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1.
This study compares different simple mixing schemes for one-dimensional models and then focuses on the two-scale mixing approach. Two-scale mixing consists of local diffusion between adjacent grid levels and nonlocal mixing over the bulk of the boundary layer (nonlocal mixing). The latter represents nonlocal mixing by the boundary-layer scale eddies. A common example of two-scale mixing is the formulation of the turbulent heat transport in terms of an eddy diffusivity to represent small-scale diffusion and a countergradient correction to represent boundary-layer scale transport. Most existing two-scale approaches are applied to heat and moisture transport while momentum transport is simultaneously parameterized only in terms of a local diffusivity without nonlocal mixing. This study attempts to correct this inconsistency.The resulting model is compared with Lidar observations of spatially averaged winds which are found to be superior to radiosonde and aircraft data for determining the mean structure. The two-scale mixing correctly predicts the observed well mixed conditions for momentum while the original model based on a local diffusivity for momentum fails to produce a well mixed state. Unfortunately, the best value for the adjustable coefficient in the nonlocal mixing part of the two-scale approach appears to depend on baroclinity in a way which can not be completely resolved from existing data. 相似文献
2.
Modelling the arctic convective boundary-layer with different turbulence parameterizations 总被引：1，自引：1，他引：0
Different parameterizations of subgrid-scale fluxes are utilized in a nonhydrostatic and anelastic mesoscale model to study their influence on simulated Arctic cold air outbreaks. A local closure, a profile closure and two nonlocal closure schemes are applied, including an improved scheme, which is based on other nonlocal closures. It accounts for continuous subgrid-scale fluxes at the top of the surface layer and a continuous Prandtl number with respect to stratification. In the limit of neutral stratification the improved scheme gives eddy diffusivities similar to other parameterizations, whereas for strong unstable stratifications they become much larger and thus turbulent transports are more efficient. It is shown by comparison of model results with observations that the application of simple nonlocal closure schemes results in a more realistic simulation of a convective boundary layer than that of a local or a profile closure scheme. Improvements are due to the nonlocal formulation of the eddy diffusivities and to the inclusion of heat transport, which is independent of local gradients (countergradient transport). 相似文献
3.
Based on gradient transport theory or K-theory, turbulent transport in the atmosphere has long been parameterized using the
eddy diffusivity. Due to its simplicity, this approach has often been applied in many numerical models but rarely tested with
observations. Here, the widely used O’Brien cubic polynomial approach has been validated together with an exponential approach
against eddy diffusivity profiles determined from measurements and from large-eddy simulation data in stable conditions. Verification
is completed by analyzing the variability effects on pollutant concentrations of two different vertical diffusion (K(z)) schemes incorporated in an atmospheric chemical model. It is shown that the analytical, exponential solution agrees better
with observations than the O’Brien profile and should be used henceforth in practical applications. 相似文献
4.
Summary The results of incorporating a nonlocal boundary-layer diffusion scheme in a forecast model over Indian region are discussed.
The simple formulation of atmospheric boundary layer height in the nonlocal diffusion scheme is examined in detail to understand
how far the model simulated boundary layer height is realistic. Analyses of the temporal and spatial variability of the boundary
height for three cases representing premonsoon, active monsoon and post monsoon conditions over Indian region show that it
is comparable with the observational evidence. Further, for a case of active monsoon condition over Indian region, comparison
of precipitation forecasts with the nonlocal scheme and the control local boundary-layer scheme clearly indicated that the
model run with the nonlocal scheme is significantly more accurate in forecasting the intense precipitation locations.
Received November 16, 2001 Revised December 28, 2001 相似文献
5.
The oceanic bottom boundary-layer model of Weatherly and Martin (1978) is used to study the vertical structure of the eddy diffusivity in a region with initially imposed bottom mixed-layer thickness. Because of near-bottom oceanic features, such as the Cold Filament (Weatherly and Kelley, 1982) and cold eddies (Ebbesmeyer et al., 1988), the bottom mixed-layer thickness is not the sole result of boundary-layer mixing; this is the incentive for this study. For a given geostrophic forcing and imposed mixed-layer depth, a formula for the eddy diffusion coefficient is found. This parameterization of the eddy diffusivity improves previous formulas used in oceanic and atmospheric boundary layers in the upper portion of the boundary layer. A simple model of a Cold Filament-like feature demonstrates the structure of the bottom boundary layer, the bottom mixed layer, and the relation between the two. A lens-like cross section of cold blobs, often used in analytical models, may be inappropriate if bottom friction is important. 相似文献
6.
