Search Results
You are looking at 1 - 10 of 31 items for
- Author or Editor: Peter A. Taylor x
- Refine by Access: All Content x
Abstract
Wakes behind 2D fences and 3D obstacles are reviewed with special emphasis on reduced mean wind speeds and sheltering effects. Based partly on Perera's study of wakes behind 2D fences, and assuming a Gaussian spread for wakes behind 3D obstacles, a shelter model is proposed and tested. The shelter produced depends on a wake moment coefficient C̃k which appears to be significantly less for 3D obstacles than for 2D fences. The model provides a simple basis on which to “correct” anemometer data for sheltering effects associated with upstream obstacles. Such corrections are an important step in the generation of improved surface wind climatologies and wind atlases.
Abstract
Wakes behind 2D fences and 3D obstacles are reviewed with special emphasis on reduced mean wind speeds and sheltering effects. Based partly on Perera's study of wakes behind 2D fences, and assuming a Gaussian spread for wakes behind 3D obstacles, a shelter model is proposed and tested. The shelter produced depends on a wake moment coefficient C̃k which appears to be significantly less for 3D obstacles than for 2D fences. The model provides a simple basis on which to “correct” anemometer data for sheltering effects associated with upstream obstacles. Such corrections are an important step in the generation of improved surface wind climatologies and wind atlases.
Abstract
A simple three-dimensional linear model of planetary boundary-layer flow is developed based on the Mixed Spectral Finite-Difference model of Beljaars et al. using the full second-order turbulence closure of Launder et al. The model uses a steady-state solution to the fully nonlinear model equations over a horizontally homogeneous surface to specify the zero-order, upwind profiles of mean and turbulent quantities. Due to the complexity of the closure and linearization of the model equations, the authors use an automated method of source code generation to produce both the time-dependent model used to generate the zero-order solution and the 3D model. Model results are presented for flow over a simple isolated hill.
Abstract
A simple three-dimensional linear model of planetary boundary-layer flow is developed based on the Mixed Spectral Finite-Difference model of Beljaars et al. using the full second-order turbulence closure of Launder et al. The model uses a steady-state solution to the fully nonlinear model equations over a horizontally homogeneous surface to specify the zero-order, upwind profiles of mean and turbulent quantities. Due to the complexity of the closure and linearization of the model equations, the authors use an automated method of source code generation to produce both the time-dependent model used to generate the zero-order solution and the 3D model. Model results are presented for flow over a simple isolated hill.
Abstract
The relationship between low-level mesoscale boundaries and convective storm initiation was investigated in southwestern Ontario, Canada. The influence of lake-breeze fronts, a type of boundary that frequently affects this region of the Great Lakes watershed in summer, presented a particular interest.
Radar data were processed using thunderstorm cell identification and tracking algorithms. The distances between the locations of storm cells reaching an intensity of 40 dBZ and the closest low-level mesoscale boundary were measured. Considering only days not influenced by a warm front, more than 75% of cells developed within 30 km of a low-level mesoscale boundary. Further examination by boundary type showed that cell initiations associated with moving boundaries and storm gust fronts occurred most often 0–5 km behind the boundaries. However, cell initiations associated with lake-breeze fronts most often occurred 0–5 km ahead of the boundaries. The analysis also suggested that lake-breeze fronts would often initiate the first storms of the day, which in turn generated gust fronts that could initiate subsequent storms.
Overall, the results were similar to a previous study investigating storm initiation in the vicinity of low-level mesoscale boundaries in eastern Colorado and include some new findings in relation to lake-breeze fronts. The findings can be used by forecasters as well as automated nowcasting algorithms in order to improve predictions of storm initiation.
Abstract
The relationship between low-level mesoscale boundaries and convective storm initiation was investigated in southwestern Ontario, Canada. The influence of lake-breeze fronts, a type of boundary that frequently affects this region of the Great Lakes watershed in summer, presented a particular interest.
