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- Author or Editor: Michel Béland x
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Abstract
The fully nonlinear barotropic vorticity equation is integrated in time to study the development of a Rossby wave critical level. The initial conditions consist of a hyperbolic tangent shear flow and a steady forced wave at the northern boundary; a radiation condition is used at the southern boundary. Linear, quasi-linear and nonlinear integrations are made, and the results are compared with previous studies.
For small values of the perturbation amplitude, such that nonlinear interactions are important only in the critical layer, a steady state is obtained in the outer domain, in which the Reynolds stress vanishes both above and below the critical level, and the forced wave is totally reflected as suggested by previous analytical nonlinear steady-state solutions; the approach to that steady state and the structure of the critical layer, however, are quite different from the quasi-linear integrations performed by other authors. Some conclusions are drawn with respect to the forcing of the equatorial regions by meridionally propagating mid-latitude Rossby waves.
Abstract
The fully nonlinear barotropic vorticity equation is integrated in time to study the development of a Rossby wave critical level. The initial conditions consist of a hyperbolic tangent shear flow and a steady forced wave at the northern boundary; a radiation condition is used at the southern boundary. Linear, quasi-linear and nonlinear integrations are made, and the results are compared with previous studies.
For small values of the perturbation amplitude, such that nonlinear interactions are important only in the critical layer, a steady state is obtained in the outer domain, in which the Reynolds stress vanishes both above and below the critical level, and the forced wave is totally reflected as suggested by previous analytical nonlinear steady-state solutions; the approach to that steady state and the structure of the critical layer, however, are quite different from the quasi-linear integrations performed by other authors. Some conclusions are drawn with respect to the forcing of the equatorial regions by meridionally propagating mid-latitude Rossby waves.
Abstract
The time-dependent nonlinear Rossby wave equation is solved numerically in order to study the evolution of a forced wave on a parallel flow in the presence of a critical level. Inviscid and viscous integrations are performed, the latter yielding steady-state solutions in the critical layer as well as away from it. These solutions are shown to be in excellent agreement with Haberman's (1972), obtained from a similar steady-state equation. The implications for planetary-wave propagation and/or structure studies are further discussed.
Abstract
The time-dependent nonlinear Rossby wave equation is solved numerically in order to study the evolution of a forced wave on a parallel flow in the presence of a critical level. Inviscid and viscous integrations are performed, the latter yielding steady-state solutions in the critical layer as well as away from it. These solutions are shown to be in excellent agreement with Haberman's (1972), obtained from a similar steady-state equation. The implications for planetary-wave propagation and/or structure studies are further discussed.
Abstract
A finite element formulation for the vertical discretization of a global spectral model is presented. Results obtained from a linearized version of the model are compared with both exact analytical solutions and those of a vertically staggered finite-difference scheme. A series of seven-day global integrations using the fully nonlinear model and simple physics is presented and compared with the corresponding series obtained using a vertically staggered finite-difference model. The finite-element version of the model seems to give better performance, particularly at medium range. The new formulation tested here is also shown to be free of a noise problem present in an older version of the model.
Abstract
A finite element formulation for the vertical discretization of a global spectral model is presented. Results obtained from a linearized version of the model are compared with both exact analytical solutions and those of a vertically staggered finite-difference scheme. A series of seven-day global integrations using the fully nonlinear model and simple physics is presented and compared with the corresponding series obtained using a vertically staggered finite-difference model. The finite-element version of the model seems to give better performance, particularly at medium range. The new formulation tested here is also shown to be free of a noise problem present in an older version of the model.
Abstract
The linear stability of vertical discretization schemes for semi-implicit primitive-equation models is thoroughly investigated. The equations are linearized about a stationary rotating basic state atmosphere that has a vertically shearing zonal wind. The amplification matrix of the finite-element model is constructed and its eigenvalues examined for possible instability. Investigating, the small time step limit of that matrix, we identify two operators whose eigenvectors are the “physical” and “computational” modes of the semi-implicit method, respectively, and whose eigenvalues are their frequencies. It is further shown that if the frequencies are real then the respective modes are stable. Switching off the rotation and horizontal advection in the above operators, we are able to state conditions on the implicit and explicit temperature profiles such that the unconditional instability is avoided (e.g., the so-called semi-implicit instability). These stability criteria may be easily extended to any type of vertical discretization.
Abstract
The linear stability of vertical discretization schemes for semi-implicit primitive-equation models is thoroughly investigated. The equations are linearized about a stationary rotating basic state atmosphere that has a vertically shearing zonal wind. The amplification matrix of the finite-element model is constructed and its eigenvalues examined for possible instability. Investigating, the small time step limit of that matrix, we identify two operators whose eigenvectors are the “physical” and “computational” modes of the semi-implicit method, respectively, and whose eigenvalues are their frequencies. It is further shown that if the frequencies are real then the respective modes are stable. Switching off the rotation and horizontal advection in the above operators, we are able to state conditions on the implicit and explicit temperature profiles such that the unconditional instability is avoided (e.g., the so-called semi-implicit instability). These stability criteria may be easily extended to any type of vertical discretization.
