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Bin Wang

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Bin Wang

Abstract

The low-level cyclonic vortices which form over the Tibetan Plateau in the summer monsoon season are major rain-producing systems and have the potential to trigger cyclogenesis on the lee side when they move off the plateau. Two cases of the plateau vortices which occurred in July 1979 are studied. The characteristics of the vertical structure in their developing stage, and the circulation condition favorable for the eastward movement in the mature stage are diagnosed and presented by use of FGGE IIIb datasets. Numerical simulations with real data were performed using the GFDL limited-area mesoscale simulation model. Results suggest that the latent heating is an essential driving force for the development of the vortices studied here.

The analysis of a continuous CISK model with a basic state resembling that actually observed over the summer plateau shows that the predicted unstable mode has a preferred scale, growth rate and vertical structure, all of which are qualitatively comparable to observations. The instability in the plateau environment is mainly attributed to 1) the relatively shallow vertical extent of heating located in the upper troposphere in which the heat capacity of the air column per unit surface area is relatively small; 2) the dramatic reduction of the static stability due to surface sensible heat flux; and 3) the significant increase of moisture content in the plateau boundary layer due to surface evaporation and monsoon transport of water vapor. Most of these favorable conditions are referred to as the dynamic and thermal effects of the elevated plateau terrain. In this sense, the development of the plateau vortices during the rainy season may be regarded as resulting from the interaction between the lane-sca1e circulation and the plateau topographic effect and from the release of convective latent heat.

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Bin Wang

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The latest two Pacific basinwide warm episodes (1982–83 and 1986–87) exhibit some common features in their development and vertical structure. These features are examined by multivariate empirical orthogonal function analysis of the interannual variability of the ocean-atmosphere system along equatorial Indian and Pacific oceans.

The updraft and downdraft branches of the anomalous Walker circulation originate over the western Pacific and the eastern Indian Ocean, respectively. The early development of basinwide warming is characterized by the strengthening of a cross-equatorial low-level southerly component over the eastern Pacific and the enhancement of convection and boundary-layer westerlies over the western Pacific.

The structure of the ENSO anomaly mode changes from the cold to the warm phase of the Southern Oscillation. This is attributed to its eastward migration and the intrinsic longitudinal dependence of the vertical structure. The latter results from the east–west contrast of the air–sea interaction processes. Over Indonesia and the western Pacific, the land–sea thermal contrast and high SST maintain a semipermanent convective action center, whose intensity is sensitively modulated by small SST fluctuation. Since moist static ability is small, the surface pressure responds sensitively to the heating, so that the anomalous low pressure and associated zonal wind convergence in the boundary layer are in phase with the enhanced convection. In contrast, over the central-eastern Pacific, large SST gradient-induced pressure gradient force drives boundary-layer flows whose beta convergence determines atmospheric heating, while the feedback of the free atmosphere to boundary-layer flows is weak due to large static stability. The enhanced convection is thus nearly in phase with anomalous boundary-layer westerlies, positive zonal SST gradient, and negative zonal surface pressure gradient.

It is possible that an individual ENSO event may result from different combinations of various sets of coupled processes, especially with regard to those that work in the eastern Pacific cold tongue and those in the western Pacific warm pool.

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Bin Wang

Abstract

In spite of the fundamental difficulties in interpreting the growth of tropical storms, the basic idea of CISK remains valuable in understanding the instability resulting from the interaction between cumulus convection and large-scale flows. A generalized solution of a quasi-balanced continuous model, which can be applied to various types of vertical heating distribution and basic-state stratification, is derived and used to explore the behaviors of the CISK mode.

In the absence of cumulus momentum mixing, the CISK solution exhibits, in general, a scale selection. However, two types of unbounded growth rates associated with different closure assumptions may exist. Both of them take place in a common situation that is characterized by local warming at the top of the moist convergence layer in the area of rising motion. In these circumstances, the direct coupling between the heating and the large-scale moisture supply through the divergent wind component dominates over the indirect coupling through the rotational component. It is suggested that, for a feasible Ekman CISK mechanism, the dominant feedback of the convective heating to the low-level moisture convergence must be of an indirect nature. In this feedback process, planetary vorticity and/or preexisting relative vorticity play an essential role in converting heating-induced divergent kinetic energy to rotational kinetic energy, thus accelerating the spin-up of a large-scale vortex.

