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1.
A prolonged down-valley flow and low-level jet were observed throughout the Enhanced Observing Period 4 (April 28–29) of the 2006 Terrain-induced Rotor Experiment, held in the Owens Valley of California near the town of Independence. The low-level jet was strongest during the nocturnal hours, and special field observations captured important details of the event lifecycle. High-resolution simulations using the Weather Research and Forecasting numerical weather prediction model were generated, with underlying assumptions being that model resolution, boundary layer physics, and nesting configuration would be dominant controlling factors in reproducing the jet. The large-scale conditions were dry throughout the event, so moist physics were not a significant forcing consideration. For the control simulation, a two-nest (4.5 and 1.5 km grid spacing) configuration with 90 vertical levels was applied. Additionally, the Quasi-Normal Scale Elimination planetary boundary and surface layer option were selected due to its published performance under conditions of stable stratification. Three other sensitivity simulations were run for comparison, differing from the control just in the choice of vertical resolution (60 versus 90 levels with Quasi-Normal Scale Elimination) and planetary boundary/surface layer physics (90 levels/Mellor-Yamada-Jancic; 90 levels/Yonsei State University). Although the gross evolution (location, height, and timing) of the low-level jet is captured by all model runs (with the 1.5 km inner nest providing the more accurate details), there were at times large underestimations of the nocturnal jet speed max in each simulation (approaching 100 % error, or up to almost 10 m s?1). Overall, the variations of vertical resolution and planetary boundary/surface physics against the control seemed to (1) yield little overall improvement to statistical or subjective evaluations; (2) do little to improve deficiencies in reproducing the magnitude strength of the nocturnal down-valley low-level jet. Since the cold-start simulations spanned 36 h (including a 12-h spin-up period), it was suspected that the lateral boundary conditions imposed on the outermost 4.5 km nest might negatively impact the interior model solutions in the Owens Valley. To investigate this possibility, an additional simulation was executed by adding two extra nests to the control configuration: an outer 13.5 km and an inner 500 m. This simulation produced a better evolution of the nocturnal low-level jet and especially the speed max. The addition of the larger 13.5 km nest appears more critical to this improvement than that of the extra spatial resolution provided by the inner 500 m nest, which supports the idea that accurate capturing of the large-scale synoptic condition was critical in reproducing important details of this down-valley low-level jet event. The extra 500-m resolution did seem to improve the morning valley cold pool forecast.  相似文献   

2.
A realistic simulation of katabatic flows is not a straightforward task for numerical models. One complicating factor is that katabatic flows develop within a stably stratified boundary layer, which is poorly resolved and described in many numerical models. To capture the jet-shaped shallow flow a model set-up with high vertical resolution is also required. In this study, ‘a state of the art’ mesoscale numerical model is applied in a simulation of katabatic flow over a melting glacier. A basic agreement between observations and model results is found. From scale analysis, it is concluded that the simulated flow can be classified as katabatic. Although the background flow varies in strength and direction, the simulated katabatic flow over Breidamerkurjökull is persistent. Two factors vital for this persistence are identified. First, the melting snow maintains the surface temperature close to 0 °C while the air temperature warms adiabatically as it descends the slope. This provides a ‘self enhanced’ negative buoyancy that drives the flow to a balance with local friction. Second, the jet-like shape of the resulting flow gives rise to a large ‘curvature term’ in the Scorer parameter, which becomes negative in the upper jet. This prevents vertical wave propagation and isolates the katabatic layer of the influence from the free troposphere aloft. Our results suggest that the formation of local microclimates dominated by katabatic flow is a general feature over melting glaciers. The modelled turbulence structure illustrates the importance of non-local processes. Neglecting the vertical transport of turbulence in katabatic flows is not a valid assumption. It is also found that the local friction velocity remains larger than zero through the katabatic jet, due to directional shear where the scalar wind speed approaches its maximum.  相似文献   

