Charney’s Model—the Renowned Prototype of Baroclinic Instability—Is Barotropically Unstable As Well

The Charney model is reexamined using a new mathematical tool, the multiscale window transform(MWT), and the MWT-based localized multiscale energetics analysis developed by Liang and Robinson to deal with realistic geophysical fluid flow processes. Traditionally, though this model has been taken as a prototype of baroclinic instability, it actually undergoes a mixed one. While baroclinic instability explains the bottom-trapped feature of the perturbation, the second extreme center in the perturbation field can only be explained by a new barotropic instability when the Charney–Green number γ 1, which takes place throughout the fluid column, and is maximized at a height where its baroclinic counterpart stops functioning.The giving way of the baroclinic instability to a barotropic one at this height corresponds well to the rectification of the tilting found on the maps of perturbation velocity and pressure. Also established in this study is the relative importance of barotropic instability to baroclinic instability in terms of γ. When γ 1, barotropic instability is negligible and hence the system can be viewed as purely baroclinic;when γ 1, however, barotropic and baroclinic instabilities are of the same order;in fact, barotropic instability can be even stronger. The implication of these results has been discussed in linking them to real atmospheric processes.

Some Aspects of the Characteristics of Monsoon Disturbances Using a Combined Barotropic－Baroclinic Model

A standing Rossby wave of wavelength 30°longitude with a finite amplitude along the meridional direction is superimposed on the zonal mean component of the monsoon flow and the stability of such a flow is examined by a quasi-geostrophic barotropic.as well as by a quasi-geostrophic combined barotropic and baroclinic model on a beta plane centered at 18°N latitude. It is found that the growth of synoptic scale disturbance increases with the amplitude of the meridional wind. The barotropic stability analysis at 700 hPa pressure level shows that there is a critical value (Umaz=11 m/s) of the maximum mean zonal wind below which the computed disturbance moves to the west due to the wave-wave superposition. For maximum mean zonal wind greater that 11m /s, the westerly wind dominates and the disturbance moves to the east. In another analysis the stability of the zonally asymmetric basic now is studied with a combined barotropic-barotropic model where cumulus heating is included. The growth rate, intensity, horizontal scale and the westward velocity of computed disturbances reasonably agree with those of observed monsoon disturbances. The fastest growing mode has a horizontal wavelength of 2000 kms and the e-folding time is about 3 days,when the meridional amplitude of the Rossby wave is 4m/ s at 700 hPa pressure level. When cumulus heating is included in the analysis the intensity of geopotential perturbation at 700 hPa disturbance is -304 m2 / s2. Energy calculations show that the kinetic energy of the mean zonal flow is the main source of energy for the perturbation to grow.It is also t'ound that the contribution of the kinetic energy of the basic Rossby wave to the growth of perturbation is more in comparison to the available potential energy.

Conversion Characteristics between Barotropic and Baroclinic Circulations of the SAH in Its Seasonal Evolution

In the context of 1958–1997 NCEP/ NCAR re-analyses, the South Asia high (SAH) was divided into two components, barotropic and baroclinic, the former based on mass weighed vertical integration and the latter on the difference between the measured circulation and the barotropic component counterpart, whereupon the barotropic and baroclinic circulation conversion features were addressed of the research SAH during its seasonal variation. Evidence suggests that i) in summer (winter), the SAH is a thermal (dynamical) system, with dominant baroclinicity (barotropicity), either of the components accounting for approximately 70% of the total contribution; ii) as time progresses from winter to summer, accompanied by the barotropic SAH evolving into its baroclinic analog, the SAH is moving under the “ thermal guidance” of its baroclinic component circulation, suggesting that the component circulation precedes the system itself in variation; iii) the reversal happens when it goes from summer to winter, with the SAH displacement under the “ dynamic steering” of its barotropic component circulation. Key words SAH (South Asia high) - Barotropic circulation - Baroclinic circulation - Seasonal variation (1)This work is supported by the National Natural Sciences Foundation of China under Grant No.49735170.

