A comparative study of implicit and explicit initialization for a tropical barotropic model

The explicit nonlinear normal mode initialization (ENMI) scheme is applied to a tropical barotropic limited area shallow water model in spherical coordinates. The model is formulated by considering potential enstrophy conserving finite difference scheme. It is seen from the results of this study that the ENMI scheme is fully capable of filtering out the spurious gravity wave oscillations. The results are compared with those using an implicit nonlinear normal mode initialization (INMI). The latter scheme gives equally satisfactory results, requiring less computational time than the explicit scheme.     

正压大气有利发展扰动型与遥相关型的建立

本文讨论了夏季纬向不对称气候平均流下通过正压大气内部动力过程建立遥相关型的问题。首先详细描述和讨论了有利发展扰动型的概念及其计算方法。作为对比，对正规模不稳定进行了分析。结果表明，正规模的不稳定增长率很小，难以用于解释遥相关型的发展。而有利发展扰动型的计算分析表明，最有利发展的那些扰动型的振幅增长率同实际大气遥相关型的增长率相一致，并且这些有利发展扰动型都将演变为同实际大气相一致的遥相关型结构。从而提出了，至少部分实际大气遥相关型可以不依赖于异常外源强迫，而仅依赖于大气内部的正压过程即能量转换过程而建立起来。     

The dynamics of barotropic vortex merging

The merging of multiple vortices is a fundamental process of the dynamics of Earth’s atmosphere and oceans. In this study, the interaction of like-signed vortices is analytically and numerically examined in a framework of two-dimensional inviscid barotropic flows. It is shown that barotropic vortex interaction turns out to be more intricate than simple merging scenarios often assumed in previous studies. Some particular configurations exist in which the vortex merging process is never complete despite strong interaction of like-signed vortices, regardless of the strengths or distances between the vortices. While the conditions for a complete vortex merging process introduced in this study appear to be too strict for most practical applications, this study suggests that careful criteria for vortex mergers should be properly defined when simulating the interaction of vortices, because the merging may not always result in a final enhanced circulation at the end of the interaction, as usually assumed in the literature.     

EFFECT OF THE NONLINEAR TERM ON MOVEMENT AND DEVELOPMENT OF TROPICAL CYCLONE

The dynamics of tropical cyclone is investigated in a nondivergent barotropic model with nobasic flow. The effect of nonlinear term on the movement and development of tropical cyclone isemphatically demonstrated. The advection of asymmetric vorticity by the symmetric flow (AAVS)produces the small-scale gyres (SSGs). The SSGs counterclockwise rotate around the tropicalcyclone center. The interaction of SSGs with the large-scale beta gyres (LSBGs) leads to theoscillation in translation speed and vacillation in translation direction for tropical cyclone. Theadvection of symmetric vorticity by the asymmetric flow (ASVA) steers the symmetric circulationof tropical cyclone. The ventilation flow vector determined by the asymmetric flow is closecorrelated with the motion vector of tropical cyclone. The nonlinear advection of relative vorticityis an order of magnitude greater than the linear advection of planetary vorticity, However, theasymmetric circulation created by the planetary vorticity advection provides a background conditionfor anomalous motions of the tropical cyclone. The combination of the linear and nonlinear effectsresults in accelerated, decelerated, changing direction and/or counterclockwise looping motions ofthe tropical cyclone.     

Baroclinic Planetary Boundary-Layer Model for Neutral and Stable Stratification Conditions

The temperature and wind profiles in abaroclinic atmospheric boundary layer (ABL) are investigated.Assuming stationary conditions, the turbulent state in the ABL forstable and neutral conditions is uniquely determined by the Rossbynumber, the external stratification parameter and two externalbaroclinic parameters. A simple two-layer baroclinic model isdeveloped. It consists of a surface layer (SL) and overlyingEkman-type layer. The system of dynamic and heat transfer equations isclosed using K-theory. In the SL the turbulent exchangecoefficient is consistent with the results of similarity theorywhile in the Ekman layer it is assumed constant. The universalfunctions in the resistance, heat and humidity transfer laws arededuced from the analytical solutions for the wind and temperatureprofiles. The solutions of the ABL resistance laws for theinternal ABL parameters, necessary for the calculations of the ABLprofiles, are approximated in terms of the external ABLparameters. Favourable agreement of model results with theavailable experimental data is demonstrated.     

