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Tianming Li and Bin Wang

Abstract

The development and movement of the tropical intraseasonal system (TIS) exhibit remarkable annual variations. It was hypothesized that spatial and temporal variation in sea surface temperature (SST) is one of the primary climatic factors that are responsible for the annual variation of TISs. This paper examines possible influences of SST on the TIS through numerical experiments with a 2.5-layer atmospheric model on an equatorial β plane, in which SST affects atmospheric heating via control of the horizontal distribution of moist static energy and the degree of convective instability.

The gradient of the antisymmetric (with respect to the equator) component of SST causes a southward propagation of the model TIS toward northern Australia in boreal winter and a northward propagation over the Indian and western Pacific Oceans in boreal summer. The phase speed of the meridional propagation increases with the magnitude, of antisymmetric SST gradients. The poleward propagation of the equatorial disturbance takes the form of moist antisymmetric Rossby modes and influences the summer monsoon.

During May when SST is most symmetric in the western Pacific, a disturbance approaching the date line may evolve into westward-moving, double cyclonelike, symmetric Rossby modes due to the suppression of the moist Kelvin mode by the cold ocean surface cast of the date line. The disturbance over the equatorial Indian Ocean, however, may evolve into an eastward-moving, moist Kelvin–Rossby wave packet; meanwhile, a cyclonic circulation may be induced over the Gulf of Thailand and Malaysia, drifting slowly westward into the Indian subcontinent.

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Bin Wang Xiaofan Li

Abstract

The beta effect on translation of cyclonic and anticyclonic vortices with height-dependent circulation (the beta-drift problem) is investigated via numerical experiments using a dry version of a multilevel primitive equation model (Florida State University model).

The vertical structure of vortex circulation influences steady translation in a manner similar to that of the horizontal structure. Both spatially change the mean relative angular momentum (MRAM) of the vortex. The translation speed and its meridional component are both approximately proportional to the square root of the magnitude of MRAM of the initial (or quasi-steady-state) symmetric circulation. The latitude is another important factor controlling the speed of the beta drift. The meridional component decreases by about 45% when the central latitude of the vortex increases from 10° to 30°N.

The beta-drift speed is intimately related to the axially asymmetric pressure field. During quasi-steady vortex translation the asymmetric pressure field maintains a stationary wavenumber 1 pattern in azimuthal direction with a high in the northeast and a low in the southwest quadrant of a Northern Hemisphere cyclone. The beta-drift velocity is approximately equal to the geostrophic flow implied by the asymmetric pressure gradient at the vortex center. If the Rossby number associated with the asymmetric flow is small, to the lowest order, the asymmetric pressure gradient force at the vortex center is balanced by the Coriolis force associated with the beta drift of the vortex.

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Bin Wang and Xiaowei Tan

Abstract

An ensemble-based approach is proposed to obtain conditional nonlinear optimal perturbation (CNOP), which is a natural extension of linear singular vector to a nonlinear regime. The new approach avoids the use of adjoint technique during maximization and is thus more attractive. Comparisons among CNOPs of a simple theoretical model generated by the ensemble-based, adjoint-based, and simplex-search methods, respectively, not only show potential equivalence of the first two approaches in application according to their very similar spatial structures and time evolutions of the CNOPs, but also reveal the limited performance of the third measure, an existing adjoint-free algorithm, due to its inconsistent spatial distribution and weak net growth ratio of norm square of CNOP comparing with the results of the first two methods. Because of its attractive features, the new approach is likely to make it easier to apply CNOP in predictability or sensitivity studies using operational prediction models.

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Bin Wang and Isidoro Orlanski

Abstract

A case of the heavy rain vortex which occurred during the period 14–15 July 1979 is studied using a limited-area mesoscale numerical model. This is a representative example of a group of warm southwest vortices that often form over the eastern flank of the Tibetan Plateau after the onset of the summer Indian monsoon.

Some common features of the dynamic structures exhibited both by the simulation and by observations are discussed. The developing vortex is noticeably detached from the polar frontal zone. A 180° phase shift exists between the upper and lower layer vorticity fields. In the boundary layer, a pronounced northward transport of mass and moisture is connected with an intense upward motion near and to the east of the 700-mb vortex center, whole the southward cold advection is insignificant.

The vortex originated and rapidly developed in a stagnation region on the lee side of the plateau. The presence of the stagnation region not only removes local dynamical energy sources from the environmental flow, but also diminishes topographic generation of vorticity by reducing the vortex stretching in the wind component flowing over the plateau and the horizontal convergence in the component moving around the plateau. Without latent heating, dynamic instability and/or forcing of the large-scale flow interacting with the Tibetan Plateau is not sufficient to generate the observed disturbance.

