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Wenshou Tian, Douglas J. Parker, Stephen Mobbs, Martin Hill, Charles A. D. Kilburn, and Darcy Ladd

Abstract

In this paper, high-frequency pressure time series measured by microbarographs are used to extract information on the existence and characteristics of convective rolls in the convective boundary layer. Rolls are identified in radar and satellite data, and it is shown that the pressure signals associated with the rolls have been detected in an array of microbarographs. The methodology of obtaining further information on roll characteristics from the array, notably orientation and drift velocity, is discussed in some detail. It is shown that the pressure time series contain signals representing the roll motion, approximately normal to the mean wind, and signals representing turbulent structures that drift along the mean wind direction. As the along-wind signals may dominate the time series, care is needed to identify the roll motion. Filtering of the higher-frequency along-wind signals can isolate the roll motion details. Also, a new approach using “beam-steering diagrams” to discriminate rolls from gravity waves and turbulent eddies is tested in both a numerical model and an observational case. In the beam-steering diagram, multiple centers of signal cross correlation can be used to identify different features in a single set of time series from an array of stations. The observations and model show that an array of microbarographs are able to resolve rolls if they are properly distributed with their spacing being tuned according to roll wavelength.

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Sarah-Jane Lock, Heinz-Werner Bitzer, Alison Coals, Alan Gadian, and Stephen Mobbs

Abstract

Advances in computing are enabling atmospheric models to operate at increasingly fine resolution, giving rise to more variations in the underlying orography being captured by the model grid. Consequently, high-resolution models must overcome the problems associated with traditional terrain-following approaches of spurious winds and instabilities generated in the vicinity of steep and complex terrain.

Cut-cell representations of orography present atmospheric models with an alternative to terrain-following vertical coordinates. This work explores the capabilities of a cut-cell representation of orography for idealized orographically forced flows. The orographic surface is represented within the model by continuous piecewise bilinear surfaces that intersect the regular Cartesian grid creating cut cells. An approximate finite-volume method for use with advection-form governing equations is implemented to solve flows through the resulting irregularly shaped grid boxes.

Comparison with a benchmark orographic test case for nonhydrostatic flow shows very good results. Further tests demonstrate the cut-cell method for flow around 3D isolated hills and stably resolving flows over very steep orography.

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Ioana Colfescu, Joseph B. Klemp, Massimo A. Bollasina, Stephen D. Mobbs, and Ralph R. Burton

Abstract

On 20 October 2016, aircraft observations documented a significant train of lee waves above and downstream of the Snæfellsnes Peninsula on the west coast of Iceland. Simulations of this event with the Weather Research and Forecasting (WRF) Model provide an excellent representation of the observed structure of these mountain waves. The orographic features producing these waves are characterized by the isolated Snæfellsjökull volcano near the tip of the peninsula and a fairly uniform ridge along its spine. Sensitivity simulations with the WRF Model document that the observed wave train consists of a superposition of the waves produced individually by these two dominant orographic features. This behavior is consistent with idealized simulations of a flow over an isolated 3D mountain and over a 2D ridge, which reproduce the essential behavior of the observed waves and those captured in the WRF simulations. Linear analytic analysis confirms the importance of a strong inversion at the top on the boundary layer in promoting significant wave activity extending far downstream of the terrain. However, analysis of the forced and resonant modes for a two-layer atmosphere with a capping inversion suggest that this wave train may not be produced by resonant modes whose energy is trapped beneath the inversion. Rather, these appear to be vertically propagating modes with very small vertical group velocity that can persist far downstream of the mountain. These vertically propagating waves potentially provide a mechanism for producing near-resonant waves farther aloft due to interactions with a stable layer in the midtroposphere.

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THE TERRAIN-INDUCED ROTOR EXPERIMENT

A Field Campaign Overview Including Observational Highlights

Vanda Grubišić, James D. Doyle, Joachim Kuettner, Stephen Mobbs, Ronald B. Smith, C. David Whiteman, Richard Dirks, Stanley Czyzyk, Stephen A. Cohn, Simon Vosper, Martin Weissmann, Samuel Haimov, Stephan F. J. De Wekker, Laura L. Pan, and Fotini Katopodes Chow

The Terrain-Induced Rotor Experiment (T-REX) is a coordinated international project, composed of an observational field campaign and a research program, focused on the investigation of atmospheric rotors and closely related phenomena in complex terrain. The T-REX field campaign took place during March and April 2006 in the lee of the southern Sierra Nevada in eastern California. Atmospheric rotors have been traditionally defined as quasi-two-dimensional atmospheric vortices that form parallel to and downwind of a mountain ridge under conditions conducive to the generation of large-amplitude mountain waves. Intermittency, high levels of turbulence, and complex small-scale internal structure characterize rotors, which are known hazards to general aviation. The objective of the T-REX field campaign was to provide an unprecedented comprehensive set of in situ and remotely sensed meteorological observations from the ground to UTLS altitudes for the documentation of the spatiotemporal characteristics and internal structure of a tightly coupled system consisting of an atmospheric rotor, terrain-induced internal gravity waves, and a complex terrain boundary layer. In addition, T-REX had several ancillary objectives including the studies of UTLS chemical distribution in the presence of mountain waves and complex-terrain boundary layer in the absence of waves and rotors. This overview provides a background of the project including the information on its science objectives, experimental design, and observational systems, along with highlights of key observations obtained during the field campaign.

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