共查询到20条相似文献，搜索用时 31 毫秒
1.
The balance and redistribution of potential vorticity in a turbulent, inhomogeneous ocean is examined analytically. In the first part of the paper, we derive a vorticity conservation law describing the way in which vorticity is lost and gained in a closed ocean basin. We show that the potential vorticity within the ocean can only be altered by molecular diffusion at the bounding surfaces, or by the advective input of vorticity via the turbulent velocity at the free upper surface. An imbalance between these two effects within the body of the ocean leads to a net temporal change in the total potential vorticity, which we are able to express in terms of the temporal changes in the magnitude and orientation of a vector formed by the surface velocity and density and the earth's rotation. Finally, the steady-state balance of integrated vorticity is found to be between the surface wind-stress curl and the molecular diffusion at the solid boundaries.We then consider the localized redistribution of potential vorticity in detail. In recent papers, Green (1970) and Welander (1973) have purported to show that an internal redistribution of potential vorticity can give rise to important forces in the atmosphere (Green) and the ocean (Welander). Using their recipe leads in some cases to such unphysical results as a breakdown of the laws of conservation of angular momentum. Such consequences cause us to question the validity of the argument, and lead us to a close examination' of Welander's mechanism. (Green's argument is more general and does not lend itself to such close examination.) Our investigation reveals important flaws not only in Welander's argument, but also in the classical mixing-length theory of Prandtl (1925) and of Taylor (1915) on which it is modelled. More specifically, the internal redistribution process presented in these theories is found to be incomplete since it fails to take into account the lift force exerted on individual parcels of fluid as they are transported by the turbulent motions. Inclusion of these lift forces not only permits the unification of the classical mixing-length theories, but also shows that the forces claimed by Welander do not in fact arise. 相似文献
2.
Several numerical experiments are conducted to examine the influence of mesoscale, bottom topography roughness on the inertial circulation of a wind-driven, mid-latitude ocean gyre. The ocean model is based on the quasi-geostrophic formulation, and is eddy-resolving as it features high vertical and horizontal resolutions (six layers and a 10 km grid). An antisymmetrical double-gyre wind stress curl forces the baroclinic modes and generates a strong surface jet. In the case of a flat bottom, inertia and inverse energy cascade force the barotropic mode, and the resulting circulation features strong, barotropic, inertial gyres. The sea-floor roughness inhibits the inertial circulation in the deep layers; the barotropic component of the flow is then forced by eddy-topography interactions, and its energy concentrates at the scales of the topography. As a result, the baroclinicity of the flow is intesified: the barotropic mode is reduced with regard to the baroclinic modes, and the bottom flow (constrained by the mesoscale sea-floor roughness) is decoupled from the surface flow (forced by the gyre-scale wind). Rectified, mesoscale bottom circulation induces an interfacial form stress at the thermocline, which enhances horizontal shear instability and opposes the eastward penetration of the jet. The mean jet is consequently shortened, but the instantaneous jet remains very turbulent, with meanders of large meridional extent. The sea-floor roughness modifies the energy pathways, and the eddies have an even more important role in the establishment of the mean circulation: below the thermocline, rectification processes are dominant, and eddies transfer energy toward permanent mesoscale circulations strongly correlated with topography, whereas above the thermocline mean flow and eddy generation are influenced by the mean bottom circulation through interfacial stress. The topography modifies the vorticity of the barotropic and highest baroclinic modes. Vorticity accumulates at the small topographic scales, and the vorticity content of the highest modes, which is very weak in the flat-bottom case, increases significantly. Few changes occur in surface-intensified modes. In the deep layers of the model, the inverse correlation between relative vorticity and topography at small scales ensures the homogenization of the potential vorticity, which mainly retains the largest scales of the bottom flow and the scale of β. 相似文献
3.
