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1.
Summary. ?The airflow over an idealized orography with two mountain peaks and a valley between is investigated using a non-linear numerical model. The flow is assumed to be two-dimensional and nonrotational. Surface friction is neglected. This setup is a first step in studying the modifications a finely structured “real” topography introduces to the well-studied flow over one isolated obstacle. The sensitivity of the flow behavior to the valley width is examined for the case of specified mountain volume as well as constant non-dimensional mountain height. Flow patterns for linear, weakly nonlinear, wave breaking and upstream blocking cases are examined. Whereas the nondimensional mountain height is still the main measure of the nonlinearity of the flow, the differing steepness of upslope and downslope caused by the separating valley, strengthens nonlinear effects. It also modifies wave breaking and upstream blocking. For wide enough valleys wave breaking regions can form above both peaks. Received January 20, 1999/Revised June 28, 1999  相似文献   

2.
Data collected during the SHEBA and CASES-99 field programs are employed to examine the flux–gradient relationship for wind speed and temperature in the stably stratified boundary layer. The gradient-based and flux-based similarity functions are assessed in terms of the Richardson number Ri and the stability parameter z/Λ*, z being height and Λ* the local Obukhov length. The resulting functions are expressed in an analytical form, which is essentially unaffected by self-correlation, when thermal stratification is strong. Turbulence within the stably stratified boundary layer is classified into four regimes: “nearly-neutral” (0 < z/Λ* < 0.02), “weakly-stable” (0.02 < z/Λ* < 0.6), “very-stable” (0.6 < z/Λ* < 50), and “extremely-stable” (z/Λ* > 50). The flux-based similarity functions for gradients are constant in “nearly-neutral” conditions. In the “very-stable” regime, the dimensionless gradients are exponential, and proportional to (z/Λ*)3/5. The existence of scaling laws in “extremely-stable” conditions is doubtful. The Prandtl number Pr decreases from 0.9 in nearly-neutral conditions and to about 0.7 in the very-stable regime. The necessary condition for the presence of steady-state turbulence is Ri < 0.7.  相似文献   

3.
A number of authors have reported the problem of unrealistic velocities (“rogue trajectories”) when computing the paths of particles in a turbulent flow using modern Lagrangian stochastic (LS) models, and have resorted to ad hoc interventions. We suggest that this problem stems from two causes: (1) unstable modes that are intrinsic to the dynamical system constituted by the generalized Langevin equations, and whose actual triggering (expression) is conditional on the fields of the mean velocity and Reynolds stress tensor and is liable to occur in complex, disturbed flows (which, if computational, will also be imperfect and discontinuous); and, (2) the “stiffness” of the generalized Langevin equations, which implies that the simple stochastic generalization of the Euler scheme usually used to integrate these equations is not sufficient to keep round-off errors under control. These two causes are connected, with the first cause (dynamical instability) exacerbating the second (numerical instability); removing the first cause does not necessarily correct the second, and vice versa. To overcome this problem, we introduce a fractional-step integration scheme that splits the velocity increment into contributions that are linear (U i ) and nonlinear (U i U j ) in the Lagrangian velocity fluctuation vector U, the nonlinear contribution being further split into its diagonal and off-diagonal parts. The linear contribution and the diagonal part of the nonlinear contribution to the solution are computed exactly (analytically) over a finite timestep Δt, allowing any dynamical instabilities in the system to be diagnosed and removed, and circumventing the numerical instability that can potentially result in integrating stiff equations using the commonly applied explicit Euler scheme. We contrast results using this and the primitive Euler integration scheme for computed trajectories in a drastically inhomogeneous urban canopy flow.  相似文献   

