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Ivana Stiperski
and
Vanda Grubišić
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Vanda Grubišić
and
Ivana Stiperski

Abstract

Lee-wave resonance over double bell-shaped obstacles is investigated through a series of idealized high-resolution numerical simulations with the nonhydrostatic Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) model using a free-slip lower boundary condition. The profiles of wind speed and stability as well as terrain derive from observations of lee-wave events over the Sierra Nevada and Inyo Mountains from the recently completed Terrain-Induced Rotor Experiment (T-REX).

Numerical experiments show that double bell-shaped obstacles promote trapped lee waves that are in general shorter than those excited by an isolated ridge. While the permissible trapped lee-wave modes are determined by the upstream atmospheric structure, primarily vertical wind shear, the selected lee-wave wavelengths for two obstacles that are close or equal in height are dictated by the discrete terrain spectrum and correspond to higher harmonics of the primary orographic wavelength, which is equal to the ridge separation distance. The exception is the smallest ridge separation distance examined, one that corresponds to the Owens Valley width and is closest to the wavelength determined by the given upstream atmospheric structure, for which the primary lee-wave and orographic wavelengths were found to nearly coincide.

The influence two mountains exert on the overall lee-wave field is found to persist at very large ridge separation distances. For the nonlinear nonhydrostatic waves examined, the ridge separation distance is found to exert a much stronger control over the lee-wave wavelengths than the mountain half-width. Positive and negative interferences of lee waves, which can be detected through their imprint on wave drag and wave amplitudes, were found to produce appreciable differences in the flow structure mainly over the downstream peak, with negative interference characterized by a highly symmetric flow pattern leading to a low drag state.

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Ivana Stiperski
and
Vanda Grubišić

Abstract

Trapped lee wave interference over double bell-shaped obstacles in the presence of surface friction is examined. Idealized high-resolution numerical experiments with the nonhydrostatic Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) are performed to examine the influence of a frictional boundary layer and nonlinearity on wave interference and the impact of interference on wave-induced boundary layer separation and the formation of rotors.

The appearance of constructive and destructive interference, controlled by the ratio of the ridge separation distance to the intrinsic horizontal wavelength of lee waves, is found to be predicted well by linear interference theory with orographic adjustment. The friction-induced shortening of intrinsic wavelength displays a strong indirect effect on wave interference. For twin peak orography, the interference-induced variation of wave amplitude is smaller than that predicted by linear theory. The interference is found to affect the formation and strength of rotors most significantly in the lee of the downstream peak; destructive interference suppresses the formation and strength of rotors there, whereas results for constructive interference closely parallel those for a single mountain. Over the valley, under both constructive and destructive interference, rotors are weaker compared to those in the lee of a single ridge while their strength saturates in the finite-amplitude flow regime.

Destructive interference is found to be more susceptible to nonlinear effects, with both the orographic adjustment and surface friction displaying a stronger effect on the flow in this state. “Complete” destructive interference, in which waves almost completely cancel out in the lee of the downstream ridge, develops for certain ridge separation distances but only for a downstream ridge smaller than the upstream one.

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Ronald B. Smith
and
Vanda Grubišić

Abstract

Under the influence of the east-northeasterly trade winds, the island of Hawaii generates a wake that extends about 200 km to the west-southwest. During the HaRP project in July and August 1990, five wake surveys were carried out by the NCAR Electra. The patterns of wind and aerosol concentration revealed by these flights suggest that Hawaii's wake consists of two large quasi-steady counterrotating eddies. The southern clockwise-rotating eddy carries a heavy aerosol load due to input from the Kī volcano. At the eastern end of the wake, the eddies are potentially warmer and more humid than the surrounding trade wind air. Several other features are discussed: sharp shear lines near the northern and southern tips of the island, dry and warm air bands along the shear lines, a small embedded wake behind the Kohala peninsula, wake centerline clouds, hydraulic jumps to the north and south of the island, a descending inversion connected with accelerating trade winds, and evidence for side-to-side wake movement.

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Vanda Grubišić
and
Mitchell W. Moncrieff

Abstract

The organization of convection by vertical shear strongly affects the transport of horizontal momentum, yet the concept of organization has received little attention in convective parameterization. Here the focus is on open-cellular convection in cold-air outbreaks under strongly baroclinic conditions that occur to the rear of midlatitude cyclones. The principles derived herein are envisaged to apply to baroclinic advection at large in similar shear flows.