《Dynamics of Atmospheres and Oceans》1999,29(2-4):335-364
Stochastic wind forcing of ocean gyre circulations is examined using the ideas of generalized linear stability theory applied to the barotropic vorticity equation of a idealized ocean. The barotropic vorticity equation is linearized about a time-evolving basic state flow, and the spatial patterns of stochastic surface wind stress curl that are optimal for increasing the variability of the ocean are computed. The most disruptive pattern of stochastic forcing is found to be insensitive to: measures of variance, the optimization time, the temporal decorrelation time of the stochastic forcing, the time evolution of the basic state flow, the stability of the basic state flow, basin size, gyre symmetry, and the presence of bathymetry. In addition, the most disruptive pattern of wind stress curl is reminiscent of that which would be associated with individual large-scale weather systems in the atmosphere, and changes in the amplitude of the atmospheric teleconnection patterns. The response of a nonlinear model to stochastic forcing described by the optimal patterns is examined, and the dynamics of the response discussed. 相似文献
4.
Island wakes in shallow water are investigated using the Regional Ocean Modeling System (ROMS). In contrast to deep water where bottom stress can be neglected in island wakes, shallow water implies that inhomogeneity in the bottom stress plays an important role in the wake vorticity generation. A series of numerical experiments are conducted to investigate wake formation and evolution in shallow water. It is found that the vertical structures of shallow-water and deep-water wakes are significantly different because of the presence of a density frontal jet, which results from the interaction between stratification and bottom topography. The frontal jet reaches its maximum within the bottom boundary layer over the shelf, giving rise to vorticity. The potential vorticity (PV) balance analysis reveals that frictional and diapycnal processes play different roles in the PV anomalies. With the absence of lateral stress (i.e., a sea mountain case), the surface vorticity becomes much weaker than that in the presence of an island. 相似文献
5.
D.E. Harrison 《Dynamics of Atmospheres and Oceans》1982,6(3):135-152
Mesoscale resolution ocean general circulation model (EGCM) experiments have been carried out under a variety of different model physical assumptions, and the different model systems often produce very different deep mean flow fields. The flat bottom, rectangular basin experiments exhibit two distinct types of deep mean flow, which are here called “corotating” and “counterrotating”. Counterrotating deep flow, in which two adjacent deep gyres, with circulation of opposite senses, underlie the upper ocean eastward jet and its recirculation, has been found only in models with adiabetic two-layer model physics. None of the more complex model systems exhibit counterrotating deep flows; this type of flow is apparently restricted to a particular range of forcing/dissipation parameter space and/or particular model physical assumptions.Since the deep flow in these EGCM systems is generally weak, geostrophic dynamics provides the basic deep flow interior balance and the mean vertical velocity field, through the lower layer vorticity equation, largely determines the deep interior flow. The dynamical constraints on the mean vertical velocity field introduced by different model physical equations are reviewed and the adiabatic quasi-geostrophic (QG) two-layer model system is shown to be strongly constrained in several respects. In particular, the idea that eddy and mean heat flux divergence (or “layer thickness flux divergence”) drive the mean vertical velocity does not generalize to more complicated dynamical systems in which there is the possibility of altering the mean vertical density profile and/or in which the horizontal flow can be divergent. As a consequence of the constraints, there can be no basin net vorticity input to the lower layer via vortex stretching in the QG system.Because of the adiabatic QG constraints and the particular parametric regime in which the published adiabatic QG EGCM experiments exist, a very plausible explanation can be found for the existence of the deep cyclonic circulation of the model subtropical gyre. It is this cyclonic circulation that causes these deep flows to differ so dramatically from those of the more physically complex model systems. Because all the published adiabatic QG experiments that have non-trivial deep flows exhibit the counterrotating behavior, and because available ocean data do not support the existence of such a gyre in the North Atlantic, it seems important to thoroughly understand the reasons for the existence or absence of the deep cyclonic circulations. If they are an invitable feature of adiabatic QG systems, these models may need to be treated with caution as tools for understanding the mean ocean circulation. 相似文献
6.