4.
A comprehensive model for the prediction of concentration fluctuations in plumes dispersing in the complex and highly disturbed wind flows in an urban environment is formulated. The mean flow and turbulence fields in the urban area are obtained using a Reynolds-averaged Navier-Stokes (RANS) flow model, while the standard k-ϵ turbulence model (k is the turbulence kinetic energy and ϵ is the viscous dissipation rate) is used to close the model. The RANS model provides a specification of the velocity statistics of the highly disturbed wind flow in the urban area, required for the solution of the transport equations for the mean concentration and concentration variance (both of which are formulated in the Eulerian framework). A physically-based formulation for the scalar dissipation time scale t d , required for the closure of the transport equation for , is presented. This formulation relates t d to an inner time scale corresponding to “internal” concentration fluctuation associated with relative dispersion, rather than an outer time scale associated with the entire portion of the fluctuation spectrum. The two lowest-order moments of concentration ( and ) are used to determine the parameters of a pre-chosen functional form for the concentration probability density function (clipped-gamma distribution). Results of detailed comparisons between a water-channel experiment of flow and dispersion in an idealized obstacle array and the model predictions for mean flow, turbulence kinetic energy, mean concentration, concentration variance, and concentration probability density function are presented.  相似文献   

5.
Mixing depth structure and its evolution have been diagnosed from radar wind profiler data in the Chamonix and the Maurienne valleys (France) during summer 2003. The behaviour of refractive index structure parameter C n 2 peaks coupled with the vertical velocity variance σ w 2 was used to estimate the height of the mixed layer. Tethersonde vertical profiles were carried out to investigate the lower layers of the atmosphere in the range of approximately 400–500 m above ground level. The tethersonde device was especially useful to study the reversal of the valley wind system during the morning transition period. Specific features such as wind reversal and the convective mixed layer up to approximately the altitude of the surrounding mountains were documented. The wind reversal was observed to be much more sudden in the Maurienne valley than in the Chamonix valley  相似文献   

6.
Summary Flow in long and deep main valleys with tributaries is studied for constant surface heating switched on att=0. The valley flows are obtained from a numerical model which combines slope wind layer equations with equations for the valley flow off the slopes. Much simpler linear models are used for the intepretation of the model results. If there are no sidevalleys an up-valley wind regime evolves in the main valley after the switch-on of the heating which protrudes towards the head. It is shown that the topographic amplification factor which captures the geometry of the valley and stratification are important factors in determining the intensity of the along-valley flow. However the up-valley winds are also quite sensitive to the specification of the boundary conditions at the upper end of the slope wind layers. If sidevalleys are added strong inflow to these tributaries is found only if their topographic amplification factors are larger than that of the main valley. This flow into the tributaries is mainly balanced by downward motion on top of the main valley but flow entering through the mounth of the main valley can contribute as well. Tributaries can induce flow in the main valley long before the main valley's own up-valley wind regime has reached the location of the tributary.With 10 Figures  相似文献   

7.
Summary Features of the mean flow structure in a small valley system in the Rosalian mountain range are discussed using data from a wind measurement network. Tethered balloon measurements during periods of clear sky form the basic dataset for the analysis of drainage winds and temperature inversions. During periods of weak ambient winds the existence of a pure thermally driven nocturnal valley wind system is shown. With strong ambient winds opposing the drainage flow, a reduced drainage height but the same jet maximum as with weak ambient winds is found. On the other hand with aiding flow the drainage winds are suppressed and flow reversal can occur. This strong valley flow interaction with the ambient wind indicates considerable dynamic influence on the evolution of drainage winds and on the breakup of temperature inversion structure for small valleys.With 15 Figures  相似文献   

8.
Flow splitting in the Rhine valley has been observed with a transportable wind lidar (TWL) during a shallow-foehn event in the framework of the Mesoscale Alpine Programme (MAP). The Doppler lidar recorded in detail flow splitting, foehn wind gusts, and flow reversal. Such structures have not previously been observed with comparable detail by conventional in-situinstruments. In addition to the TWL, boundary-layer processes have been documented by means of rawinsondes and surface stations. This paper presents an analysis of the processes giving birth to flow splitting between the Seez and Rhine valleys during Intensive Observation Period (IOP) 5 (1–3 October 1999) by combining the collected data with hydraulic theory. The study shows thatthe splitting of the channelled flow is associated with (1) the existence of a stagnation point at the intersection of the Seez and Rhine valleys, and (2) the deflection of the flow by the lateral sidewalls of the valleys.  相似文献   

