共查询到20条相似文献，搜索用时 31 毫秒
1.
Stefan Zacharias Mark Reyers Joaquim G. Pinto Jan H. Schween Susanne Crewell Michael Kerschgens 《Meteorology and Atmospheric Physics》2012,117(1-2):47-61
High-resolution simulations with a mesoscale model are performed to estimate heat and moisture budgets of a well-mixed boundary layer. The model budgets are validated against energy budgets obtained from airborne measurements over heterogeneous terrain in Western Germany. Time rate of change, vertical divergence, and horizontal advection for an atmospheric column of air are estimated. Results show that the time trend of specific humidity exhibits some deficiencies, while the potential temperature trend is matched accurately. Furthermore, the simulated turbulent surface fluxes of sensible and latent heat are comparable to the measured fluxes, leading to similar values of the vertical divergence. The analysis of different horizontal model resolutions exhibits improved surface fluxes with increased resolution, a fact attributed to a reduced aggregation effect. Scale-interaction effects could be identified: while time trends and advection are strongly influenced by mesoscale forcing, the turbulent surface fluxes are mainly controlled by microscale processes. 相似文献
2.
本文选取了7个实例,分析冷锋前浙江大范围冰雹天气的发生条件。大范围冰雹天气发生前,必须具有较大的位势不稳定能和较强的边界层辐合。强的风速垂直切变并不是必要条件。冰雹天气发生前6—12小时,对流层中、下部不一定有大尺度辐合上升。冰雹发生之前,边界层内暖湿空气十分强盛,而中层的冷干空气活动一般并不明显。因此不稳定能的形成主要是由于边界层内暖湿平流的结果。冰雹天气过程的水汽通量散度,与一般暴雨过程不同,前者的水汽辐合几乎全部集中在边界层内。冷锋位置、边界层辐合线、潜在不稳定度和对流不稳定度这四个因子相结合,可能是预报冷锋前有无大范围冰雹天气发生的有效信息。 相似文献
3.
The viscous semigeostrophic solutions obtained for the baroclinic Eady wave fronts are analyzed for the generation of the cross-frontal temperature gradient in the boundary layer. In the case of free-slip boundaries, the cross-frontal gradient is maximally generated at the surface by meridional temperature advection. In the case of no-slip boundaries, surface friction reduces the meridional temperature advection in the boundary layer: The maximum generation occurs above the surface layer and the temperature gradient at the surface is maintained by vertical diffusion. The no-slip solution is compared with the Ekman-layer model solution. Errors are quantified for the use of the Ekman-layer model in the mature state of frontogenesis.The surface frontogenesis is found to be affected by diffusivity both directly and indirectly. The direct effect of diffusivity is represented explicitly by the diffusion term in the potential temperature equation. The indirect effect of diffusivity is related implicitly to the temperature advection caused by the viscous part of the ageostrophic motion whose horizontal velocity component is defined by the frictional wind deflection (away from the geostrophy). The direct effect of diffusivity is frontolytical, whilst theindirect effect of diffusivity is frontogenetic in the mesoscale vicinity of the front. The indirect effect of diffusivity contributes dominantly to the mesoscale surface frontogenesis for the free-slip case, but it is offset by the divergence of the dynamic part of the ageostrophic motion at the surface level for the non-slip case. 相似文献
4.
The South China Sea(SCS) is an eddy-active area. Composite analyses based on 438 mesoscale ocean eddies during 2000–2012 revealed the status of the atmospheric boundary layer is influenced remarkably by such eddies. The results showed cold-core cyclonic(warm-core anticyclonic) eddies tend to cool(warm) the overlying atmosphere and cause surface winds to decelerate(accelerate). More than 5% of the total variance of turbulent heat fluxes, surface wind speed and evaporation rate are induced by mesoscale eddies. Furthermore, mesoscale eddies locally affect the columnar water vapor, cloud liquid water, and rain rate. Dynamical analyses indicated that both variations of atmospheric boundary layer stability and sea level pressure are responsible for atmospheric anomalies over mesoscale eddies. To reveal further details about the mechanisms of atmospheric responses to mesoscale eddies, atmospheric manifestations over a pair of cold and warm eddies in the southwestern SCS were simulated. Eddy-induced heat flux anomalies lead to changes in atmospheric stability. Thus, anomalous turbulence kinetic energy and friction velocity arise over the eddy dipole, which reduce(enhance) the vertical momentum transport over the cold(warm) eddy, resulting in the decrease(increase) of sea surface wind. Diagnoses of the model's momentum balance suggested that wind speed anomalies directly over the eddy dipole are dominated by vertical mixing terms within the atmospheric boundary layer, while wind anomalies on the edges of eddies are produced by atmospheric pressure gradient forces and atmospheric horizontal advection terms. 相似文献
5.