Open-cellular convection, forced by surface fluxes of sensible and latent heat and organized by unidirectional shear, is simulated using a three-dimensional cloud-resolving numerical model. The effects of the in-cloud pressure field, organization, and shear on momentum transport are quantified for the simulated clouds. A simple analytic model of blocked overturning provides dynamical insight into downgradient momentum transport by the simulated three-dimensional cumulonimbus ensemble. The overturning circulation having vorticity of the same sign as the ambient shear maintains the far-field flow, while the blocking effect decelerates in-cloud momentum. The combined effect is to maintain the mean flow somewhat below its undisturbed value.

The numerical results and analytic predictions are used to evaluate and interpret two parameterization schemes for convective momentum transport used in operational global weather prediction and climate models at the U.K. Meteorological Office and the European Centre for Medium-Range Weather Forecasts. Finally, the authors comment on how the downgradient momentum transport by three-dimensional cumulus convection contrasts with momentum-mixing concepts as well as countergradient momentum transport by two-dimensional squall lines.

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Vanda Grubišić
and
Piotr K. Smolarkiewicz

Abstract

The effect of a critical level on airflow past an isolated axially symmetric obstacle is investigated in the small-amplitude hydrostatic limit for mean flows with linear negative shear. Only flows with mean Richardson numbers (Ri) greater or equal to ¼ are considered. The authors examine the problem using the linear, steady-state, inviscid, dynamic equations, which are well known to exhibit a singular behavior at critical levels, as well as a numerical model that has the capability of capturing both nonlinear and dissipative effects where these are significant.

Linear theory predicts the 3D wave pattern with individual waves that are confined to paraboloidal envelopes below the critical level and strongly attenuated and directionally filtered above it. Asymptotic solutions for the wave field far from the mountain and below the critical level show large shear-induced modifications in the proximity of the critical level, where wave envelopes quickly widen with height. Above the critical level, the perturbation field consists mainly of waves with wavefronts perpendicular to the mean flow direction. A closed-form analytic formula for the mountain-wave drag, which is equally valid for mean flows with positive and negative shear, predicts a drag that is smaller than in the uniform wind case. In the limit of Ri d⃗ ¼, in which linear theory predicts zero drag for an infinite ridge, drag on an axisymmetric mountain is nonzero.

Numerical simulations with an anelastic, nonhydrostatic model confirm and qualify the analytic results. They indicate that the linear regime, in which analytic solutions are valid everywhere except in the vicinity of the critical level, exists for a range of mountain heights given Ri > 1. For Ri d⃗ ¼ this same regime is difficult to achieve, as the flow is extremely sensitive to nonlinearities introduced through the lower boundary forcing that induce strong nonlinear effects near the critical level. Even well within the linear regime, flow in the vicinity of a critical level is dissipative in nature as evidenced by the development of a potential vorticity doublet.

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Vanda Grubišić
and
Brian J. Billings

Abstract

A large-amplitude lee-wave rotor event observationally documented during Sierra Rotors Project Intensive Observing Period (IOP) 8 on 24–26 March 2004 in the lee of the southern Sierra Nevada is examined. Mountain waves and rotors occurred over Owens Valley in a pre-cold-frontal environment. In this study, the evolution and structure of the observed and numerically simulated mountain waves and rotors during the event on 25 March, in which the horizontal circulation associated with the rotor was observed as an opposing, easterly flow by the mesonetwork of surface stations in Owens Valley, are analyzed.

The high-resolution numerical simulations of this case, performed with the Coupled Ocean–Atmosphere Mesoscale Prediction System (COAMPS) run with multiple nested-grid domains, the finest grid having 333-m horizontal spacing, reproduced many of the observed features of this event. These include small-amplitude waves above the Sierra ridge decoupled from thermally forced flow within the valley, and a large-amplitude mountain wave, turbulent rotor, and strong westerlies on the Sierra Nevada lee slopes during the period of the observed surface easterly flow. The sequence of the observed and simulated events shows a pronounced diurnal variation with the maximum wave and rotor activity occurring in the early evening hours during both days of IOP 8.

