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Glenn Shutts

Abstract

A large-amplitude lee-wave event detected in radiosonde ascents during a field experiment in the Welsh mountains is described and wave characteristics are deduced. The synoptic-flow pattern accompanying this wave field is used to initialize the U.K. Meteorological Office's nonhydrostatic mesoscale model (with a 3-km horizontal grid length) in an attempt to simulate the gravity-wave response over the Welsh mountains. After 4 h of integration time the resulting flow held is quasi-study and exhibits a fairly regular wave field with a dominant wavelength at about 22 km and nearly vertical phase lines. Verification is achieved by comparing rate-of-ascent variations with model vertical velocity variations along the trajectory of a radiosonde: the agreement is very good.

The vertical structure equation for linear gravity waves is solved using the smoothed wind and temperature fields obtained from the sondes. A “leaky” resonance is identified at the dominant wavelength of the model simulation. Limited confirmation of the horizontal wavelength comes from satellite imagery.

One implication of this study is that nonhydrostatic, operational weather forecasting models are likely to exhibit useful predictability even at the scale of orographically forced trapped lee waves and could assist aviation forecasters in predicting hazardous wave-induced turbulence. They can also be used to diagnose wave momentum transfer and pressure drag for the purposes of refining gravity-wave-drag parameterization schemes.

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Glenn Shutts

Abstract

Linearized solutions are derived for the case of constant (negative) shear flow over an isolated, circularly symmetric hill and are evaluated for three characteristic wave regimes. Of particular interest is the three-dimensional structure of the resulting field of inertia–gravity waves near the lower of two levels where the magnitude of the intrinsic frequency is equal to the Coriolis parameter—the inertia critical levels. In contrast to the equivalent nonrotating problem, the critical level height is a function of the horizontal wavenumber for each Fourier mode representing the disturbance. Furthermore the wave field, which consists of a downstream train of inertia waves, exhibits an azimuthal asymmetry about the direction of the flow. When the Rossby number near the mountain is less than unity the wave response consists of an evanescent disturbance and a pattern of neutral baroclinic lee waves that radiate horizontally away from the hill. These too show some cross-flow asymmetry, though in the opposite sense to that found with the inertia–gravity wave field.

The spectral distribution of the wave stress is found to be symmetric about the flow direction, and the total stress averaged over the finite domain area decays with height as wave pseudomomentum is carried downstream in the form of quasi-inertial waves.

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Glenn Shutts

Abstract

Analytical solutions are obtained to the linearized equations describing a particular class of directionally sheared flow over isolated hills or ridges. The flows are characterized by constant buoyancy frequency and vertical wind shear, though the wind direction changes with height due to the presence of a constant, horizontal wind component normal to the wind shear vector. The inclusion of a constant Coriolis parameter permits the formation of inertia wave trains and neutral baroclinic wave trains. A particular focus of this study is the nature of the selective critical level absorption process that results from varying wind direction with height in the presence of an azimuthal spectrum of forced wave modes. It will be shown that the stationary disturbance generated by circular and elliptical hills of mesoscale dimensions comprises an inertia wave train that trails downwind at all heights in the form of a horizontal wind perturbation, and a baroclinic wave train that extends in the direction of the surface flow. Unlike the equivalent nonrotating problem, these calculations suggest that the kinetic energy density of the inertia wave train does not decay downstream but asymptotes to a constant value, as recently inferred by Broutman. The form of the downstream wave train is also shown to be very different in the rotating and nonrotating cases, implying that wave breakdown will occur much nearer to the orographic wave source in the presence of rotation (though still potentially a long way from the source). The results support the general notion that three-dimensionality, directional wind shear, and rotation promote horizontal energy dispersion into a background inertia–gravity wave “soup.”

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John Marshall and Glenn Shutts

Abstract

If the deviation of mean flow from mean temperature contours is small, it is shown that a part of the eddy heat flux can be separated out which circulates around eddy potential energy contours, and has a component up/down the mean temperature gradient if there is flow advection of eddy potential energy into/out of the region. If the mean flow is strong, this rotational flux is large and results in regions of up- and downgradient flux. It is a prominent feature of maps of geostrophic eddy fluxes in the ocean and atmosphere.

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Warren J. Tennant, Glenn J. Shutts, Alberto Arribas, and Simon A. Thompson

Abstract

An improved stochastic kinetic energy backscatter scheme, version 2 (SKEB2) has been developed for the Met Office Global and Regional Ensemble Prediction System (MOGREPS). Wind increments at each model time step are derived from a streamfunction forcing pattern that is modulated by a locally diagnosed field of likely energy loss due to numerical smoothing and unrepresented convective sources of kinetic energy near the grid scale. The scheme has a positive impact on the root-mean-square error of the ensemble mean and spread of the ensemble. An improved growth rate of spread results in a better match with ensemble-mean forecast error at all forecast lead times, with a corresponding improvement in probabilistic forecast skill from a more realistic representation of model error. Other examples of positive impact include improved forecast blocking frequency and reduced forecast jumpiness. The paper describes the formulation of the SKEB2 and its assessment in various experiments.

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Andrew C. Bushell, Neal Butchart, Stephen H. Derbyshire, David R. Jackson, Glenn J. Shutts, Simon B. Vosper, and Stuart Webster

Abstract

Analysis of a high-resolution, convection-permitting simulation of the tropical Indian Ocean has revealed empirical relationships between precipitation and gravity wave vertical momentum flux on grid scales typical of earth system models. Hence, the authors take a rough functional form, whereby the wave flux source spectrum has an amplitude proportional to the square root of total precipitation, to represent gravity wave source strengths in the Met Office global model’s spectral nonorographic scheme. Key advantages of the new source are simplicity and responsiveness to changes in convection processes without dependence upon model-specific details of their representation. Thus, the new source scheme is potentially a straightforward adaptation for a class of spectral gravity wave schemes widely used for current state-of-the-art earth system models. Against an invariant source, the new parameterized source generates launch-level flux amplitudes with greater spatial and temporal variability, producing probability density functions for absolute momentum flux over the ocean that have extended tails of large-amplitude, low-occurrence events. Such distributions appear more realistic in comparison with reported balloon observations. Source intermittency at the launch level affects mean fluxes at higher levels in two ways: directly, as a result of upward propagation of the new source variation, and indirectly, through changes in filtering characteristics that arise from intermittency. Initial assessment of the new scheme in the Met Office global model indicates an improved representation of the quasi-biennial oscillation and sensitivity that offers potential for further impact in the future.

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