orographic Flow Speed-Up

Boundary-Layer Meteorology, 2017

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Research paper thumbnail of Drag produced by trapped lee waves and propagating mountain waves in a two-layer atmosphere

Quarterly Journal of the Royal Meteorological Society, 2012

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Research paper thumbnail of The importance of friction in mountain wave drag amplification by Scorer parameter resonance

Quarterly Journal of the Royal Meteorological Society, 2012

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Research paper thumbnail of Mountain Waves in Two-Layer Sheared Flows: Critical-Level Effects, Wave Reflection, and Drag Enhancement

Journal of the Atmospheric Sciences, 2008

Internal gravity waves generated in two-layer stratified shear flows over mountains are investiga... more Internal gravity waves generated in two-layer stratified shear flows over mountains are investigated here using linear theory and numerical simulations. The impact on the gravity wave drag of wind profiles with constant unidirectional or directional shear up to a certain height and zero shear above, with and without critical levels, is evaluated. This kind of wind profile, which is more realistic than the constant shear extending indefinitely assumed in many analytical studies, leads to important modifications in the drag behavior due to wave reflection at the shear discontinuity and wave filtering by critical levels. In inviscid, nonrotating, and hydrostatic conditions, linear theory predicts that the drag behaves asymmetrically for backward and forward shear flows. These differences primarily depend on the fraction of wavenumbers that pass through their critical level before they are reflected by the shear discontinuity. If this fraction is large, the drag variation is not too dif...

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Research paper thumbnail of Orographic drag associated with lee waves trapped at an inversion

Journal of the Atmospheric Sciences, 2013

The drag produced by 2D orographic gravity waves trapped at a temperature inversion and waves pro... more The drag produced by 2D orographic gravity waves trapped at a temperature inversion and waves propagating in the stably stratified layer existing above are explicitly calculated using linear theory, for a two-layer atmosphere with neutral static stability near the surface, mimicking a well-mixed boundary layer. For realistic values of the flow parameters, trapped-lee-wave drag, which is given by a closed analytical expression, is comparable to propagating-wave drag, especially in moderately to strongly nonhydrostatic conditions. In resonant flow, both drag components substantially exceed the single-layer hydrostatic drag estimate used in most parameterization schemes. Both drag components are optimally amplified for a relatively low-level inversion and Froude numbers Fr ≈ 1. While propagating-wave drag is maximized for approximately hydrostatic flow, trapped-lee-wave drag is maximized for l2a = O(1) (where l2 is the Scorer parameter in the stable layer and a is the mountain width). ...

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Research paper thumbnail of Estimation of the Friction Velocity in Stably Stratified Boundary-Layer Flows Over Hills

Boundary-Layer Meteorology, 2008

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Research paper thumbnail of A linear study of high-drag states and flow stagnation produced by mountain waves

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Research paper thumbnail of Orographic gravity wave drag amplification by parametric resonance

ABSTRACT One of the mechanisms identified by Wells and Vosper (2010) for the amplification of gra... more ABSTRACT One of the mechanisms identified by Wells and Vosper (2010) for the amplification of gravity wave drag produced in flow over mountains, which makes this force substantially exceed its linear estimate even for very low mountain heights, requires a Scorer parameter profile that oscillates with height with a wavelength half that of the primary waves generated by the vertically-averaged Scorer parameter. This mechanism, first mentioned by Phillips (1968) in an oceanic context, relies on a resonance process, and Wells and Vosper claim that this resonance is intrinsically nonlinear. In the present study (see Teixeira et al. 2012), we examine this mechanism in more detail, considering flow over an isolated mountain with a Scorer parameter that is constant in the vertical apart from a superposed small-amplitude sinusoidal oscillation. This problem is treated both using an analytical perturbation method and numerical simulations with a mesoscale model. It is shown that the processes leading to drag enhancement (or reduction) are linear, since they result from the interaction between the primary wave and the Scorer parameter oscillation. These processes may therefore be captured in a linear framework, by expanding the solution to the Taylor-Goldstein equation in powers of a small parameter proportional to the amplitude of the Scorer parameter oscillation. A semi-analytical model based on this approach produces results that are qualitatively similar to those obtained in the numerical simulations. The drag normalized by its value for a constant Scorer parameter is studied as a function of the wavelength and phase of the Scorer parameter oscillation. Only higher-order effects may be attributed to nonlinearity. However, it is shown that the results are quite sensitive to friction, in particular no drag enhancement is produced for some values of the phase of the Scorer parameter oscillation unless friction is included in some form. This implies that, in numerical simulations, the drag behaviour should be sensitive to the numerical diffusion inherent to the discretization schemes and to the type of turbulence closure employed in the numerical model. In our study, the semi-analytical model uses a simple Rayleigh damping coefficient to represent friction, and the value of this parameter is estimated from the numerical simulations. While most of the numerical simulations are nominally inviscid, a first assessment of the effect of physical friction, using simulations with turbulence closures, is also carried out. Additionally, non-hydrostatic effects are assessed, and found to have an important impact on the drag amplification, weakening the resonance process due to wave dispersion.