3.
Summary. ?A hydrostatic numerical model is used to simulate the lee wave event IOP3 (0000 GMT to 1200 GMT 15th October 1990) from the PYREX mountain experiment. Results from integrations at different horizontal resolutions are used to investigate the effect on surface pressure drag and the vertical flux of horizontal momentum due to orographically forced gravity waves. In particular, results showing the dependence on resolution of the partitioning between resolved and parametrized wave drag and fluxes are presented. With the model horizontal gridlength changing from 50 km to 10 km the majority of wave momentum flux changes from being parametrized to becoming resolved. More significantly, there is a change in the profile of flux with height. At 50 km resolution the largest inferred mean flow decelerations are at lower stratospheric level due to the parametrization scheme. At 10 km resolution this is shifted, with less deceleration high up and more wave deceleration lower down in the troposphere. Numerical weather prediction models are now beginning to take account of such low level drag with beneficial results. Received March 2, 1999/Revised July 15, 1999  相似文献   

4.
The influence of increased horizontal resolution on regional climate models simulations of 1961–1990 period was investigated with a focus on precipitation. The main attention was paid to the annual cycle of precipitation described by a special characteristic, precipitation half-time. Two models (RegCM3 and ALADIN-CLIMATE/CZ), both of them in two horizontal resolutions (25 and 10 km), were used. An evaluation of model simulations with 25 km resolution on the European domain is presented as well as a more detailed evaluation of both 25 and 10 km versions on the area of the Czech Republic. Generally, the effects of increased horizontal resolution vary with climate model and evaluated characteristic. For the precipitation amount and the dependence of precipitation amount on altitude, the increase in horizontal resolution decreases the accuracy of results in both models. For the simulation of annual precipitation cycle and the precipitation half-time, RegCM3 results improved with the increased horizontal resolution, whereas ALADIN-CLIMATE/CZ results worsened.  相似文献   

5.
A hierarchical modeling approach is used to study the process by which interactions of easterly waves with the background flow can result in a reduction in the longitudinal and vertical scale of the waves. Theory suggests that in flows that possess a negative longitudinal gradient (U x  < 0) there is a reduction of longitudinal and vertical group speeds and an increase in regional wave action density (or “wave energy”). Relative vorticity increases locally leading to an increase in the likelihood of tropical cyclogenesis near the wave axis. Opposite impacts on the structure of the waves is expected in a U x  > 0 domain. In the simplified framework of a free-surface and divergent shallow water model, Rossby wave properties are tracked through a range of background flow scenarios to determine the important scales of interaction. The importance of wave energy accumulation for tropical cyclogenesis is then studied in a full physics and dynamics model using a nested regional climate model simulation, at 12 km horizontal grid spacing, over the tropical North Atlantic region for the entire 2005 hurricane season. The dynamical environment within which 70% of easterly waves formed tropical cyclones exhibits coherent regions in which easterly winds increase towards the east, consistent with the occurrence of wave energy accumulation.  相似文献   

6.
Interaction of a zonal jet and small-amplitude Rossby-wave turbulence is studied within the framework of the barotropic β-plane model. It is demonstrated that turbulent-laminar interaction in this case transfers energy from the wave turbulence to the laminar flow (the effect of negative friction). We derive a conclusion that, as the geophysical turbulence is determined partly by wave turbulence and none of the traditional heuristic models can adequately describe the effect of negative friction associated with wave turbulence, the application of these models to the ‘real’ ocean and atmosphere is unreliable.It is also demonstrated that, as they are affected by the turbulence, all westward jets slowly expand without strengthening. Each jet has a core, within the limits of which the velocity of the fluid is constant. In some cases, the core expands faster than the jet periphery, resulting in jumps on the profile of the flow. All eastward jets are steady irrespective of their profiles.  相似文献   