Radar data were processed using thunderstorm cell identification and tracking algorithms. The distances between the locations of storm cells reaching an intensity of 40 dBZ and the closest low-level mesoscale boundary were measured. Considering only days not influenced by a warm front, more than 75% of cells developed within 30 km of a low-level mesoscale boundary. Further examination by boundary type showed that cell initiations associated with moving boundaries and storm gust fronts occurred most often 0–5 km behind the boundaries. However, cell initiations associated with lake-breeze fronts most often occurred 0–5 km ahead of the boundaries. The analysis also suggested that lake-breeze fronts would often initiate the first storms of the day, which in turn generated gust fronts that could initiate subsequent storms.
Overall, the results were similar to a previous study investigating storm initiation in the vicinity of low-level mesoscale boundaries in eastern Colorado and include some new findings in relation to lake-breeze fronts. The findings can be used by forecasters as well as automated nowcasting algorithms in order to improve predictions of storm initiation.
Abstract
Results from an analysis of the Southampton Oceanography Centre (SOC) global air–sea heat flux climatology, which has been calculated using in situ weather reports from voluntary observing ships covering the period 1980–93, are presented. Systematic errors in the fluxes arising from differences in observing procedure have been quantified and corrected; the magnitude of these errors is up to 15 W m−2 with strong seasonal and regional variations. Despite these corrections, closure of the ocean heat budget is not obtained as the global mean net heat flux is an oceanic gain of 30 W m−2. The validity of closing the heat budget by global scaling of the flux components is assessed by comparison of the SOC flux fields with Woods Hole Oceanographic Institute research buoy measurements. The level of agreement between the two is found to vary from one site to another. Thus, closure of the ocean heat budget requires regional adjustments to the flux components in order to avoid significant biases in the adjusted fields. Close agreement is found for several buoys deployed in the Subduction Array off the coast of northwest Africa. However, at other buoy deployment sites in the western equatorial Pacific warm pool and south of Bermuda in the North Atlantic, the flux adjustment improves the estimate of the net heat exchange. Further evidence for regional biases is obtained from a comparison of box mean surface heat fluxes derived from hydrographic section data with the corresponding SOC values in the Atlantic and North Pacific. The climatological heat loss is found to be an underestimate in those boxes containing the strongest surface flux expression of the major western boundary currents.
Abstract
Results from an analysis of the Southampton Oceanography Centre (SOC) global air–sea heat flux climatology, which has been calculated using in situ weather reports from voluntary observing ships covering the period 1980–93, are presented. Systematic errors in the fluxes arising from differences in observing procedure have been quantified and corrected; the magnitude of these errors is up to 15 W m−2 with strong seasonal and regional variations. Despite these corrections, closure of the ocean heat budget is not obtained as the global mean net heat flux is an oceanic gain of 30 W m−2. The validity of closing the heat budget by global scaling of the flux components is assessed by comparison of the SOC flux fields with Woods Hole Oceanographic Institute research buoy measurements. The level of agreement between the two is found to vary from one site to another. Thus, closure of the ocean heat budget requires regional adjustments to the flux components in order to avoid significant biases in the adjusted fields. Close agreement is found for several buoys deployed in the Subduction Array off the coast of northwest Africa. However, at other buoy deployment sites in the western equatorial Pacific warm pool and south of Bermuda in the North Atlantic, the flux adjustment improves the estimate of the net heat exchange. Further evidence for regional biases is obtained from a comparison of box mean surface heat fluxes derived from hydrographic section data with the corresponding SOC values in the Atlantic and North Pacific. The climatological heat loss is found to be an underestimate in those boxes containing the strongest surface flux expression of the major western boundary currents.
