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1.
Summary Hydrostatic and nonhydrostatic simulation models are employed to study the intensification of a terrain drag-induced dryline. The study develops a multi-stage theory for the evolution of the dryline including the concentration of potential vorticity accompanying meso-gamma scale dryline bulges.The numerical simulations indicate three fundamental stages of dryline intensification all of which are either directly or indirectly a result of the terrain-drag on the mid/upper-tropospheric jet stream by the Front Range of the Colorado Rocky Mountains. The first stage involves the downward momentum flux accompanying a large amplitude hydrostatic mountain wave which induces a downslope windstorm along the lee slopes. The surge of momentum (i.e., the dry, warm air associated with the downslope windstorm) propagates down the leeslope and modifies an existing weak dryline boundary. As the downslope windstorm initiates an undular bore along the lee slopes, the high momentum gradient which propagates downstream accompanying the bore, as well as the strong lower tropospheric sinking motions ahead of the bore, contract the scale of the surface moisture boundary between the dry air from above the leeslope and the moist air over the High Plains. This process further strengthens the dryline.The second stage involves the coupling of the terrain drag-induced along-stream ageostrophic front within the midtroposphere to the boundary layer through a thermally-indirect circulation. As the along-stream ageostrophic circulation intensifies within the middle troposphere down-stream from the mountain wave, sinking air parcels originating above 40 kPa descend to below 60 kPa over the High Plains where surface pressures are, only 85 kPa. These descending air parcels within the upstream branch of the along-stream ageostrophic thermally-indirect circulation contain high values of momentum and very low dewpoint values. As the planetary boundary layer (PBL) deepens due to surface warming during the morning hours, momentum and dry air from the midtropospheric along-stream ageostrophic front are entrained into the PBL. This process amplifies the bore-induced hydrostatic dryline bulge via low-level ageostrophic confluence.Finally, regions of low Richardson number (arising from strong vertical shears) within the amplifying midtropospheric along-stream ageostrophic thermally-indirect circulation become preferred regions for the development of non-hydrostatic evanescent internal gravity waves. These waves are embedded within the hydrostatic along-stream front above the low-level dryline and are accomapanied by very significant values of vertical momentum flux which act to focus the meso-gamma scale structure of the dryline into smaller scale bulges where low-level winds and vorticities are very high. This meso-gamma scale process follows the hydrostatic tilting and vortex tube stretching which creates meso-beta scale maxima of mid-lower tropospheric vorticity. The turbulent momentum fluxes accompanying wavebreaking within the nonhydrostatic dryline bulge create very large (i.e., stratospheric values of) potential vorticity near 70 kPa due to the nonconservation of potential vorticity on isentropic surfaces.With 30 Figures 相似文献
2.
A nonhydrostatic model based on a new approach 总被引：4，自引：0，他引：4
Z. I. Janjic 《Meteorology and Atmospheric Physics》2003,82(1-4):271-285
Summary ?The nonhydrostatic Meso model developed at NCEP (Janjic et al, 2001) is based on a new approach. Namely, a hydrostatic NWP
model using mass based vertical coordinate has been extended to include the nonhydrostatic motions. In this way favorable
features of the hydrostatic formulation have been preserved. This procedure did not require any linearization or approximation.
The nonhydrostatic dynamics has been introduced through an add-on module. The nonhydrostatic module can be turned on and off,
so that easy comparison can be made of hydrostatic and nonhydrostatic solutions.
Here, the basic philosophy behind the discretization methods applied in the model, and not covered by Janjic et al (2001),
is discussed, and the latest developments are reviewed. The forecast examples shown indicate that significant differences
between hydrostatic and nonhydrostatic forecasts may develop even at relatively coarse resolution of 8 km. Possible future
developments are considered.
Received May 7, 2001; revised October 15, 2001 相似文献
3.
A barotropic model of the Ekman planetary boundary layer based on the geostrophic momentum approximation 总被引：2，自引：0，他引：2
A model of a stationary planetary boundary layer is proposed based on the equation of motion with the advective term retained. The latter is modeled by means of the so-called geostrophic momentum approximation in two versions — original and modified. New expressions for the vertical velocity W
at the top of the boundary layer are derived and analyzed. They underestimate W
compared to the classical expression. 相似文献
4.
