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6598 KiB

Open AccessFeature PaperArticle

The Generation and Propagation of Atmospheric Internal Waves Caused by Volcanic Eruptions

by Peter G. Baines and Selwyn Sacks

Atmosphere 2017, 8(3), 60; https://doi.org/10.3390/atmos8030060 - 21 Mar 2017

Cited by 4 | Viewed by 4894

Observations from the island of Montserrat in the Caribbean have shown that volcanic eruptions (particularly explosive ones) can generate internal waves in the atmosphere that can be observed by microbarographs at ground level. It is possible that observations of such waves may give [...] Read more.

Observations from the island of Montserrat in the Caribbean have shown that volcanic eruptions (particularly explosive ones) can generate internal waves in the atmosphere that can be observed by microbarographs at ground level. It is possible that observations of such waves may give early information about volcanic eruptions when other methods are unavailable (because of bad weather, nocturnal eruptions, and poor visibility or remoteness), if it is possible to interpret them. This paper describes a dynamical model of the forcing of internal waves in which the eruption is modelled as a turbulent plume, forced by a source of buoyancy at ground level that specifies the total height and relevant properties of the eruption. Specifically, the rising plume entrains environmental air from ground level to 70% of its maximum height z_M, and above 0.7z_M the rising fluid spreads radially. During the eruption, this flow forces horizontal motion in the surrounding fluid that generates internal waves, which may be computed by assuming that this is due to a linear dynamical process. Properties of the resulting waves are described for a variety of parameters that include the strength and height of the eruption, the effect of the tropopause, generation in the stratosphere for large eruptions, and the differing effects of the duration of the eruption. Implications for characterising eruptions from observations of these properties are discussed. Full article

(This article belongs to the Special Issue Atmospheric Gravity Waves)

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7467 KiB

Open AccessArticle

On the Interpretation of Gravity Wave Measurements by Ground-Based Lidars

by Andreas Dörnbrack, Sonja Gisinger and Bernd Kaifler

Atmosphere 2017, 8(3), 49; https://doi.org/10.3390/atmos8030049 - 01 Mar 2017

Cited by 21 | Viewed by 5719

Abstract

This paper asks the simple question: How can we interpret vertical time series of middle atmosphere gravity wave measurements by ground-based temperature lidars? Linear wave theory is used to show that the association of identified phase lines with quasi-monochromatic waves should be considered [...] Read more.

This paper asks the simple question: How can we interpret vertical time series of middle atmosphere gravity wave measurements by ground-based temperature lidars? Linear wave theory is used to show that the association of identified phase lines with quasi-monochromatic waves should be considered with great care. The ambient mean wind has a substantial effect on the inclination of the detected phase lines. The lack of knowledge about the wind might lead to a misinterpretation of the vertical propagation direction of the observed gravity waves. In particular, numerical simulations of three archetypal atmospheric mountain wave regimes show a sensitivity of virtual lidar observations on the position relative to the mountain and on the scale of the mountain. Full article

(This article belongs to the Special Issue Atmospheric Gravity Waves)

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13398 KiB

Open AccessFeature PaperArticle

Profiling Radar Observations and Numerical Simulations of a Downslope Wind Storm and Rotor on the Lee of the Medicine Bow Mountains in Wyoming

by Binod Pokharel, Bart Geerts, Xia Chu and Philip Bergmaier

Atmosphere 2017, 8(2), 39; https://doi.org/10.3390/atmos8020039 - 15 Feb 2017

Cited by 9 | Viewed by 5267

Abstract

This study describes a downslope wind storm event observed over the Medicine Bow range (Wyoming, USA) on 11 January 2013. The University of Wyoming King Air (UWKA) made four along-wind passes over a five-hour period over the mountain of interest. These passes were [...] Read more.

This study describes a downslope wind storm event observed over the Medicine Bow range (Wyoming, USA) on 11 January 2013. The University of Wyoming King Air (UWKA) made four along-wind passes over a five-hour period over the mountain of interest. These passes were recognized as among the most turbulent ones encountered in many years by crew members. The MacCready turbulence meter aboard the UWKA measured moderate to severe turbulence conditions on each pass in the lee of the mountain range, with eddy dissipation rate values over 0.5 m^2/3 s⁻¹. Three rawinsondes were released from an upstream location at different times. This event is simulated using the non-hydrostatic Weather Research and Forecast (WRF) model at an inner- domain resolution of 1 km. The model produces a downslope wind storm, notwithstanding some discrepancies between model and rawinsonde data in terms of upstream atmospheric conditions. Airborne Wyoming Cloud Radar (WCR) vertical-plane Doppler velocity data from two beams, one pointing to the nadir and one pointing slant forward, are synthesized to obtain a two-dimensional velocity field in the vertical plane below flight level. This synthesis reveals the fine-scale details of an orographic wave breaking event, including strong, persistent downslope acceleration, a strong leeside updraft (up to 10 m·s⁻¹) flanked by counter-rotating vortices, and deep turbulence, extending well above flight level. The analysis of WCR-derived cross-mountain flow in 19 winter storms over the same mountain reveals that cross-mountain flow acceleration and downslope wind formation are difficult to predict from upstream wind and stability profiles. Full article