7.
Summary In this paper, we will focus on the real-time prediction of environments that are predisposed to producing moderate-severe (hazardous) aviation turbulence. We will describe the numerical model and its postprocessing system that is designed for said prediction of environments predisposed to severe aviation turbulence as well as presenting numerous examples of its utility. The purpose of this paper is to demonstrate that simple hydrostatic precursor circulations organize regions of preferred wave breaking and turbulence at the nonhydrostatic scales of motion. This will be demonstrated with a hydrostatic numerical modeling system, which can be run in real time on a very inexpensive university computer workstation employing simple forecast indices. The forecast system is designed to efficiently support forecasters who are directing research aircraft to measure the environment immediately surrounding turbulence. The numerical model is MASS version 5.13, which is integrated over three different grid matrices in real-time on a university workstation in support of NASA-Langley’s B-757 turbulence research flight missions. The model horizontal resolutions are 60, 30, and 15 km and the grids are centered over the region of operational NASA-Langley B-757 turbulence flight missions. The postprocessing system includes several turbulence-related products including four turbulence forecasting indices, winds, streamlines, turbulence kinetic energy, and Richardson numbers. Additionally there are convective products including precipitation, cloud height, cloud mass fluxes, lifted index, and K-index. Furthermore, soundings, sounding parameters, and Froude number plots are also provided. The horizontal cross section plot products are provided from 16,000–46,000 feet in 2,000 feet intervals. Products are available every three hours at the 60 and 30 km grid interval and every 1.5 hours at the 15 km grid interval. The model is initialized from the NWS ETA analyses and integrated two times a day.  相似文献   

8.
About 75 % of the Antarctic surface mass gain occurs over areas below 2,000 m asl, which cover 40 % of the grounded ice-sheet. As the topography is complex in many of these regions, surface mass balance modelling is highly dependent on horizontal resolution, and studying the impact of Antarctica on the future rise in sea level requires physical approaches. We have developed a computationally efficient, physical downscaling model for high-resolution (15 km) long-term surface mass balance (SMB) projections. Here, we present results of this model, called SMHiL (surface mass balance high-resolution downscaling), which was forced with the LMDZ4 atmospheric general circulation model to assess Antarctic SMB variability in the twenty first and the twenty second centuries under two different scenarios. The higher resolution of SMHiL better reproduces the geographical patterns of SMB and increase significantly the averaged SMB over the grounded ice-sheet for the end of the twentieth century. A comparison with more than 3200 quality-controlled field data shows that LMDZ4 and SMHiL reproduce the observed values equally well. Nevertheless, field data below 2,000 m asl are too scarce to efficiently show the added value of SMHiL and measuring the SMB in these undocumented areas should be a future scientific priority. Our results suggest that running LMDZ4 at a finer resolution (15 km) may give a future increase in SMB in Antarctica that is about 30 % higher than by using its standard resolution (60 km) due to the higher increase in precipitation in coastal areas at 15 km. However, a part (～15 %) of these discrepancies could be an artefact from SMHiL since it neglects the foehn effect and likely overestimates the precipitation increase. Future changes in the Antarctic SMB at low elevations will result from the competition between higher snow accumulation and runoff. For this reason, developing downscaling models is crucial to represent processes in sufficient detail and correctly model the SMB in coastal areas.  相似文献   

9.
Over a range of incidence angles the energy of an internal wave propagating onto a sloping boundary is concentrated in a boundary layer on the slope. As a wave propagates upslope the change in its amplitude and interaction with the downslope flow remaining from previous waves results in the wave breaking and the generation of turbulence and mixing in the boundary layer. Measurements of the overturning and buoyancy scales on the slope show that turbulence is generated and decays during each wave cycle and that much of the energy input to mixing scales is extracted from density inversions generated by the wave-induced mean flow. A comparison with decaying turbulence behind a grid in a stratified water tunnel suggests that the criterion for the extinction of the buoyancy flux is similar in the two cases.  相似文献   

10.
基于不可压缩流体运动方程组研究了急流附近重力惯性波及其不稳定,结合飞机风速变化方程,分析指出重力波失稳破碎为湍流是飞机颠簸产生的可能机制。斜压大气在急流轴北侧气旋切变区是惯性稳定的,当满足条件σ=f[f-/y] N2m2/n2<0时,由于天气尺度对流不稳定发展而引起重力惯性波不稳定破碎为湍流,可能是急流北侧气旋切变区对流性天气引起飞机颠簸产生的一种机制。但是在急流轴南侧反气旋切变区是惯性不稳定的,当满足条件σ=f[f-/y] N2m2/n2<0时,可能由于惯性不稳定的作用,急流重力波不稳定发展破碎为湍流,可能是急流南侧反气旋气流中晴空湍流和飞机颠簸产生和发展的一种机制。揭示了急流附近晴空湍流和飞机颠簸产生的物理本质,有利于增强航空飞行颠簸的预测能力。  相似文献   