Xiaohua Yang 《Boundary-Layer Meteorology》1991,54(1-2):183-208
Nonhydrostatic effects in two-dimensional mesoscale sea breeze systems are investigated by numerical simulations. It is shown that nonhydrostatic effects are directly contributed by the vertical gradients of the vertical velocity variance as well as by the vertical accelerations. It is also shown that a K-type turbulence closure is not suitable in a nonhydrostatic primitive equation model, and a higher-order closure scheme should therefore be used. Results from hydrostatic and fully-nonhydrostatic models are compared for various surface and atmospheric background conditions, such as scale and strength of surface heating, geostrophic wind, stability, surface roughness contrast, Coriolis effect, etc. It is found that for strongly developed sea breeze cases, vertical gradients of vertical velocity variance contribute most to nonhydrostatic forcing in the lower layers, and that the resultant nonhydrostatic pressure gradient acts against the hydrostatic pressure gradient, so that nonhydrostatic simulations produce weaker systems than hydrostatic ones. For weak sea breeze systems, the difference between the two models tends to be small. 相似文献
5.
Modeling nonhydrostatic atmospheric flow requires the solution of the vertical equation of motion and a prognostic or diagnostic equation for pressure. If the nonhydrostatic components of the flow are relatively small, they can be approximated and incorporated into a purely hydrostatic model, which usually is conceptually simpler and computationally more efficient. A method to do this for a linear model of local thermally-induced circulations is further developed and adapted to a non-linear numerical model of the neutral atmospheric boundary layer. A hydrostatic model and the quasi-nonhydrostatic version were used to simulate neutral flow over simple terrain features. One set of observations taken over a simple change in roughness and another set taken over a change in both roughness and terrain were simulated by both models to assess the capabilities of the quasi-nonhydrostatic technique.It is found that (as expected) the pressure deviation from the hydrostatic state is negligible for the roughness change, but it is an important aspect of neutral flow over terrain. Thus, for flow encountering a simple roughness change, the hydrostatic approximation is good, even for small horizontal scales. However, the quasi-nonhydrostatic model qualitatively produces the features in the observations for flow over a terrain change that the hydrostatic model cannot produce.Journal Paper No. J-12737 of the Iowa Agriculture and Home Economics Experiment Station, Ames, Iowa. Project No. 2779. 相似文献
6.
建立了一个二维非静力平衡模式，通过坐标变换的方法使得模式低层有较高的分辨率。对重力流的模拟结果显示；本模式模拟出的重力流冷锋主要特征，如抬高的头部、头部较强的上升运动以及等位温线密集带与实际大气观测很一致。此外，模式还成功地模拟出了重力流头部后面底层的高速潜流层。应用模拟结果与理论结果对重力流的移动速度进行了对比分析。当考虑地形时，重力流在山前受阻，其头部比不考虑地形时高，且产生了向上游传播的“涌浪”；重力流过山后其头部降低，位温梯度减弱，头部后面的水平速度减小。此外，还对非静力平衡与静力平衡条件下重力流的差异进行了分析讨论。 相似文献
7.
T. Kato 《Meteorology and Atmospheric Physics》1997,63(1-2):39-51
Summary Quantitatively comparative experiments of moist convection using hydrostatic and non-hydrostatic models are reviewed and a further study is made of the suitability of the hydrostatic approximation for a high-resolution model when the grid size falls below 20 km. Idealized moist convection is treated, and then the torrential rain that occurred on 6 August 1993 in Kagoshima, southern Kyushu, Japan is simulated by each model. An explicit warm-rain process predicting cloud water and rainwater and the scheme of moist convective adjustment are individually or conjunctively employed in the model. The effect of hydrostatic water loading is also examined in detall.For the simulation of idealized convection, the hydrostatic simulation tends to overestimate and overexpand precipitation in comparison with the non-hydrostatic counterpart, and the drag effect of hydrostatic water loading is more significant for convective development than the non-hydrostatic effect. In the 20-km simulations, however, the hydrostatic simulation with hydrostatic water loading produces results that are comparable to the nonhydrostatic counterpart. For the simulation with real data, the comparative results well correspond to those of idealized convection. Furthermore, the 5 km hydrostatic simulation overestimates total precipitation more than that of dealized convection. On the basis of these results, when developing 1020 km numerical weather prediction (NWP) models, hydrostatic water loading should be evaluated in preference to adopting non-hydrostatic models, and a non-hydrostatic model with hydrostatic water loading is thought to be recommendable for a high-resolution NWP model.With 7 Figures 相似文献
8.
