Search Results
You are looking at 21 - 30 of 73 items for
- Author or Editor: Dale R. Durran x
- Refine by Access: All Content x
Abstract
The development of shallow cellular convection in warm orographic clouds is investigated through idealized numerical simulations of moist flow over topography using a cloud-resolving numerical model. Buoyant instability, a necessary element for moist convection, is found to be diagnosed most accurately through analysis of the moist Brunt–Väisälä frequency (N
m
) rather than the vertical profile of θ
e
. In statically unstable orographic clouds (
Abstract
The development of shallow cellular convection in warm orographic clouds is investigated through idealized numerical simulations of moist flow over topography using a cloud-resolving numerical model. Buoyant instability, a necessary element for moist convection, is found to be diagnosed most accurately through analysis of the moist Brunt–Väisälä frequency (N
m
) rather than the vertical profile of θ
e
. In statically unstable orographic clouds (
Abstract
The spectral turbulence model of Lorenz, as modified for surface quasigeostrophic dynamics by Rotunno and Snyder, is further modified to more smoothly approach nonlinear saturation. This model is used to investigate error growth starting from different distributions of the initial error. Consistent with an often overlooked finding by Lorenz, the loss of predictability generated by initial errors of small but fixed absolute magnitude is essentially independent of their spatial scale when the background saturation kinetic energy spectrum is proportional to the −5/3 power of the wavenumber. Thus, because the background kinetic energy increases with scale, very small relative errors at long wavelengths have similar impacts on perturbation error growth as large relative errors at short wavelengths. To the extent that this model applies to practical meteorological forecasts, the influence of initial perturbations generated by butterflies would be swamped by unavoidable tiny relative errors in the large scales.
The rough applicability of the authors’ modified spectral turbulence model to the atmosphere over scales ranging between 10 and 1000 km is supported by the good estimate that it provides for the ensemble error growth in state-of-the-art ensemble mesoscale model simulations of two winter storms. The initial-error spectrum for the ensemble perturbations in these cases has maximum power at the longest wavelengths. The dominance of large-scale errors in the ensemble suggests that mesoscale weather forecasts may often be limited by errors arising from the large scales instead of being produced solely through an upscale cascade from the smallest scales.
Abstract
The spectral turbulence model of Lorenz, as modified for surface quasigeostrophic dynamics by Rotunno and Snyder, is further modified to more smoothly approach nonlinear saturation. This model is used to investigate error growth starting from different distributions of the initial error. Consistent with an often overlooked finding by Lorenz, the loss of predictability generated by initial errors of small but fixed absolute magnitude is essentially independent of their spatial scale when the background saturation kinetic energy spectrum is proportional to the −5/3 power of the wavenumber. Thus, because the background kinetic energy increases with scale, very small relative errors at long wavelengths have similar impacts on perturbation error growth as large relative errors at short wavelengths. To the extent that this model applies to practical meteorological forecasts, the influence of initial perturbations generated by butterflies would be swamped by unavoidable tiny relative errors in the large scales.
The rough applicability of the authors’ modified spectral turbulence model to the atmosphere over scales ranging between 10 and 1000 km is supported by the good estimate that it provides for the ensemble error growth in state-of-the-art ensemble mesoscale model simulations of two winter storms. The initial-error spectrum for the ensemble perturbations in these cases has maximum power at the longest wavelengths. The dominance of large-scale errors in the ensemble suggests that mesoscale weather forecasts may often be limited by errors arising from the large scales instead of being produced solely through an upscale cascade from the smallest scales.
Abstract
The internal structure and dynamics of rotors that form in the lee of topographic ridges are explored using a series of high-resolution eddy-resolving numerical simulations. Surface friction generates a sheet of horizontal vorticity along the lee slope that is lifted aloft by the mountain lee wave at the boundary layer separation point. Parallel-shear instability breaks this vortex sheet into small intense vortices or subrotors.
The strength and evolution of the subrotors and the internal structure of the main large-scale rotor are substantially different in 2D and 3D simulations. In 2D, the subrotors are less intense and are ultimately entrained into the larger-scale rotor circulation, where they dissipate and contribute their vorticity toward the maintenance of the main rotor. In 3D, even for flow over a uniform infinitely long barrier, the subrotors are more intense, and primarily are simply swept downstream past the main rotor along the interface between that rotor and the surrounding lee wave. The average vorticity within the interior of the main rotor is much weaker and the flow is more chaotic.