Abstract
Results from an analysis of the Southampton Oceanography Centre (SOC) global wind stress climatology, which is based on in situ reports for the period 1980–93, are presented. The accuracy of the SOC stresses has been assessed at several locations by comparison of individual monthly means with measurements from Woods Hole Oceanographic Institution research buoy deployments. For the subduction buoy array, situated in the subtropical North Atlantic, the random error in the SOC individual monthly mean wind stress ranges from 0.004 to 0.008 N m−2, which corresponds to between 5% and 10% of the mean stress depending on which buoy is considered. The large-scale characteristics of the SOC fields are compared with those of the NCEP–NCAR and ECMWF atmospheric model reanalyses, and the in situ observation based on the University of Wisconsin—Milwaukee/Comprehensive Ocean–Atmosphere Dataset (UWM/COADS) and Hellerman and Rosenstein (HR) climatologies. The NCEP–NCAR fields show noticeably weaker wind stress forcing in the Tropics than SOC, while ECMWF and UWM/COADS are in good agreement. From the Tropics to the midlatitudes, the HR stresses tend to be stronger than SOC and the other recent climatologies. At higher latitudes, differences in the spatial structure of the Northern Hemisphere subpolar gyres in SOC and HR are found that are consistent with variations in the state of the North Atlantic and North Pacific Oscillations within the periods on which the climatologies are based. A detailed comparison of the wind-driven response of the ocean is presented for SOC and HR. The North Atlantic subpolar gyre is more intense in SOC than HR and this leads to a doubling in the strength of the Ekman suction. January mean upwelling velocities in this region deduced from the two datasets are 18.9 and 8.6 m month−1, respectively. In the North Pacific a single large-scale subpolar gyre is evident in SOC compared with two smaller gyres in HR. Seasonal to interannual variability in the wind-driven ocean response is quantified using an extended version of the SOC dataset covering the period 1980–97. Significant variability in the Ekman transport across several latitudes that correspond to WOCE hydrographic sections is observed and related to the major atmospheric pressure oscillations.
Abstract
Results from an analysis of the Southampton Oceanography Centre (SOC) global wind stress climatology, which is based on in situ reports for the period 1980–93, are presented. The accuracy of the SOC stresses has been assessed at several locations by comparison of individual monthly means with measurements from Woods Hole Oceanographic Institution research buoy deployments. For the subduction buoy array, situated in the subtropical North Atlantic, the random error in the SOC individual monthly mean wind stress ranges from 0.004 to 0.008 N m−2, which corresponds to between 5% and 10% of the mean stress depending on which buoy is considered. The large-scale characteristics of the SOC fields are compared with those of the NCEP–NCAR and ECMWF atmospheric model reanalyses, and the in situ observation based on the University of Wisconsin—Milwaukee/Comprehensive Ocean–Atmosphere Dataset (UWM/COADS) and Hellerman and Rosenstein (HR) climatologies. The NCEP–NCAR fields show noticeably weaker wind stress forcing in the Tropics than SOC, while ECMWF and UWM/COADS are in good agreement. From the Tropics to the midlatitudes, the HR stresses tend to be stronger than SOC and the other recent climatologies. At higher latitudes, differences in the spatial structure of the Northern Hemisphere subpolar gyres in SOC and HR are found that are consistent with variations in the state of the North Atlantic and North Pacific Oscillations within the periods on which the climatologies are based. A detailed comparison of the wind-driven response of the ocean is presented for SOC and HR. The North Atlantic subpolar gyre is more intense in SOC than HR and this leads to a doubling in the strength of the Ekman suction. January mean upwelling velocities in this region deduced from the two datasets are 18.9 and 8.6 m month−1, respectively. In the North Pacific a single large-scale subpolar gyre is evident in SOC compared with two smaller gyres in HR. Seasonal to interannual variability in the wind-driven ocean response is quantified using an extended version of the SOC dataset covering the period 1980–97. Significant variability in the Ekman transport across several latitudes that correspond to WOCE hydrographic sections is observed and related to the major atmospheric pressure oscillations.
