ON THE BAROTROPIC INSTABILITY OF LARGE SCALE VORTICES

By using the linearized barotropic vorticity equation in polar coordinates the stability of pertur-bations on a large scale circular basic flow is transformed into a generalized eigenvalue problem,yielding the relationship between the growth rate of the amplitude of perturbations and the az-imuthal wave number. Then, numerical experiments whose integration time is 60 model hours areperformed in terms of a quasi-geostrophic barotropic model in Cartesian coordinates using the per-turbation stream function field of unstable mode superimposed on a strong and weak circular basicflows as the initial fields. The experimental results reveal that the amplitudes of the initial pertur-bations in the model atmosphere grow with time. The amplitude of the perturbations superimposedon the strong circular basic flow grows quicker and forms a spiral-band-like structure.     

初始场结构对台风路径影响的数值试验

设计了一个套网格的准地转正压模式，对模式的可用性进行了检验。用该模式实施了两组不同初始场的试验，分析了初始场结构对台风路径的影响，指出台风移动具有内在随机性的特征。     

亚欧及北太平洋相当正压层高度的分析及数值模拟

本文利用相当正压层方法,分析了亚欧及北太平洋中纬度地区47个站点的相当正压层高度,其结果与文献[1]类似。计算表明:亚欧及北太平洋的相当正压层高度平均位于359hPa,略去高原站点后,平原地区的平均高度为374hPa。相当正压层是一个波动很大的不同气压的曲面,并随地区和季节变化较大。高原和西风急流的强弱可能都是其影响因子,尤其是高原的作用值得进一步探讨。数值模拟也证实了300hPa上Rossby波比500hPa效果好。     

Study on the Dynamic Process of the Onset of South China Sea Summer Monsoon

By using the NCEP/NCAR reanalysis data from 1958 to 1997, we first looked into the atmospheric flow conditions in the one month immediately prior to the onset of the South China Sea summer monsoon （SCSSM） each year. A monthly-averaged zonal basic flow of 40-yr composite was then calculated. The stability of Rossby wave in the basic flow was studied based on the spherical barotropic vorticity equation. Furthermore, the spectral function expansion method was adopted to define and compute the evolvement of a developing wave packet. The results indicate that there exists barotropic instability of spherical Rossby wave in the climatically-averaged flow field before the SCSSM onset. The instability is triggered by the westerly jet stream in the Southern Hemisphere, and the strongest instable perturbation lies to the south of the westerly jet stream. The peak of the developing spherical Rossby wave packet propagates from mid and high latitudes to low latitudes, though not crossing the equator, spurring the cumulus convection in the tropical zones. The eruption of the cumulus convection and its spread to monsoon regions help to speed up the adjustment of the general circulation and the SCSSM onset. It is concluded that elements that contribute to the SCSSM onset are on global scale, albeit the onset itself looks like a local phenomenon.     

Mesoscale Barotropic Instability of Vortex Rossby Waves in Tropical Cyclones

In this study, the barotropic stability of vortex Rossby waves (VRWs) in 2D inviscid tropical cyclone (TC)-like vortices is explored in the context of rotational dynamics on an f-plane. Two necessary instable conditions are discovered: (a) there must be at least one zero point of basic vorticity gradient in the radial scope; and (b) the relative propagation velocity of perturbations must be negative to the basic vorticity gradient, which reflects the restriction relationship of instable energy. The maximum ...     

