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- Author or Editor: Marc Georgelin x
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Abstract
The intensive observing period (IOP) 6 of the Pyrenees Experiment (PYREX) has been simulated with a hydrostatic three-dimensional model. The PYREX IOP 6 was devoted to the observation of a regional wind, the tramontana, which blows in the vicinity of the Pyrenees Mountains at the French–Spanish border. Under northerly synoptic wind conditions, the low-level flow frequently splits around the Pyrenees barrier (400 km long and 3000 m high), with the eastern branch of the flow, favored by Coriolis effect, forming the tramontana.
Model results are consistent with the tramontana climatology and in good agreement with the PYREX observations at all the different stages of the tramontana development: in the blocking zone where the model reproduces the observed reversed flow as well as its disappearance later in the course of the simulation, in the acceleration zone where the model gives accurate wind intensity and direction, at the land–sea transition where the development of an internal boundary layer is well predicted, and above the Mediterranean Sea where the spatial structure of the tramontana is well reproduced.
Abstract
The intensive observing period (IOP) 6 of the Pyrenees Experiment (PYREX) has been simulated with a hydrostatic three-dimensional model. The PYREX IOP 6 was devoted to the observation of a regional wind, the tramontana, which blows in the vicinity of the Pyrenees Mountains at the French–Spanish border. Under northerly synoptic wind conditions, the low-level flow frequently splits around the Pyrenees barrier (400 km long and 3000 m high), with the eastern branch of the flow, favored by Coriolis effect, forming the tramontana.
Model results are consistent with the tramontana climatology and in good agreement with the PYREX observations at all the different stages of the tramontana development: in the blocking zone where the model reproduces the observed reversed flow as well as its disappearance later in the course of the simulation, in the acceleration zone where the model gives accurate wind intensity and direction, at the land–sea transition where the development of an internal boundary layer is well predicted, and above the Mediterranean Sea where the spatial structure of the tramontana is well reproduced.
Abstract
The airplane data collected between 4 and 12 km above the Pyrénées during the intensive observation period (IOP) 3 of the Pyrénées Experiment (PYREX) are analyzed again. A spectral analysis of the velocity and potential temperature series shows that the mountain waves are dominated by two oscillations with well-defined horizontal wavenumbers. At nearly all altitudes, at least one among these two oscillations can be extracted: the short oscillation dominates the signal below 6 km and the long one above. These two oscillations contribute to the Reynolds stress below 5 km and not above.
Linear steady nondissipative simulations show that the short oscillation is a trapped resonant mode and the long one a leaking, or partially leaking, resonant mode of the background flow. Pseudo-momentum flux budgets show that the short resonant mode only contributes to the Reynolds stress at low level (here below 3 to 4 km typically) while the long one contributes to the Reynolds stress at all levels. At low level, (below 4 to 6 km typically), the long mode can induce a decay of the Reynolds stress amplitude, when it partially leaks toward the stratosphere. Various tests, changing the incident flow profiles within limits provided by the different soundings available this day, reveal, on the one hand, that the above findings are quite robust. On the other hand, they reveal that the resonant modes response is very sensitive to the background flow and orography specifications.
In some of the steady linear simulations, the long resonant oscillation has a Reynolds stress that is constant with altitude. In all of them the downwind extent of the lee waves is overestimated and the waves amplitude is too large. To explain these mismatches with the observations, we present simulations that last 3 h only, so the resonant modes patterns are everywhere unsteady. They show that during their build-up phase, all the leaking modes can make the Reynolds stress amplitude decays with altitude at low level (here below 4 to 6 km, typically). At this time, the downstream extent of the waves is also correctly predicted. These linear unsteady simulations also give realistic waves amplitude and Reynolds stress profiles if the mountain is cut off to parameterize nonlinear low-level flow splitting.
By using a nonlinear model, the simulated waves are matched to that observed through an adjustment of the parameters of the turbulent diffusion parameterization scheme: with enough dissipation, the model response can become quite realistic. In these nonlinear simulations, the background flow is chosen so that there is only one resonant mode and this mode does not contribute much to the Reynolds stress in the inviscid case. When increasing the mountain height and the dissipation, the overall structure of that mode stays unchanged, and it never contributes much to the Reynolds stress. This indicates that the dissipative and nonlinear processes alone are not likely to produce the observed low-level stress variations associated with the resonant modes.
Abstract
The airplane data collected between 4 and 12 km above the Pyrénées during the intensive observation period (IOP) 3 of the Pyrénées Experiment (PYREX) are analyzed again. A spectral analysis of the velocity and potential temperature series shows that the mountain waves are dominated by two oscillations with well-defined horizontal wavenumbers. At nearly all altitudes, at least one among these two oscillations can be extracted: the short oscillation dominates the signal below 6 km and the long one above. These two oscillations contribute to the Reynolds stress below 5 km and not above.