《Dynamics of Atmospheres and Oceans》2002,35(2):97-130
This work is an attempt to simulate the Mediterranean Sea general circulation with a Spectral Finite Element Model. This numerical technique associates the geometrical flexibility of the finite elements for the proper coastline definition with the precision offered by spectral methods. The model is reduced gravity and we study the wind-driven ocean response in order to explain the large scale sub-basin gyres and their variability. The study period goes from January 1987 to December 1993 and two forcing data sets are used. The effect of wind variability in space and time is analyzed and the relationship between wind stress curl and ocean response is stressed. Some of the main permanent structures of the general circulation (Gulf of Lions cyclonic gyre, Rhodes gyre, Gulf of Syrte anticylone) are shown to be induced by permanent wind stress curl structures. The magnitude and spatial variability of the wind is important in determining the appearance or disappearance of some gyres (Tyrrhenian anticyclonic gyre, Balearic anticyclonic gyre, Ionian cyclonic gyre). An EOF analysis of the seasonal variability indicates that the weakening and strengthening of the Levantine basin boundary currents is a major component of the seasonal cycle in the basin.The important discovery is that seasonal and interannual variability peak at the same spatial scales in the ocean response and that the interannual variability includes the change in amplitude and phase of the seasonal cycle in the sub-basin scale gyres and boundary currents. The Coriolis term in the vorticity balance seems to be responsible for the weakening of anticyclonic structures and their total disappearance when they are close to a boundary.The process of adjustment to winds produces a train of coastally trapped gravity waves which travel around the eastern and western basins, respectively in approximately 6 months. This corresponds to a phase velocity for the wave of about 1 m/s, comparable to an average velocity of an internal Kelvin wave in the area. 相似文献
7.
The dynamics of the North Atlantic subpolar gyre (SPG) are assessed under present and glacial boundary conditions by investigating the SPG sensitivity to surface wind-stress changes in a coupled climate model. To this end, the gyre transport is decomposed in Ekman, thermohaline, and bottom transports. Surface wind-stress variations are found to play an important indirect role in SPG dynamics through their effect on water-mass densities. Our results suggest the existence of two dynamically distinct regimes of the SPG, depending on the absence or presence of deep water formation (DWF) in the Nordic Seas and a vigorous Greenland?CScotland ridge (GSR) overflow. In the first regime, the GSR overflow is weak and the SPG strength increases with wind-stress as a result of enhanced outcropping of isopycnals in the centre of the SPG. As soon as a vigorous GSR overflow is established, its associated positive density anomalies on the southern GSR slope reduce the SPG strength. This has implications for past glacial abrupt climate changes, insofar as these can be explained through latitudinal shifts in North Atlantic DWF sites and strengthening of the North Atlantic current. Regardless of the ultimate trigger, an abrupt shift of DWF into the Nordic Seas could result both in a drastic reduction of the SPG strength and a sudden reversal in its sensitivity to wind-stress variations. Our results could provide insight into changes in the horizontal ocean circulation during abrupt glacial climate changes, which have been largely neglected up to now in model studies. 相似文献
8.
Nicolas Barrier Anne-Marie Treguier Christophe Cassou Julie Deshayes 《Climate Dynamics》2013,41(5-6):1159-1171
Interannual variability of subtropical sea-surface-height (SSH) anomalies, estimated by satellite and tide-gauge data, is investigated in relation to wintertime daily North-Atlantic weather regimes. Sea-level anomalies can be viewed as proxies for the subtropical gyre intensity because of the intrinsic baroclinic structure of the circulation. Our results show that the strongest correlation between SSH and weather regimes is found with the so-called Atlantic-Ridge (AR) while no significant values are obtained for the other regimes, including those related to the North Atlantic Oscillation (NAO), known as the primary actor of the Atlantic dynamics. Wintertime AR events are characterized by anticyclonic wind anomalies off Europe leading to a northward shift of the climatological wind-stress curl. The latter affects subtropical SSH annual variability by altered Sverdrup balance and ocean Rossby wave dynamics propagating westward from the African coast towards the Caribbean. The use of a simple linear planetary geostrophic model allows to quantify those effects and confirms the primary importance of the winter season to explain the largest part of SSH interannual variability in the Atlantic subtropical gyre. Our results open new perspectives in the comprehension of North-Atlantic Ocean variability emphasizing the role of AR as a driver of interannual variability at least of comparable importance to NAO. 相似文献
9.