Comparison of Flow Characteristics Around Refractive and Right-angled Groins in Barotropic and Baroclinic Conditions

Groins are employed to prevent nearshore areas from erosion and to control the direction of flow. However, the groin structure and its associated flow characteristics are the main causes of local erosion. In this study, we investigate the flow patterns around refractive and right-angle groins. In particular, we analytically compare the flow characteristics around a refractive groin and study the degree of accuracy that can be achieved by using a right-angle groin of various projected lengths. To compare the flow characteristics, we replaced the right-angle groin with an approximation of a refractive groin. This replacement had the least effect on the maximum velocity of flow in the channel. Moreover, we investigated the distribution of the density variables of temperature and salinity, and their effects on the flow characteristics around the right-angle groin. A comparison of the flow analysis results in baroclinic and barotropic conditions reveals that the flow characteristic values are very similar for both the refractive and right-angle groins. The geometry of the groin, i.e., right-angle or refractive, has little effect on the maximum speed to relative average speed. Apart from the angular separation, the arm length of the groin in downstream refractive groins has less effect on other flow characteristics than do upstream refractive groins. We also correlated a number of non-dimensional variables with respect to various flow characteristics and groin geometry. These comparisons indicate that the correlation between the thalweg height and width of the channel and groin arm＇s length to projection length have been approximated using linear and nonlinear formulas regardless of inner velocity in the subcritical flow.

The Role of the Halted Baroclinic Mode at the Central Equatorial Pacific in El Nifn Event

The role of halted ＂baroclinic modes＂ in the central equatorial Pacific is analyzed. It is found that dominant anomaly signals corresponding to ＂baroclinic modes＂ occur in the upper layer of the equatorial Pacific, in a two-and-a-half layer oceanic model, in assimilated results of a simple OGCM and in the ADCP observation of TAO. A second ＂baroclinic mode＂ is halted in the central equatorial Pacific corresponding to a positive SST anomaly while the first ＂baroclinic mode＂ propagates eastwards in the eastern equatorial Pacific. The role of the halted second ＂baroclinic mode＂ in the central equatorial Pacific is explained by a staged ocean-atmosphere interaction mechanism in the formation of El Nifio： the westerly bursts in boreal winter over the western equatorial Pacific generate the halted second ＂baroclinic mode＂ in the central equatorial Pacific, leading to the increase of heat content and temperature in the upper layer of the central Pacific which induces the shift of convection from over the western equatorial Pacific to the central equatorial Pacific; another wider, westerly anomaly burst is induced over the western region of convection above the central equatorial Pacific and the westerly anomaly burst generates the first ＂baroclinic mode＂ propagating to the eastern equatorial Pacific, resulting in a warm event in the eastern equatorial Pacific. The mechanism presented in this paper reveals that the central equatorial Pacific is a key region in detecting the possibility of ENSO and, by analyzing TAO observation data of ocean currents and temperature in the central equatorial Pacific, in predicting the coming of an El Nino several months ahead.

A 3-dimensional baroclinic circulation model of the South China Sea

The summer and winter circulations in the South China Sea (SCS) including the surface elevation and water temperature are simulated using the model described by Cai and Li (1996) with the monthly mean wind stress and air temperature field at the 1000 mb level from the European Centre for Medium-Range Weather Forecasts as inputs. The boundary conditions at Bashi Channel and Taiwan Strait are taken from the simulation results of the Kuroshio using the same numerical model with a grid size of 0.5°×0.5° and the results of Cai and Li (1996) as boundary conditions. The computational domain for the present paper is between 100°E and 123°E and between 4.5°N and 27°N. The horizontal resolution is 0.25°×0.25° and the vertical variations of the velocity components are resolved by 6 layers The computed steady flow, temperature and elevation fields are consistent with the corresponding fields observed. In particular, the temperature and elevation fields of the South China Sea Warm Current (SCSWC) have been successfully simulated. The paths of the branch of the Kuroshio entering the South China Sea (SCSBK) through Bashi Channel in winter and summer are discussed It is found that the SCSBK flows southward to the southern SCS from the coast of the Guangdong Province. A portion of the SCSBK returns to the Bashi Channel and subdivides again in deep waters in winter with a branch flows to the south along the coast of the Philippines instead of flowing back to the Pacific In addition, our results confirm the existence of a eastward current to the northeast of Dongsha in summer with the Kuroshio as its source as suggested by Huang et al. Since the value of the eddy viscosity adopted for the simulation of the Kuroshio is on the high side, resulting in a weaker west boundary current in the western Pacific as the boundary conditions for the present simulations, some deviations from the actual situations are expected although the results are in general consistent with observations.

