Topographically induced waves within the stable boundary layer (original) (raw)
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Quarterly Journal of the Royal Meteorological Society, 2002
This study examines the generation of gravity waves by the dry convective boundary layer in two dimensions. In particular, a number of cases are considered which range from zero background-flow conditions to conditions with strong wind shear. These experiments agree with previous studies that identified a distinct horizontal-scale selection of the gravity waves due to changes in wind speed. Detailed linear analysis is completed which attributes the horizontal-scale selection to two processes. The first process is the dissipation of downstream-propagating gravity waves by critical levels, and the second process is the trapping of upstream-propagating waves. The result is a narrow region of the generated spectrum of gravity waves that are able to propagate vertically. The experiments also agree with previous studies that have observed a resonant interaction between the boundary-layer thermals and the gravity waves. This resonance is described as a result of the trapped gravity waves, which form a resonant cavity near the top of the mixed layer. Finally, the experiments are used to evaluate the role of shear in the gravitywave generation. In cases with little resonance, shear is shown to play a minor role in the wave generation. In these cases, changes in the wave's amplitude spectrum are mostly due to linear wave refraction and the aforementioned selection mechanism.
The effect of wind shear and curvature on the gravity wave drag produced by a ridge
Journal of the atmospheric Sciences, 2004
The analytical model proposed by Teixeira, Miranda, and Valente is modified to calculate the gravity wave drag exerted by a stratified flow over a 2D mountain ridge. The drag is found to be more strongly affected by the vertical variation of the background velocity than for an axisymmetric mountain. In the hydrostatic approximation, the corrections to the drag due to this effect do not depend on the detailed shape of the ridge as long as this is exactly 2D. Besides the drag, all the perturbed quantities of the flow at the surface, including the pressure, may be calculated analytically.
2010
A b s t r a c t This paper addresses the quantification of gravity wave drag due to small hills in the stable boundary layer. A single column atmospheric model is used to forecast wind and temperature profiles in the boundary layer. Next, these profiles are used to calculate vertical profiles of gravity wave drag. Climatology of wave drag magnitude and "wave drag events" is presented for the CASES-99 experimental campaign. It is found that gravity wave drag events occur for several relatively calm nights, and that the wave drag is then of equivalent magnitude as the turbulent drag. We also illustrate that wave drag events modify the wind speed sufficiently to substantially change the surface sensible heat flux.
Numerical Exploration of the Stable Boundary Layer
2008 DoD HPCMP Users Group Conference, 2008
Simulating the stable atmospheric boundary-layer (SABL) presents a significant challenge to numerical models due to the interactions of several processes with widely varying scales. For example, at larger scales, gravity waves can extract energy and momentum from the mean flow and efficiently transport them between regions in the atmosphere. Simulation of gravity waves requires a large spatial domain. Meanwhile, at small-scales, turbulent motions exhibit a complicated, time-dependent structure requiring sophisticated numerical treatment and a very small grid spacing. How these two processes interact to generate relatively strong turbulent mixing in environments ostensibly hostile to turbulent production remains a mystery. While focusing on idealized simulations to make the problem more tractable, a Reynolds Stress turbulence closure scheme was incorporated into the National Taiwan University/Purdue University Non-hydrostatic model, in order to more accurately simulate shear instability, one of the most important processes in the SABL. Shear instability is the most common mechanism for gravity wave generation, a mechanism for gravity wave absorption, and the dominant turbulence production process. The primary instability and cross-flow vorticity are accurately reproduced. In addition, secondary instabilities appear to be occurring, such as vortex pairing and knotting, which leads to the generation of stream-wise and vertical vorticity. In addition, gravity waves are generated via non-linear interactions between the Kelvin-Helmholtz modes directly produced by the shear. The resulting waves have an order of magnitude longer wavelength than the Kelvin-Helmholtz waves and a significantly greater phase speed in agreement with the literature.