Abstract
A field study on visibility during Arctic blowing snow events over sea ice in Franklin Bay, Northwest Territories, Canada, was carried out from mid-January to early April 2004 during the Canadian Arctic Shelf Exchange Study (CASES) 2003–04 expedition. Visibilities at two heights, wind and temperature profiles, plus blowing and drifting snow particle flux at several heights were monitored continually during the study period. Good relations between visibility and wind speed were found in individual events of ground blowing snow with coefficients of determination >0.9. Regression equations relating 1.5-m height visibility to 10-m wind speed can be used for predicting visibility with a mean relative error in the range of 19%–32%. Similar regression functions obtained from the data for observed visibility of less than 1 km could predict visibilities more accurately for more extreme visibility reductions and wind speeds (>9.5 m s−1) with mean relative error ranging from 15% to 26%. For the event of ground blowing snow, a simple power law relationship between wind speed and visibility is sufficient for operational purposes. A poorer relationship was observed in the event of blowing snow with concurrent precipitating snow. A theoretical visibility model developed by Pomeroy and Male fit well with observed visibilities if using a mean radius of 50 μm and an alpha value of 10. The predicted visibility had a mean relative error of 30.5% and root-mean-square error of 1.3 km. The observed visibility at 1.5 m had a strong relation with particle counter readings, with an R 2 of 0.92, and was consistent among all events.
Abstract
A field study on visibility during Arctic blowing snow events over sea ice in Franklin Bay, Northwest Territories, Canada, was carried out from mid-January to early April 2004 during the Canadian Arctic Shelf Exchange Study (CASES) 2003–04 expedition. Visibilities at two heights, wind and temperature profiles, plus blowing and drifting snow particle flux at several heights were monitored continually during the study period. Good relations between visibility and wind speed were found in individual events of ground blowing snow with coefficients of determination >0.9. Regression equations relating 1.5-m height visibility to 10-m wind speed can be used for predicting visibility with a mean relative error in the range of 19%–32%. Similar regression functions obtained from the data for observed visibility of less than 1 km could predict visibilities more accurately for more extreme visibility reductions and wind speeds (>9.5 m s−1) with mean relative error ranging from 15% to 26%. For the event of ground blowing snow, a simple power law relationship between wind speed and visibility is sufficient for operational purposes. A poorer relationship was observed in the event of blowing snow with concurrent precipitating snow. A theoretical visibility model developed by Pomeroy and Male fit well with observed visibilities if using a mean radius of 50 μm and an alpha value of 10. The predicted visibility had a mean relative error of 30.5% and root-mean-square error of 1.3 km. The observed visibility at 1.5 m had a strong relation with particle counter readings, with an R 2 of 0.92, and was consistent among all events.
Abstract
A 34% reduction in 10-m wind speeds at Sudbury Airport in Ontario, Canada, over the period 1975–95 appears to be a result of significant changes in the surface roughness of the surrounding area that are due to land restoration and reforestation following historical environmental damage caused by high sulfur dioxide and other industrial emissions. Neither 850-hPa winds extracted from the NCEP–NCAR reanalysis database nor wind measurements at meteorological stations 200 km to the north and 120 km to the east of Sudbury show the same decrease. To assess these changes in observed wind speed quantitatively, geostrophic drag laws were employed to illustrate potential changes in near-surface wind speeds in areas surrounding the airport. A model of the internal boundary layer flow adjustment associated with changes in the surface roughness length between the surroundings and the grass or snow surface of the airport was then applied to compute expected annual average wind speeds at the airport site itself. The estimates obtained with this relatively simple procedure match the observations and confirm that reforestation is likely the major cause of the reduced wind speeds. This finding bears economic, social, and ecological importance, because it will influence wind energy potential, wind loads on structures, wind chill, and home heating costs through to the biology of small- to medium-sized lakes.
Abstract
A 34% reduction in 10-m wind speeds at Sudbury Airport in Ontario, Canada, over the period 1975–95 appears to be a result of significant changes in the surface roughness of the surrounding area that are due to land restoration and reforestation following historical environmental damage caused by high sulfur dioxide and other industrial emissions. Neither 850-hPa winds extracted from the NCEP–NCAR reanalysis database nor wind measurements at meteorological stations 200 km to the north and 120 km to the east of Sudbury show the same decrease. To assess these changes in observed wind speed quantitatively, geostrophic drag laws were employed to illustrate potential changes in near-surface wind speeds in areas surrounding the airport. A model of the internal boundary layer flow adjustment associated with changes in the surface roughness length between the surroundings and the grass or snow surface of the airport was then applied to compute expected annual average wind speeds at the airport site itself. The estimates obtained with this relatively simple procedure match the observations and confirm that reforestation is likely the major cause of the reduced wind speeds. This finding bears economic, social, and ecological importance, because it will influence wind energy potential, wind loads on structures, wind chill, and home heating costs through to the biology of small- to medium-sized lakes.