11.
Li  Yana  Lau  Ngar-Cheung  Tam  Chi-Yung  Cheung  Ho-Nam  Deng  Yi  Zhang  Henian 《Climate Dynamics》2021,56(11):4013-4026

Summer monsoonal rainfall over East Asia is dominated by precipitation associated with the East Asian summer monsoonal front (EASMF). A Community Atmospheric Model (CAM5.1) with a high horizontal resolution of 50 km is employed in this study to investigate the interannual variability as well as projected future trends in the EASMF under the Representative Concentration Pathway 8.5 scenario. Seasonal march of the EASMF is reproduced reasonably well in the model’s present-day simulation despite a northward shift of the simulated front from its observed position. Based upon a suite of objectively-defined daily indices of the EASMF, we show that the EASMF in the late twenty-first century will be more intense and displaced eastward and southward from its present-day mean location. Moreover, EASMF events will exhibit a wider meridional expansion and a longer duration. Monsoonal precipitation over East Asia is particularly sensitive to the meridional displacements of EASMF. In conjunction with the projected southward shift of EASMF, an enhanced rain band is seen to extend northeastward from southern China to the northwestern Pacific south of Japan. This precipitation feature is associated with strengthened and southward-shifted westerly jet streams at 250 and 700 hPa, which are respectively linked to tropical warming in the upper troposphere and warming over the South China Sea in the lower troposphere during the twenty-first century. Within the latitudinal “gap” south of the upper-level jet and north of the lower-level jet, the local vorticity tendencies are maintained by upper-level divergence and lower-level convergence, thus accompanied by enhanced upward motion and precipitation. The site at which this “jet stream-precipitation” relationship prevails is notably modulated by long-term trends in the temperature and circulation patterns associated with climate change.

  相似文献   

12.
We present an analysis of data from a nearly 1-year measurement campaign performed at Høvsøre, Denmark, a coastal farmland area where the terrain is flat. Within the easterly sector upstream of the site, the terrain is nearly homogenous. This topography and conditions provide a good basis for the analysis of vertical wind-speed profiles under a wide range of atmospheric stability, turbulence, and forcing conditions. One of the objectives of the campaign was to serve as a benchmark for flow over flat terrain models. The observations consist of combined wind lidar and sonic anemometer measurements at a meteorological mast. The sonic measurements cover the first 100 m and the wind lidar measures above 100 m every 50 m in the vertical. Results of the analysis of observations of the horizontal wind-speed components in the range 10–1200 m and surface turbulence fluxes are illustrated in detail, combined with forcing conditions derived from mesoscale model simulations. Ten different cases are presented. The observed wind profiles approach well the simulated gradient and geostrophic winds close to the simulated boundary-layer height during both barotropic and baroclinic conditions, respectively, except for a low-level jet case, as expected. The simulated winds are also presented for completeness and show good agreement with the measurements, generally underpredicting the turning of the wind in both barotropic and baroclinic cases.  相似文献   

13.
Summary ¶This study presents a numerical simulation of the bora wind as it occurs in form of a severe wind blowing down coastal mountains and over the Adriatic Sea. A typical cyclonic bora event, occurring during the period January 3–6, 1995, is simulated using a nested limited area model. An integration, with horizontal resolution of about 14km, and a nested one, with higher resolution, about 5.5km, are presented. The 1997 version of the Eta Model is used for both the lower resolution and the higher resolution runs. Numerous details of the simulation are found to be in good agreement with the understanding as well as the observational knowledge of the bora, thus supporting confidence in the realism of the results. In particular, features of the simulated flow are seen strongly indicative of some basic characteristics of the hydraulic model of the phenomenon, such as the mountain wave breaking and the upstream flow acceleration. Moreover, the increase in horizontal resolution, in combination with an improvement of the coastal SST information, led to a still improved realism of the low-level wind representation over the Adriatic Sea.Received December 31, 2001; revised March 25, 2002; accepted July 19, 2002 Published online: February 20, 2003  相似文献   