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Research paper thumbnail of Resonant gravity-wave drag enhancement in linear stratified flow over mountains

Quarterly Journal of the Royal Meteorological Society, 2005

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Research paper thumbnail of Neutral and stably-stratified turbulent boundary layer flows past topography

In this work, we study neutral and stably-stratified turbulent boundary layer flows over orograph... more In this work, we study neutral and stably-stratified turbulent boundary layer flows over orography using a non-hydrostatic microscale-mesoscale numerical model, which has been validated previously with observations. To allow for a simulation of flows over complex topography, the ...

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Research paper thumbnail of Trapped lee wave drag in two-layer atmospheres

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Research paper thumbnail of Estimating the friction velocity in stable boundary layers

... JL Argain, University of Algarve, Faro, Portugal; and PMA Miranda and MAC Teixeira. A new met... more ... JL Argain, University of Algarve, Faro, Portugal; and PMA Miranda and MAC Teixeira. A new method is proposed for the calculation of friction velocity in stable turbulent boundary layer flows, where gravity waves propagation is possible. ...

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Research paper thumbnail of The dependence of mountain wave reflection on the abruptness of atmospheric profile variations

Quarterly Journal of the Royal Meteorological Society, 2020

It is known from geometric optics that a change in refractive index is potentially reflective if ... more It is known from geometric optics that a change in refractive index is potentially reflective if it occurs over scales much smaller than the wavelength of the incident waves. The limitations of this assumption for hydrostatic orographic gravity waves are tested here using linear theory and a method recently developed by the authors to evaluate the reflection coefficient, based on the wave drag. Two atmospheric profiles optimally suited to this method are adopted, the first with piecewise constant static stability (representative of a tropopause), and the second with constant wind speed near the surface, and a linearly decreasing wind aloft below a critical level (relevant to downslope windstorms). Both profiles consist of two atmospheric layers separated by a transition layer with controllable thickness, where the parameters vary continuously. The variation of the reflection coefficient between its maximum (for a zero‐thickness transition layer) and zero, as the ratio of the thickne...

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Research paper thumbnail of Reflection of nonlinear mountain waves by critical levels: behaviour of the reflection coefficient

Quarterly Journal of the Royal Meteorological Society

Critical levels, where the wind vanishes in the atmosphere, are of key importance for gravity wav... more Critical levels, where the wind vanishes in the atmosphere, are of key importance for gravity wave drag parametrization. The reflectivity of these levels to mountain waves is investigated here using a combination of high-resolution numerical simulations and insights from linear theory. A methodology is developed for relating the reflection coefficient R of 2D hydrostatic orographic gravity waves to the extrema of the associated drag as a function of an independent flow parameter. This method is then used to infer the variation of the reflection coefficient with flow nonlinearity. To isolate the effect of critical levels, a wind profile with negative shear is adopted, which is characterized by its Richardson number Ri and the dimensionless mountain height Nh0/U0, based on the mountain height h0, Brunt-Vaisala frequency N and surface incoming wind speed U0. Subject to the assumptions of linear theory, the drag is shown to be modified by wave refraction and reflection. The modulation of the drag by wave reflection is used to derive the reflection coefficient from the drag diagnosed from the numerical simulations. Despite considerable uncertainty, the critical level is found to have an R that first increases with Nh0/U0 for low values of this parameter, and for stronger nonlinearity saturates to a value of about 0.6. The flow configuration in this saturated regime is characterized in the case of high-drag states by constructive wave interference, resembling downslope windstorms. Wave reflection by critical levels enhances the flow nonlinearity and the associated drag amplification, more than doubling it for values of Nh0/U0 as low as 0.12. These results emphasize the need to represent this process in orographic gravity wave drag parametrizations, and suggest a possible way of doing it using a prescribed critical level reflection coefficient, derived using the present methodology.

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