The cumulus momentum mixing destabilizes short waves by enhancing cyclonic circulation at the top of the Ekman layer and by reducing the vertical extent of the temperature disturbance; meanwhile, it stabilizes long waves by weakening the anticyclonic circulation in the upper levels.

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Bin Wang

Abstract

Stability of the equatorial atmosphere to a quasi-zonal, low frequency (order of 10−6 s−1) disturbance is investigated, using a model that consists of a two-layer free atmosphere and well-mixed boundary layer. The inclusion of boundary layer convergence leads to a circulation-dependent heating more nearly in accord with the behavior of numerically simulated low-frequency waves.

Slowly eastward moving unstable waves were found in a parameter regime stable to inviscid wave-CISK. The instability depends crucially upon the vertical distribution of the moist static energy of the basic state. A derived instability criterion suggests that amplification occurs when the condensational heating supported jointly by interior wave convergence and frictional convergence dominates over the dissipations due to longwave radiation and boundary layer viscosity.

The unstable waves exhibit preferred planetary scales. Since the vertical distribution of moist static energy of the basic state is closely related to sea surface temperature (SST), with increasing SST the growth rate increases for all wavelength, but the preferred scales shifts to shorter wavelength. The boundary layer convergence plays an important role in spatial scale selection. It not only suppresses unbounded growth of short waves, but also couples barotropic and baroclinic components in such a way that the generation of wave available potential energy is most efficient for planetary scales.

In the presence of boundary layer friction, the amplitude of an unstable wave in the zonal wind (or geopotential) at the upper level is significantly larger than that at the lower level. The phases between the two levels differ by about 180°. The slow eastward movement results mainly from the interior wave-induced reduction of static stability and thermal damping.

Behaviors of model unstable waves appear to resemble the observed 40–50 day mode in many aspects, such as vertical structure, energy source, zonal phase propagations, characteristic zonal and meridional scales, and its relation to SST. Limitations of the model are also discussed.

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Risheng Wang and Bin Wang

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The variability of El Niño–La Niña events was analyzed in a low-dimensional phase space, a concept derived from dynamic system theory. The space–time extended EOFs derived from the observed monthly mean SST field over tropical Pacific were used as the basis of the phase space that describes the time evolution of ENSO signals. It was shown that the essential features of the ENSO variability, such as the irregular oscillation, the phase locking to the annual cycle, and the interdecadal changes in its propagation and onset, can be effectively represented by a three-dimensional phase space. The typical El Niño–La Niña life cycle is four years with its mature phases in boreal winter. The intensity of the ENSO signal within one life cycle is closely linked to the frequency of its occurrence (onset). The interdecadal variability of the ENSO signals is characterized by both the intensity and the frequency of occurrence, displaying an irregularity with the gross feature comparable to the regime behavior and intermittency of some low-dimensional chaotic systems.

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Bin Wang and Albert Barcilon

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Cold season atmospheric observations of vacillation point to a wave-mean flow interaction of baroclinic, planetary waves with their mean flow, and the observational data show that wave 2 is the largest contributor to the energetics and the heat flux. To verify this hypothesis we present a weakly nonlinear analysis of the evolution of a single, most unstable Green mode interacting with mean zonal flow in the presence of internal and Ekman layer dissipations, the former being larger than the latter.

The derived amplitude equations for the wave and the mean fields transform into a Lorenz set of equations that admits stable, finite amplitude wave gates. No stable limit cycle or aperiodic solutions were found in the realistic parameter ranges that typify atmospheric winter conditions. When the system is disturbed away from these gable states, there is a monotonic or vacillators approach to equilibrium. Damped vacillation occurs when the internal dissipative time scale is longer than the efolding time scale of the inviscid, Green mode, a condition realized in the winter atmosphere. During the vacillation, due to the presence of the internal dissipation the tilt of the constant phase line may remain westward, and the horizontal he-at flux may be poleward throughout most of the cycle.