Two Modes of the Silk Road Pattern and Their Interannual Variability Simulated by LASG/IAP AGCM SAMIL2.0

In this study, two modes of the Silk Road pattern were investigated using NCEP2 reanalysis data and the simulation produced by Spectral Atmospheric Circulation Model of IAP LASG, Version 2 （SAMIL2.0） that was forced by SST observation data. The horizontal distribution of both modes were reasonably reproduced by the simulation, with a pattern correlation coefficient of 0.63 for the first mode and 0.62 for the second mode. The wave train was maintained by barotropic energy conversion （denoted as CK） and baroclinic energy conversion （denoted as CP） from the mean flow. The distribution of CK was dominated by its meridional component （CKy） in both modes. When integrated spatially, CKx was more efficient than its zonal component （CKx） in the first mode but less in the second mode. The distribution and efficiency of CK were not captured well by SAMIL2.0. However, the model performed reasonably well at reproducing the distribution and efficiency of CP in both modes. Because CP is more efficient than CK, the spatial patterns of the Silk Road pattern were well reproduced. Interestingly, the temporal phase of the second mode was well captured by a single-member simulation. However, further analysis of other ensemble runs demonstrated that the successful reproduction of the temporal phase was a result of internal variability rather than a signal of SST forcing. The analysis shows that the observed temporal variations of both CP and CK were poorly reproduced, leading to the low accuracy of the temporal phase of the Silk Road pattern in the simulation.

Vertical Structure of Baroclinic Currents over Northern South China Sea Continental Slope

The 28.6 d time series of ADCP currents at 27 depth levels, (11,15,…,115m) which are obtained from a mooring station at the northern South China Sea continental slope, have been decomposed into barotropic and baroclinic components. The emphasis in this paper is on the analysis of the time series of baroclinic currents by means of several methods,such as the tidal harmonic analysis, the power spectra and the kinetic energy estimation.The major results are as follows: (1) In respect of the baroclinic currents, the values of several parameters first decrease and then increase with depth. These parameters include the temporal mean value of the cross-shelf component, the standard deviations, the amplitudes of K1 constituents, the mean eddy kinetic energy, as well as the significant peaks of the power spectra of the cross-shelf components. (2) The diurnal period of the baroclinic currents is dominant. The diumal tidal current rotates clockwise and the major axis of current ellipse is located along the directions of NW-SE. The vertical distribution of the phases of the diurnal constituent varies with the different water layers. Around the 67m depth level, the phase changes very much.At those layers far away from the 67m depth level, the vertical distribution of the phase is relatively stable, but with opposite phases in the upper and lower water layers. For the upper layers between the surface and 67m,the phases are around 300°; for the lower layers between 67m and 115m, the phases are around 120° .(3) The mean eddy kinetic energy of the baroclinic current is quite large, accounting for 41% of the mean kinetic energy of the measured currents. The cross-shelf component is larger than the along-shelf one. The two baroclinic current components correspond to the major and minor axes of the current ellipse of the diurnal constituent respectively. (4) The power spectra of the baroclinic currents show a singnificant period of about 24h, with 23.6h at both 19m and 99m and 24.4h at 55m. The vertical distributions of the significant spectra-peak values of the power spectra of both the crossshelf and along-shelf components of both the baroclinic currents are different. The former increases with depth, then decreases and finally increases again while the latter decreases with depth.

BAROTROPIC AND BAROCLINIC ENSTROPHY EQUATIONS WITH THEIR APPLICATIONS TO A BLOCKING CIRCULATION

After the manner for studying atmospheric kinetic energy,concepts of atmospheric enstrophy (ζ~2/2)_m and barotropic and baroclinic enstrophy (ζ_m^2/2,ζ_s^2/2) are developed with their relations investigated,whereupon are established,separately,equations for the 1000- 100 hPa extent- averaged ζ_m^2/2 and ζ_s^2/2 over a limited area and on a local basis.Study shows that controlling their changes are the following factors:the terms of their fluxes (viz.,divergences).β effect,their mutual conversions,production and dissipation.Analysis is undertaken of these terms-dependent physical mechanisms for the variations in barotropic and baroclinic enstrophy and by means of the equations,calculation is conducted of the terms during the development of an Okhotsk blocking circulation,indicating that the total,harotropic and haroclinic enstrophies experience noticeable variations,from which we see that the latter two factors can really characterize the development as a whole,thus revealing the mechanisms at different stages of the circulation history.

ANALYSIS OF THE INTER-TRANSFOMATION OF EQUIVALENT BAROTROPIC AND BAROCLINIC ROSSBY WAVES

By using the low-spectral model simplified from quasi-geostrophic approximation two-level model,we obtained the result that the equivalent barotropic state is an equilibrium state of the atmosphere in that model,and discussed the conditions needed for existing the Rossby wave possessing the equivalent barotropic structure.And then we derived the oscillation equation reflecting the phase change of Rossby wave between higher and lower levels from the basic state of equivalent barotropic stationary Rossby wave,indicating the relations between the change of difference of phase at higher and lower levels and the disturbance of vertical shear,the amplitude disturbance of the part of waves of stream function and thermal wind stream function on the average level (A′ and B′).It is concluded that the inter-transformation exists between Rossby waves possessing the equivalent barotropic and baroclinic structure.