The lee-wave response, and thus indirectly the appearance of lee-wave rotor during the core IOP 8 period, is found to be strongly controlled by temporal changes in the upstream ambient wind and stability profiles. The downstream mountain range exerts strong control over the lee-wave horizontal wavelength during the strongest part of this event, thus exhibiting the control over the cross-valley position of the rotor and the degree of strong downslope wind penetration into the valley.

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Lukas Strauss
,
Stefano Serafin
, and
Vanda Grubišić

Abstract

The conceptual model of an atmospheric rotor is reexamined in the context of a valley, using data from the Terrain-Induced Rotor Experiment (T-REX) conducted in 2006 in the southern Sierra Nevada and Owens Valley, California. All T-REX cases with strong mountain-wave activity have been investigated, and four of them (IOPs 1, 4, 6, and 13) are presented in detail. Their analysis reveals a rich variety of rotorlike turbulent flow structures that may form in the valley during periods of strong cross-mountain winds. Typical flow scenarios in the valley include elevated turbulence zones, downslope flow separation at a valley inversion, turbulent interaction of in-valley westerlies and along-valley flows, and highly transient mountain waves and rotors. The scenarios can be related to different stages of the passage of midlatitude frontal systems across the region. The observations from Owens Valley show that the elements of the classic rotor concept are modulated and, at times, almost completely offset by dynamically and thermally driven processes in the valley. Strong lee-side pressure perturbations induced by large-amplitude waves, commonly regarded as the prerequisite for flow separation, are found to be only one of the factors controlling rotor formation and severe turbulence generation in the valley. Buoyancy perturbations in the thermally layered valley atmosphere appear to play a role in many of the observed cases. Based on observational evidence from T-REX, extensions to the classic rotor concept, appropriate for a long deep valley, are proposed.

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Vanda Grubišić
,
Ronald B. Smith
, and
Christoph Schär

Abstract

The effect of bottom friction on fluid flow past an isolated obstacle is investigated in the shallow-water framework. The controlling parameter for this effect is the nondimensional bottom friction number, defined as a ratio of friction to inertia. With the bottom stress related to the horizontal wind via standard bulk aerodynamic formula, the friction number is proportional to the surface roughness, the horizontal scale of the obstacle, and the inverse of the upstream fluid depth. Thus, under otherwise identical conditions, the flow past larger obstacles will be more “viscous.” Bottom friction modifies the vorticity generation in several ways, but under normal conditions, the wake formation remains dominated by a pseudoinviscid process related to the presence of hydraulic jumps. However, friction strongly controls the velocity-deficit region of the wake and thus influences the stability of the steady-state wakes. Predictions of the linear stability analysis are compared with numerical simulations of the eddy-shedding development under the stabilizing effect of friction. For the values of friction parameter for which linear theory predicts that the flow should be absolutely stable, fully nonlinear numerical evolutions indeed reach a stable quasi-stationary state. For a realistic value of bottom friction, the simulation of the flow past the island of Hawaii produces a wake that is consistent with the recent observations.

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Vanda Grubišić
,
Johannes Sachsperger
, and
Rui M. A. Caldeira

Abstract

The island of Madeira is well known for giving rise to atmospheric wakes. Strong and unsteady atmospheric wakes, resembling a von Kármán vortex street, are frequently observed in satellite images leeward of Madeira, especially during summer months, when conditions favoring the formation of atmospheric wakes occur frequently under the influence of the Azores high.

Reported here is the analysis of the first airborne measurements of Madeira’s wake collected during the 2010 Island-induced Wake (I-WAKE) campaign. High-resolution in situ and remote sensing data were collected in the I-WAKE by a research aircraft. The measurements reveal distinctive wake signatures, including strong lateral wind shear zones and warm and dry eddies downwind of the island. A strong anticorrelation of the horizontal wind speed and sea surface temperature (SST) was found within the wake.

High-resolution numerical simulations with the Weather Research and Forecasting (WRF) Model were used to study the dynamics of the wake generation and its temporal evolution. The comparison of the model results and observations reveals a remarkable fidelity of the simulated wake features within the marine boundary layer (MBL). Strong potential vorticity (PV) anomalies were found in the simulated MBL wake, emanating from the flanks of the island. The response of the wake formation within the MBL to surface friction and enhanced thermal forcing is explored through the model sensitivity analyses.

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