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- Author or Editor: Dale R. Durran x
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Abstract
The factor that contribute to the development of the sharp edge along the poleward boundary of the jet-stream cirrus are examined in three cast studies, using wind field and thermodynamic information from the FGGE dataset as input to a numerical model. The model generated a cloud field that satisfactorily reproduced the cirrus cloud distributions shown on satellite photos. Trajectory calculations, together with an examination of the vertical velocity field, suggest that the cloud boundary is not directly produced by differential vertical motions (with sinking on the clear side of the cloud edge and rising motion on the cloudy side). In each case, significant ascent was found in the clear air on the poleward side of the cloud boundary. The cloud. boundary appears to develop when a preexisting moisture gradient experiences relatively uniform lifting. Differential vertical motions (together with horizontal confluence) were found to play a significant role in generating the initial moisture gradient.
Abstract
The factor that contribute to the development of the sharp edge along the poleward boundary of the jet-stream cirrus are examined in three cast studies, using wind field and thermodynamic information from the FGGE dataset as input to a numerical model. The model generated a cloud field that satisfactorily reproduced the cirrus cloud distributions shown on satellite photos. Trajectory calculations, together with an examination of the vertical velocity field, suggest that the cloud boundary is not directly produced by differential vertical motions (with sinking on the clear side of the cloud edge and rising motion on the cloudy side). In each case, significant ascent was found in the clear air on the poleward side of the cloud boundary. The cloud. boundary appears to develop when a preexisting moisture gradient experiences relatively uniform lifting. Differential vertical motions (together with horizontal confluence) were found to play a significant role in generating the initial moisture gradient.
Abstract
Wind profiler data from Lay Creek, Colorado, along with stability data from the Lander and Grand Junction rawinsonde observations, were examined in an attempt to link various parameters in the upstream flow to the onset of strong downslope winds in Boulder. Some correlation was found between the occurrence of high surface winds at Boulder and the upstream wind direction, upper tropospheric wind shear and the vertical phase shift across the troposphere. However, these parameters alone were not able to distinguish between windstorm and nonwindstorm events. It is likely that the remaining ambiguity could be eliminated with information on the location and strength of inversions in the upstream flow.
Abstract
Wind profiler data from Lay Creek, Colorado, along with stability data from the Lander and Grand Junction rawinsonde observations, were examined in an attempt to link various parameters in the upstream flow to the onset of strong downslope winds in Boulder. Some correlation was found between the occurrence of high surface winds at Boulder and the upstream wind direction, upper tropospheric wind shear and the vertical phase shift across the troposphere. However, these parameters alone were not able to distinguish between windstorm and nonwindstorm events. It is likely that the remaining ambiguity could be eliminated with information on the location and strength of inversions in the upstream flow.
Abstract
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Abstract
Seven-year averaged values of percent frequency of occurrence of highly reflective cloud for the months June, July, and August indicate that offshore convection is a major component of the cloudiness of the southwest monsoon. Principal areas of convection occur off of the western coats of India, Burma, Thailand, and the Philippines. This study concentrates on the area upstream of the Western Ghats Mountains of India. Analysis of a special boundary layer mission flown during the WMO/ICSU Summer Monsoon Experiment leads us to believe that partial deceleration of the monsoon flow by upstream blocking effects of the mountains initiates and maintains a vertical and horizontal motion field that could support the observed convection. Data obtained on this mission allow a large-scale momentum budget computation for the subcloud layer, which shows pressure deceleration to be significant. The budget, dominated by advection, predicts an increase of average wind speed which is observed. The pressure deceleration result is further explored by applying an idealized monsoon flow to an analytical, nonliner, two-dimensional mountain-flow interaction model using a smoothed profile of the Western Ghats Mountains. The model qualitatively agrees with aircraft observations taken in the subcloud layer, and predicts large vertical wind shears over the coastal area and mountain crest which would inhibit deep convection. These shears are confirmed by earlier observations.