FEATURES OF VARIATION IN TOTAL,BAROTROPIC AND BAROCLINIC KINETIC ENERGY WITH THE MECHANISM EXPLORED AROUND THE 1998 SCS SUMMER MONSOON ONSET

This paper concerns atmospheric kinetic energy variation related to the onset of summer monsoon in May,1998 over the SCS (South China Sea).Results show that around the onset, noticeable conversion occurs in atmospheric total,barotropic and baroclinic kinetic energy (KE) at 300-1000 hPa;three days before the onset,total and barotropic KE are already growing markedly and baroclinic KE increases simultaneously with the onset:the monsoon's onset is associated closely to the meridional propagation of barotropic and baroclinic KE in the SCS region and study of equations of barotropie/baroclinic KE indicates that the net production term is the dominant factor for the change of the two forms of KE,the term of fluxes plays a part in weakening the energies and the conversion term is responsible for transforming barotropic into baroclinic KE.

BAROTROPIC AND BAROCLINIC PATTERNS OF ATMOSPHERIC CIRCULATIONS RELATED TO MONSOON TYPES

In the context of 1982—1994 NCEP/NCAR wind at 12-level isobaric surfaces on a global basis calculation is made of the barotropic(mass-weighed vertical mean)and baroclinic components (difference between the actual wind at each level and barotropic component)of atmospheric flow fields,followed by dealing with the distribution features of barotropic and baroelinie patterns globally in winter and summer,alongside with the classification of global monsoons according to the surface barotropic/baroclinic patterns.Evidence suggests that the seasonal variation of both components will lead to the reversal of a prevailing wind between winter and summer,thus causing a related monsoon:the baroclinie flow pattern is indicative of a thermal circulation driven by atmospheric inhomogeneous heating chiefly from land-sea thermal contrast whilst the barotropic counterpart represents the result mainly from dynamic effects,which is helpful to the understanding of monsoon nature.And further study shows that the classical monsoon regions in tropical Asia,Africa and South America fall into a baroclinic category,those in the bi-hemispherie subtropical Pacific into a barotropic type and the East Asian subtropical monsoon generated under the joint action of both the patterns falls into a mixed category.

BAROTROPIC AND BAROCLINIC DIVERGENCE QUASI ENERGY EQUATIONS AND THEIR APPLICATION TO A BLOCKING EVENT

As in the case of the expression for enstrophy (1/2ζ~2) and equations for barotropic and baroclinic enstrophy,this paper presents an expression of divergence assumed to have its quasi energy because of flows (DQE,or 1/2 D^2) with which to establish the equations of barotropie and baroclinic DQE,each consisting of five factors responsible for changes in these equations:the terms of energy flux,production,conversion,geostrophic effect and dissipation,and also a case study of a blocking event over the Sea of Okhotsk whose development is characterized largely by changes in barotropic and baroclinic DQE's on the strength of upper-level strong divergence and low-level vigorous convergence.During the event's strengthening,the baroclinic net production (inclusive of geostrophie effect) mechanism acts as the dominant factor for the enhancement of the baroclinic DQE,with its net transport outward offsetting part of its growth but the conversion mechanism augmenting its growth,and during the weakening the outward net transport of baroclinic DQE is the predominant factor of its enfeebling,baroclinic net DQE production (inclusive of geostrophic effect) and the conversion mechanisms make the baroclinic DQE increased,thereby alleviating the weakening in intensity.

Equivalent and Nonequivalent Barotropic Modes for Rotating Stratified Flows

All the possible equivalent barotropic (EB) laminar solutions are firstly explored,and all the possible non-EB elliptic circulations and hyperbolic laminar modes of rotating stratified fluids are discovered in this paper.The EB circulations (including the vortex streets and hurricane like vortices) possess rich structures,because either the arbitrary solutions of arbitrary nonlinear Poisson equations can be used or an arbitrary two-dimensional stream function is revealed which may be broadly applied in atmospheric and oceanic dynamics,plasma physics,astrophysics and so on.The discovery of the non-EB modes disproves a known conjecture.

