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James D. Doyle
,
Qingfang Jiang
,
Ronald B. Smith
, and
Vanda Grubišić

Abstract

Measurements from the National Science Foundation/National Center for Atmospheric Research (NSF/NCAR) Gulfstream V (G-V) obtained during the recent Terrain-Induced Rotor Experiment (T-REX) indicate marked differences in the character of the wave response between repeated flight tracks across the Sierra Nevada, which were separated by a distance of approximately 50 km. Observations from several of the G-V research flights indicate that the vertical velocities in the primary wave exhibited variations up to a factor of 2 between the southern and northern portions of the racetrack flight segments in the lower stratosphere, with the largest amplitude waves most often occurring over the southern flight leg, which has a terrain maximum that is 800 m lower than the northern leg. Multiple racetracks at 11.7- and 13.1-km altitudes indicate that these differences were repeatable, which is suggestive that the deviations were likely due to vertically propagating mountain waves that varied systematically in amplitude rather than associated with transients. The cross-mountain horizontal velocity perturbations are also a maximum above the southern portion of the Sierra Nevada ridge.

Real data and idealized nonhydrostatic numerical model simulations are used to test the hypothesis that the observed variability in the wave amplitude and characteristics in the along-barrier direction is a consequence of blocking by the three-dimensional Sierra Nevada and the Coriolis effect. The numerical simulation results suggest that wave launching is sensitive to the overall three-dimensional characteristics of the Sierra Nevada barrier, which has an important impact on the wave amplitude and characteristics in the lower stratosphere. Real-time high-resolution Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) forecasts successfully capture the along-barrier variations in the wave amplitude (using vertical velocity as a proxy) as well as skillfully distinguishing between large- and small-amplitude stratospheric wave events during T-REX.

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Qingfang Jiang
,
Ronald B. Smith
, and
James D. Doyle

Abstract

Interaction between mountain waves and the atmospheric boundary layer (BL) has been investigated using a mesoscale model with BL parameterizations and analytical BL models. The impact of the BL diagnosed from the mesoscale model simulations is in qualitative agreement with the analytical BL models. In general, for stratified flow over a mesoscale ridge, the influence of the BL tends to shift wave patterns upstream, decrease the wave drag by up to 60% and the momentum flux above the BL top by up to 80%, and significantly delay the onset of wave breaking.

It has been demonstrated that the BL effect is governed by four nondimensional parameters, namely, the nondimensional BL depth, BL shape factor, nonhydrostatic parameter, and frictional adjustment parameter. The BL effect is stronger over a rougher surface, which creates a deeper BL with a smaller shape factor (i.e., slower BL flow). The scale dependence of the BL effect is governed by the nonhydrostatic and frictional adjustment parameters. For narrow terrain, the BL reduces mountain drag through rapid inertial adjustment and behaves like an inviscid shear layer, and the momentum reduction across the BL is negligible. The attenuation of the momentum flux across the BL is more substantial over a wider ridge because of the increasing importance of frictional adjustment. The BL effect decreases with increasing ridge height and for 3D terrain.

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Ronald B. Smith
,
Qingfang Jiang
, and
James D. Doyle

Abstract

A one-layer model of the atmospheric boundary layer (BL) is proposed to explain the nature of lee-wave attenuation and gravity wave absorption seen in numerical simulations. Two complex coefficients are defined: the compliance coefficient and the wave reflection coefficient. A real-valued ratio of reflected to incident wave energy is also useful. The key result is that, due to horizontal friction, the wind response in the BL is shifted upstream compared to the phase of disturbances in the free atmosphere. The associated flow divergence modulates the thickness of the BL so that it partially absorbs incident gravity waves. A simple expression is derived relating the reflection coefficient to the attenuation and wavelength shift of trapped lee waves. Results agree qualitatively with the numerical simulations, including the effects of increased surface roughness and heat flux.