利用常规地面、高空观测、雷达及ERA5再分析等资料，对山东初冬一次极端降水、大风天气成因分析，结果表明：低槽东移发展，冷空气南压，低空切变线配合东北、西北地区地面高压坝形成的“阻挡”形势利于极端降水的产生。本次水汽条件具有较强的极端性，水汽通量辐合远强于气候平均态，925 hPa和700 hPa水汽通量辐合大值区分别与雨、雪区域配合较好。降雨时垂直上升运动中心在边界层，升至600 hPa时转为降雪，降雨时低层辐合、高层辐散，降雪时由低到高呈辐散-辐合-辐散分布。冷锋过境条件对称不稳定触发产生对流，随后在冷锋后侧逆温层上由锋生过程的上升支环流强迫产生高架对流。强冷空气扰动从内蒙古高原下滑至华北平原，与近地面冷平流汇合增强，产生较强变压风，同时促进了势能向动能转换和动量下传。地形强迫造成下沉运动增强，华北地区低层形成锋面次级环流，环流前部锋区暖界面为地转偏差辐合，冷界面为地转偏差辐散。环流内有水平动能和地转偏差大值区，偏北气流和下沉运动使水平动能向南、向下输送，导致地面极端大风。 相似文献
6.
梅雨期西南低涡的东移发展与长江中下游降水的加强有密切关系,作者采用中尺度模式对一次西南低涡及其伴随低空急流的发展演变进行了数值模拟.模拟结果表明:在长江中下游大巴山地区低空急流先于西南涡东移发展;西南低涡及低空急流的生成发展在开始阶段与中层(400 hPa)的弱辐散密切相关;南风分量在西南低空急流的演变发展过程中起着更为主动的作用;南风分量增大中心位于南风分量中心的前方,促使南风分量中心东移;南风分量的动量方程收支分析表明水平平流项和产生项是促使南风分量变化的主要作用项,水平平流项和垂直平流项大部分相互抵消,科里奥利项的作用不可忽视,而其他项的值较小,在个别阶段和地区行星边界层项的作用在低层也较大. 相似文献
7.
A three-dimensional model of the mesoscale surface boundary layer of the open ocean is developed through scale analysis of the primitive equations with mixing included. A set of surface boundary-layer equations appropriate for a broad range of oceanic and atmospheric scales is thereby derived. The essential basis of the model is a coupling between quasigeostrophic dynamics away from the boundary layer and arbitrary mixing models within the mixed layer. The coupling consists of advection of the boundary layer by the horizontal and vertical components of the interior quasigeostrophic flow and forcing of the interior by the boundary layer in the form of divergence within the boundary layer which leads to vortex stretching/compression in the interior. The divergence is generalized for mesoscale wind-driven flows and includes nonlinear interaction between the directly wind-driven boundary-layer flow and the interior flow in the form of interior relative vorticity advection by the wind-driven flow. The nature of the equations leads us to apply a numerical algorithm to their solution. This algorithm is calibrated through application to idealized problems to determine the temporal and spatial grid requirements. The model is initialized with a realistic ocean flow having the properties of the Gulf Stream. 相似文献
8.
G. Pigeon A. Lemonsu C. S. B. Grimmond P. Durand O. Thouron V. Masson 《Boundary-Layer Meteorology》2007,124(2):269-290
This study quantifies the processes that take place in the layer between the mean building height and the measurement level
of an energy balance micrometeorological tower located in the dense old core of a coastal European city. The contributions
of storage, vertical advection, horizontal advection and radiative divergence for heat are evaluated with the available measurements
and with a three-dimensional, high-resolution meteorological simulation that had been evaluated against observations. The
study focused on a summer period characterized by sea-breeze flows that affect the city. In this specific configuration, it
appears that the horizontal advection is the dominant term. During the afternoon when the sea breeze is well established,
correction of the sensible heat flux with horizontal heat advection increases the measured sensible heat flux up to 100 W m−2. For latent heat flux, the horizontal moisture advection converted to equivalent latent heat flux suggests a decrease of
50 W m−2. The simulation reproduces well the temporal evolution and magnitude of these terms. 相似文献
9.