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Liguang Wu and Bin Wang

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The influence of convective heating on movement and vertical coupling of tropical cyclones (TCs) is investigated using a hurricane model with different environmental flows. The authors identify two processes by which convective heating may affect TC motion. One is the advection of symmetric potential vorticity (PV) by heating-induced asymmetric flow. The other is the direct generation of a positive PV tendency by asymmetric heating, which acts to shift a TC to the region of maximum downward gradient of asymmetric heating. A steering level exists that is located at the level where the direct influence of asymmetric heating vanishes, normally in the lower troposphere. At that level, a TC moves with the asymmetric flow averaged within a radius of 200 km, because the influence of asymmetric flows on TC motion is weighted by the horizontal PV gradient that is primarily confined within the TC core. Although the vertical shear in the asymmetric flow (including environmental and heating-induced flows) could tilt the vortex, the influence of asymmetric heating tends to offset the vertical tilt caused by the vertical shear through a fast adjustment between the asymmetric wind and diabatic heating. Therefore, diabatic heating enhances the vertical coupling.

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Bin Wang and Yan Xue

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The effects of nonlinear (positive only or conditional) heating on moist Kelvin waves are examined with a simple equatorial zonal-plane model describing the gravest baroclinic mode.

The unstable perturbation subject to nonlinear beating emerges as a wave packet. A typical amplifying, eastward-moving wave packet is characterized by an asymmetric structure: 1) the ascending branch (wet region) is much narrower than the two descending ones (dry regions); and 2) the circulation cell to the east of the wet region center is smaller and stronger than its counterpart to the west of the center. The wet-dry asymmetry is primarily caused by the nonlinear beating effect, while the east-west asymmetry is a result of the movement of the wave packet relative to mean flow. The existence of Newtonian cooling and Rayleigh friction enhances the structural asymmetries.

The unstable wave packet is characterized by two zonal length scales: the ascending branch length (ABL) and total circulation extent (TCE). For a given basic state, the growth rate of a wave packet increases with decreasing ABL or TCE. However, up to a moderate growth rate (order of day−1) the energy spectra of all wave packets are dominated by zonal wavenumber one regardless of ABL size. In particular, the slowly growing (low frequency) wave packets normally exhibit TCEs of planetary scale and ABLs of synoptic scale.

Observed equatorial intraseasonal disturbances often display a narrow convection region in between two much broader dry regions and a total circulation of planetary scale. These structure and scale characteristics are caused by the effects of nonlinear heating and the cyclic geometry of the equator. It is argued that the unstable disturbance found in numerical experiments (e.g., Lau and Peng; Hayashi and Sumi) is a manifestation of the nonlinear wave packet.

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Bin Wang and Albert Barcilon

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The moist stability of a midlatitude zonal flow with a conditionally unstable layer in the presence of an Ekman layer is investigated. The vertical velocity employed in a simplified Kuo's parameterization is sustained by baroclinic wave forcing, diabatic heating and Ekman pumping. A general dispersion relation and eigenfunction are derived analytically for a class of flows with various vertical heating profiles.

The moist unstable mode may be regarded as a baroclinic wave modified by the bulk effect of the convective heating, for which the fundamental dependences of the baroclinic growth rate on the Burger number and vertical shear remain qualitatively valid. Waves longer than the Rossby radius of deformation are not appreciably affected, while the shorter waves are significantly destablized by the convective heating. The growth rates and wavelengths of the most unstable modes are nonlinear functions of the averaged specific humidity of the moist layer, and there is an optimum specific humidity that minimizes the preferred wavelength, this value being proportional to the static stability for a representative heating profile. The quasi-geostrophic constraints and baroclinity appear to be decisive factors that suppress short waves and lead to a finite preferred wavelength.

The destabilizing effect of the convective heating is considerably enhanced by the reduction of the static stability. Among the other influential parameters that affect the growth rate, relatively lower cloud top and a deep moist layer have a profound effect an the stability. Because of the cooperative interactions between favorable factors, the simultaneous occurrence of several of the mechanisms listed above may produce explosive-like growth. The relatively shallow convention and the Ekman layer will slow down the wave propagation speed.

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