When an isolated peak is added to a 3D ridge, systematic along-ridge velocity perturbations create regions of preferential vortex stretching at the leading edge of the rotor. Subrotors passing through such regions are intensified by stretching and may develop values of the ridge-parallel vorticity component well in excess of those in the parent, shear-generated vortex sheet. Because of their intensity, such subrotor circulations likely pose the greatest hazard to aviation.
Abstract
The internal structure and dynamics of rotors that form in the lee of topographic ridges are explored using a series of high-resolution eddy-resolving numerical simulations. Surface friction generates a sheet of horizontal vorticity along the lee slope that is lifted aloft by the mountain lee wave at the boundary layer separation point. Parallel-shear instability breaks this vortex sheet into small intense vortices or subrotors.
The strength and evolution of the subrotors and the internal structure of the main large-scale rotor are substantially different in 2D and 3D simulations. In 2D, the subrotors are less intense and are ultimately entrained into the larger-scale rotor circulation, where they dissipate and contribute their vorticity toward the maintenance of the main rotor. In 3D, even for flow over a uniform infinitely long barrier, the subrotors are more intense, and primarily are simply swept downstream past the main rotor along the interface between that rotor and the surrounding lee wave. The average vorticity within the interior of the main rotor is much weaker and the flow is more chaotic.
When an isolated peak is added to a 3D ridge, systematic along-ridge velocity perturbations create regions of preferential vortex stretching at the leading edge of the rotor. Subrotors passing through such regions are intensified by stretching and may develop values of the ridge-parallel vorticity component well in excess of those in the parent, shear-generated vortex sheet. Because of their intensity, such subrotor circulations likely pose the greatest hazard to aviation.
Abstract
Numerical simulations of nonrotating flow with uniform basic wind and stability past long three-dimensional (3D) ridges are compared to the corresponding two-dimensional (2D) limit to reveal the importance of 3D effects. For mountain heights smaller than the threshold for breaking waves, the low-level flow over the interior of the ridge is well described by 2D theory when the horizontal aspect ratio β is roughly 10 or greater. By contrast, in flows with wave breaking significant discrepancies between 2D and 3D results remain apparent even for β = 12.
It is found that the onset of wave breaking and the transition to the high-drag state is accompanied in 3D by an abrupt increase in deflection of the low-level flow around the ridge. The increased flow deflection is produced at least in part by upstream-propagating columnar disturbances forced by the transition to the high-drag state. The deflection of the incident flow reduces the amplitude of the mountain wave aloft relative to 2D and acts as a negative feedback on the surface form drag. As a result, the nonlinear enhancement of the surface drag associated with wave breaking for a ridge with β = 7.5 is found to be roughly half the enhancement obtained for a 2D ridge.
Abstract
Numerical simulations of nonrotating flow with uniform basic wind and stability past long three-dimensional (3D) ridges are compared to the corresponding two-dimensional (2D) limit to reveal the importance of 3D effects. For mountain heights smaller than the threshold for breaking waves, the low-level flow over the interior of the ridge is well described by 2D theory when the horizontal aspect ratio β is roughly 10 or greater. By contrast, in flows with wave breaking significant discrepancies between 2D and 3D results remain apparent even for β = 12.
It is found that the onset of wave breaking and the transition to the high-drag state is accompanied in 3D by an abrupt increase in deflection of the low-level flow around the ridge. The increased flow deflection is produced at least in part by upstream-propagating columnar disturbances forced by the transition to the high-drag state. The deflection of the incident flow reduces the amplitude of the mountain wave aloft relative to 2D and acts as a negative feedback on the surface form drag. As a result, the nonlinear enhancement of the surface drag associated with wave breaking for a ridge with β = 7.5 is found to be roughly half the enhancement obtained for a 2D ridge.