9.
The popular method of presenting wavenumber–frequency power spectrum diagrams for studying tropical large-scale waves in the literature is shown to give an incomplete presentation of these waves. The so-called “convectively coupled Kelvin (mixed Rossby-gravity) waves” are presented as existing only in the symmetric (anti-symmetric) component of the diagrams. This is obviously not consistent with the published composite/regression studies of “convectively coupled Kelvin waves,” which illustrate the asymmetric nature of these waves. The cause of this inconsistency is revealed in this note and a revised method of presenting the power spectrum diagrams is proposed. When this revised method is used, “convectively coupled Kelvin waves” do show anti-symmetric components, and “convectively coupled mixed Rossby-gravity waves (also known as Yanai waves)” do show a hint of symmetric components. These results bolster a published proposal that these waves should be called “chimeric Kelvin waves,” “chimeric mixed Rossby-gravity waves,” etc. This revised method of presenting power spectrum diagrams offers an additional means of comparing the GCM output with observations by calling attention to the capability of GCMs to correctly simulate the asymmetric characteristics of equatorial waves.  相似文献   

10.
The effect of variable roughness length upon the flow characteristics over hills is investigated. The changes considered herein cover a range of flow configurations such as the change from a forested (rough) valley with a moderately smooth hilltop to a grassy valley (smooth) with a “spiky” (rough) mountain top. The effect of moving the roughness with respect to the hill is also considered. Although many of the flow features change when the position of the roughness change is varied with respect to the hill these changes have very little impact upon the global properties used within orographic drag parametrization schemes.  相似文献   

11.
Numerical convergence of the dynamics of a GCM   总被引：1，自引：0，他引：1  
 Atmospheric general circulation models (GCMs) are characterized by many features but especially by: (1) the manner of discretizing the governing equations and of representing the variables involved at a given resolution, and (2) the manner of parameterizing unresolved physical processes in terms of those resolved variables. These two aspects of model formulation are not independent and it is difficult to untangle their intertwined effects when assessing model performance. The attempt here is to separate these aspects of GCM behaviour and to ask, “Given a perfect parameterization of the physical processes in a model, what resolution is needed to capture the dominant dynamical aspects of the atmospheric climate?” Alternatively, “At what resolution do the dynamics of a GCM converge”? The perfect parameterization approach assumes that the calculation of the physical terms returns the “correct” result at all resolutions. In the idealized case, a time-independent forcing is one of the simplest that satisfies this condition. However, experiments show that it is difficult for the dynamics of a GCM to balance a time-independent forcing with atmosphere-like flows and structures. The model requires, and the atmosphere presumably includes, physical feedback mechanisms which act so as to maintain the kinds of flows and structures that are observed. Resolution experiments are performed with a simplified forcing function for the thermodynamic equation which combines a dominant time-independent specified forcing with a weak linear relaxation feedback. These experiments show that the dynamics of the GCM have essentially converged at T32 and certainly by T63 which is the next resolution considered. This is shown by the constancy of structures, variances, covariances, transports and energy budgets with increasing resolution. Experiments with an alternative forcing proposed by Held and Suarez, which has the form of a linear relaxation, show somewhat less evidence of convergence at these resolutions. In both cases the “physics” are known by assumption. However, the form and nature of the forcing is different, as is the behaviour with resolution. The implication for the real system is that the resolution required for simulating the dynamical aspects of climate is rather modest. The simulated climate does, however, apparently depend on the ability to correctly and consistently parameterize the physical processes in a GCM, involving both forcing and feedback mechanisms, as a function of resolution. Received 19 January 1996/Accepted 22 August 1996  相似文献   