Soundings in the stably-stratified boundary layer were executed over the rough terrain of Northern Germany during a night with a low-level jet (LLJ) development. Vertical wind and temperature profiles were obtained at 5 m height intervals using a tethersonde transported up and down along a 300 m high radio tower by an elevator. From these profiles, turbulent fluxes of heat and momentum, coefficients of eddy diffusivity and boundary-layer parameters were estimated. The nocturnal mean state analysis agrees well with the second-order model results of Brost and Wyngaard (1978) and our own first-order numerical testing while the time histories of different profile groups are in accordance with the observations of Izumi and Barad (1963). 相似文献
7.
Evaluation of the Weather Research and Forecasting Mesoscale Model for GABLS3: Impact of Boundary-Layer Schemes,Boundary Conditions and Spin-Up 总被引：3，自引：3，他引：0
Michal A. Kleczek Gert-Jan Steeneveld Albert A. M. Holtslag 《Boundary-Layer Meteorology》2014,152(2):213-243
We evaluated the performance of the three-dimensional Weather Research and Forecasting (WRF) mesoscale model, specifically the performance of the planetary boundary-layer (PBL) parametrizations. For this purpose, Cabauw tower observations were used, with the study extending beyond the third GEWEX Atmospheric Boundary-Layer Study (GABLS3) one-dimensional model intercomparison. The WRF model (version 3.4.1) contains 12 different PBL parametrizations, most of which have been only partially evaluated. The GABLS3 case offers a clear opportunity to evaluate model performance, focusing on time series of near-surface weather variables, radiation and surface flux budgets, vertical structure and the nighttime inertial oscillation. The model results revealed substantial differences between the PBL schemes. Generally, non-local schemes tend to produce higher temperatures and higher wind speeds than local schemes, in particular, for nighttime. The WRF model underestimates the 2-m temperature during daytime (about \(2\) K) and substantially underestimates it at night (about \(4\) K), in contrast to the previous studies where modelled 2-m temperature was overestimated. Considering the 10-m wind speed, during the night turbulent kinetic energy based schemes tend to produce lower wind speeds than other schemes. In all simulations the sensible and latent heat fluxes were well reproduced. For the net radiation and the soil heat flux we found good agreement with daytime observations but underestimations at night. Concerning the vertical profiles, the selected non-local PBL schemes underestimate the PBL depth and the low-level jet altitude at night by about 50 m, although with the correct wind speed. The latter contradicts most previous studies and can be attributed to the revised stability function in the Yonsei University PBL scheme. The local, turbulent kinetic energy based PBL schemes estimated the low-level jet altitude and strength more accurately. Compared to the observations, all model simulations show a similar structure for the potential temperature, with a consistent cold bias ( \(\approx \) 2 K) in the upper PBL. In addition to the sensitivity to the PBL schemes, we studied the sensitivity to technical features such as horizontal resolution and domain size. We found a substantial difference in the model performance for a range of 12, 18 and 24 h spin-up times, longer spin-up time decreased the modelled wind speed bias, but it strengthened the negative temperature bias. The sensitivity of the model to the vertical resolution of the input and boundary conditions on the model performance is confirmed, and its influence appeared most significant for the non-local PBL parametrizations. 相似文献
8.
The impact of different vertical diffusion schemes in a three-dimensional oil spill model in the Bohai Sea 总被引：2，自引：0，他引：2
Vertical transport is critical to the movement of oil spills in seawater. Breaking waves play an important role by developing a well-defined mixing layer in the upper part of the water column. A three-dimensional （3-D） Lagrangian random walk oil spill model was used here to study the influence of sea surface waves on the vertical turbulence movement of oil particles. Three vertical diffusion schemes were utilized in the model to compare their impact on oil dispersion and transportation. The first scheme calculated the vertical eddy viscosity semi-empirically. In the second scheme, the vertical diffusion coefficient was obtained directly from an Eulerian hydrodynamic model （Princeton Ocean Model, POM2k） while considering wave- caused turbulence. The third scheme was formulated by solving the Langevin equation. The trajectories, percentages of oil particles intruding into water, and the vertical distribution structures of oil particles were analyzed for a series of numerical experiments with different wind magnitudes. The results showed that the different vertical diffusion schemes could generate different horizontal trajectories and spatial distributions of oil spills on the sea surface. The vertical diffusion schemes caused different water-intruding and resurfacing oil particle behaviors, leading to different horizontal transport of oil particles at the surface and subsurface of the ocean. The vertical diffusion schemes were also applied to a realistic oil spill simulation, and these results were compared to satellite observations. All three schemes yielded acceptable results, and those of the third scheme most closely simulated the observed data. 相似文献
9.