Linear steady nondissipative simulations show that the short oscillation is a trapped resonant mode and the long one a leaking, or partially leaking, resonant mode of the background flow. Pseudo-momentum flux budgets show that the short resonant mode only contributes to the Reynolds stress at low level (here below 3 to 4 km typically) while the long one contributes to the Reynolds stress at all levels. At low level, (below 4 to 6 km typically), the long mode can induce a decay of the Reynolds stress amplitude, when it partially leaks toward the stratosphere. Various tests, changing the incident flow profiles within limits provided by the different soundings available this day, reveal, on the one hand, that the above findings are quite robust. On the other hand, they reveal that the resonant modes response is very sensitive to the background flow and orography specifications.
In some of the steady linear simulations, the long resonant oscillation has a Reynolds stress that is constant with altitude. In all of them the downwind extent of the lee waves is overestimated and the waves amplitude is too large. To explain these mismatches with the observations, we present simulations that last 3 h only, so the resonant modes patterns are everywhere unsteady. They show that during their build-up phase, all the leaking modes can make the Reynolds stress amplitude decays with altitude at low level (here below 4 to 6 km, typically). At this time, the downstream extent of the waves is also correctly predicted. These linear unsteady simulations also give realistic waves amplitude and Reynolds stress profiles if the mountain is cut off to parameterize nonlinear low-level flow splitting.
By using a nonlinear model, the simulated waves are matched to that observed through an adjustment of the parameters of the turbulent diffusion parameterization scheme: with enough dissipation, the model response can become quite realistic. In these nonlinear simulations, the background flow is chosen so that there is only one resonant mode and this mode does not contribute much to the Reynolds stress in the inviscid case. When increasing the mountain height and the dissipation, the overall structure of that mode stays unchanged, and it never contributes much to the Reynolds stress. This indicates that the dissipative and nonlinear processes alone are not likely to produce the observed low-level stress variations associated with the resonant modes.
Abstract
Two-dimensional numerical simulations of mountain waves observed during the Pyrenees Experiment have been performed. Two intensive observing periods (IOP) have been simulated, IOP 3, which lasted less than one day, and IOP 9, which lasted two and one-half days. The time evolution of the large-scale flow was incorporated in the model through time-dependent boundary conditions that were updated using the closet upwind sounding. The numerically simulated mountain waves agree well with the available aircraft observations. Good agreement is also obtained between the simulated and observed vertical momentum flux profiles. In addition, the model-generated cross-mountain pressure drag accurately follows the time evolution of the observed drag. To get such a good agreement between observations and computations, it has been necessary to take into account in the model surface layer the effects of subgrid-scale orographic elements.
Abstract
Two-dimensional numerical simulations of mountain waves observed during the Pyrenees Experiment have been performed. Two intensive observing periods (IOP) have been simulated, IOP 3, which lasted less than one day, and IOP 9, which lasted two and one-half days. The time evolution of the large-scale flow was incorporated in the model through time-dependent boundary conditions that were updated using the closet upwind sounding. The numerically simulated mountain waves agree well with the available aircraft observations. Good agreement is also obtained between the simulated and observed vertical momentum flux profiles. In addition, the model-generated cross-mountain pressure drag accurately follows the time evolution of the observed drag. To get such a good agreement between observations and computations, it has been necessary to take into account in the model surface layer the effects of subgrid-scale orographic elements.
Abstract
The intensive observing period (IOP) 4 of the Pyrenees Experiment (PYREX) has been simulated with a hydrostatic three-dimensional model. The PYREX IOP 4 was devoted to the observation of the orographically generated flow above and around the Pyrenees Mountains located at the French–Spanish border. Two simulations, with or without surface thermal forcing, have been performed. Their results are described and compared with the PYREX observations. The influence of thermal forcing is assessed on the main characteristics of the flow, including the upwind flow reversal, the vertically propagating mountain wave, the regional winds created by the flow deviation around the mountain, and the lee vortices. Prominent alterations of the flow are found downwind of the mountain where the diurnal cooling induces the disappearance of the lee vortices and the heating significantly reduces the intensity of the low-level deviated flow.