Abstract
The performance of a pressure transducer, with its inlet attached to differing hydromechanical front ends, has been evaluated in flow flume and wave flume experiments in which laminar and turbulent flows, and regular progressive gravity waves and combinations of flows and waves, were generated. For steady laminar flows, and for waves propagating on quiescent waters, the transducer’s performance improved when the inlet was at the center and flush with a large, thin, and smooth circular horizontal end plate. This enhancement is likely to have been achieved because of the isolation of the pressure inlet from the separated flows and vortices generated by the transducer housing. Flow disturbances, generated by nearby solid structures, deteriorated the performance of the pressure transducer. However, its performance could be significantly improved by protecting the pressure inlet by a sturdy, curved perforated shield. The dynamic pressure error in this case was 2 mb at 100 cm s−1, compared to 8 mb in the absence of the shield. For turbulent flows less than 100 cm s−1, a pair of thin, circular, parallel plates, with a diameter three to four times that of the transducer housing and separation equal to the housing diameter, led to a much improved horizontal azimuthal response. At this speed the spread in the dynamic pressure, ΔP, was less than 1 mb compared to 6 mb without a plate. Beyond this speed the transducer’s horizontal azimuthal response deteriorated faster. For combinations of waves and flows a relatively small ΔP was found. This result is of special significance to tidal measurements of coastal waters, in which waves propagate on tidal currents.
Abstract
The performance of a pressure transducer, with its inlet attached to differing hydromechanical front ends, has been evaluated in flow flume and wave flume experiments in which laminar and turbulent flows, and regular progressive gravity waves and combinations of flows and waves, were generated. For steady laminar flows, and for waves propagating on quiescent waters, the transducer’s performance improved when the inlet was at the center and flush with a large, thin, and smooth circular horizontal end plate. This enhancement is likely to have been achieved because of the isolation of the pressure inlet from the separated flows and vortices generated by the transducer housing. Flow disturbances, generated by nearby solid structures, deteriorated the performance of the pressure transducer. However, its performance could be significantly improved by protecting the pressure inlet by a sturdy, curved perforated shield. The dynamic pressure error in this case was 2 mb at 100 cm s−1, compared to 8 mb in the absence of the shield. For turbulent flows less than 100 cm s−1, a pair of thin, circular, parallel plates, with a diameter three to four times that of the transducer housing and separation equal to the housing diameter, led to a much improved horizontal azimuthal response. At this speed the spread in the dynamic pressure, ΔP, was less than 1 mb compared to 6 mb without a plate. Beyond this speed the transducer’s horizontal azimuthal response deteriorated faster. For combinations of waves and flows a relatively small ΔP was found. This result is of special significance to tidal measurements of coastal waters, in which waves propagate on tidal currents.
Abstract
Atmospheric modeling with element-based high-order Galerkin methods presents a unique challenge to the conventional physics–dynamics coupling paradigm, due to the highly irregular distribution of nodes within an element and the distinct numerical characteristics of the Galerkin method. The conventional coupling procedure is to evaluate the physical parameterizations (physics) on the dynamical core grid. Evaluating the physics at the nodal points exacerbates numerical noise from the Galerkin method, enabling and amplifying local extrema at element boundaries. Grid imprinting may be substantially reduced through the introduction of an entirely separate, approximately isotropic finite-volume grid for evaluating the physics forcing. Integration of the spectral basis over the control volumes provides an area-average state to the physics, which is more representative of the state in the vicinity of the nodal points rather than the nodal point itself and is more consistent with the notion of a “large-scale state” required by conventional physics packages. This study documents the implementation of a quasi-equal-area physics grid into NCAR’s Community Atmosphere Model Spectral Element and is shown to be effective at mitigating grid imprinting in the solution. The physics grid is also appropriate for coupling to other components within the Community Earth System Model, since the coupler requires component fluxes to be defined on a finite-volume grid, and one can be certain that the fluxes on the physics grid are, indeed, volume averaged.