A model study of the Little Ice Age and beyond: changes in ocean heat content,hydrography and circulation since 1500 总被引：1，自引：1，他引：0
The Earth System Climate Model from the University of Victoria is used to investigate changes in ocean properties such as
heat content, temperature, salinity, density and circulation during 1500 to 2000, the time period which includes the Little
Ice Age (LIA) (1500–1850) and the industrial era (1850–2000). We force the model with two different wind-stress fields which
take into account the North Atlantic Oscillation. Furthermore, temporally varying radiative forcings due to volcanic activity,
insolation changes and greenhouse gas changes are also implemented. We find that changes in the upper ocean (0–300 m) heat
content are mainly driven by changes in radiative forcing, except in the polar regions where the varying wind-stress induces
changes in ocean heat content. In the full ocean (0–3,000 m) the wind-driven effects tend to reduce, prior to 1700, the downward
trend in the ocean heat content caused by the radiative forcing. Afterwards no dynamical effect is visible. The colder ocean
temperatures in the top 600 m during the LIA are caused by changes in radiative forcing, while the cooling at the bottom is
wind-driven. The changes in salinity are small except in the Arctic Ocean. The reduced salinity content in the subsurface
Arctic Ocean during the LIA is a result from reduced wind-driven inflow of saline water from the North Atlantic. At the surface
of the Arctic Ocean the changes in salinity are caused by changes in sea–ice thickness. The changes in density are a composite
picture of the temperature and salinity changes. Furthermore, changes in the meridional overturning circulation (MOC) are
caused mainly by a varying wind-stress forcing; the additional buoyancy driven changes due to the radiative forcings are small.
The simulated MOC is reduced during the LIA as compared to the industrial era. On the other hand, the ventilation rate in
the Southern Ocean is increased during the LIA. 相似文献
10.
A high-resolution ocean general circulation model （OGCM） is used to investigate the Kuroshio path variations south of Japan. The model reproduces many important features of the Kuroshio system including its interannual bimodal variability south of Japan. A decreasing trend of the spatial averaged relative vorticity is detected when the Kuroshio takes the non-large meander （NLM） path, and during the transition period from the NLM to the large meander （LM）, a sudden release of velocity shear corresponds well to the weakening of the Shikoku recirculation gyre （SRG）, which plays a key role in modulating the Kuroshio path variations. Analysis of eddy energetics indicates that baroclinic instability is mainly responsible for the formation of the LM. In addition, further analysis shows that the strength of the SRG could be largely influenced by the baroclinic Rossby wave adjustment process, forced by the wind stress curl anomalies in the North Pacific basin, based on the model investigation. It is suggested that the cyclonic disturbances might account for the weakening of the SRG, and act as a remote trigger for the baroclinic instability of the Kuroshio south of Japan. 相似文献
11.