(This article belongs to the Special Issue Atmospheric Gravity Waves)

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11835 KiB

Open AccessArticle

Role of Wind Filtering and Unbalanced Flow Generation in Middle Atmosphere Gravity Wave Activity at Chatanika Alaska

by Colin C. Triplett, Richard L. Collins, Kim Nielsen, V. Lynn Harvey and Kohei Mizutani

Atmosphere 2017, 8(2), 27; https://doi.org/10.3390/atmos8020027 - 26 Jan 2017

Cited by 9 | Viewed by 5071

Abstract

The meteorological control of gravity wave activity through ﬁltering by winds and generation by spontaneous adjustment of unbalanced ﬂows is investigated. This investigation is based on a new analysis of Rayleigh LiDAR measurements of gravity wave activity in the upper stratosphere-lower mesosphere (USLM,40–50km)on [...] Read more.

The meteorological control of gravity wave activity through ﬁltering by winds and generation by spontaneous adjustment of unbalanced ﬂows is investigated. This investigation is based on a new analysis of Rayleigh LiDAR measurements of gravity wave activity in the upper stratosphere-lower mesosphere (USLM,40–50km)on 152 nights at Poker Flat Research Range (PFRR), Chatanika, Alaska (65◦ N, 147◦ W), over 13 years between 1998 and 2014. The LiDAR measurements resolve inertia-gravity waves with observed periods between 1 h and 4 h and vertical wavelengths between 2 km and 10 km. The meteorological conditions are deﬁned by reanalysis data from the Modern-Era Retrospective Analysis for Research and Applications (MERRA). The gravity wave activity shows large night-to-night variability, but a clear annual cycle with a maximum in winter,and systematic interannual variability associated with stratospheric sudden warming events. The USLM gravity wave activity is correlated with the MERRA winds and is controlled by the winds in the lower stratosphere through ﬁltering by critical layer ﬁltering. The USLM gravity wave activity is also correlated with MERRA unbalanced ﬂow as characterized by the residual of the nonlinear balance equation. This correlation with unbalanced ﬂow only appears when the wind conditions are taken into account, indicating that wind ﬁltering is the primary control of the gravity wave activity. Full article

(This article belongs to the Special Issue Atmospheric Gravity Waves)

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6833 KiB

Open AccessArticle

Mountain Waves in High Resolution Forecast Models: Automated Diagnostics of Wave Severity and Impact on Surface Winds

by Peter Sheridan, Simon Vosper and Philip Brown

Atmosphere 2017, 8(1), 24; https://doi.org/10.3390/atmos8010024 - 23 Jan 2017

Cited by 8 | Viewed by 6320

Abstract

An automated method producing a diagnostic of the severity of lee waves and their impacts on surface winds as represented in output from a high resolution linear numerical model (3D velocities over mountains (3DVOM)) covering several areas of the U.K. is discussed. Lee [...] Read more.

An automated method producing a diagnostic of the severity of lee waves and their impacts on surface winds as represented in output from a high resolution linear numerical model (3D velocities over mountains (3DVOM)) covering several areas of the U.K. is discussed. Lee waves involving turbulent rotor activity or downslope windstorms represent a hazard to aviation and ground transport, and summary information of this kind is highly valuable as an efficient ‘heads-up’ for forecasters, for automated products or to feed into impact models. Automated diagnosis of lee wave surface effects presents a particular challenge due to the complexity of turbulent zones in the lee of irregular terrain. The method proposed quantifies modelled wind perturbations relative to those that would occur in the absence of lee waves for a given background wind, and diagnoses using it are found to be quite consistent between cases and for different ranges of U.K. hills. A recent upgrade of the operational U.K. limited area model, the U.K. Variable Resolution Model (UKV) used for general forecasting at the Met Office means that it now resolves lee waves, and its performance is here demonstrated using comparisons with aircraft- and surface-based observations and the linear model. In the future, automated diagnostics may be adapted to use its output to routinely produce contiguous mesoscale maps of lee wave activity and surface impacts over the whole U.K. Full article

(This article belongs to the Special Issue Atmospheric Gravity Waves)

