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doi: 10.1002/qj.49712051503pmid: N/A
Large‐eddy simulation is being increasingly used as a means of both predicting the properties of specific turbulent flows and providing flow details which can be used like data to test and refine other turbulence‐closure models. Although such simulations derive their credibility from the explicit resolution of large‐scale turbulent eddies, they depend upon a small‐scale turbulence closure and must to some degree inherit the many uncertainties associated with turbulence closure. Most results obtained to date have been very encouraging, with examples of good agreement with observations and insensitivity to the small‐scale turbulence closure. However, there are some counter examples of incorrect prediction in circumstances where the small‐scale closure is critical. The evidence from the simulations suggests that this critical behaviour may only occur in the regions of large‐eddy simulation where matches to boundaries, or regions of statically stable fluid, are encountered. In such regions an empirical approach dominates, and there is evidence that the small‐scale turbulence closure must represent stochastic fluctuations of the unresolved stresses. Although there is a need to refine the small‐scale turbulence models, it is clear from the promise of current applications that, with the expected growth in computer power, this technique will have increasing breadth of application. Here the underlying assumptions and current practical implementations of the technique are critically reviewed.
doi: 10.1002/qj.49712051504pmid: N/A
Wind profiles through the stable boundary layer are presented. The data were collected at a site located in a shallow river valley and show evidence of upstream effects near the edge of the valley. The wind‐speed profiles are consistent with a non‐dimensional shear Øm = 1 + 3z/L for z/L < 0.5. Changes in wind speed and direction are found to occur over a greater depth than the depth of the turbulent nocturnal boundary layer estimated from various diagnostic formulae.
Gryning, Sven‐Erik; Batchvarova, Ekaterina
doi: 10.1002/qj.49712051505pmid: N/A
A new parametrization for the depth of the daytime entrainment zone is presented. It is derived from a simple energy budget, in which the eddy velocity in the mixed layer penetrating the overlying stable entrainment zone is taken to be proportional to the characteristic top‐down velocity scale. It gives a better ordering of the data of two earlier studies by Deardorff et al. and Boers and Eloranta as compared with traditional parcel theory based on the bottom‐up velocity scale. Furthermore, it can be seen that the data from the two studies belong to different regimes of the characteristic scaling parameter.
Shutts, G. J.; Healey, P.; Mobbs, S. D.
doi: 10.1002/qj.49712051506pmid: N/A
Radiosondes released in rapid succession, with a time separation of a few minutes, were launched during a field experiment in central Wales, U.K. to study orographic gravity waves. Rate of ascent fluctuations were used to infer the existence of vertical‐velocity fluctuations associated with orographic gravity waves, and differences between consecutive ascents provided information on the steadiness and phase‐line slope of the waves.
doi: 10.1002/qj.49712051507pmid: N/A
Forced flow over the asymmetric topography of the Darling Scarp, Western Australia is modelled, using a nonhydrostatic model, and compared with observations. Drag histories and sensitivity tests indicate that it is possible for the flow to be dominated by hydrostatic downslope windstorms even if the hydrostatic index indicates nonhydrostatic dominance. Experiments on a wide range of one‐ or two‐layered flows suggest that nonhydrostatic effects on windstorm events are very small. However, the development and location of trapped lee waves can be significantly affected if there is sufficient hydrostatic forcing in the flow. For hydrostatic forcing to dominate over trapped lee waves it is necessary for reflection, from a region of wave breaking or a critical layer, to occur at the right height.
doi: 10.1002/qj.49712051508pmid: N/A
Radiative parametrizations for both ice and water clouds are developed in terms of liquid/ice water content, based on Mie scattering theory. For ice crystals the application of Mie theory is guided by the hexagonal‐crystal/equivalent‐spheres comparison of Takano and Liou. These parametrizations are extensively tested against measurements from aircraft and are shown to perform satisfactorily, although corrections for unobserved small crystals and the effect of crystal shape are large and not currently well defined. The parametrizations are then used to investigate the effect of mixed‐phase clouds on radiative transfer. It is found that, because the radiative properties of ice crystals and liquid droplets are significantly different, the radiative properties of mixed‐phase clouds cannot be simulated successfully if the ice in clouds is converted into liquid water. Both the albedo and the rate of change of albedo with ice fraction are significantly dependent on the method by which the phases are mixed; these factors may be of especial importance in climate‐sensitivity experiments that incorporate mixed‐phase clouds. The presence of ice in clouds below the cirrus level is often ignored in climate‐model and radiation‐budget studies. The calculations presented here indicate that this neglect may lead to a serious bias in cloud albedo for a given path of condensed water.
doi: 10.1002/qj.49712051509pmid: N/A
Results are presented from a numerical mesoscale modelling study of the effects of meteorological phenomena on the distribution of contaminants, both airborne and deposited onto the surface, arising from hypothetical idealized release episodes. An Eulerian transport/diffusion/removal model is coupled to the Meteorological Office non‐hydrostatic mesoscale model, with removal tuned to represent the wet deposition of pollutants likely to be emitted during a nuclear power station accident.
Giorgi, Filippo; Hostetler, Steven W.; Brodeur, Christine Shields
doi: 10.1002/qj.49712051510pmid: N/A
This paper discusses the surface hydrology of a multi‐year simulation of present day climate over the United States (US) conducted with a regional climate model (RegCM) nested within a general circulation model (GCM). The RegCM, which is run with a 60 km gridpoint spacing is interactively coupled with a state‐of‐the‐art surface physics package that includes full surface hydrology calculations (the Biosphere‐Atmosphere Transfer Scheme or BATS). The hydrologic budgets of ten regional drainage basins in the US are analysed. Model results are compared with available observations and with results from previous modelling experiments to evaluate the feasibility of using nested RegCM/GCM models for hydrology studies. In our experiment, the model captures the basic seasonality of the basin hydrologic budgets, although the simulated precipitation amounts are too high over the western US and too low over the eastern US. As a result, runoff, snow cover and soil water content are underestimated over the eastern US basins, while evaporation and runoff are overestimated in some of the western US basins. Topographically induced characteristics of precipitation, snow cover and runoff are well simulated over the mountainous western regions. Also well captured is the inter‐basin variation of hydrologic budgets which occurs in response to different climatic settings. The springtime snowmelt and peak runoff season generally occurs in the model earlier in the year than is observed. Although our work indicates that the coupled regional modelling system can be useful in applications to hydrological studies, results from this experiment indicate that better accuracy in the simulation of regional climatic variables and more detailed representation of some hydrologic processes would be required before the coupled modelling system could be used to provide accurate assessments of hydrologic responses to climate change.
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