12.
In order to investigate the upper troposphere/lower stratosphere (UTLS) region of the earth's atmosphere, ESA/ESTEC (European space agency) is considering the opportunity to develop the spaceborne limb sounding millimeter sensor “MASTER” (millimeter wave acquisitions for stratosphere/troposphere exchange research). This instrument is part of the “atmospheric composition explorer for chemistry and climate interactions” (ACECHEM) project. In addition, ESA/ESTEC is developing the “MARSCHALS” (millimeter-wave airborne receiver for spectroscopic characterization of atmospheric limb sounding) airborne instrument which will demonstrate the feasibility of MASTER. The present paper describes the line-by-line database which was generated in order to meet at best the needs of the MASTER (or MARSCHALS) instrument. The linelist involves line positions, line intensities, line broadening and line shift parameters in the 294–305, 316–325, 342–348, 497–506 and 624–626 GHz spectral microwindows. This database was first generated for the target molecules for MASTER (H2O, O3, N2O, CO, O2, HNO3, HCl, ClO, CH3Cl, BrO). In addition, ten additional molecules (SO2, NO2, OCS, H2CO, HOCl, HCN, H2O2, COF2, HO2 and HOBr) had also to be considered in the database as “possible interfering species” for the retrieval of the target molecules of MASTER. The line parameters were derived, depending on their estimated accuracy, (i) from a combination of spectral parameters included in the JPL and HITRAN catalogs (ii) from data taken into the literature or (iii) using data obtained through experimental measurements (and/or) calculations performed during the present study.  相似文献   

13.
A comparison of estimates of the root-mean-square error (RMSE) and potential predictability index (PPI) is carried out between experiments with observed and “persistent” anomalies of sea surface temperature (SST). The results obtained point to a possible significant bias of seasonal forecasting results in some regions when boundary conditions are introduced by a “persistence” procedure, particularly for summer T 850. Indirect evidence of the influence of extratropical SST anomalies points to their possible role in seasonal forecasts, which is more substantial in the summer season. Although the conclusions should rather be regarded as preliminary ones because of a limited size of the sample, it is nonetheless certain that the influence of boundary conditions governing the signal becomes more significant in summer because of a decrease in the instability of the internal atmospheric dynamics.  相似文献   

14.
Numerical analysis of flux footprints for different landscapes   总被引：5，自引：0，他引：5  
Summary A model for the canopy – planetary boundary layer flow and scalar transport based on E- closure was applied to estimate footprint for CO2 fluxes over different inhomogeneous landscapes. Hypothetical heterogeneous vegetation patterns – forest with clear-cuts as well as hypothetical heterogeneous relief – a bell-shaped valley and a ridge covered by forest were considered. The distortions of airflow caused by these heterogeneities are shown – the upwind deceleration of the flow at the ridge foot and above valley, acceleration at the crest and the flow separation with the reversed flow pattern at lee slopes of ridge and valley. The disturbances induce changes in scalar flux fields within the atmospheric surface layer comparing to fluxes for homogeneous conditions: at a fixed height the fluxes vary as a function of distance to disturbance. Correspondingly, the flux footprint estimated from model data depends on the location of the point of interest (flux measurement point) and may significantly deviate from that for a flat terrain. It is shown that proposed method could be used for the choice of optimal sensor position for flux measurements over complex terrain as well as for the interpretation of data for existing measurement sites. To illustrate the latter the method was applied for experimental site in Solling, Germany, taking into account the complex topography and vegetation heterogeneities. Results show that in certain situations (summer, neutral stratification, south or north wind) and for a certain sensor location the assumptions of idealized air flow structure could be used for measurement interpretation at this site, though in general, extreme caution should be applied when analytical footprint models are used in the interpretation of flux measurements over complex sites.  相似文献   

15.
Summary An unusually strong nocturnal downvalley wind can be regularly observed in the upper Isar Valley close to Mittenwald (Bavarian Alps) when a high-pressure system is located over Central Europe or when ambient southerly winds are present. Due to the structure of the local topography, this downvalley wind has foehn-like properties in the sense that the breakthrough of the flow into the valley is characterized by a strong increase in temperature and a decrease in relative humidity. Therefore the author called this flow Minifoehn. In fact, wind speeds are low in comparison to deep foehn, but gusts may reach values up to 20ms–1, even under the influence of high pressure systems with weak atmospheric pressure gradients. To investigate the Minifoehn, surface stations have been installed for collecting temperature, humidity, wind and pressure data. Measurements have shown that the Minifoehn represents the upper part of one of the drainage currents which flows over a mountain ridge into the valley at Mittenwald. Nocturnally cooled air drains from a plateau south of Mittenwald through different valleys which merge again near Mittenwald. It seems that the forcing of the nocturnal currents is dominated by the temperature difference between this plateau and the free atmosphere above Mittenwald at the same level. Strong temperature differences are found during clear winter nights and in case of subsidence inversions. Moreover, the appearance of the Minifoehn in autumn and winter is so frequent that we even may find a climatic effect: the upper Isar Valley is usually free of fog during these seasons and nocturnal temperatures are often considerably higher than in other Bavarian Alpine valleys at comparable altitude.  相似文献   