Previous numerical studies have focused on the combined effect of momentum and scalar eddy diffusivity on the intensity and structure of tropical cyclones. The separate impact of eddy diffusivity estimated by planetary boundary layer(PBL) parameterization on the tropical cyclones has not yet been systematically examined. We have examined the impacts of eddy diffusion of moisture on idealized tropical cyclones using the Advanced Research Weather Research and Forecasting model with the Yonsei University PBL scheme. Our results show nonlinear effects of moisture eddy diffusivity on the simulation of idealized tropical cyclones. Increasing the eddy diffusion of moisture increases the moisture content of the PBL, with three different effects on tropical cyclones:(1) an decrease in the depth of the PBL;(2) an increase in convection in the inner rain band and eyewall; and(3) drying of the lowest region of the PBL and then increasing the surface latent heat flux. These three processes have different effects on the intensity and structure of the tropical cyclone through various physical mechanisms. The increased surface latent heat flux is mainly responsible for the decrease in pressure. Results show that moisture eddy diffusivity has clear effects on the pressure in tropical cyclones, but contributes little to the intensity of wind. This largely influences the wind–pressure relationship, which is crucial in tropical cyclones simulation. These results improve our understanding of moisture eddy diffusivity in the PBL and its influence on tropical cyclones, and provides guidance for interpreting the variation of moisture in the PBL for tropical cyclone simulations. 相似文献
10.
Joseph P. pandolfo 《Boundary-Layer Meteorology》1971,1(3):277-289
The basic numerical air-sea boundary-layer model described in Pandolfo (1969a, b) was varied to produce a set of models with differing atmospheric boundary-layer formulas, four of which are discussed here. Model I is the basic model itself, with stability and sea-state dependent eddy viscosity, conductivity and diffusivity which may, in certain ranges ofRi, be unequal. This model is applied on a relatively fine grid. Model II, applied on the same grid, uses formulas which yield equal eddy conductivity, diffusivity, and viscosity. The calculated eddy coefficients depend only on the height and wind shear. Model III uses the same exchange coefficient formulas as Model II. However, the surface-layer eddy flux in Model III is calculated by assuming that logarithmic profiles of the transported variables are present in this layer. Model IV is the same as Model III in these respects, but employs a relatively coarse vertical grid. This model, therefore, includes boundary layer formulas most like those conventionally used in large scale atmospheric models (e.g. Miyakoda, 1969).The four models were integrated numerically with identical inputs of initial, boundary, and auxiliary data prepared from observations made over the eastern half of the BOMEX observational area during June 21–25, 1969.Models I and IV are, in general, in better agreement with each other than either is with Model II. This is true for the model-generated upper and lower boundary fluxes of mean momentum and latent heat; and for the internal boundary layer production of mean kinetic energy by the cross-isobaric flow component. Model I agrees, on balance, about as well with Model IV as does Model III. The solutions for Models I, III, and IV are also, in general, more consistent with observed data, viz. 5-day average temperature profiles in the layer from the surface to 1000 meters, and 5-day averages of sea surface temperature and of surface-layer atmospheric humidity. Solutions for Model I are in better overall agreement with the observed data, and with the average observed surface-layer wind.The results show that, under the limitations implicit in these preliminary experiments, accurate simulations of observed data are possible with boundary-layer formulas of the type used in Model IV, and even more accurate simulation with the modest refinements represented by Model I. Piecemeal imposition of such refinements could, however, lead to models, like Model II, with significantly different energetic properties and less simulative accuracy. Specifically, the results support the speculation (Miyakodaet al., 1969) that the shallowness of the simulated Trades noted in some large-scale models is due to deficiencies in the boundary-layer eddy stress formulations used. 相似文献
11.
Summary ?A time-dependent semi-geostrophic Ekman boundary-layer model (SG), including slowly varying eddy diffusivity with height
and inertial term effects, is developed to investigate the diurnal wind variation in the planetary boundary layer (PBL). An
approximate analytical solution of this model is derived by using the WKB method, which extends the Tan and Farahani (1998)’s
solution by including the vertical variable eddy viscosity.