Ankur R. Desai Kenneth J. Davis Christoph J. Senff Syed Ismail Edward V. Browell David R. Stauffer Brian P. Reen 《Boundary-Layer Meteorology》2006,119(2):195-238
The atmospheric boundary-layer (ABL) depth was observed by airborne lidar and balloon soundings during the Southern Great Plains 1997 field study (SGP97). This paper is Part I of a two-part case study examining the relationship of surface heterogeneity to observed ABL structure. Part I focuses on observations. During two days (12–13 July 1997) following rain, midday convective ABL depth varied by as much as 1.5 km across 400 km, even with moderate winds. Variability in ABL depth was driven primarily by the spatial variation in surface buoyancy flux as measured from short towers and aircraft within the SGP97 domain. Strong correlation was found between time-integrated buoyancy flux and airborne remotely sensed surface soil moisture for the two case-study days, but only a weak correlation was found between surface energy fluxes and vegetation greenness as measured by satellite. A simple prognostic one-dimensional ABL model was applied to test to what extent the soil moisture spatial heterogeneity explained the variation in north–south ABL depth across the SGP97 domain. The model was able to better predict mean ABL depth and variations on horizontal scales of approximately 100 km using observed soil moisture instead of constant soil moisture. Subsidence, advection, convergence/divergence and spatial variability of temperature inversion strength also contributed to ABL depth variations. In Part II, assimilation of high-resolution soil moisture into a three-dimensional mesoscale model (MM5) is discussed and shown to improve predictions of ABL structure. These results have implications for ABL models and the influence of soil moisture on mesoscale meteorology 相似文献
11.
引发暴雨天气的中尺度低涡的数值研究 总被引：1，自引：1，他引：0
2008年7月17—19日发生在山东的大到暴雨天气是由“海鸥”台风和副热带高压共同向山东输送水汽,与弱冷空气相互作用造成的。对流层低层的中尺度低涡是暴雨天气的直接制造者。利用常规观测资料和中尺度模式WRF（Weather Research and Forecasting）的模拟资料对该中尺度低涡的结构及形成机制进行了分析研究。结果表明,数值模拟可以清楚地捕捉到中尺度低涡东移过程中有新的涡旋中心形成,并与原来的涡旋中心合并的过程,而不是简单的沿切变线东移。中尺度低涡形成在增温增湿明显、上升运动为主的对流区内;中尺度低涡形成后其中心转为下沉运动,对流区东移,降水区位于低涡的东北和东南象限。中尺度低涡上空近地面层的冷池、600～400hPa的弱冷空气堆、900～850hPa的弱风区及高低空急流耦合发展是中尺度低涡形成和发展阶段的重要特征。中尺度低涡减弱阶段,下沉运动变强,低空急流和高空出流都明显减弱。涡度方程的收支表明,对流层低层的散度项、倾侧项及对流层中层的水平平流项和铅直输送项是正涡度的主要贡献者。中低层的水平辐合、涡度由低层向高层的垂直输送都有利于中尺度低涡的形成和发展。倾侧项对中尺度低涡的形成也有重要贡献。中尺度低涡形成后期,低层辐合、高层辐散及垂直输送的减弱导致正涡度制造的减弱,从而使中尺度低涡减弱。 相似文献
12.
地气通量中存贮和平流项计算方案的探讨 总被引：1，自引：0，他引：1
从物质收支方程出发,推导了一个包含物质存贮、水平平流输送、垂直对流输送以及传统的涡度相关项的地气通量计算方程。平流项本质上是地表非均匀性的结果,不同下垫面的感热和潜热通量也不同,将会产生中尺度环流,使得辐合辐散过程得以维持,从而将体元内的物质输送到体元以外,因此可以通过计算水汽和感热的存贮,间接求出物质的水平平流输送。量纲分析和实际的资料应用都表明,存贮和平流的通量贡献是非常小的。尤其是在均匀下垫面下,方程中的存贮和平流项的通量贡献可以忽略,因此估算地气通量时仅需考虑涡度相关项和Webb修正项即可。而在非均匀下垫面下,在1 d以上的时间尺度上,为方便计算,可以忽略存贮和平流的通量贡献;而在小时这样的时间尺度上,从物质能量收支守恒的角度考虑,估算地气通量需要包括存贮和平流的通量贡献。 相似文献
13.
S. Khodayar N. Kalthoff M. Fiebig-Wittmaack M. Kohler 《Meteorology and Atmospheric Physics》2008,99(3-4):181-198
Summary The boundary-layer structure of the Elqui Valley is investigated, which is situated in the arid north of Chile and extends
from the Pacific Ocean in the west to the Andes in the east. The climate is dominated by the south-eastern Pacific subtropical
anticyclone and the cold Humboldt Current. This combination leads to considerable temperature and moisture gradients between
the coast and the valley and results in the evolution of sea and valley wind systems. The contribution of these mesoscale
wind systems to the heat and moisture budget of the valley atmosphere is estimated, based on radiosoundings performed near
the coast and in the valley.