Abstract
Using extensive observations collected from various platforms around the Brenner Pass in the Austrian Alps during the Mesoscale Alpine Programme, a detailed description of the kinematic and thermodynamic structure of the shallow-foehn event that occurred on 20 October 1999 in the Wipp Valley is constructed. Downstream of the gap the flow develops a well-mixed surface layer capped by a relatively strong temperature inversion of 5–6 K. Such inversions are often assumed to be kinematically similar to the free surface at the top of a liquid; however, the data suggest the presence of strong subsidence through the mean position of the inversion layer capping the flow. Such subsidence is supported by in situ aircraft observations and Doppler lidar measurements but is not consistent with the observed turbulent heat fluxes, which are too small to account for the diabatic heating required by the isentrope-relative downward velocities. The 1-Hz time resolution of the P3 data may, however, be too coarse to correctly capture the full turbulent heat flux.
Abstract
Using extensive observations collected from various platforms around the Brenner Pass in the Austrian Alps during the Mesoscale Alpine Programme, a detailed description of the kinematic and thermodynamic structure of the shallow-foehn event that occurred on 20 October 1999 in the Wipp Valley is constructed. Downstream of the gap the flow develops a well-mixed surface layer capped by a relatively strong temperature inversion of 5–6 K. Such inversions are often assumed to be kinematically similar to the free surface at the top of a liquid; however, the data suggest the presence of strong subsidence through the mean position of the inversion layer capping the flow. Such subsidence is supported by in situ aircraft observations and Doppler lidar measurements but is not consistent with the observed turbulent heat fluxes, which are too small to account for the diabatic heating required by the isentrope-relative downward velocities. The 1-Hz time resolution of the P3 data may, however, be too coarse to correctly capture the full turbulent heat flux.
Abstract
The sensitivity of downslope wind forecasts to small changes in initial conditions is explored by using 70-member ensemble simulations of two prototypical windstorms observed during the Terrain-Induced Rotor Experiment (T-REX). The 10 weakest and 10 strongest ensemble members are composited and compared for each event.
In the first case, the 6-h ensemble-mean forecast shows a large-amplitude breaking mountain wave and severe downslope winds. Nevertheless, the forecasts are very sensitive to the initial conditions because the difference in the downslope wind speeds predicted by the strong- and weak-member composites grows to larger than 28 m s−1 over the 6-h forecast. The structure of the synoptic-scale flow one hour prior to the windstorm and during the windstorm is very similar in both the weak- and strong-member composites.
Wave breaking is not a significant factor in the second case, in which the strong winds are generated by a layer of high static stability flowing beneath a layer of weaker mid- and upper-tropospheric stability. In this case, the sensitivity to initial conditions is weaker but still significant. The difference in downslope wind speeds between the weak- and strong-member composites grows to 22 m s−1 over 12 h. During and one hour before the windstorm, the synoptic-scale flow exhibits appreciable differences between the strong- and weak-member composites. Although this case appears to be more predictable than the wave-breaking event, neither case suggests that much confidence should be placed in the intensity of downslope winds forecast 12 or more hours in advance.
Abstract
The sensitivity of downslope wind forecasts to small changes in initial conditions is explored by using 70-member ensemble simulations of two prototypical windstorms observed during the Terrain-Induced Rotor Experiment (T-REX). The 10 weakest and 10 strongest ensemble members are composited and compared for each event.
In the first case, the 6-h ensemble-mean forecast shows a large-amplitude breaking mountain wave and severe downslope winds. Nevertheless, the forecasts are very sensitive to the initial conditions because the difference in the downslope wind speeds predicted by the strong- and weak-member composites grows to larger than 28 m s−1 over the 6-h forecast. The structure of the synoptic-scale flow one hour prior to the windstorm and during the windstorm is very similar in both the weak- and strong-member composites.
Wave breaking is not a significant factor in the second case, in which the strong winds are generated by a layer of high static stability flowing beneath a layer of weaker mid- and upper-tropospheric stability. In this case, the sensitivity to initial conditions is weaker but still significant. The difference in downslope wind speeds between the weak- and strong-member composites grows to 22 m s−1 over 12 h. During and one hour before the windstorm, the synoptic-scale flow exhibits appreciable differences between the strong- and weak-member composites. Although this case appears to be more predictable than the wave-breaking event, neither case suggests that much confidence should be placed in the intensity of downslope winds forecast 12 or more hours in advance.