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Qingfang Jiang
,
James D. Doyle
, and
Ronald B. Smith

Abstract

The absorption of trapped lee waves by the atmospheric boundary layer (BL) is investigated based on numerical simulations and theoretical formulations. It is demonstrated that the amplitude of trapped waves decays exponentially with downstream distance due to BL absorption. The decay coefficient, α, defined as the inverse of the e-folding decay distance, is found to be sensitive to both surface momentum and heat fluxes. Specifically, α is larger over a rougher surface, associated with a more turbulent BL. On the other hand, the value of α decreases with increasing surface heating and increases with increasing surface cooling, implying that a stable nocturnal BL is more efficient in absorbing trapped waves than a typically deeper and more turbulent convective BL. A stagnant layer could effectively absorb trapped waves and increase α. Over the range of parameters examined, the absorption coefficient shows little sensitivity to wave amplitude. A relationship is derived to relate the surface reflection factor and the wave decay coefficient. Corresponding to wave absorption, there are positive momentum and negative energy fluxes across the boundary layer top, indicating that an absorbing BL serves as a momentum source and energy sink to trapped waves. Wave reflection by a shallow viscous layer with a linear shear is examined using linear theory, and its implication on BL wave absorption is discussed.

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Qingfang Jiang
,
James D. Doyle
,
Shouping Wang
, and
Ronald B. Smith

Abstract

The onset of boundary layer separation (BLS) forced by gravity waves in the lee of mesoscale topography is investigated based on a series of numerical simulations and analytical formulations. It is demonstrated that BLS forced by trapped waves is governed by a normalized ratio of the vertical velocity maximum to the surface wind speed; other factors such as the mountain height, mountain slope, or the leeside speedup factor are less relevant. The onset of BLS is sensitive to the surface sensible heat flux—a positive heat flux tends to increase the surface wind speed through enhancing the vertical momentum mixing and accordingly inhibits the occurrence of BLS, and a negative heat flux does the opposite. The wave forcing required to cause BLS decreases with an increase of the aerodynamical roughness z o; a larger z o generates larger surface stress and weaker surface winds and therefore promotes BLS. In addition, BLS shows some sensitivity to the terrain geometry, which modulates the wave characteristics. For a wider ridge, a higher mountain is required to generate trapped waves with a wave amplitude comparable to that generated by a lower but narrower ridge. The stronger hydrostatic waves associated with the wider and higher ridge play only a minor role in the onset of BLS.

It has been demonstrated that although hydrostatic waves generally do not directly induce BLS, undular bores may form associated with wave breaking in the lower troposphere, which in turn induce BLS. In addition, BLS could occur underneath undular jump heads or associate with trapped waves downstream of a jump head in the presence of a low-level inversion.

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Qingfang Jiang
,
James D. Doyle
,
Vanda Grubišić
, and
Ronald B. Smith

Abstract

Characteristics of turbulence in the lower and middle troposphere over Owens Valley have been examined using aircraft in situ measurements obtained from the Terrain-Induced Rotor Experiment. The two events analyzed in this study are characterized by a deep turbulent layer from the valley floor up to the midtroposphere associated with the interaction between trapped waves and an elevated shear layer. Kelvin–Helmholtz (KH) instability develops above the mountaintop level and often along the wave crests where the Richardson number is reduced. The turbulence induced by KH instability is characterized by a progressive downscale energy cascade, a well-defined inertial subrange up to 1 km, and large eddies with vertical to horizontal aspect ratios less than unity. The turbulence below the mountaintop level is largely shear induced, associated with wave steepening and breaking, and is more isotropic. Evaluation of structure functions indicates that while the turbulence energy cascade is predominately downscale, upscale energy transfer exists with horizontal scales from a few hundred meters to a few kilometers because of the transient energy dispersion of large eddies generated by KH instability and gravity wave steepening or breaking.

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Qingfang Jiang
,
Peter Sullivan
,
Shouping Wang
,
James Doyle
, and
Linwood Vincent

Abstract

The impact of fast-propagating swell on the air–sea momentum exchange and the marine boundary layer is examined based on multiple large-eddy simulations over a range of wind speed and swell parameters in the light-wind–fast-wave regime. A wave-driven supergeostrophic jet forms near the top of the wave boundary layer when the forwarding-pointing (i.e., negative) form drag associated with fast wind-following swell overpowers the positive surface shear stress. The magnitude of the form drag increases with the wavelength and slope and decreases with increasing wind speed, and the jet intensity in general increases with the magnitude of the surface form drag. The resulting negative vertical wind shear above the jet in turn enhances the turbulence aloft. The level of the wind maximum is found to be largely determined by the wavenumber and the ratio of the surface shear stress and form drag: the larger the magnitude of this ratio, the higher the altitude of the wind maximum.