Sea Surface Gravity Wave-wind Interaction in the Marine Atmospheric Boundary Layer
Energy Procedia, 2014
In this study, we investigate the turbulence structure over idealized wind-generated surface gravity waves with varying wave age using a wave-modified one-dimensional boundary layer model. To prescribe the shape of the water wave and the associated orbital velocities, we employ an empirical expression for the wave energy spectrum without assigning a prognostic equation for modelling wave evolution under the action of wind. The key element in this model is the the work done by the wave-induced momentum flux on the atmosphere in the presence of waves. This is incorporated into the airflow using an exponential decay function. Finally, we conduct a series of numerical experiments to identify wave effects on the airflow over a wavy moving interface as a function of wave age, and to check the skill of the present model in capturing wave-induced processes in the marine atmospheric boundary layer (MABL). The results obtained confirm again the significant role of wave-induced processes in influencing the MABL, for example, in modifying the wind profile. Meanwhile, it is shown that the modified one-dimensional model is sensitive to wave parameterizations and the wave energy spectrum. However, a number of uncertainties remain for further investigation, such as the choice of wave energy spectrum, wave forcing parametrization, and surface boundary conditions for momentum and energy.
Three-dimensional numerical experiments on convectively forced internal gravity waves
Quarterly Journal of the Royal Meteorological Society, 1989
Results are presented of thermally forced dry and moist convection and the associated gravity wave fields from three-dimensional numerical simulations using a non-hydrostatic anelastic model. This paper extends earlier two-dimensional simulations to include effects of the third spatial dimension employing a very similar environmental speed-shear case for the study. The present simulations produce scattered fair weather cumuli in agreement with observations. In many important respects, the physical response is quite similar to that obtained in the earlier two-dimensional calculations. The near-uniform surface sensible heat flux results in Rayleigh modes filling the convective boundary layer (CBL) to begin with, whereas later, after convective motions start interacting with the overlying stable layer, larger horizontal scale deep modes become evident and in some cases dominant. The eigenfunction structure of these dominant forced normal modes consists of boundary layer eddies in the lower levels and gravity waves above. They are important organizers of the cumulus convection. As in the earlier two-dimensional simulations, the efficiency of gravity wave excitation was found to be very sensitive to the mean wind shear in the region spanning the CBL and the overlying stable layer. The dominant horizontal wavelength in the shear direction ranges between 10 and 15 km in the free atmosphere whereas it peaks at about 6 km in the CBL.
The effect of a stable boundary layer on orographic gravity wave drag
Quarterly Journal of the Royal Meteorological Society, 2020
Numerical simulations are carried out using the WRF model to explicitly calculate the ratio of orographic gravity wave drag (GWD) in the presence of a stable boundary layer (BL) to the inviscid drag in its absence, either obtained from inviscid WRF simulations or estimated using an analytical linear model. This ratio is represented as a function of three scaling variables defined as ratios of the BL depth to the orography width, height, and stability height scale of the atmosphere. All results suggest that the GWD affected by the stable BL, D_BL, is inversely proportional to the BL depth h_BL, roughly following D_BL ~ h_BL^(-2). The scaling relations are calibrated and tested using a multilinear regression applied to data from the WRF simulations, for idealised orography and inflow atmospheric profiles derived from reanalysis, representative of Antarctica in austral winter, where GWD is expected to be especially strong. These comparisons show that the scaling relations where the drag is normalised by the analytical inviscid estimate work best. This happens because stable BL effects reduce the amplitude of the waves above the BL, making their dynamics more linear. Knowledge of the BL depth and orography parameters is sufficient to obtain a reasonable correction to the inviscid drag without needing additional information about the wind and stability profiles. Since the drag currently available from numerical weather prediction model parametrizations comes from linear theory uncorrected for BL effects, the results reported here may be applied straightforwardly to improve those parametrizations.
Determining Wave–Turbulence Interactions in the Stable Boundary Layer
Bulletin of the American Meteorological Society, 2014
What: Fifty scientists from nine countries gathered to address theoretical frameworks and observational strategies to understand how gravity waves are generated in the boundary layer and how they in turn generate and interact with turbulence. A major goal was to compare between theories and observations and to apply knowledge of interactions between gravity waves and turbulence to improve parameterizations in numerical models.
2009
This paper addresses the quantification of gravity wave drag due to small hills in the stable boundary layer. A single column atmospheric model is used to forecast wind and temperature profiles in the boundary layer. Next, these profiles are used to calculate vertical profiles of gravity wave drag. Climatology of wave drag magnitude and “wave drag events” is presented for the CASES-99 experimental campaign. It is found that gravity wave drag events occur for several relatively calm nights, and that the wave drag is then of equivalent magnitude as the turbulent drag. We also illustrate that wave drag events modify the wind speed sufficiently to substantially change the surface sensible heat flux.