Abstract
Atmospheric modeling with element-based high-order Galerkin methods presents a unique challenge to the conventional physics–dynamics coupling paradigm, due to the highly irregular distribution of nodes within an element and the distinct numerical characteristics of the Galerkin method. The conventional coupling procedure is to evaluate the physical parameterizations (physics) on the dynamical core grid. Evaluating the physics at the nodal points exacerbates numerical noise from the Galerkin method, enabling and amplifying local extrema at element boundaries. Grid imprinting may be substantially reduced through the introduction of an entirely separate, approximately isotropic finite-volume grid for evaluating the physics forcing. Integration of the spectral basis over the control volumes provides an area-average state to the physics, which is more representative of the state in the vicinity of the nodal points rather than the nodal point itself and is more consistent with the notion of a “large-scale state” required by conventional physics packages. This study documents the implementation of a quasi-equal-area physics grid into NCAR’s Community Atmosphere Model Spectral Element and is shown to be effective at mitigating grid imprinting in the solution. The physics grid is also appropriate for coupling to other components within the Community Earth System Model, since the coupler requires component fluxes to be defined on a finite-volume grid, and one can be certain that the fluxes on the physics grid are, indeed, volume averaged.
Abstract
An algorithm to consistently couple a conservative semi-Lagrangian finite-volume transport scheme with a spectral element (SE) dynamical core is presented. The semi-Lagrangian finite-volume scheme is the Conservative Semi-Lagrangian Multitracer (CSLAM), and the SE dynamical core is the National Center for Atmospheric Research (NCAR)’s Community Atmosphere Model–Spectral Elements (CAM-SE). The primary motivation for coupling CSLAM with CAM-SE is to accelerate tracer transport for multitracer applications. The coupling algorithm result is an inherently mass-conservative, shape-preserving, and consistent (for a constant mixing ratio, the CSLAM solution reduces to the SE solution for air mass) transport that is efficient and accurate. This is achieved by first deriving formulas for diagnosing SE airmass flux through the CSLAM control volume faces. Thereafter, the upstream Lagrangian CSLAM areas are iteratively perturbed to match the diagnosed SE airmass flux, resulting in an equivalent upstream Lagrangian grid that spans the sphere without gaps or overlaps (without using an expensive search algorithm). This new CSLAM algorithm is not specific to airmass fluxes provided by CAM-SE but applies to any airmass fluxes that satisfy the Lipshitz criterion and for which the Courant number is less than one.
Abstract
An algorithm to consistently couple a conservative semi-Lagrangian finite-volume transport scheme with a spectral element (SE) dynamical core is presented. The semi-Lagrangian finite-volume scheme is the Conservative Semi-Lagrangian Multitracer (CSLAM), and the SE dynamical core is the National Center for Atmospheric Research (NCAR)’s Community Atmosphere Model–Spectral Elements (CAM-SE). The primary motivation for coupling CSLAM with CAM-SE is to accelerate tracer transport for multitracer applications. The coupling algorithm result is an inherently mass-conservative, shape-preserving, and consistent (for a constant mixing ratio, the CSLAM solution reduces to the SE solution for air mass) transport that is efficient and accurate. This is achieved by first deriving formulas for diagnosing SE airmass flux through the CSLAM control volume faces. Thereafter, the upstream Lagrangian CSLAM areas are iteratively perturbed to match the diagnosed SE airmass flux, resulting in an equivalent upstream Lagrangian grid that spans the sphere without gaps or overlaps (without using an expensive search algorithm). This new CSLAM algorithm is not specific to airmass fluxes provided by CAM-SE but applies to any airmass fluxes that satisfy the Lipshitz criterion and for which the Courant number is less than one.