Suryachandra A. Rao Ashish R. Dhakate Subodh K. Saha Somnath Mahapatra Hemantkumar S. Chaudhari Samir Pokhrel Sobhan K. Sahu 《Climatic change》2012,110(3-4):709-719
Observations have shown that the Indian Ocean is consistently warming and its warm pool is expanding, particularly in the recent decades. This paper attempts to investigate the reason behind these observations. Under global warming scenario, it is expected that the greenhouse gas induced changes in air–sea fluxes will enhance the warming. Surprisingly, it is found that the net surface heat fluxes over Indian Ocean warm pool (IOWP) region alone cannot explain the consistent warming. The warm pool area anomaly of IOWP is strongly correlated with the sea surface height anomaly, suggesting an important role played by the ocean advection processes in warming and expansion of IOWP. The structure of lead/lag correlations further suggests that Oceanic Rossby waves might be involved in the warming. Using heat budget analysis of several Ocean data assimilation products, it is shown that the net surface heat flux (advection) alone tends to cool (warm) the Ocean. Based on above observations, we propose an ocean-atmosphere coupled positive feedback mechanism for explaining the consistent warming and expansion of IOWP. Warming over IOWP induces an enhancement of convection in central equatorial Indian ocean, which causes anomalous easterlies along the equator. Anomalous easterlies in turn excite frequent Indian ocean Dipole events and cause anti-cyclonic wind stress curl in south-east and north-east equatorial Indian ocean. The anomalous wind stress curl triggers anomalous downwelling oceanic Rossby waves, thereby deepening the thermocline and resulting in advection of warm waters towards western Indian ocean. This acts as a positive feedback and results in more warming and westward expansion of IOWP. 相似文献
12.
利用热带太平洋地区 2层区域海洋模式和再分析资料探讨了大气 /海洋的季节背景对ENSO时间尺度上海温异常的增幅或减幅作用。结果表明 :不论什么季节背景下 ,就海洋变化而言 ,El Nino/La Nina事件均可存在 ,说明 El Nino/La Nina事件的存在和维持并没有季节性选择 ;模式海洋的西太平洋赤道地区次表层海水温度异常变化位相明显超前于东太平洋SSTA且在空间上自西向东传播 ,同时有位相超前的量还包括模式第 1层厚度异常及热容量异常 ,这些对 ENSO的预测和机制研究具有重要意义。 SSTA的振幅在不同的季节背景下可受到不同的调节。以春季为背景 ,同样的异常风应力作用于海洋 ,可使 NINO3区 SSTA较正常季节背景下该区的 SSTA振幅明显增大 ;而在冬季背景下可使 SSTA受到一定程度的减幅 ,这说明 El Nino/L a Nina现象的发生和消亡有季节选择倾向。与春季背景下 NINO 3区SSTA的增幅倾向相反 ,模式第 1层厚度异常的振幅则受到削弱。而西太平洋赤道地区 ,模式海洋混合层的厚度异常则有所增强。这种与 SSTA增 /减幅反相关的现象需要进一步研究 相似文献
13.
K. Haines P. Malanotte-Rizzoli R.E. Young W.R. Holland 《Dynamics of Atmospheres and Oceans》1993,17(2-3)
Two simple methods for assimilating ocean surface pressure data into a three-layer adiabatic primitive equation model are tested and compared using a twin-experiment approach. One of the methods is based on a recently presented direct insertion scheme using a quasi-geostrophic model. The method explicitly avoids making changes to the potential vorticity fields in the lower layers and the vertical current structure after assimilation is determined by this criterion. The modification required for a primitive equation framework are discussed and a comparison is made with the more familiar ‘nudging’ method of assimilation. We use root mean square errors to quantify the response of the model to the assimilation of a single complete surface pressure field and also to repeated or intermittent data available every 20 days for a period of 1 year. The two methods are almost equally effective in the short term, although the direct insertion scheme appears to be more effective in the longer term. It is suggested that this is because insertion maintains a geostrophic balance at all levels and thus avoids the generation of internal gravity waves which are needed when nudging is used to restore geostrophic currents in the deeper layers. 相似文献
14.