The features of the diurnal wind variation in the PBL mainly depend on three factors: the latitude, horizontal momentum advection
and eddy viscosity. The vertical variable eddy viscosity has little influence on diurnal wind variation in the PBL at the
low latitude, however its effect may be exacerbated in the mid- and high latitudes. In comparing with the constant eddy viscosity
case, the decreasing (increasing) with height eddy viscosity produces a large (small) maximum wind speed (MWS) in the PBL,
however, the eddy viscosity that has a mid-layer peak in the vertical gives rise to a higher height of occurrence of MWS.
For the boundary-layer wind structure, there is a singular point when the modified SG inertial oscillation frequency η equals
the forcing frequency ω. The isotachs of boundary-layer wind speed have almost no tilt to left or right relative to time evolution
and the occurrence time of the MWS is the earliest at the singular point. The feature will be enhanced in the decreasing with
height eddy viscosity case and weakened in the eddy viscosity initially increasing with height case.
Received April 6, 2001; accepted December 27, 2001 相似文献
12.
The difficulties associated with the parameterization of turbulence in the stable nocturnal planetary boundary layer (PBL)
have been a great challenge for the nighttime predictions from mesoscale meteorological models such as MM5. As such, there
is a general consensus on the need for better stable boundary-layer parameterizations. To this end, two new turbulence parameterizations
based on the measurements of the Vertical Transport and Mixing (VTMX) field campaign were implemented and evaluated in MM5.
A unique aspect of this parameterization is the use of a stability-dependent turbulent Prandtl number that allows momentum
to be transported by the internal waves, while heat diffusion is impeded by the stratification. This improvement alleviates
the problem of over-prediction of heat diffusion under stable conditions, which is a characteristic of conventional atmospheric
boundary-layer schemes, such as the Medium Range Forecast (MRF) and Blackadar schemes employed in MM5. The predictions made
with the new PBL scheme for the complex terrain airshed of Salt Lake City were compared with those made with a default scheme
of MM5, and with observations made during the VTMX campaign. The new schemes showed an improvement in predictions, particularly
for the nocturnal near-surface temperature. Surface wind predictions also improved slightly, but not to the extent of temperature
predictions. The default MRF scheme showed a significantly higher surface temperature than observed, which could be attributed
to the enhanced vertical heat exchange brought about by its turbulence parameterization. The modified parameterizations reduced
the surface sensible heat flux, thus enhancing the strength of the near-surface inversion and lowering the temperature towards
the observed values. 相似文献
13.
Helmut Kraus 《Boundary-Layer Meteorology》1971,1(4):385-388
The paper deals with the role of the vertical flux of turbulent energy in the atmospheric boundary layer. The influence of the divergence of this flux on the vertical wind profile and on the eddy diffusivity is shown by theoretical considerations. 相似文献
14.
A new algebraic turbulent length scale model is developed, based on previous one-equation turbulence modelling experience in atmospheric flow and dispersion calculations. The model is applied to the neutral Ekman layer, as well as to fully-developed pipe and channel flows. For the pipe and channel flows examined the present model results can be considered as nearly equivalent to the results obtained using the standard k– model. For the neutral Ekman layer, the model predicts satisfactorily the near-neutral Cabauw friction velocities and a dependence of the drag coefficient versus Rossby number very close to that derived from published (G. N. Coleman) direct numerical simulations. The model underestimates the Cabauw cross-isobaric angles, but to a less degree than the cross-isobar angle versus Rossby dependence derived from the Coleman simulation. Finally, for the Cabauw data, with a geostrophic wind magnitude of 10 ms–1, the model predicts an eddy diffusivity distribution in good agreement with semi-empirical distributions used in current operational practice. 相似文献
15.
The Role of Planetary Boundary-Layer Parameterizations in the Air Quality of an Urban Area with Complex Topography 总被引：3，自引：1，他引：2
Elizabeth Bossioli Maria Tombrou Aggeliki Dandou Eleni Athanasopoulou Kostas V. Varotsos 《Boundary-Layer Meteorology》2009,131(1):53-72
The effect of different planetary boundary-layer (PBL) parameterization schemes on the spatial distribution of atmospheric
pollution over the complex topography of the greater Athens area is investigated. Four PBL schemes originally implemented
in a numerical meteorological model and a fifth one simulating the urban effect are examined. Two different atmospheric conditions
are analyzed; a typical summer and a typical winter pollution episode. The relative importance of chemical and physical processes
of the pollution predictions is discussed using process analysis. It is revealed that, for primary pollutants, a local scheme
seems more adequate to represent the maximum observed concentrations while, completely different in structure, a non-local
scheme reproduces the mean observed values in the basin. Concerning secondary pollutants, peak concentration differences,
due to the different PBL schemes, are smoothed out. Nevertheless, the PBL scheme selection shapes the horizontal and the vertical
extension of maximum values. The non-local and semi non-local schemes are superior to the others, favouring strong vertical
mixing and transport towards the surface. The stronger turbulence accommodated effectively by the semi non-local urban scheme
enhances ozone production along the sea-breeze axis and preserves the high ozone concentrations during the nighttime hours
in the urban core. 相似文献
16.