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7318 KiB

Open AccessArticle

Water Tank Experiments on Stratified Flow over Double Mountain-Shaped Obstacles at High-Reynolds Number

by Ivana Stiperski, Stefano Serafin, Alexandre Paci, Hálfdán Ágústsson, Anne Belleudy, Radiance Calmer, Kristian Horvath, Christoph Knigge, Johannes Sachsperger, Lukas Strauss and Vanda Grubišić

Atmosphere 2017, 8(1), 13; https://doi.org/10.3390/atmos8010013 - 13 Jan 2017

Cited by 8 | Viewed by 7736

Abstract

In this article, we present an overview of the HyIV-CNRS-SecORo (Hydralab IV-CNRS-Secondary Orography and Rotors Experiments) laboratory experiments carried out in the CNRM (Centre National de Recherches Météorologiques) large stratified water flume. The experiments were designed to systematically study the influence of double [...] Read more.

In this article, we present an overview of the HyIV-CNRS-SecORo (Hydralab IV-CNRS-Secondary Orography and Rotors Experiments) laboratory experiments carried out in the CNRM (Centre National de Recherches Météorologiques) large stratified water flume. The experiments were designed to systematically study the influence of double obstacles on stably stratified flow. The experimental set-up consists of a two-layer flow in the water tank, with a lower neutral and an upper stable layer separated by a sharp density discontinuity. This type of layering over terrain is known to be conducive to a variety of possible responses in the atmosphere, from hydraulic jumps to lee waves and highly turbulent rotors. In each experiment, obstacles were towed through the tank at a constant speed. The towing speed and the size of the tank allowed high Reynolds-number flow similar to the atmosphere. Here, we present the experimental design, together with an overview of laboratory experiments conducted and their results. We develop a regime diagram for flow over single and double obstacles and examine the parameter space where the secondary obstacle has the largest influence on the flow. Trapped lee waves, rotors, hydraulic jumps, lee-wave interference and flushing of the valley atmosphere are successfully reproduced in the stratified water tank. Obstacle height and ridge separation distance are shown to control lee-wave interference. Results, however, differ partially from previous findings on the flow over double ridges reported in the literature due to the presence of nonlinearities and possible differences in the boundary layer structure. The secondary obstacle also influences the transition between different flow regimes and makes trapped lee waves possible for higher Froude numbers than expected for an isolated obstacle. Full article

(This article belongs to the Special Issue Atmospheric Gravity Waves)

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1722 KiB

Open AccessArticle

Diagnosing Lee Wave Rotor Onset Using a Linear Model Including a Boundary Layer

by Miguel A. C. Teixeira

Atmosphere 2017, 8(1), 5; https://doi.org/10.3390/atmos8010005 - 07 Jan 2017

Cited by 6 | Viewed by 4487 | Correction

Abstract

A linear model is used to diagnose the onset of rotors in flow over 2D hills, for atmospheres that are neutrally stratified near the surface and stably stratified aloft, with a sharp temperature inversion in between, where trapped lee waves may propagate. This [...] Read more.

A linear model is used to diagnose the onset of rotors in flow over 2D hills, for atmospheres that are neutrally stratified near the surface and stably stratified aloft, with a sharp temperature inversion in between, where trapped lee waves may propagate. This is achieved by coupling an inviscid two-layer mountain-wave model and a bulk boundary-layer model. The full model shows some ability to diagnose flow stagnation associated with rotors as a function of key input parameters, such as the Froude number and the height of the inversion, in numerical simulations and laboratory experiments carried out by previous authors. While calculations including only the effects of mean flow attenuation and velocity perturbation amplification within the surface layer represent flow stagnation fairly well in the more non-hydrostatic cases, only the full model, taking into account the feedback of the surface layer on the inviscid flow, satisfactorily predicts flow stagnation in the most hydrostatic case, although the corresponding condition is unable to discriminate between rotors and hydraulic jumps. Versions of the model not including this feedback severely underestimate the amplitude of trapped lee waves in that case, where the Fourier transform of the hill has zeros, showing that those waves are not forced directly by the orography. Full article

(This article belongs to the Special Issue Atmospheric Gravity Waves)

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3 pages, 825 KiB

Open AccessCorrection

Correction: Teixeira, M.A.C. Diagnosing Lee Wave Rotor Onset Using a Linear Model Including a Boundary Layer. Atmosphere 2017, 8, 5

by Miguel A. C. Teixeira

Atmosphere 2018, 9(12), 491; https://doi.org/10.3390/atmos9120491 - 11 Dec 2018

Viewed by 2241

Abstract

The author would like to correct a published article by Teixeira [1], in which there is a factor of 2 missing from his Equation (24).[...] Full article

(This article belongs to the Special Issue Atmospheric Gravity Waves)

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