When the lifting predicted by the model is applied to mean dropwindsonde soundings, well upstream of the coast, for days with and without offshore convection, deep convection is predicted for the mean sounding associated with offshore convection. The mean sounding for days without deep convection shows more offshore lifting is needed to produce convection; even if the lifting were applied, the convection would not be very deep due to a cooler surface layer and a dry layer above the boundary layer which may have originated from the desert areas to the west and/or upper tropospheric downward motion. We conclude that the mountains, though not very high, play an important role in overall monsoon convection for India. It is suggested that, given the climatic character of offshore monsoon convection, interaction of the low-level flow with the western coastal mountains of India, Burma, Thailand, and the Philippines should be considered a factor in monsoon climatology.
Abstract
Seven-year averaged values of percent frequency of occurrence of highly reflective cloud for the months June, July, and August indicate that offshore convection is a major component of the cloudiness of the southwest monsoon. Principal areas of convection occur off of the western coats of India, Burma, Thailand, and the Philippines. This study concentrates on the area upstream of the Western Ghats Mountains of India. Analysis of a special boundary layer mission flown during the WMO/ICSU Summer Monsoon Experiment leads us to believe that partial deceleration of the monsoon flow by upstream blocking effects of the mountains initiates and maintains a vertical and horizontal motion field that could support the observed convection. Data obtained on this mission allow a large-scale momentum budget computation for the subcloud layer, which shows pressure deceleration to be significant. The budget, dominated by advection, predicts an increase of average wind speed which is observed. The pressure deceleration result is further explored by applying an idealized monsoon flow to an analytical, nonliner, two-dimensional mountain-flow interaction model using a smoothed profile of the Western Ghats Mountains. The model qualitatively agrees with aircraft observations taken in the subcloud layer, and predicts large vertical wind shears over the coastal area and mountain crest which would inhibit deep convection. These shears are confirmed by earlier observations.
When the lifting predicted by the model is applied to mean dropwindsonde soundings, well upstream of the coast, for days with and without offshore convection, deep convection is predicted for the mean sounding associated with offshore convection. The mean sounding for days without deep convection shows more offshore lifting is needed to produce convection; even if the lifting were applied, the convection would not be very deep due to a cooler surface layer and a dry layer above the boundary layer which may have originated from the desert areas to the west and/or upper tropospheric downward motion. We conclude that the mountains, though not very high, play an important role in overall monsoon convection for India. It is suggested that, given the climatic character of offshore monsoon convection, interaction of the low-level flow with the western coastal mountains of India, Burma, Thailand, and the Philippines should be considered a factor in monsoon climatology.
Abstract
A radiative upper boundary condition is proposed for numerical mesoscale models which allows vertically propagating internal gravity waves to pass out of the computational domain with minimal reflection. In this formulation, the pressure along the upper boundary is determined from the Fourier transform of the vertical velocity at that boundary. This boundary condition can easily be incorporated in a wide variety of models and requires little additional computation. The radiation boundary condition is derived from the linear, hydrostatic, Boussinesq equations of motion, neglecting Coriolis effects. However, tests of this radiation boundary condition in the presence of nonhydrostatic, Coriolis, nonlinear and non-Boussinesq effects suggest that it would be effective in many mesoscale modeling applications.
Abstract
A radiative upper boundary condition is proposed for numerical mesoscale models which allows vertically propagating internal gravity waves to pass out of the computational domain with minimal reflection. In this formulation, the pressure along the upper boundary is determined from the Fourier transform of the vertical velocity at that boundary. This boundary condition can easily be incorporated in a wide variety of models and requires little additional computation. The radiation boundary condition is derived from the linear, hydrostatic, Boussinesq equations of motion, neglecting Coriolis effects. However, tests of this radiation boundary condition in the presence of nonhydrostatic, Coriolis, nonlinear and non-Boussinesq effects suggest that it would be effective in many mesoscale modeling applications.
Abstract
A two-dimensional, nonlinear, nonhydrostatic model is described which allows the calculation of moist airflow in mountainous terrain. The model is compressible, uses a terrain-following coordinate system, and employs lateral and upper boundary conditions which minimize wave reflections.