Sulochana Gadgil 《Dynamics of Atmospheres and Oceans》1976,1(2):127-161
The effects of baroclinic instability of a broad ocean current, flowing in an ocean basin with a plane sloping bottom, on the path of the current are studied. The set of equations governing this path and its variation with depth are the vorticity equation and the heat equation. It is assumed that the vertical and horizontal temperature contrasts are comparable as suggested by observations of the Gulf Stream. When quasi-geostrophy is assumed in addition, this implies that the leading contribution to the heat equation does not contain the vertical advection of the basic stratification. This corresponds to the long-wave approximation of the usual baroclinic-instability problem. The heat equation determines the vertical variation of the path and when this is combined with the vorticity equation, the equation governing the path at one level is obtained. The path equation requires a specification of the direction and curvature at the inlet and these conditions are taken to be time-dependent. When these conditions contain frequencies for which the current is unstable, meanders in the path of the current increase in amplitude downstream of the inlet. When the path at the inlet changes suddenly from one parallel to the isobaths to one making a small angle with them, the region of instability in which the amplitude of the meanders increases, is confined to a restricted segment of the path, at soms distance from the inlet. This region becomes advected with the basic current, and its extent increases with time. The amplitude of the meanders in this region increases while their wavelength decreases in time because the shorter waves are unstabler. The increase in amplitude and decrease in wavelength in a restricted segment of the path could lead to eddy formation in a finite-amplitude model and may therefore suggest a mechanism for eddy formation in the Gulf Stream. 相似文献
15.
The mesoscale balance and imbalance and the corresponding potential vorticity inversion from the view of helicity 
下载免费PDF全文
下载免费PDF全文 The static balance and the geostrophic balance are the common balances in meteorology.All the synoptic systems and most of the mesoscale systems satisfy the above two balances.However,due to the strong convection and non-geostrophic feature,many mesoscale systems usually present as static imbalance,and the quasi-geostrophic approximation is no longer attainable.This paper tried to find out a kind of balance that exists for mesoscale convective system.To do this,the concrete mathematics definitions for balance and imbalance equations were defined.Then,it is proposed that the new balance equation should include the divergence,vorticity,and vertical motion simultaneously,and the helicity equation was a good choice for the basis.Finally,the mesoscale balance and imbalance equations were constructed,as well as a new balance model that was based on the helicity,horizontal divergence,vertical vorticity,continuity,and thermal dynamic equations under same approximations.Moreover,the corresponding potential vorticity(PV)inversion technique was introduced.It was pointed out that by using the PV conservation and the potential temperature conservation,the flows of the mesoscale balance model can be deduced,and their comparison with the real fields would give the degree of the imbalance. 相似文献
16.
《Dynamics of Atmospheres and Oceans》1988,12(1):47-70
Two local implementations of no-slip boundary conditions are investigated for both the vorticity—streamfunction and momentum—pressure formulations of the time-dependent planar incompressible Navier-Stokes equations, as applied to barotropic ocean circulation modelling. The objective is to determine the extent to which the local accuracy and numerical consistency of these conditions affects the global solution. The effects of a non-local implementation of no-slip conditions for the vorticity—streamfunction equations are also studied. In all cases, boundary condition effects are measured by comparing time-averaged dynamics of turbulent solutions of numerical models based on the two formulations.In the model interior, the energy and enstrophy conserving Arakawa Jacobian is used for the vorticity—streamfunction equations while an extension of the energy and potential enstrophy conserving Arakawa and Lamb finite difference scheme is used for the momentum-pressure equations.Numerical experiments performed with a non-linear model similar to Bryan's barotropic ocean reveal no significant differences between the time-averaged solutions obtained with either of the two formulations, with each using either of the two local boundary conditions. A simple one-dimensional analogue of the vorticity—streamfunction equations is solved algebraically to explain the experimental results. A similar analogue suggests that an apparent inconsistency in the no-slip boundary conditions within the Cox stratified, primitive equation, ocean circulation model should not affect the accuracy or convergence of the global solution. 相似文献
17.