During spring and autumn, many lakes in temperate latitudes experience intensive convective mixing in the vertical, which leads to almost isothermal conditions with depth. Thus the regime of turbulence appears to be similar with that characteristic of convective boundary layers in the atmosphere. In the present paper a simple analytical approach, based on boundary-layer theory, is applied to convective conditions in lakes. The aims of the paper are firstly to analyze in detail the temperature distribution during these periods, and secondly to investigate the current system, created by the horizontal temperature gradient and wind action. For these purposes, simple analytical solutions for the current velocities are derived under the assumption of depth-constant temperatures. The density-induced current velocities are shown to be small, in the order of a few mm/sec. The analytical model of wind-driven currents is compared with field data. The solution is in good qualitative agreement with observed current velocities under the condition that the wind field is steady for a relatively long time and that residual effects from former wind events are negligible.The effect of the current system on an approximately depth-constant temperature distribution is then checked by using the obtained current velocity fields in the heat transfer equation and deriving an analytical solution for the corrected temperature field. These temperature corrections are shown to be small, which indicates that it is reasonable to describe the temperature distribution with vertical isotherms.Notation
T
temperature
-
t
time
-
x, y, z
cartesian coordinates
-
molecular viscosity
-
h
,
v
horizontal and vertical turbulent viscosity
-
K
h
,K
v
horizontal and vertical turbulent conductivity
-
Q
heat flux through the water surface
-
D
depth
-
u, v, w
average current velocity components inx, y andz directions
-
f
Coriolis parameter
-
p
pressure
-
density
-
g
gravity acceleration
-
a
constant in the freshwater state equation
-
h
s
deviation from the average water surface elevation
-
L
*,H
*
length and depth scale
-
U
*,W
*
horizontal and vertical velocity scale
- T
temperature difference scale
-
bottom slope
-
u
*
friction velocity at the water surface
-
von Karman constant
-
L
Monin-Obukhov length scale
-
buoyancy parameter
-
l
turbulence length scale
-
C
1,C
2,C
3
dimensionless constants in the expressions for the vertical turbulent viscosity
- ,
dimensionless vertical coordinate and dimensionless local depth
-
angle between surface stress direction andx-axis
-
T
bx
,T
by
bottom stress components
-
c
bottom drag coefficient 相似文献
9.
F. Mesinger 《Meteorology and Atmospheric Physics》1997,63(1-2):3-14
Summary Following a few historical remarks, approximations used in formulating the dynamics of limited-area and variable resolution atmospheric forecasting models are reviewed. Particular attention is given to current efforts to relax or remove the hydrostatic approximation.Turning to numerical methods used in discretizing the equations, an attempt is made to record recent work and to clarify the motivation for the various approaches being followed by different modeling centers. Topics commented upon include: semi-Lagrangian methods, numerical formulation of nonhydrostatic models, resolution, the eta (step-mountain) vs sigma or isentropic/sigma vertical coordinate, choice of the vertical grid, numerics of the propagation of gravity waves, and the box-average vs pointsample treatment of predicted variables.It is finally pointed out that the extraordinary diversity of roads being taken shows that a lot remains to be discovered as to what possible rewards may be found in exploring one or the other of the principles underlying the methods being developed.This is a revised and updated text of an invited lecture given within the One-Day Intensive Course of the International Workshop on Limited-Area and Variable Resolution Models, Beijing, China, 23–27 October 1995. 相似文献
10.
11.
Zbigniew Sorbjan 《Boundary-Layer Meteorology》1996,79(1-2):181-189
Two runs of a large-eddy simulation model with Deardorff's and Schumann's subgrid parametrizations have been compared in order to analyze spurious effects at the top of the mixed layer due to a presence of a strong temperature jump. Simulations performed showed that Schumann's subgrid eddy viscosity was sufficient to spreads out sharp ripples which appeared in the numerical solution due to numerical dispersion. Deardorff's subgrid eddy viscosity was found too small near the top of the mixed layer, and as a result truncation dispersion errors caused unphysical solutions in this region. 相似文献
12.