The model's accuracy and sensitivity are examined. These tests suggest that in numerical simulations of vertically propagating, highly nonlinear mountain waves, a wave absorbing layer does not accurately mimic the effects of wave breakdown and dissipation at high levels in the atmosphere. In order to obtain a correct simulation, the region in which the waves are physically absorbed must generally be included in the computational domain (a nonreflective upper boundary condition should be used as well).
The utility of the model is demonstrated in two examples (linear waves in a uniform atmosphere and the 11 January 1972 Boulder windstorm) which illustrate how the presence of moisture can influence propagating waves. In both cases, the addition of moisture to the upstream flow greatly reduces the wave response.
Abstract
A two-dimensional, nonlinear, nonhydrostatic model is described which allows the calculation of moist airflow in mountainous terrain. The model is compressible, uses a terrain-following coordinate system, and employs lateral and upper boundary conditions which minimize wave reflections.
The model's accuracy and sensitivity are examined. These tests suggest that in numerical simulations of vertically propagating, highly nonlinear mountain waves, a wave absorbing layer does not accurately mimic the effects of wave breakdown and dissipation at high levels in the atmosphere. In order to obtain a correct simulation, the region in which the waves are physically absorbed must generally be included in the computational domain (a nonreflective upper boundary condition should be used as well).
The utility of the model is demonstrated in two examples (linear waves in a uniform atmosphere and the 11 January 1972 Boulder windstorm) which illustrate how the presence of moisture can influence propagating waves. In both cases, the addition of moisture to the upstream flow greatly reduces the wave response.
Abstract
Strong downslope windstorms can cause extensive property damage and extreme wildfire spread, so their accurate prediction is important. Although some early studies suggested high predictability for downslope windstorms, more recent analyses have found limited predictability for such winds. Nevertheless, there is a theoretical basis for expecting higher downslope wind predictability in cases with a mean-state critical level, and this is supported by one previous effort to forecast actual events. To more thoroughly investigate downslope windstorm predictability, we compare archived simulations from the NCAR ensemble, a 10-member mesoscale ensemble run at 3-km horizontal grid spacing over the entire contiguous United States, to observed events at 15 stations in the western United States susceptible to strong downslope winds. We assess predictability in three contexts: the average ensemble spread, which provides an estimate of potential predictability; a forecast evaluation based upon binary-decision criteria, which is representative of operational hazard warnings; and a probabilistic forecast evaluation using the continuous ranked probability score (CRPS), which is a measure of an ensemble’s ability to generate the proper probability distribution for the events under consideration. We do find better predictive skill for the mean-state critical-level regime in comparison to other downslope windstorm–generating mechanisms. Our downslope windstorm warning performance, calculated using binary-decision criteria from the bias-corrected ensemble forecasts, performed slightly worse for no-critical-level events, and slightly better for critical-level events, than National Weather Service high-wind warnings aggregated over all types of high-wind events throughout the United States and annually averaged for each year between 2008 and 2019.
Abstract
Strong downslope windstorms can cause extensive property damage and extreme wildfire spread, so their accurate prediction is important. Although some early studies suggested high predictability for downslope windstorms, more recent analyses have found limited predictability for such winds. Nevertheless, there is a theoretical basis for expecting higher downslope wind predictability in cases with a mean-state critical level, and this is supported by one previous effort to forecast actual events. To more thoroughly investigate downslope windstorm predictability, we compare archived simulations from the NCAR ensemble, a 10-member mesoscale ensemble run at 3-km horizontal grid spacing over the entire contiguous United States, to observed events at 15 stations in the western United States susceptible to strong downslope winds. We assess predictability in three contexts: the average ensemble spread, which provides an estimate of potential predictability; a forecast evaluation based upon binary-decision criteria, which is representative of operational hazard warnings; and a probabilistic forecast evaluation using the continuous ranked probability score (CRPS), which is a measure of an ensemble’s ability to generate the proper probability distribution for the events under consideration. We do find better predictive skill for the mean-state critical-level regime in comparison to other downslope windstorm–generating mechanisms. Our downslope windstorm warning performance, calculated using binary-decision criteria from the bias-corrected ensemble forecasts, performed slightly worse for no-critical-level events, and slightly better for critical-level events, than National Weather Service high-wind warnings aggregated over all types of high-wind events throughout the United States and annually averaged for each year between 2008 and 2019.