A three-dimensional coastal barotropic model in generalized curvilinear grids

INTRODUCTIONThemethodofautomaticallygeneratinggeneralizedcurvilinearmeshesisaneffectivetoolforobtainingnumericalsolutionsofpartialdifferentialequationsinaregionwitharbitraryboundaries.Thegrids,calledadaptivemesh ,canbeadaptedtotheshapeoftheboundaryortothespatialdis tributionofthesimulatedfield .Insteadofthetraditionalequidistantgrids,automaticallyproducedcurvilineargrids,whichcanbeadaptedtotheequationsolutions’featuresandtotheirregularshapeofthelateralboundary,areused .Thenumericalcalcul…     

Observations of Eddies in the Japan Basin Interior

Eddy features in the Japan Basin have been studied by combining satellite-derived sea surface temperature (SST) images and WOCE drifter tracks with recent current meter data from a deep mooring in the interior of the Basin. SST images indicate that anticyclonic eddies often appear around the Subpolar Front in cold seasons and move into the northern cold water region entraining warm water of the frontal zone. The anti-cyclonic eddies "visualized" by the entrained warm water and trajectories of some drifters are typically 30 km in radius and have rotational speeds of 0.15 to 0.3 m/s at the surface. On the other hand, the current meter data of 3-year duration show that vertically coherent eddy-like currents of the order of 0.1 m/s occur every year in cold seasons in the deep (1000 to 3000 m) layer of the Japan Basin interior. An important finding is that available time series of SST patterns are well correlated to the vertically coherent deep currents. This correlation suggests that the anticyclonic eddies indicated by both SST images and drifter tracks are actually barotropic or quasi-barotropic, extending from the surface to the bottom. It is argued that the unique current features in the deep layer of the Japan Basin can be explained in terms of barotropic eddies. A brief discussion is also made of the possible source of the eddy kinetic energy. This revised version was published online in August 2006 with corrections to the Cover Date.     

Potential enstrophical diagnostic analyses on the mechanism of change of tropical cyclone intensity

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INFLUENCE OF THE INTERANNUAL VARIATION OF CROSS-EQUATORIAL FLOW ON TROPICAL CYCLOGENESIS OVER THE WESTERN NORTH PACIFIC

The influence of the interannual variation of cross-equatorial flow(CEF) on tropical cyclogenesis over the western North Pacific(WNP) is examined in this paper by using the tropical cyclone(TC) best track data from the Joint Typhoon Warning Center and the JRA-25 reanalysis dataset. The results showed that the number of TCs forming to the east of 140°E over the southeastern part of the western North Pacific(WNP) is in highly positive correlation with the variation of the CEF near 125° E and 150° E, i.e., the number of tropical cyclogeneses increases when the cross-equatorial flows are strong. Composite analyses showed that during the years of strong CEF, the variations of OLR, vertical wind shear between 200-850 h Pa, 850 h Pa relative vorticity and 200 h Pa divergence are favorable for tropical cyclogenesis to the east of 140°E over the tropical WNP, and vice versa. Moreover, it is also discussed from the view of barotropic energy conversion that during the years of strong CEF, an eastward-extended monsoon trough leads to the rapid growth of eddy kinetic energy over the eastern part of WNP, which is favorable for tropical cyclogenesis;but during the years of weak CEF, the monsoon trough is located westward in the western part of the WNP, consistent with the growth area of eddy kinetic energy. As a result, there are fewer TC geneses over the eastern part of WNP.Besides, the abrupt strengthening of a close-by CEF 2-4 days before tropical cyclogenesis may be the one of its triggers.     

A one-dimensional turbulence model; Enstrophy cascades and small-scale intermittency in decaying turbulence

Abstract

Severe unidirectional Fourier truncation of the equations for 2-D incompressible flow leads to a system of three coupled PDEs in one space dimension with the same quadratic invariants as the original set (i.e. energy and enstrophy). Numerically generated equilibria for inviscid, truncated versions of the reduced system are well approximated by Kraichnan's energy-enstrophy equipartition spectra. Viscous calculations for decaying turbulence at moderate resolution (1024 degrees of freedom) also appear to be consistent with a direct, k ^?3, enstrophy cascading inertial range when the dissipation is small. Dissipation range intermittency in the form of spatially intermittent enstrophy dissipation with occasional strong bursts producing linear phase locking is also observed. In contrast to full 2-D simulations, no tendency towards the emergence of isolated, coherent vorticity structures is observed. The model consequently mimics some, but not all, of the properties of the full 2-D set.