Near the coast, a well-mixed cloud-topped boundary layer exists. Both, the temperature and the specific humidity do not change
considerably during the day. In the stratus layer the potential temperature increases, while the specific humidity decreases
slightly with height. The top of the thin stratus layer, about 300 m in depth, is marked by an inversion. Moderate sea breeze
winds of 3–4 m s−1 prevail in the sub-cloud and cloud layer during daytime, but no land breeze develops during the night.
The nocturnal valley atmosphere is characterized by a strong and 900 m deep stably stratified boundary layer. During the day,
no pronounced well-mixed layer with a capping inversion develops in the valley. Above a super-adiabatic surface layer of about
150 m depth, a stably stratified layer prevails throughout the day. However, heating can be observed within a layer above
the surface 800 m deep. Heat and moisture budget estimations show that sensible heat flux convergence exceeds cold air advection
in the morning, while both processes compensate each other around noon, such that the temperature evolution stagnates. In
the afternoon, cold air advection predominates and leads to net cooling of the boundary layer. Furthermore, the advection
of moist air results in the accumulation of moisture during the noon and afternoon period, while latent heat flux convergence
is of minor relevance to the moisture budget of the boundary layer.
Correspondence: Norbert Kalthoff, Institut für Meteorologie und Klimaforschung, Universit?t Karlsruhe/Forschungszentrum Karlsruhe,
Postfach 3640, 76021 Karlsruhe, Germany 相似文献
14.
Michiel R. Van Den Broeke Peter G. Duynkerke Edwin A. C. Henneken 《Boundary-Layer Meteorology》1994,71(4):393-413
The budgets of momentum, heat and moisture of the atmospheric boundary layer overlying the melting zone of the west Greenland ice sheet during an 8-day period in summer are calculated. To do so, the governing budget equations are derived and presented in terms of vertically averaged quantities. Moreover, stationarity is assumed in the present study. Measurements collected during the GIMEX-91 experiment are used to calculate the contribution of the different terms in the equations to the budget.During summer, a well developed katabatic wind system is present over the melting zone of the Greenland ice sheet. The budgets show that advection in the katabatic layer is small for momentum, heat and humidity, when the horizontal length scale of the integration area is sufficiently large (>50 km). This indicates that in principle one-dimensional atmospheric models can be used to study the boundary layer over the melting zone of the Greenland ice sheet. The background stratification plays a crucial role in the heat and moisture budget. Vertical divergence of longwave radiation provides one-third and the turbulent flux of sensible heat the rest of the cooling of the boundary layer. Moisture is added to the boundary layer by evaporation which is a significant term in the moisture budget. Negative buoyancy (katabatic forcing) dominates the momentum budget in the downslope direction. Coriolis forcing is important, stressing the large spatial scale of the katabatic winds on the Greenland ice sheet. 相似文献
15.
Effects of mesoscale sea-surface temperature fronts on the marine atmospheric boundary layer 总被引：1，自引：0，他引：1
Eric D. Skyllingstad Dean Vickers Larry Mahrt Roger Samelson 《Boundary-Layer Meteorology》2007,123(2):219-237
A numerical modelling study is presented focusing on the effects of mesoscale sea-surface temperature (SST) variability on
surface fluxes and the marine atmospheric boundary-layer structure. A basic scenario is examined having two regions of SST
anomaly with alternating warm/cold or cold/warm water regions. Conditions upstream from the anomaly region have SST values
equal to the ambient atmosphere temperature, creating an upstream neutrally stratified boundary layer. Downstream from the
anomaly region the SST is also set to the ambient atmosphere value. When the warm anomaly is upstream from the cold anomaly,
the downstream boundary layer exhibits a more complex structure because of convective forcing and mixed layer deepening upstream
from the cold anomaly. An internal boundary layer forms over the cold anomaly in this case, generating two distinct layers
over the downstream region. When the cold anomaly is upstream from the warm anomaly, mixing over the warm anomaly quickly
destroys the shallow cold layer, yielding a more uniform downstream boundary-layer vertical structure compared with the warm-to-
cold case. Analysis of the momentum budget indicates that turbulent momentum flux divergence dominates the velocity field
tendency, with pressure forcing accounting for only about 20% of the changes in momentum. Parameterization of surface fluxes
and boundary-layer structure at these scales would be very difficult because of their dependence on subgrid-scale SST spatial
order. Simulations of similar flow over smaller scale fronts (<5 km) suggest that small-scale SST variability might be parameterized
in mesoscale models by relating the effective heat flux to the strength of the SST variance. 相似文献
16.