16.
The 4-year data sets (1998–2001) of PM10 and other gaseous pollutants at four rural and urban monitoring sites provided by Environmental Protection Department of Hong Kong have been analyzed for days of extremely high and low PM10 levels. The annual means of PM10 concentrations are between 37 and 57 μg/m3. The level of high PM10 concentration is defined from the comparison of local and international standards. Episode days are mainly controlled by different meteorological conditions: the continental outflow, the land-sea breeze/weak synoptic forcings and the approaching tropical cyclones. Integrated approaches have been used to distinguish between the predominantly “territory wide” and “long-range transport” (LRT) episode days. Case studies of these types of episodes are presented and the number of episode days per year for each type has been estimated. It is found that the LRT contributions are significant and account for ∼66% of the PM10 episode days. Very high correlations between CO and PM10 concentrations, and between SO2 and PM10 concentrations, can be found during the “territory wide” episode days which implies the important contributions of fossil fuel combustion to the PM10 episodes. The number of “low level” PM10 days per year has decreased by a factor near 3 from 1998 to 2001. Precipitation scavenging is the major process causing low levels of PM10, irrespective of the associated weather systems. The regional annual background level is ∼9 μg/m3. With the exception of seasalt components, the average elemental concentrations of major inorganic species are similar for all of the sites during LRT events and constitute representative compositions of PM10 during such events.  相似文献   

17.
Summary During the Hartheim Experiment (HartX) 1992 conducted in the upper Rhine Valley, Germany, three different methods were used to measure sap flow in Scots pine trees via heating of water transported in the xylem: (1) constant heating applied radially in the sapwood (Granier-system-G), (2) constant heating of a stem segment (ermák-system-C), and (3) regulated variable heating of a stem segment that locally maintains a constant temperature gradient in the trunk (ermák/Schulze-system-CS). While the constant heating methods utilize changes in the induced temperature gradient to quantify sap flux, the CS-system estimates water flow from the variable power requirement to maintain a 2 or 3 degree Kelvin temperature gradient over a short distance between inserted electrodes and reference point. The C- and CS-systems assume that all transported water is encompassed and equally heated by the electrodes. In this case, flux rate is determined from temperature difference or energy input and the heat capacity of water. Active sapwood area need not be determined exactly. In contrast, the G-system requires an empirical calibration of the sensors that allows conversion of temperature difference into sap flow density. Estimates of sapwood area are used to calculate the total flux. All three methods assume that the natural fluctuation in temperature of the trunk near the point of insertion of heating and sensing elements is the same as that where reference thermocouples are inserted.Using all three systems, 24 trees were simultaneously monitored during the HartX campaign. Tree size within the stand ranged between 18 and 61 cm circumference at breast height, while sample trees ranged between 24 and 55 cm circumference. The smallest trees could only be measured by utilizing the G-system. Sap flow rates of individual trees measured at breast height increased rapidly in the morning along with increases in irradiance and vapor pressure deficit (D), decreased slowly during the course of the afternoon with continued increase inD, and decreased more slowly during the night.Ignoring potential effects introduced by the different methods, maximum flow rates of individual trees ranged between 0.5 and 2.5 kg H2O h–1 tree–1 or 0.3 and 0.6 mm h–1 related to projected crown area of trees and daily sums of sap flow for individual trees varied between 4.4 and 24 kg H2O tree–1 d–1 or 1.1 and 6.0 mm d–1. Maximum sap flow rates per sapwood area of trees varied least for the G-system (11–17 g cm–2 h–1) and was of similar magnitude as the C- (8–21 g cm–2 h–1) and CS-system (4–14 g cm–2 h–1).Regressions of total tree conductance (g t ) derived from sap flow estimates demonstrated the same linear increase of conductance with increasing irradiance, however decrease of conductance with increasingD under non-limiting light conditions was different for the three systems with strongest reduction ofg t measured with the CS-system followed by the C- and G-system. This led to different estimates of daily sap flow rates especially during the second part of the measurement period.Variation in sap flow rates is explained on the basis of variation in leaf area index of individual trees, heterogeneity in soil conditions, and methodological differences in sap flow measurements. Despite the highly uniform plantation forest at the scale of hectares, the heterogeneity in tree size and soil depth at the scale of square meters still make it difficult to appropriately and efficiently select sample trees and to scale-up water flux from individual trees to the stand level.With 5 Figures  相似文献   