Abstract
The intensive observing period (IOP) 4 of the Pyrenees Experiment (PYREX) has been simulated with a hydrostatic three-dimensional model. The PYREX IOP 4 was devoted to the observation of the orographically generated flow above and around the Pyrenees Mountains located at the French–Spanish border. Two simulations, with or without surface thermal forcing, have been performed. Their results are described and compared with the PYREX observations. The influence of thermal forcing is assessed on the main characteristics of the flow, including the upwind flow reversal, the vertically propagating mountain wave, the regional winds created by the flow deviation around the mountain, and the lee vortices. Prominent alterations of the flow are found downwind of the mountain where the diurnal cooling induces the disappearance of the lee vortices and the heating significantly reduces the intensity of the low-level deviated flow.
Abstract
A subgrid-scale orography parameterization based upon the use of an effective roughness length has been implemented in a mesobeta-scale model. The impact of such a parameterization is investigated in the framework of orographic flow simulations. Three mountain flow situations observed during the Pyrenees Experiment (PYREX) are studied. When subgrid-scale orography is accounted for, the mountain wave amplitude is reduced, the blocking is increased, the leeside low-level turbulence is intensified, and the regional wind characteristics are modified. Detailed comparisons made with the PYREX data indicate that the inclusion of subgrid orography yields a significant improvement in the model results.
Abstract
A subgrid-scale orography parameterization based upon the use of an effective roughness length has been implemented in a mesobeta-scale model. The impact of such a parameterization is investigated in the framework of orographic flow simulations. Three mountain flow situations observed during the Pyrenees Experiment (PYREX) are studied. When subgrid-scale orography is accounted for, the mountain wave amplitude is reduced, the blocking is increased, the leeside low-level turbulence is intensified, and the regional wind characteristics are modified. Detailed comparisons made with the PYREX data indicate that the inclusion of subgrid orography yields a significant improvement in the model results.
Abstract
The main objective of the present paper is the use of a constant volume balloon (CVB) as a tool to (i) study trapped lee waves and (ii) assess the forecasting capability of a nonhydrostatic numerical model. Then, CVB data obtained during the Pyrénées Experiment (PYREX) are compared with nonhydrostatic two-dimensional trapped lee waves simulated by the Meso-NH model. This model is a community research model based on the Lipps–Hemler form of the anelastic system, which has been recently developed by the CNRM of Météo-France and the Laboratoire d’Aérologie of Université Paul Sabatier in France.
To analyze how the CVB responds to lee waves, a simple CVB model is first applied to academic atmospheric stationary wave flows, analogous to those encountered during PYREX. This model takes into account the vertical velocity of the surrounding air, geometrical parameters of the balloon, and the atmospheric heating processes. Results show that the CVB reacts well to the atmospheric wave period, with a phase delay of only a few minutes.
Three CVB trajectories obtained during the third Intensive Observation Period of PYREX are then computed within Meso-NH 2D simulations from the balloon’s starting point, using the CVB model. The simulated quantities are compared to the experimental CVB data, focusing especially on the lee-wave vertical movements. The simulated lee-wave vertical velocities and amplitude are found to be in good agreement with the observations, as shown by the statistical analysis. The computed CVB heights deviate by less than 13% from the altitude of the measured trajectories. This comparison of the model output to the CVB experimental data demonstrates the good performance of the Meso-NH model in the prediction of the vertical excursions of the lee waves.
Abstract
The main objective of the present paper is the use of a constant volume balloon (CVB) as a tool to (i) study trapped lee waves and (ii) assess the forecasting capability of a nonhydrostatic numerical model. Then, CVB data obtained during the Pyrénées Experiment (PYREX) are compared with nonhydrostatic two-dimensional trapped lee waves simulated by the Meso-NH model. This model is a community research model based on the Lipps–Hemler form of the anelastic system, which has been recently developed by the CNRM of Météo-France and the Laboratoire d’Aérologie of Université Paul Sabatier in France.
To analyze how the CVB responds to lee waves, a simple CVB model is first applied to academic atmospheric stationary wave flows, analogous to those encountered during PYREX. This model takes into account the vertical velocity of the surrounding air, geometrical parameters of the balloon, and the atmospheric heating processes. Results show that the CVB reacts well to the atmospheric wave period, with a phase delay of only a few minutes.
Three CVB trajectories obtained during the third Intensive Observation Period of PYREX are then computed within Meso-NH 2D simulations from the balloon’s starting point, using the CVB model. The simulated quantities are compared to the experimental CVB data, focusing especially on the lee-wave vertical movements. The simulated lee-wave vertical velocities and amplitude are found to be in good agreement with the observations, as shown by the statistical analysis. The computed CVB heights deviate by less than 13% from the altitude of the measured trajectories. This comparison of the model output to the CVB experimental data demonstrates the good performance of the Meso-NH model in the prediction of the vertical excursions of the lee waves.