Dr. D. P. Jorgensen T. Matejka J. D. DuGranrut 《Meteorology and Atmospheric Physics》1996,59(1-2):83-104
Summary Two techniques for deriving horizontal and vertical air motions using vertically scanning airborne Doppler radar data are presented and discussed. These techniques make use of the scanning ability of the NOAA P-3 tail-mounted radar antenna to view a region of space from at least two vantage points during a straight-line flight track. The scanning methodology is termed the Fore/Aft Scanning Technique or FAST because the antenna is alternately scanning forward and then aft of the flight track. The major advantages of FAST over flying two quasi-orthogonal flight tracks with the antenna scanning normal to the flight track are that the data are collected in roughly half the time and the aircraft does not have to execute a right-angle turn. However, accuracy of the resulting wind field is compromised slightly because the beam intersection angle is reduced from 90° to about 50°. The reduction of area covered because of large drift angles is also discussed.A three-dimensional wind field can be constructed using the dual-Doppler equations from FAST data using the two radial velocity estimates and vertical integration of the continuity equation with a boundary condition of no vertical motion at cloud top and the Earth's surface. To keep errors in the calculated winds acceptably small, the elevation angles are typically restricted to ±45° from the horizontal to minimize contamination of the horizontal wind by terminal fallspeeds.A different, and perhaps more believable vertical velocity, can be derived using a second technique that utilizes two (or more) airborne Doppler radar equipped aircraft each using FAST to observe the echo-top vertical velocity at common point (e.g., two aircraft flying parallel flight, paths, or by using an L-shaped flight track with a single aircraft). This technique results in 4 (or more) radial velocity estimates at each point (hence is called the quad-Doppler technique). Horizontal winds can be derived using either an overdetermined three-equation solution or an overdetermined dual-Doppler solution, whichever is more accurate. For the calculation of vertical velocity a new approach is proposed that utilizes the overdetermined triple-Doppler solution for vertical particle motion near cloud top, minus an estimate of terminal fallspeeds, as a top boundary condition for the downward vertical divergence integration to derive vertical air velocity elsewhere in the domain. In addition, this approach allows measurements at steep elevation angles allowing for more depth of coverage for a given range.To show the utility of the method, analyses of data collected using FAST are compared to conventional dual-Doppler-derived wind fields constructed from data collected simultaneously by S-band ground-based Doppler radars. An example of the quad-Doppler technique is also presented from the recently completed Tropical Oceans/Global Atmospheres Coupled Ocean/Atmosphere Response Experiment (TOGA/COARE). Comparisons of quad-Doppler vertical velocity are made with in-situ derived vertical air motions collected by the NASA DC-8 to judge the quality of the approach.With 9 Figures 相似文献
13.
Thomas K. Flesch 《Boundary-Layer Meteorology》1996,78(3-4):399-404
Backward Lagrangian stochastic models calculate particle trajectories in the atmosphere upstream of an observation point. A feature of these models is the ease with which the vertical flux contribution from surface area sources can be calculated. The flux contribution at an observation point P is found by summing the ratio of the particle vertical velocity at P to the touchdown velocity for particles which impact the ground within the source boundary. 相似文献
14.
An isentropic vertical coordinate model: Design and application to atmospheric frontogenesis studies
Summary The isentropic vertical coordinate model developed at UCLA is briefly reviewed. The review includes an outline of the approach used to overcome technical difficulties in handling model layers with small mass.The model's performance is demonstrated by simulating the evolution of a middle-latitude baroclinic disturbance. During the evolution of the disturbance, sharp frontal zones are generated in the upper and middle troposphere with realistic tropopause folding. The extent to which different dynamical processes contribute to frontogenesis is analyzed.While the model successfully simulates frontogenesis in the upper and middle troposphere, it has a difficulty in simulating surface fronts. The difficulty arises due to the lack of degrees of freedom in surface temperatures since an isentropic vertical coordinate model requires a large number ofvertical layers to obtain a highhorizontal resolution at the lower boundary. This suggests the potential of a hybrid vertical coordinate, which approaches at upper levels and at lower levels.With 12 Figures 相似文献
15.