18.
Summary This paper presents idealized numerical simulations of the valley wind circulation in the Alpine Inn Valley, which are compared with existing data and are used to improve our dynamical understanding of the valley wind. The simulations have been performed with the Penn State/NCAR mesoscale model MM5. They use a high-resolution realistic topography but idealized large-scale conditions without any synoptic forcing to focus on the thermally induced valley wind system. The comparison with the available observations shows that this simplified set-up is sufficient to reproduce the essential features of the valley wind.The results show that the tributaries of the Inn Valley have a considerable impact on the along-valley mass fluxes associated with the valley wind circulation. The upvalley mass flux is found to increase where tributaries enter the Inn Valley from the north, that is, from the direction where the Alpine foreland is located. On the other hand, the upvalley mass flux is reduced at the junctions with southern tributaries because part of the upvalley flow is deflected into these tributaries. For the downvalley flow, the situation is essentially reversed, but the influence of the valley geometry on the flow structure is larger than for the upvalley flow. The most important feature is a lateral valley contraction near the valley exit into the Alpine foreland. It reduces the downvalley mass flux at low levels, so that the wind maximum in the interior of the valley is shifted to a fairly large distance from the ground. North of the valley contraction, however, the downvalley flow strongly accelerates and forms a pronounced low-level jet. A dynamical analysis indicates that this acceleration can be interpreted as a transition from subcritical to supercritical hydraulic flow. Another interesting feature is that the low-level jet maintains its structure for several tenths of kilometres into the Alpine foreland. This appears to be related to the fact that the lateral wind shear on the flanks of the jet is associated with a strong dipole of potential vorticity (PV). Due to the conservation properties of the PV, the downstream advection of the PV dipole leads to the formation of a band-like feature that decays fairly slowly.  相似文献   

19.
Using an incomplete third-order cumulant expansion method (ICEM) and standard second-order closure principles, we show that the imbalance in the stress contribution of sweeps and ejections to momentum transfer (ΔS o ) can be predicted from measured profiles of the Reynolds stress and the longitudinal velocity standard deviation for different boundary-layer regions. The ICEM approximation is independently verified using flume data, atmospheric surface layer measurements above grass and ice-sheet surfaces, and within the canopy sublayer of maturing Loblolly pine and alpine hardwood forests. The model skill for discriminating whether sweeps or ejections dominate momentum transfer (e.g. the sign of ΔS o ) agrees well with wind-tunnel measurements in the outer and surface layers, and flume measurements within the canopy sublayer for both sparse and dense vegetation. The broader impact of this work is that the “genesis” of the imbalance in ΔS o is primarily governed by how boundary conditions impact first and second moments.  相似文献   

20.
 The inter-annual variability and potential predictability of 850 hPa temperature (T 850), 500 hPa geopotential (φ500) and 300 hPa stream function (ψ300) simulated by the models participating in the Atmospheric Model Intercomparison Project (AMIP) are examined. The total inter-annual variability is partitioned into a potentially predictable component which arises from the forcing implied by the prescribed SST and sea-ice evolution, or from sources internal to the simulated climate, and an unpredictable low frequency component induced by “weather noise”. There is wide variation in the ability to simulate observed inter-annual variability, both total and weather-noise induced. A majority of models under simulate seasonal mean φ500 variability in DJF and JJA and over simulate ψ300 variability in JJA. All but one model simulates less T 850 total inter-annual variability than in the analysed data. There is little apparent connection between gross model characteristics and the corresponding ability to simulate observed variability, with the possible exceptions of resolution. Received: 7 July 1996 / Accepted: 8 January 1998  相似文献   

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