Although the simulated wind profile often closely follows the log law in the wave boundary layer, the surface stress derived from the logarithmic wind profile is significantly larger than the actual total surface stress in the presence of swell. Therefore, the Monin–Obukhov similarity theory is generally invalid over swell-dominated ocean. This is attributed to the wave-induced contribution to momentum flux, which decays roughly exponentially in the vertical and is largely independent of local wind shear.

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Eric A. Hendricks
,
James D. Doyle
,
Stephen D. Eckermann
,
Qingfang Jiang
, and
P. Alex Reinecke

Abstract

During austral winter, and away from orographic maxima or “hot spots,” stratospheric gravity waves in both satellite observations and Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data reveal enhanced amplitudes in a broad midlatitude belt extending across the Southern Ocean from east of the Andes to south of New Zealand. The peak latitude of this feature slowly migrates poleward from 50° to 60°S. Wave amplitudes are much weaker across the midlatitude Pacific Ocean. These features of the wave field are in striking agreement with diagnostics of baroclinic growth rates in the troposphere associated with midlatitude winter storm tracks and the climatology of the midlatitude jet. This correlation suggests that these features of the stratospheric gravity wave field are controlled by geographical variations of tropospheric nonorographic gravity wave sources in winter storm tracks: spontaneous adjustment emission from the midlatitude winter jet, frontogenesis, and convection.

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Qingfang Jiang
,
James D. Doyle
,
Alex Reinecke
,
Ronald B. Smith
, and
Stephen D. Eckermann

Abstract

Large-amplitude stratospheric gravity waves over the southern Andes and Drake Passage, as observed by the Atmospheric Infrared Sounder (AIRS) on 8–9 August 2010, are modeled and studied using a deep (0–70 km) version of the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model. The simulated tropospheric waves are generated by flow over the high central Andes ridge and the Patagonian peaks in the southern Andes. Some waves emanating from Patagonia propagate southeastward across Drake Passage into the stratosphere over a horizontal distance of more than 1000 km. The wave momentum flux is characterized by a tropospheric maximum over Patagonia that splits into two comparable maxima in the stratosphere: one located directly over the terrain and the other tilting southward with altitude.

Using spatial ray-tracing techniques and flow conditions derived from the numerical simulation, the authors find that waves that originate from the high ridge in the Central Andes are absorbed by a critical level in the lower stratosphere. The three-dimensional waves originating from Patagonia could be separated into three families—namely, a northeast-propagating family, which is absorbed by a critical level between 15 and 20 km; a localized family, which breaks down in the stratosphere and lower mesosphere directly above Patagonia; and a southeast-propagating family, which forms the observed linear stratospheric wave patterns oriented across Drake Passage. The southward group propagation, assisted by lateral wave refraction due to persistent meridional shear of the zonal winds, leads to stratospheric wave breaking and drag near 60°S, well south of the parent orography.

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James D. Doyle
,
Melvyn A. Shapiro
,
Qingfang Jiang
, and
Diana L. Bartels

Abstract

A large-amplitude mountain wave generated by strong southwesterly flow over southern Greenland was observed during the Fronts and Atlantic Storm-Track Experiment (FASTEX) on 29 January 1997 by the NOAA G-IV research aircraft. Dropwindsondes deployed every 50 km and flight level data depict a vertically propagating large-amplitude wave with deep convectively unstable layers, potential temperature perturbations of 25 K that deformed the tropopause and lower stratosphere, and a vertical velocity maximum of nearly 10 m s−1 in the stratosphere. The wave breaking was associated with a large vertical flux of horizontal momentum and dominated by quasi-isotropic turbulence. The Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) nonhydrostatic model with four-nested grid meshes with a minimum resolution of 1.7 km accurately simulates the amplitude, location, and timing of the mountain wave and turbulent breakdown. Finescale low-velocity plumes that resemble wakelike structures emanate from highly dissipative turbulent regions of wave breaking in the lower stratosphere. Idealized adiabatic three-dimensional simulations suggest that steep terrain slopes increase the effective Rossby number of the relatively wide Greenland plateau, decrease the sensitivity of the wave characteristics to rotation, and ultimately increase the tendency for wave breaking. Linear theory and idealized simulations indicate that diabatic cooling within the boundary layer above the Greenland ice sheet augments the effective mountain height and increases the wave amplitude and potential for wave breaking for relatively wide obstacles such as Greenland.

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