14.
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15.
索马里低空急流的数值模拟   总被引：1，自引：1，他引：0  
本文首先将5°×5°P—σ五层原始方程粗网格模式改进为2.5°×2.5°的细网格模式,进而设计了粗细网格的嵌套模式。用嵌套模式对索马里低空急流进行了数值模拟,其结果表明:模式的水平分辨率对急流的模拟效果影响很大,模拟的急流轴上最大风速为13米/秒,与急流实际强度较为接近。 我们用六月纬向平均风场作初值,进行了10天的数值积分。根据风速的时间变化将急流的演变分为形成、加强及维持三个阶段,对急流各个阶段的流场及结构特征作了详细讨论。索马里沿岸低空气流由南向北越赤道气流的上空500百帕和300百帕层正好相反,气流由北向南穿越赤道。还通过控制性试验讨论了索马里低空急流向北发展的情况。   相似文献   

16.
Using large-eddy simulation (LES), the effects of mesoscale local surface heterogeneity on the temporal evolution of low-level flows in the convective boundary layer driven by two-dimensional surface heat-flux variations are investigated at a height of about 100 m over flat terrain. The surface variations are prescribed with sinusoids of wavelength 32 km and varying amplitudes of 0, 50, 100, and 200 W m $^{-2}$ . The Weather Research and Forecasting numerical model is used as a mesoscale-domain LES model that has a grid spacing fine enough to explicitly resolve energy-containing turbulent eddies and a model domain large enough to include mesoscale circulations. Mesoscale circulations induced by the two-dimensional surface heterogeneity may undergo a flow transition and an associated spectral energy cascade, which has been found previously but only with one-dimensional surface heat-flux variations. Over a strongly heterogeneous surface prescribed with a two-dimensional sinusoid of amplitude 200 W m $^{-2}$ , the domain-averaged variance of the horizontal wind component initially grows rapidly, then undergoes a flow transition and subsequently rapidly decays. With a background wind, the induced mesoscale circulations are inhibited in the streamwise direction. However in the spanwise direction, somewhat stronger mesoscale circulations are induced, compared with those with no background wind. The background wind attenuates the significant reduction of the low-level temperature gradient by the fully-developed mesoscale horizontal flow. Spectral decomposition reveals that this rapid transition also exists in the mesoscale horizontal flows induced by the intermediate surface heterogeneity prescribed with a sinusoid of amplitude 100 W m $^{-2}$ . However the transition is masked by continuously growing turbulence.  相似文献   

17.
A one-dimensional Explicit Time-dependent Tilting cloud Model (ETTM) that separates updraft and downdraft columns and takes into account the effect of cloud tilting on precipitation is introduced and incorporated into the Advanced Regional Prediction System (ARPS). Results of the stand-alone ETTM are compared with that of cloud resolving simulations using the ARPS mesoscale model. Inter-comparison is performed by qualitative examination of simulated parameters such as vertical distribution of fluxes of mass, heat, and moisture. Although there is a great degree of similarity between the vertical profiles, ETTM systematically underestimates magnitudes of all fluxes. Sensitivity tests carried with ETTM show that the effect of varying cloud radius and tilting angle is considerable on the simulated cloud behavior. Increasing the cloud radius, results in a corresponding increase in fluxes of mass, heat, and moisture, while increasing the cloud tilt angle has the opposite effect. Since ETTM showed promise as a suitable sub-grid cumulus parameterization scheme; it was incorporated into ARPS as an additional cumulus parameterization scheme (CPS) to be available for the wider community. Results of simulations using ETTM and other CPSs already available in ARPS were compared for 2, 4 and 10 km grid spacings to assess its utility. Simulation results of the 2 km grid showed that at this resolution, the simulated time series of updraft velocities using the new scheme (ETTM) compared well with the results of other schemes in the ARPS model. The simulations with horizontal resolution of 4 km that was compared with the convection resolving reference run (No-CPS-2KM) showed almost consistent results for all schemes except for one using KF scheme. The results of the simulation with the ETTM scheme and other schemes in the model with resolution of 10 km showed that at this resolution, there is not significant difference between the uses of these schemes.  相似文献   