V. HAMZA C. A. BABU T. P. SABIN 《大气科学进展》2007,24(4):631-643
The marine atmospheric boundary layer （MABL） plays a vital role in the transport of momentum and heat from the surface of the ocean into the atmosphere. A detailed study on the MABL characteristics was carried out using high-resolution surface-wind data as measured by the QuikSCAT （Quick scatterometer） satellite. Spatial variations in the surface wind, frictional velocity, roughness parameter and drag coefficient for the different seasons were studied. The surface wind was strong during the southwest monsoon season due to the modulation induced by the Low Level Jetstream. The drag coefficient was larger during this season, due to the strong winds and was lower during the winter months. The spatial variations in the frictional velocity over the seas was small during the post-monsoon season （-0.2 m s^-1）. The maximum spatial variation in the frictional velocity was found over the south Arabian Sea （0.3 to 0.5 m s^-1） during the southwest monsoon period, followed by the pre-monsoon over the Bay of Bengal （0.1 to 0.25 m s^-1）. The mean wind-stress curl during the winter was positive over the equatorial region, with a maximum value of 1.5×10^-7 N m^-3, but on either side of the equatorial belt, a negative wind-stress curl dominated. The area average of the frictional velocity and drag coefficient over the Arabian Sea and Bay of Bengal were also studied. The values of frictional velocity shows a variability that is similar to the intraseasonal oscillation （ISO） and this was confirmed via wavelet analysis. In the case of the drag coefficient, the prominent oscillations were ISO and quasi-biweekly mode （QBM）. The interrelationship between the drag coefficient and the frictional velocity with wind speed in both the Arabian Sea and the Bay of Bengal was also studied. 相似文献
18.
巢纪平 《Acta Meteorologica Sinica》2001,(1)
Some dynamic constrains in the process of the tropical baroclinic geostrophic adjustment arediscussed.The dispersion equation of three-dimensional inertia-gravitational wave and the equation oftemporal conservation of potential vorticity are given,without considering the gradient of planetarypotential vorticity.It indicates that the motions will be horizontal,meaning that Taylor-Proudmantheorem is still right for tropics.At the same time,the semi-geostrophic balance is easier to appear inthe tropical belt.Therefore the motions are generally horizontal and non-divergent,but still arestratified after the geostrophic balance establishes. 相似文献
19.
CHAO Jiping 《Acta Meteorologica Sinica》2001,15(1):40-48
Some dynamic constrains in the process of the tropical baroclinic geostrophic adjustment are discussed.The dispersion equation of three-dimensional inertia-gravitational wave and the equation of temporal conservation of potential vorticity are given,without considering the gradient of planetary potential vorticity.It indicates that the motions will be horizontal,meaning that Taylor-Proudman theorem is still right for tropics.At the same time,the semi-geostrophic balance is easier to appear in the tropical belt.Therefore the motions are generally horizontal and non-divergent,but still are stratified after the geostrophic balance establishes. 相似文献
20.
在初步明确东亚沙尘气溶胶对流层-平流层输送监测事实的基础上,利用观测资料、NCEP再分析资料以及基于中尺度天气模式MM5的数值模拟方法,对一次蒙古气旋沙尘暴过程中沙尘对流层-平流层输送问题进行了初步分析.结果表明:斜压不稳定是本次蒙古气旋发展的主要强迫要素,伴随气旋发展成熟,高空切断低涡的形成引导高空急流下落并诱发对流层顶折叠和高空位涡下传.对流层顶折叠区呈漏斗状,底部达500 hPa左右.高空急流产生近似垂直的下落,并在高空切断低涡的南侧和东侧达到最强.在对流层顶折叠区周边的300-500 hPa,上升气流与低涡区偏西、偏南、偏东气流叠加,或水平横穿折叠的对流层顶,或斜升并准垂直地穿过下落的对流层顶到达平流层,且随时间的推移,空气质点能够进一步抵达平流层中部(100 hPa).轨迹分析表明,沙尘天气区对流层低层的空气质点在气旋涡旋上升气流的驱动下呈气旋式盘旋上升,并在对流层高层形成分支,一支穿过对流层顶到达平流层,并在平流层向下游进行反气旋式螺旋运动,另一支则留在对流层高层并向下游进行准水平的气旋式螺旋运动.在高空位涡下传过程中,主要产生平流层到对流层的净输送;高空位涡停止下传之后则出现对流层到平流层的净输送,且强度随时间呈指数型增长.这一特征有利于形成更强的沙尘对流层平流层输送. 相似文献