Abstract
The accuracy of a slightly modified version of the finite-element vertical discretization scheme first described in Staniforth and Daley is studied with respect to a set of Rossby and gravity analytical normal modes obtained as solutions of a linearized primitive equation model. The scheme is also compared to a second-order, staggered, finite-difference vertical discretization scheme. The results of these comparisons are in favor of the finite-element method as far as accuracy is concerned. In terms of computation time, both methods are identical.
Abstract
The accuracy of a slightly modified version of the finite-element vertical discretization scheme first described in Staniforth and Daley is studied with respect to a set of Rossby and gravity analytical normal modes obtained as solutions of a linearized primitive equation model. The scheme is also compared to a second-order, staggered, finite-difference vertical discretization scheme. The results of these comparisons are in favor of the finite-element method as far as accuracy is concerned. In terms of computation time, both methods are identical.
Abstract
Many studies have demonstrated the importance of land surface schemes in climate change studies using general circulation models (GCMs). However, there have not been many studies that explore the role of land surface schemes in the context of short-range and high spatial resolution precipitation forecasts. The motivation of this study is to examine the sensitivity of simulated precipitation, and sensible and latent heat fluxes, to the use of different land surface schemes at two different spatial resolutions. The meteorological model used is the Mesoscale Compressible Community (MC2) model, and the land surface schemes are the force–restore method and the Canadian Land Surface Scheme (CLASS). Parallel runs have been performed using MC2/CLASS and MC2/force–restore at spatial resolutions of 10 and 5 km to simulate the severe precipitation case of 19–21 July 1996 in the Saguenay region of Québec, Canada. Comparisons of the simulated precipitation time series and the simulated 48-h accumulated precipitation at different spatial resolutions with rain gauges indicate that MC2/CLASS at 5-km resolution gives the best simulated precipitation. The comparison results show the model accuracy of MC2/CLASS at 10 km is comparable to the accuracy of MC2/force–restore at 5 km. The mechanism responsible for this is that CLASS represents the land surface vegetation characteristics in a more sophisticated manner than the force–restore method. Furthermore, in CLASS, each grid square is divided into a maximum of four separate subareas, and subvariations of the grid surface vegetation characteristics are taken into account. Therefore, for a grid square containing different types of vegetation, the subgrid-scale information can be used by CLASS, and the computed effective variables that are fed back to MC2 on a 10 × 10 km2 grid are equivalent to computing them at a higher effective resolution than 10 km. This higher effective resolution for surface characteristics is not found in the force–restore method. The total simulated domain-averaged precipitation, and the sum of sensible and latent heat fluxes from MC2/CLASS and MC2/force–restore at different spatial resolutions, are similar. The major difference is in the partitioning of the simulated sensible and latent heat fluxes. The positioning of the simulated precipitation has been improved by using CLASS. The overall results suggest that the impact of land surface schemes is indeed significant in a short-range precipitation forecast, especially in regions with complicated vegetation variations.
Abstract
Many studies have demonstrated the importance of land surface schemes in climate change studies using general circulation models (GCMs). However, there have not been many studies that explore the role of land surface schemes in the context of short-range and high spatial resolution precipitation forecasts. The motivation of this study is to examine the sensitivity of simulated precipitation, and sensible and latent heat fluxes, to the use of different land surface schemes at two different spatial resolutions. The meteorological model used is the Mesoscale Compressible Community (MC2) model, and the land surface schemes are the force–restore method and the Canadian Land Surface Scheme (CLASS). Parallel runs have been performed using MC2/CLASS and MC2/force–restore at spatial resolutions of 10 and 5 km to simulate the severe precipitation case of 19–21 July 1996 in the Saguenay region of Québec, Canada. Comparisons of the simulated precipitation time series and the simulated 48-h accumulated precipitation at different spatial resolutions with rain gauges indicate that MC2/CLASS at 5-km resolution gives the best simulated precipitation. The comparison results show the model accuracy of MC2/CLASS at 10 km is comparable to the accuracy of MC2/force–restore at 5 km. The mechanism responsible for this is that CLASS represents the land surface vegetation characteristics in a more sophisticated manner than the force–restore method. Furthermore, in CLASS, each grid square is divided into a maximum of four separate subareas, and subvariations of the grid surface vegetation characteristics are taken into account. Therefore, for a grid square containing different types of vegetation, the subgrid-scale information can be used by CLASS, and the computed effective variables that are fed back to MC2 on a 10 × 10 km2 grid are equivalent to computing them at a higher effective resolution than 10 km. This higher effective resolution for surface characteristics is not found in the force–restore method. The total simulated domain-averaged precipitation, and the sum of sensible and latent heat fluxes from MC2/CLASS and MC2/force–restore at different spatial resolutions, are similar. The major difference is in the partitioning of the simulated sensible and latent heat fluxes. The positioning of the simulated precipitation has been improved by using CLASS. The overall results suggest that the impact of land surface schemes is indeed significant in a short-range precipitation forecast, especially in regions with complicated vegetation variations.