Abstract
The behavior of nonstationary trapped lee waves in a nonsteady background flow is studied using idealized three-dimensional (3D) numerical simulations. Trapped waves are forced by the passage of an isolated, synoptic-scale barotropic jet over a mountain ridge of finite length. Trapped waves generated within this environment differ significantly in their behavior compared with waves in the more commonly studied two-dimensional (2D) steady flow. After the peak zonal flow has crossed the terrain, two disparate regions form within the mature wave train: 1) upwind of the jet maximum, trapped waves increase their wavelength and tend to untrap and decay, whereas 2) downwind of the jet maximum, wavelengths shorten and waves remain trapped. Waves start to untrap approximately 100 km downwind of the ridge top, and the region of untrapping expands downwind with time as the jet progresses, while waves downstream of the jet maximum persist. Wentzel–Kramers–Brillouin (WKB) ray tracing shows that spatial gradients in the mean flow are the key factor responsible for these behaviors. An example of real-world waves evolving similarly to the modeled waves is presented.
As expected, trapped waves forced by steady 2D and horizontally uniform unsteady 3D flows decay downstream because of leakage of wave energy into the stratosphere. Surprisingly, the downstream decay of lee waves is inhibited by the presence of a stratosphere in the isolated-jet simulations. Also unexpected is that the initial trapped wavelength increases quasi-linearly throughout the event, despite the large-scale forcing at the ridge crest being symmetric in time about the midpoint of the isolated-jet simulation.
Abstract
The behavior of nonstationary trapped lee waves in a nonsteady background flow is studied using idealized three-dimensional (3D) numerical simulations. Trapped waves are forced by the passage of an isolated, synoptic-scale barotropic jet over a mountain ridge of finite length. Trapped waves generated within this environment differ significantly in their behavior compared with waves in the more commonly studied two-dimensional (2D) steady flow. After the peak zonal flow has crossed the terrain, two disparate regions form within the mature wave train: 1) upwind of the jet maximum, trapped waves increase their wavelength and tend to untrap and decay, whereas 2) downwind of the jet maximum, wavelengths shorten and waves remain trapped. Waves start to untrap approximately 100 km downwind of the ridge top, and the region of untrapping expands downwind with time as the jet progresses, while waves downstream of the jet maximum persist. Wentzel–Kramers–Brillouin (WKB) ray tracing shows that spatial gradients in the mean flow are the key factor responsible for these behaviors. An example of real-world waves evolving similarly to the modeled waves is presented.
As expected, trapped waves forced by steady 2D and horizontally uniform unsteady 3D flows decay downstream because of leakage of wave energy into the stratosphere. Surprisingly, the downstream decay of lee waves is inhibited by the presence of a stratosphere in the isolated-jet simulations. Also unexpected is that the initial trapped wavelength increases quasi-linearly throughout the event, despite the large-scale forcing at the ridge crest being symmetric in time about the midpoint of the isolated-jet simulation.
Abstract
Observations show that on a mountainside the boundary between snow and rain, the snow line, is often located at an elevation hundreds of meters below its elevation in the free air upwind. The processes responsible for this mesoscale lowering of the snow line are examined in semi-idealized simulations with a mesoscale numerical model and in simpler theoretical models. Spatial variations in latent cooling from melting precipitation, in adiabatic cooling from vertical motion, and in the melting distance of frozen hydrometeors are all shown to make important contributions. The magnitude of the snow line drop, and the relative importance of the responsible processes, depends on properties of the incoming flow and terrain geometry. Results suggest that the depression of the snow line increases with increasing temperature, a relationship that, if present in nature, could act to buffer mountain hydroclimates against the impacts of climate warming. The simulated melting distance, and hence the snow line, depends substantially on the choice of microphysical parameterization, pointing to an important source of uncertainty in simulations of mountain snowfall.