通过一次影响华南地区的锋面降水过程实例，进行了静力与非静力中尺度模式、不同驱动模式、不同初边值组合、不同边值更新周期、不同起报时次等27组对比试验。较全面细致地从不同侧面分析讨论了初、边值对中尺度模式的影响规律及特点，得出了一些有学术和应用价值的结论。如中尺度模式预报的总体趋势决定于其驱动模式，初值仅显著影响中尺度模式大约前10小时的预报，以后的预报趋势则主要决定于边值的信息；边值对模式内区的影响主要通过各种时间尺度天气信息(天气波动)的传播和移动来实现；土壤含水量等模式外参量初值可影响较长时效的预报；模式通过边界只能引入大于等于边值更新周期4倍的时间尺度的信息，更快的信息则被过滤掉；等等。其中一些结论对今后发展中尺度数值预报及其资料同化具有参考价值。 相似文献
16.
An explicit three-dimensional nonhydrostatic numerical simulation of a tropical cyclone 总被引：6，自引：0，他引：6
G. J. Tripoli 《Meteorology and Atmospheric Physics》1992,49(1-4):229-254
Summary A nonhydrostatic numerical simulation of a tropical cyclone is performed with explicit representation of cumulus on a meso- scale grid and for a brief period on a meso- scale grid. Individual cumulus plumes are represented by a combination of explicit resolution and a 1.5 level closure predicting turbulent kinetic energy (TKE).The results demonstrate a number of expected and unexpected important scale interaction processes. Within the central core of the developing cyclone, meso- convective regions grow and breakdown into propagating inertiagravity waves throughout the lifecycle of the cyclone. In the early stages, the amplitude of pressure fluctuations associated with the meso- scale convection exceed the central pressure of the cyclone and strongly modulate its intensity. With each meso- scale pulsation, the cyclone core increases in strength, measured by the central pressure deficit. The increasingly strong inertial frequency of the storm core acts to increasingly trap the convection induced heating within the core by balancing the tangential wind against the low central pressure, before the meso- scale convection breaks down and sends the warmth away as a propagating wave. Eventually, the slow manifold's amplitude exceeds the amplitude of the meso- scale oscillations and a stable eye region is formed. As inertial instability increases, increasingly high thermal warmth can be protected in the core, allowing persistent subsidence to form and to clear out the cyclone eye.On the outside of the eye wall, strong inertial stability gradients in the troposphere cause convective warming to split the inflow to the eye wal! and spawn outwardly propagating inertia gravity waves. These waves carry away all of the heating forced by convection that is not inertially trapped by the eye wall and act as a moderating influence on storm intensity.Inertia gravity waves are also spawned in the stratosphere at the top of the eye wall by the revolution of asymmetric cumulus structures. In all instances, the tropospheric waves are coupled to the propagating stratospheric waves which both move at 35 ms–1, although there are many instances where the stratospheric waves seem to have no tropospheric counterpart. Hence the anvil top forcing and low level breakdown are linked.The outwardly propagating inertia gravity waves act to initiate outer bands of convection. This initiation is with the assistance of low level boundary layer variations of density related to previous convection and to virga falling from the anvil which moistens and destabilizes the mid levels of
e
minimum. The convection initiated by these waves does not move substantially outward with the wave, although may appear to develop outward discontinuously.With 12 Figures 相似文献
17.
Roy M. Endlich 《Boundary-Layer Meteorology》1984,30(1-4):375-386
A diagnostic model is a relatively simple and practical tool for modeling the wind flow of the boundary layer in complex terrain. The model begins with a wind analysis based on available surface wind reports and geostrophic winds (computed from pressure data). The height of the boundary layer top (upper surface of the computational domain) is prescribed to fit local conditions. Using the continuity equation in terrain-following coordinates, the winds at mesh points are adjusted to produce nondivergence while maintaining the original vertical component of vorticity. The method of computing the nondivergent winds uses direct alterations. This method may be useful for other modeling purposes and will be described. Data for a long period (usually a year) are analyzed to obtain eigenvectors and the associated time series of their coefficients at each observation time. The model is run only for the five or six eigenvectors that explain most of the variance. The wind field at any particular time is reconstructed from the eigenvector solutions and their appropriate coefficients. Comparisons of model results with measured winds at sites representing different types of terrain will be shown. The accuracy and economy of the model make it a useful tool for estimating wind energy and also for giving wind fields for low-level diffusion models. 相似文献
18.