Scaling the Laws of Thermal Imaging–Based Whale DetectionAbstract
Marine mammals are under growing pressure as anthropogenic use of the ocean increases. Ship strikes of large whales and loud underwater sound sources including air guns for marine geophysical prospecting and naval midfrequency sonar are criticized for their possible negative effects on marine mammals. Competent authorities regularly require the implementation of mitigation measures, including vessel speed reductions or shutdown of acoustic sources if marine mammals are sighted in sensitive areas or in predefined exclusion zones around a vessel. To ensure successful mitigation, reliable at-sea detection of animals is crucial. To date, ship-based marine mammal observers are the most commonly implemented detection method; however, thermal (IR) imaging–based automatic detection systems have been used in recent years. This study evaluates thermal imaging–based automatic whale detection technology for its use across different oceans. The performance of this technology is characterized with respect to environmental conditions, and an automatic detection algorithm for whale blows is presented. The technology can detect whales in polar, temperate, and subtropical ocean regimes over distances of up to several kilometers and outperforms marine mammal observers in the number of whales detected. These results show that thermal imaging technology can be used to assist in providing protection for marine mammals against ship strike and acoustic impact across the world’s oceans.
Abstract
Marine mammals are under growing pressure as anthropogenic use of the ocean increases. Ship strikes of large whales and loud underwater sound sources including air guns for marine geophysical prospecting and naval midfrequency sonar are criticized for their possible negative effects on marine mammals. Competent authorities regularly require the implementation of mitigation measures, including vessel speed reductions or shutdown of acoustic sources if marine mammals are sighted in sensitive areas or in predefined exclusion zones around a vessel. To ensure successful mitigation, reliable at-sea detection of animals is crucial. To date, ship-based marine mammal observers are the most commonly implemented detection method; however, thermal (IR) imaging–based automatic detection systems have been used in recent years. This study evaluates thermal imaging–based automatic whale detection technology for its use across different oceans. The performance of this technology is characterized with respect to environmental conditions, and an automatic detection algorithm for whale blows is presented. The technology can detect whales in polar, temperate, and subtropical ocean regimes over distances of up to several kilometers and outperforms marine mammal observers in the number of whales detected. These results show that thermal imaging technology can be used to assist in providing protection for marine mammals against ship strike and acoustic impact across the world’s oceans.
The necessity and benefits for establishing the international Earth-system Prediction Initiative (EPI) are discussed by scientists associated with the World Meteorological Organization (WMO) World Weather Research Programme (WWRP), World Climate Research Programme (WCRP), International Geosphere–Biosphere Programme (IGBP), Global Climate Observing System (GCOS), and natural-hazards and socioeconomic communities. The proposed initiative will provide research and services to accelerate advances in weather, climate, and Earth system prediction and the use of this information by global societies. It will build upon the WMO, the Group on Earth Observations (GEO), the Global Earth Observation System of Systems (GEOSS) and the International Council for Science (ICSU) to coordinate the effort across the weather, climate, Earth system, natural-hazards, and socioeconomic disciplines. It will require (i) advanced high-performance computing facilities, supporting a worldwide network of research and operational modeling centers, and early warning systems; (ii) science, technology, and education projects to enhance knowledge, awareness, and utilization of weather, climate, environmental, and socioeconomic information; (iii) investments in maintaining existing and developing new observational capabilities; and (iv) infrastructure to transition achievements into operational products and services.
The necessity and benefits for establishing the international Earth-system Prediction Initiative (EPI) are discussed by scientists associated with the World Meteorological Organization (WMO) World Weather Research Programme (WWRP), World Climate Research Programme (WCRP), International Geosphere–Biosphere Programme (IGBP), Global Climate Observing System (GCOS), and natural-hazards and socioeconomic communities. The proposed initiative will provide research and services to accelerate advances in weather, climate, and Earth system prediction and the use of this information by global societies. It will build upon the WMO, the Group on Earth Observations (GEO), the Global Earth Observation System of Systems (GEOSS) and the International Council for Science (ICSU) to coordinate the effort across the weather, climate, Earth system, natural-hazards, and socioeconomic disciplines. It will require (i) advanced high-performance computing facilities, supporting a worldwide network of research and operational modeling centers, and early warning systems; (ii) science, technology, and education projects to enhance knowledge, awareness, and utilization of weather, climate, environmental, and socioeconomic information; (iii) investments in maintaining existing and developing new observational capabilities; and (iv) infrastructure to transition achievements into operational products and services.