Abstract
Observations show that on a mountainside the boundary between snow and rain, the snow line, is often located at an elevation hundreds of meters below its elevation in the free air upwind. The processes responsible for this mesoscale lowering of the snow line are examined in semi-idealized simulations with a mesoscale numerical model and in simpler theoretical models. Spatial variations in latent cooling from melting precipitation, in adiabatic cooling from vertical motion, and in the melting distance of frozen hydrometeors are all shown to make important contributions. The magnitude of the snow line drop, and the relative importance of the responsible processes, depends on properties of the incoming flow and terrain geometry. Results suggest that the depression of the snow line increases with increasing temperature, a relationship that, if present in nature, could act to buffer mountain hydroclimates against the impacts of climate warming. The simulated melting distance, and hence the snow line, depends substantially on the choice of microphysical parameterization, pointing to an important source of uncertainty in simulations of mountain snowfall.
Abstract
The impact of transient mountain waves on a large-scale flow is examined through idealized numerical simulations of the passage of a time-evolving synoptic-scale jet over an isolated 3D mountain. Both the global momentum budget and the spatial flow response are examined to illustrate the impact of transient mountain waves on the large-scale flow. Additionally, aspects of the spatial response are quantified by potential vorticity inversion.
Nearly linear cases exhibit a weak loss of domain-averaged absolute momentum despite the absence of wave breaking. This transient effect occurs because, over the time period of the large-scale flow, the momentum flux through the top boundary does not balance the surface pressure drag. Moreover, an adiabatic spatial redistribution of momentum is observed in these cases, which results in an increase (decrease) of zonally averaged zonal momentum south (north) of the mountain.
For highly nonlinear cases, the zonally averaged momentum field shows a region of flow deceleration downstream of the mountain, flanked by broader regions of weak flow acceleration. Cancellation between the accelerating and decelerating regions results in weak fluctuations in the volume-averaged zonal momentum, suggesting that the mountain-induced circulations are primarily redistributing momentum. Potential vorticity anomalies develop in a region of wave breaking near the mountain, and induce local regions of flow acceleration and deceleration that alter the large-scale flow.
A “perfect” conventional gravity wave–drag parameterization is implemented on a coarser domain not having a mountain, forced by the momentum flux distribution from the fully nonlinear simulation. This parameterization scheme produces a much weaker spatial response in the momentum field and it fails to produce enough flow deceleration near the 20 m s−1 jet. These results suggest that the potential vorticity sources attributable to the gravity wave–drag parameterization have a controlling effect on the longtime downstream influence of the mountain.
Abstract
The impact of transient mountain waves on a large-scale flow is examined through idealized numerical simulations of the passage of a time-evolving synoptic-scale jet over an isolated 3D mountain. Both the global momentum budget and the spatial flow response are examined to illustrate the impact of transient mountain waves on the large-scale flow. Additionally, aspects of the spatial response are quantified by potential vorticity inversion.
Nearly linear cases exhibit a weak loss of domain-averaged absolute momentum despite the absence of wave breaking. This transient effect occurs because, over the time period of the large-scale flow, the momentum flux through the top boundary does not balance the surface pressure drag. Moreover, an adiabatic spatial redistribution of momentum is observed in these cases, which results in an increase (decrease) of zonally averaged zonal momentum south (north) of the mountain.
For highly nonlinear cases, the zonally averaged momentum field shows a region of flow deceleration downstream of the mountain, flanked by broader regions of weak flow acceleration. Cancellation between the accelerating and decelerating regions results in weak fluctuations in the volume-averaged zonal momentum, suggesting that the mountain-induced circulations are primarily redistributing momentum. Potential vorticity anomalies develop in a region of wave breaking near the mountain, and induce local regions of flow acceleration and deceleration that alter the large-scale flow.
A “perfect” conventional gravity wave–drag parameterization is implemented on a coarser domain not having a mountain, forced by the momentum flux distribution from the fully nonlinear simulation. This parameterization scheme produces a much weaker spatial response in the momentum field and it fails to produce enough flow deceleration near the 20 m s−1 jet. These results suggest that the potential vorticity sources attributable to the gravity wave–drag parameterization have a controlling effect on the longtime downstream influence of the mountain.