The role of thermals in the convective boundary layer 总被引：1，自引：0，他引：1
Detailed measurements of the structure of thermals throughout the convective boundary layer were obtained from the NCAR Electra aircraft over the ocean during the Air Mass Transformation Experiment (AMTEX). Humidity was used as an indicator of thermals. The variables were first high-pass filtered with a 5 km cutoff digital filter to eliminate mesoscale variations. Segments of the 5 min (30 km length) horizontal flight legs with humidity greater than half the standard deviation of humidity fluctuations for that leg were defined as thermals. This was found to be a better indicator of thermals than temperature in the upper part of the boundary layer since the temperature in a thermal is cooler than its environment in the upper part of the boundary layer. Using mixed-layer scaling, the normalized length and number of thermals were found to scale with the 1/3 and -1/3 powers, respectively, of normalized height, while vertical velocity and temperature scaled according to similarity predictions in the free convection region of the surface layer. The observational results presented here extend throughout the entire mixed layer. Using these results in the equation for mean updraft velocity of a field of thermals, the sum of the vertical pressure gradient and edge-effect terms can be estimated. This residual term is found to be important throughout most of the boundary layer. The magnitude of the divergence of vertical velocity variance within a thermal is found to be larger than the magnitude of the mean updraft velocity term throughout most of the mixed layer.Part of this work was completed while visiting Risø National Laboratory, Denmark.The National Center for Atmospheric Research is sponsored by the National Science Foundation. 相似文献
17.
The structure of nocturnal inversions in the first 300 m of the atmosphere is analyzed using observational data from the Boulder Atmospheric Observatory (BAO) from March through June 1981. The temperature profiles show more than one inversion layer 41% of the time during the observational period. The vertical distributions of wind speed and moisture also show evidence of stratification during these multiple-layer events. The relation between the radiative cooling rate in time and height, including moisture, and the vertical structure of the multiple layers is calculated. The vertical distribution of eddy kinetic energy and the turbulent vertical fluxes of heat and momentum are also calculated. Turbulent structure in the elevated inversion layers is more complicated than that in the single-layer, stable nocturnal boundary layer. The total heat budget for a multiple-layer case is calculated, and turbulent cooling is found to be negligible relative to radiative cooling and to horizontal advection and/or horizontal divergence of heat flux. 相似文献
18.
The local advection of momentum, heat and moisture in micrometeorology due to a horizontal inhomogeneity in surface conditions is numerically investigated by a higher-order turbulence closure model which includes equations for the mean quantities, turbulent fluxes, and the viscous dissipation rate. The application of the two-dimensional model in this paper deals with the simulation of the flow from an extensive smooth dry area to a grassy wet terrain. The mean wind speed, temperature, and humidity distributions in the resulting internal boundary layer downstream of the surface discontinuity are determined such that the energy and moisture balances at the Earth's surface are satisfied.Numerical calculations of the mean temperature and humidity profiles are compared with available observed ones. The results include the advective effects on turbulent flux distributions, surface energy balance, evaporation rate, and Bowen ratio. The sensitivity of the predicted mean profiles and turbulent flux distributions to the surface relative humidity, thermal stratification, and the roughness change is discussed.NRC-NAS Resident Research Associate at AFCRL. 相似文献
19.
利用1981～2016年气象常规观测和自动站资料对南充大风的基本气候特征进行统计分析,重点探讨不同类型区域雷暴大风的天气系统配置和环境物理量基本特征。结果表明:(1)南充雷暴大风按照形成原因主要分为高空冷平流强迫类和斜压锋生类,按落区出现情况分为全市型、东部型和西部型,东部型雷暴大风主要由高空冷平流强迫所致,全市型和西部型雷暴大风过程则多为斜压锋生所造成。(2)斜压锋生类雷暴大风主要发生在显著冷暖平流导致的斜压锋生与锋面动力强迫共同作用的形势下,高空冷平流强迫类则主要是高空强干冷平流的作用。(3)雷暴大风过程发生前大气环境呈上干下湿、湿层浅薄或为“喇叭口”形态,对于不同类型雷暴大风过程发生前的环境物理结构不同,斜压锋生类雷暴大风产生时大气环境多为明显斜压特征,高空常伴有强锋区,低层不稳定能量大,因此热力因子比较重要。高空冷平流强迫类主要发生在川陕槽后强烈冷平流形势下,水平风垂直切变大、要求低层增温快,故热力和动力因子都重要。 相似文献