Large scale water movements in lakes

By large scale circulation in lakes one means motions whose characteristic length scales extend over most parts or all of the water masses in a lake or the ocean. We present the governing equations and motivate, by means of a scale analysis, the various simplified versions of model equations that are in use in computational lake dynamics. This scale analysis not only permits rational deduction of the reduced equations, it equally provides a means of estimating their limitations. These are discussed as are the difficulties and the peculiarities inherent in the proposed equation sets.Special features of external and internal wave motions are studied. For barotropic oscillations of a lake system (Lake of Lugano) it is shown that substantial water masses are exchanged between the individual basins at the resonating periods. Baroclinic seiches of a three layer model in which each layer is effective within its own domain show (for the North basin of the Lake of Lugano) that mode structures may differ from layer to layer pointing at important modifications of classical interpretations of higher baroclinic wave dynamics. And in large lakes in the equatorial belt the-effect forces modifications of the classical understanding of seiche behavior. Long periodic oscillating features may be attributed to topographic Rossby waves or higher baroclinic internal gravity waves; the observational identification is, however, difficult because lack of spatial resolution of the data makes this identification non-unique. We, finally present results of a full nonlinear numerical baroclinic circulation model and demonstrate that it is able to reproduce the gross features of the immediate response to strong storms during a few days.This paper is an extended version of a lecture with the same title, held by K. Hutter on June 14, 1989, at the Workshop Spatial and temporal scales of water bodies of the Sonderforschungsbereich Stoffhaushalt des Bodens, Universität Konstanz. The character of the paper is that of areview, however, it contains material, theoretical, computational and observational, that has never been presented before to make it sufficiently unique. Some of the new material has been collected and gathered by E. Bäuerle, G. Salvadè, C. Spinedi and F. Zamboni, who should be credited for it, even though none of these people contributed to the layout and drafting of the text, for which K. Hutter is responsible alone.We thank Professor Max Tilzer of the University Konstanz for inviting K. Hutter to the workshop and giving him the opportunity to present the material as of that time.As far as referencing current literature is concerned we are neither exhaustive nor complete and only refer to literature which is directly related to the material presented. For a list with more than 300 relevant references, the reader may consult the article Hydrodynamic Modeling of Lakes by K. Hutter in the Encyclopedia of Fluid Mechanics, Gulf Publ. Company, Houston Texas (1987). I would like to express my sincere thanks to Mrs. Danner for her diligent work in typing the text.     

Seasonal variations of the upper ocean in the western north pacific observed by an argo float

A seasonal evolution of surface mixed layer in the western North Pacific around 24°N between 143°E and 150°E was observed by using an Argo float for more than 9 months, from December 2001 through August 2002. The result showed that the mixed layer deepened gradually in the first two months. It reached its maximum depth of about 130 m at the end of January, after which the mixed layer varied largely and sometimes the pycnocline below the mixed layer was much weakened until the summer mixed layer formed in late April. The thin surface mixed layer was maintained during the rest of the observation period. Heat budget analysis suggests that the vertical and horizontal temperature advections are the two most dominant terms in the heat balance in the upper layer on time scales from a few days to a month. The vertical motions that are possibly responsible for the vertical temperature advection are discussed.     

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.     

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-hemispheric subtropical Pacific into a barotropic type and the East Asian subtropical monsoon generated underthe joint action of both the patterns falls into a mixed category.     

亚洲季风变动与大气正压/斜压运动动能变化的气候特征

使用ＮＣＥＰ／ＮＣＡＲ４０年（１９５８～１９９７年）月平均再分析资料,通过动力学论断研究了大气斜压／正压运动动能的变化及其相互转换,分析了亚洲季风变动与这两种动能变化的联系。指出：季风区大气运动动能的组成和变化具有独特的特征。冬季风时期大气斜压运动动能与正压运动动能具有正相关线性关系,斜压运动能向正压运动动能转换;春、秋季无论是东亚还是印度季风区斜压运动动能与正压运动动能之间转换都处于极小值,只是