两次北上相似路径台风导致的暴雨湿位涡诊断分析

利用2012年8月28—29日和2019年9月7—8日水平分辨率为0.25°×0.25°的ERA5再分析资料、地面观测资料和中国气象局热带气旋最佳移动路径数据,对两次北上相似移动路径的台风“布拉万”和“玲玲”导致东北地区暴雨天气过程的湿位涡进行诊断分析。结果表明:台风导致的强降水与假相当位温的密集带对应较好,降水主要位于假相当位温密集带偏暖一侧,锋区越强,降水量越大。两个台风过程均在降水前期存在对流不稳定层结,为台风降水提供初始能量积累。初始时刻出现湿对称不稳定,是台风中心西北侧暴雨出现的主要因素。降水增强时刻,大气对流不稳定再次参与,说明低层对流不稳定提供了降水快速增强的主要能量。低层湿正压项(MPV1)和湿斜压项(MPV2)的绝对值均出现先增大后减小再增大的趋势,强降水主要分布在两个极值中间过渡时段。两次北上台风引起的吉林省中部暴雨带均与低层850 hPa的MPV2负值区和MPV1的正值区有较好的对应关系。

雅浦-马里亚纳海沟连接区深层流季节内变异的特征和机制

雅浦-马里亚纳海沟连接区东深水通道是太平洋深层经向翻转环流携带南极绕极水进入西太平洋的首要入口。基于高分辨率的再分析数值模型结果,研究发现该通道深层流速和温度具有显著的40~90d周期的季节内变异,且季节内变异强度随深度的增加而增加,进一步研究揭示地形罗斯贝波(topographic Rossby waves,TRW)是引起上述季节内变异的主要原因,TRW波解可以解释深层流和温度季节内振幅的垂向变化。通过建立全水体的动力关联研究发现TRW在不同时刻具有不同的能量来源,首先次表层强涡旋经过时可以通过位涡守恒激发深层的TRW;第二,大洋中层存在的强涡旋也可激发深层的TRW;第三,深层平均流可以主要通过正压不稳定过程向季节内TRW提供能量。

基于解析四维集合变分的全球正压谱模式初始场优化研究

解析四维集合变分数据同化方法是一种无需伴随的、完全继承四维变分非线性处理能力的、“流依赖”的非顺序数据同化方法。本研究基于解析四维集合变分,开展全球正压谱模式初始场优化研究,验证解析四维集合变分的同化能力,并构建了高效的扰动集合成员生成方案,将解析四维集合变分降维到了样本空间,最后检验了解析四维集合变分对同化积分窗口长度和观测采样间隔的敏感性。试验结果表明,解析四维集合变分能优化全球正压谱模式的初始场,降维到样本空间后,只需要80个集合成员就可以取得很好的同化效果,在较长的同化积分窗口和观测采样间隔的条件下也可以达到理想的同化效果。

白鹤滩水电站坝区灾害性大风特征分析

白鹤滩水电站地处我国地势第二级阶梯与第一级阶梯交界处横断山脉,地形复杂。受多种天气系统影响,加之特殊复杂的下垫面,大风天气频繁,平均每年发生7级以上大风日数达235 d,占全年总日数的64.2%。分别从时间、空间和气候变化方面分析坝区新田自动站气象资料、NCEP格点资料大风变化规律,具体分析了9级以上各级大风个例特征、瞬时风速、10min最大风速持续时间、坝区9级以上不同等级风速持续时间等大风预报指标,旨在进一步完善9级以上灾害性大风的预报方法,提高电站灾害性大风预警预报气象服务水平。

鄂霍茨克海阻高发展过程的正斜压涡度和涡度拟能的演变特征

用正斜压分解法 ,对 1 998年发生在鄂霍茨克海的阻高进行涡度和涡度拟能分析 ,结果发现 :在阻高的不同阶段 ,正压涡度和正压涡度拟能远大于斜压涡度和斜压涡度拟能且在不同的阶段有着不同的演变过程 ;在阻塞的酝酿阶段正压和斜压涡度和涡度拟能不稳定增长 ,且向西南传播 ;在维持阶段 ,正压和斜压涡度和涡度拟能的增长达到最大值 ,阻塞形势建立并维持在阻塞区域内 ;在消亡阶段 ,正压涡度和涡度拟能与斜压涡度和涡度拟能则呈现出不稳定减少 ,向东南传播 ,阻塞崩溃 ,然后又增长。

长江口正压、斜压诊断及斜压预报模式——三维流场数值模拟

建立了σ坐标系下长江口正压、斜压诊断及斜压预报模式三维流场数学模型,采用k-kl二方程紊流闭合模型求解垂向涡粘系数。通过正压模式与斜压诊断模式、斜压诊断模式与斜压预报模式流场模拟结果的定量比较,说明了各种模式对长江口流场模拟精度影响的程度,推荐了长江口流场数值模拟采用的模式。