17.
本文选用中国科学院大气物理研究所全球海洋模式(LICOM),对中尺度涡旋参数化方案(GM90方案)中等密度扩散系数和等密度面厚度扩散系数(统称为涡旋扩散系数Aρ)对物理场及CFC-11(一氟三氯甲烷)分布的影响进行了研究。本文做了两个试验,即涡旋扩散系数采用常系数方式(对照试验)和采用在非绝热层以下Aρ随海洋浮力频率垂直变化的参数化方案(浮力试验)。模拟结果表明,依浮力频率垂直变化的方案对模式物理场的模拟能力有一定程度的提升,如南极绕极流的输送强度比常系数方案增大了约20%~30%,与观测事实更接近;浮力试验对对照试验中过强的南极中层水有一定的削弱作用,使得模式对南大洋高纬次表层位密度的模拟有一定的改善。稍有不足的是,浮力试验对南极底层水也有一定的削弱,使得2000~3000 m深度位密度模拟较常系数方案偏低。通过对CFC-11分布、存储以及输送的研究发现,次网格参数取值的不同对南大洋CFC-11模拟情况有较大影响。浮力试验加大了南北高纬CFC-11海表的吸收通量,对南极大陆周边海域向南大洋主储藏区(34°S~60°S)的CFC-11输送能力有一定的增强,使得南大洋对CFC-11储藏量增大,大部分海区与观测资料更接近。通过CFC-11断面分析,浮力试验对南大洋上层海洋位密度模拟的改善使得CFC-11垂直结构与观测更接近。 相似文献
18.
A nested grid regional model with a high vertical resolution in the atmospheric boundary layer is used to simulate various atmospheric processes during an active monsoon period. A turbulence kinetic energy closure scheme is used to predict the boundary-layer structure. Model predictions indicate different structures of the boundary layer over land and oceans, as observed. Significant diurnal variation in boundary-layer structure and associated processes is predicted over land and negligible variations over oceans. The Somali jet over the Arabian Sea is well predicted. Location of the predicted monsoon depression and the associated rainfall are in good agreement with the observations. Also, predicted rainfall and its spatial distribution along the west coast of India are in good agreement with the observations. 相似文献
19.
针对GRAPES四维变分同化系统的升级, 研究了GRAPES模式垂直扩散方案线性化问题。通过2005年8月7—27日21个个例的批量试验, 发现在GRAPES模式垂直扩散方案源代码的基础上逐句线性化得到的切线性垂直扩散方案即使能通过正确性测试试验, 在少数情况下也会存在很大误差。切线性模式计算的扰动气压场和扰动风场可能出现明显异常, 这种异常与垂直扩散方案中地表动量通量的强非线性有关。如果在切线性垂直扩散方案中忽略地表动量通量扰动, 既可以避免异常的出现, 又不影响其他正常时刻的计算精度。修改后的切线性垂直扩散方案能够在所有变量上一致地提高切线性模式的计算精度。 相似文献
20.
Two parameterization schemes for vertical eddy diffusivity were utilized to investigate their impacts on both the daily and monthly mean concentrations of ozone and NOy, which are the major fractions of the sum of all reactive nitrogen species, i.e., NOy=NO＋NO2＋HNO3＋PAN. Simulations indicate that great changes in the vertical diffusivity usually occur within the planetary boundary layer （PBL）. Daily and monthly mean concentrations of NOy are much more sensitive to changes in the vertical diffusivity than those of ozone and ozone and NOy levels only at or in （relatively） clean sites and areas, where long-range transport plays a crucial role, display roughly equivalent sensitivity. The results strongly suggest that a widely-accepted parameterization scheme be selected and the refinement of the model＇s vertical resolution in the PBL be required, even for regional and long-term studies, and ozone only being examined in an effort to judge the model＇s performance be unreliable, and NOy be included for model evaluations. 相似文献