Abstract
A parameter widely used to predict topographic flow blocking is the nondimensional mountain height or, synonymously, the inverse Froude number. Predictions using this parameter are based on the morphology of flows with uniform upstream static stability and wind speed, which rarely occur in the real world. The appropriateness of applying this theory in the presence of nontrivial background stability is therefore investigated using a numerical model. Two methods were considered to estimate the low-level stability, averaging the Brunt–Väisälä frequency below the crest and using the bulk change in θ between the ground and crest level.
No single best method emerged for estimating the upstream static stability and thereby mapping the simulations with inversions onto the set of solutions with constant stratification. Instead, the best method depended on the application at hand. To predict the onset of flow stagnation, averaging the low-level stability worked best, while to predict low-level flow diversion the bulk estimate of low-level stability was most appropriate. These results are consistent across a range of inversion thicknesses and strengths. In addition, it is shown that variations in static stability above the mountain crest have little impact on flow blocking.
Abstract
A parameter widely used to predict topographic flow blocking is the nondimensional mountain height or, synonymously, the inverse Froude number. Predictions using this parameter are based on the morphology of flows with uniform upstream static stability and wind speed, which rarely occur in the real world. The appropriateness of applying this theory in the presence of nontrivial background stability is therefore investigated using a numerical model. Two methods were considered to estimate the low-level stability, averaging the Brunt–Väisälä frequency below the crest and using the bulk change in θ between the ground and crest level.
No single best method emerged for estimating the upstream static stability and thereby mapping the simulations with inversions onto the set of solutions with constant stratification. Instead, the best method depended on the application at hand. To predict the onset of flow stagnation, averaging the low-level stability worked best, while to predict low-level flow diversion the bulk estimate of low-level stability was most appropriate. These results are consistent across a range of inversion thicknesses and strengths. In addition, it is shown that variations in static stability above the mountain crest have little impact on flow blocking.
Abstract
The three-dimensional structure of shallow orographic convection is investigated through simulations performed with a cloud-resolving numerical model. In moist flows that overcome a given topographic barrier to form statically unstable cap clouds, the organization of the convection depends on both the atmospheric structure and the mechanism by which the convection is initiated. Convection initiated by background thermal fluctuations embedded in the flow over a smooth mountain (without any small-scale topographic features) tends to be cellular and disorganized except that shear-parallel bands may form in flows with strong unidirectional vertical shear. The development of well-organized bands is favored when there is weak static instability inside the cloud and when the dry air surrounding the cloud is strongly stable. These bands move with the flow and distribute their cumulative precipitation evenly over the mountain upslope.
Similar shear-parallel bands also develop in flows where convection is initiated by small-scale topographic noise superimposed onto the main mountain profile, but in this case stronger circulations are also triggered that create stationary rainbands parallel to the low-level flow. This second dominant mode, which is less sensitive to the atmospheric structure and the strength of forcing, is triggered by lee waves that form over small-scale topographic bumps near the upstream edge of the main orographic cloud. Due to their stationarity, these flow-parallel bands can produce locally heavy precipitation amounts.
Abstract
The three-dimensional structure of shallow orographic convection is investigated through simulations performed with a cloud-resolving numerical model. In moist flows that overcome a given topographic barrier to form statically unstable cap clouds, the organization of the convection depends on both the atmospheric structure and the mechanism by which the convection is initiated. Convection initiated by background thermal fluctuations embedded in the flow over a smooth mountain (without any small-scale topographic features) tends to be cellular and disorganized except that shear-parallel bands may form in flows with strong unidirectional vertical shear. The development of well-organized bands is favored when there is weak static instability inside the cloud and when the dry air surrounding the cloud is strongly stable. These bands move with the flow and distribute their cumulative precipitation evenly over the mountain upslope.
Similar shear-parallel bands also develop in flows where convection is initiated by small-scale topographic noise superimposed onto the main mountain profile, but in this case stronger circulations are also triggered that create stationary rainbands parallel to the low-level flow. This second dominant mode, which is less sensitive to the atmospheric structure and the strength of forcing, is triggered by lee waves that form over small-scale topographic bumps near the upstream edge of the main orographic cloud. Due to their stationarity, these flow-parallel bands can produce locally heavy precipitation amounts.