The present study explores the extent to which the logarithmic region of the adiabatic atmospheric boundary layer can be modeled using a three-dimensional large eddy simulation. A value of the von Kármán constant (LES) is obtained by determining the slope of a logarithmic portion of the velocity profile. Its numerical value is found to be dependent on the value of the Smagorinsky-Model Reynolds number, ReSM: the value of LES increases with ReSM. Results indicate that LES approaches a value of 0.35 as ReSM reaches about 7.75 × 105 for the largest domain. The sensitivity of LES to the profile region over which it is evaluated has been tested. Results show that LES is not sensitive to the depth of this evaluation region when we employ five grids above the sub-grid buffer layer where sub-grid-scale effects dominate. The maximum LES is obtained when the lower boundary of the evaluation region is just above the top of the sub-grid-scale buffer layer. This result is consistent with modelled mean speed and resolved-scale shear stress profiles. 相似文献
19.
A higher order closure model is applied to simulate the dynamics in an area with a deep valley characterized by complex terrain in the southwestern US. The simulation results show generally good agreement with measured profiles at two locations within the valley. Both the measurements and the simulations indicate that the flow dynamics in the area are highly influenced by the topography and meandering of the valley, and can be resolved only by the full three-dimensional model code. The wind veering simulated over the range of the topographic elevations is often larger than 100 deg and in some cases as large as 180 deg, as a consequence of topographic forcing. In the case of an infinitely long valley, as is assumed in two-dimensional test simulations, a strong low-level jet occurs within the valley during stable conditions. The jet is mainly a consequence of the Coriolis effect. However, the jet development is significantly reduced due to asymmetric effects of the actual topography treated in the three-dimensional simulations. Tests with the two-dimensional nonhydrostatic version of the model show significant wave responses for a stable stratified flow over the valley. The structure resembles nonlinear mesoscale lee waves, which are intrinsically nonhydrostatic. However, considering the three-dimensional nature of this valley system, a better understanding and verification of the nonhydrostatic effects requires both a three-dimensional nonhydrostatic numerical model and an observational data set which is fully representative in all three dimensions.List of symbols (unless otherwise defined in the text)
B
1
closure constant
-
f
Coriolis parameter
-
g
acceleration of gravity
-
K
M
,K
H
,K
R
turbulent exchange coefficients for momentum, heat and moisture
-
k
von Karman constant
-
L
Monin-Obukhov length
-
q
2
twice the turbulent kinetic energy
-
R
specific humidity
-
s
height of the model top
-
T
g
ground surface temperature
-
t
time
-
U, V
horizontal components of wind
-
U
g
,V
g
geostrophic wind components
-
u, w
perturbation components ofU andW wind components
-
u
*
friction velocity
-
W
vertical wind component in the terrain-following coordinates
-
x, y
horizontal coordinates
-
Z
actual height above sea level
-
z
actual height above ground
-
z
0
roughness length
-
z
g
terrain height
-
z
i
depth of the convective boundary layer
-
1
closure constant
-
coefficient of thermal expansion
-
height in the terrain-following coordinate
-
master length scale in the turbulent parameterization
-
scaled pressure (Exner function)
-
potential temperature
-
m
normalized vertical wind shear 相似文献
20.
Summary Land-surface heterogeneity affects surface energy fluxes. The magnitudes of selected land-surface influences are quantified by comparing observations with model simulations of the FIFE (First ISLSCP Field Experiment) domain. Several plausible heterogeneous and homogeneous initial and boundary conditions are examined, although soilmoisture variability is emphasized. It turns out that simple spatial averages of surface variation produced biased flux values. Simulated maximum latent-heat fluxes were approximately 30 to 40 W m–2 higher, and air temperatures 0.4 °C lower (at noon), when computations were initialized with spatially averaged soil-moisture and leaf-area-index fields. The planetary boundary layer (PBL) height and turbulent exchanges were lower as well. It additionally was observed that (largely due to the nonlinear relationship between initial soil-moisture availability and the evapotranspiration rate), real latent-heat flux can be substantially less than simulated latent-heat flux using models initialized with spatially averaged soil-moisture fields. Differences between real and simulated fluxes also vary with the resolution at which real soil-moisture heterogeneity is discretized.With 8 Figures 相似文献