18.
Summary In this paper, we describe the results of 44 case study analyses of synoptic scale data sets that define the atmospheric structure prior to the development of accident-producing turbulence. First, the 44 case studies are categorized as a function of the location, altitude, time of year, time of day, and turbulence environment, i.e., in clear air, cloudiness, convection, near mountains, or in the proximity of deep convection. It is noteworthy that this later category was much more ubiquitous than was anticipated. Second, NCEP Reanalysis data sets as well as both visible and infrared satellite imagery are employed to diagnose predictor fields associated with the synoptic-scale environment preceding severe turbulence. These predictor fields are calculated based on jet stream configuration, kinematic, dynamical, and thermodynamic analyses of the synoptic-scale atmosphere.The results of these analyses indicate a prevalence of severe accident-producing turbulence within the entrance region of the polar or subtropical jet stream at the synoptic-scale. Typically, there is a region of flow curvature located just upstream within the jet entrance region, convection is present within 100km of the accident, the vertical motion is upward typically within the curved entrance region, absolute vorticity is low, the vertical wind shear is increasing with time, and horizontal cold air advection is substantial. Not all of the 44-case studies conform to this entrance region paradigm. However, most do and the most consistent predictor of severe turbulence is upstream curvature in the synoptic-scale flow. Nearby convection is the second most ubiquitous predictor field. Upward vertical motion, low absolute vorticity, and horizontal cold air advection are all typical predictors in case studies occurring both within the entrance and exit regions of the polar or subtropical jet stream.Current affiliation: USDA/Forest Service, North Central Research Station, East Lansing, MI 48823.  相似文献   

19.
In current operational numerical weather prediction models, the effect of shallow convection is parametrized. The grey zone of shallow convection is found between the horizontal resolutions of mesoscale numerical models (2–3 km) and large-eddy simulations (10–100 m or finer). At these horizontal scales the shallow convection is to some extent explicitly resolved by the model. The shallow-convection parametrization is still needed, but has to be regulated according to the model horizontal resolution. Here the behaviour of the non-hydrostatic mesoscale numerical weather prediction model Application of Research to Operations at Mesoscale is examined in the grey zone and a new scale-adaptive surface closure of its shallow-convection parametrization, dependent on horizontal resolution, is defined based on large-eddy simulation. The new closure is tested on a series of numerical experiments and validated on a 15-day-long real case period. Its impact on the development of deep convection is examined in detail. The idealized simulations show promising results, as the mean profiles of the subgrid and resolved turbulence change in the desired way. Based on the real case tests our modification has a low impact on model performance, but is part of a set of upgrades of the current parametrization that is aimed to treat the shallow convection grey zone.  相似文献   

20.
A large-eddy simulation model with rotated coordinates and an open boundary is used to simulate the characteristics of katabatic flows over simple terrain. Experiments examine the effects of cross winds on the development of the slope-flow boundary layer for a steep (20°) slope and the role of drainage winds in preventing turbulence collapse on a gentle slope (1°). For the steep flow cases, comparisons between model average boundary-layer velocity, temperature deficit, and turbulence kinetic energy budget terms and tower observations show reasonable agreement. Results for different cross slope winds show that as the cross slope winds increase, the slope flow deepens faster and behaves more like a weakly stratified, sheared boundary layer. Analysis of the momentum budget shows that near the surface the flow is maintained by a balance between downslope buoyancy forcing and vertical turbulence flux from surface drag. Above the downslope jet, the turbulence vertical momentum flux reverses sign and acceleration of the flow by buoyancy is controlled by horizontal advection of slower moving ambient air. The turbulence budget is dominated by a balance between shear production and eddy dissipation, however, buoyancy and pressure transport both are significant in reducing the strength of turbulence above the jet. Results from the gentle slope case show that even a slight terrain variation can lead to significant drainage winds. Comparison of the gentle slope case with a flat terrain simulation indicates that drainage winds can effectively prevent the formation of very stable boundary layers, at least near the top of sloping terrain.  相似文献   

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