On the other hand, the plateau blocking effect favors the establishment of a conditionally unstable environment. The simulation indicates that a sudden onset of vigorous deep convection, following by a rapid growth of relative vorticity in the lower troposphere, takes place once the dynamic forcing associated with a mesoscale plateau disturbance was positioned over the western stagnation region. Our principle result is that the warm heavy rain vortex in this can study is triggered by a migratory plateau boundary layer disturbance and basically driven by cumulus convective heating. The thermal influence of the elevated plateau topography may appreciably affect the vortex initiation through changing the intensity of the forcing associated with the triggering mechanism.

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Liguang Wu and Bin Wang

Abstract

In order to understand the roles of various physical processes in baroclinic tropical cyclone (TC) motion and the vertical coupling between the upper- and lower-level circulations, a new dynamical framework is advanced. A TC is treated as a positive potential vorticity (PV) anomaly from environmental flows, and its motion is linked to the positive PV tendency. It is shown that a baroclinic TC moves to the region where the azimuthal wavenumber one component of the PV tendency reaches a maximum, but does not necessarily follow the ventilation flow (the asymmetric flow over the TC center). The contributions of individual physical processes to TC motion are equivalent to their contributions to the wavenumber one PV component of the PV tendency. A PV tendency diagnostic approach is described based on this framework. This approach is evaluated with idealized numerical experiments using a realistic hurricane model. The approach is capable of estimating TC propagation with a suitable accuracy and determining fractional contributions of individual physical processes (horizontal and vertical advection, diabatic heating, and friction) to motion. Since the impact of the ventilation flow is also included as a part of the influence of horizontal PV advection, this dynamical framework is more general and particularly useful in understanding the physical mechanisms of baroclinic and diabatic TC motion.

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Li Dong and Bin Wang

Abstract

A Lagrangian linear advection scheme, which is called the trajectory-tracking scheme, is proposed in this paper. The continuous tracer field has been discretized as finite tracer parcels that are points moving with the velocity field. By using the inverse distance weighted interpolation, the density carried by parcels is mapped onto the fixed Eulerian mesh (e.g., regular latitude–longitude mesh on the sphere) where the result is rendered. A renormalization technique has been adopted to accomplish mass conservation on the grids. The major advantage of this scheme is the ability to preserve discontinuity very well. Several standard tests have been carried out, including 1D and 2D Cartesian cases, and 2D spherical cases. The results show that the spurious numerical diffusion has been eliminated, which is a potential merit for the atmospheric modeling.

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Xiouhua Fu and Bin Wang

Abstract

The boreal-summer intraseasonal oscillation (BSISO) simulated by an atmosphere–ocean coupled model is validated with the long-term observations [Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) rainfall, ECMWF analysis, and Reynolds' SST]. This validation focuses on the three-dimensional water vapor cycle associated with the BSISO and its interaction with underlying sea surface. The advantages of a coupled approach over stand-alone atmospheric approaches on the simulation of the BSISO are revealed through an intercomparison between a coupled run and two atmosphere-only runs.

This coupled model produces a BSISO that mimics the one presented in the observations over the Asia– western Pacific region. The similarities with the observations include 1) the coherent spatiotemporal evolutions of rainfall, surface winds, and SST associated with the BSISO; 2) the intensity and period (or speed) of the northward-propagating BSISO; and 3) the tropospheric moistening (or drying) and overturning circulations of the BSISO. However, the simulated tropospheric moisture fluctuations in the extreme phases (both wet and dry) are larger than those in the ECMWF analysis. The simulated sea surface cooling during the wet phase is weaker than the observed cooling. Better representations of the interaction between convection and boundary layer in the GCM and including salinity effects in the ocean model are expected to further improve the simulation of the BSISO.

The intercomparison between a coupled run and two atmospheric runs suggests that the air–sea coupled system is the ultimate tool needed to realistically simulate the BSISO. Though the major characteristics of the BSISO are very likely determined by the internal atmospheric dynamics, the correct interaction between the internal dynamics and underlying sea surface can only be sustained by a coupled system. The atmosphere-only approach, when forced with high-frequency (e.g., daily) SST, introduces an erroneous boundary interference on the internal dynamics associated with the BSISO. The implications for the predictability of the BSISO are discussed.

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Li Dong and Bin Wang

Abstract

A previous Lagrangian linear advection scheme (trajectory-tracking scheme) is modified to achieve local mass conservation in this paper, which is more favorable to climate modeling. The discretized tracer parcels are volumes with interfaces instead of centroids. In 2D problems, the parcels are polygons and the interfaces are described by polygonal edges with a finite number. Because polygons will deform under the background wind field, a curvature-guard algorithm (CGA) is devised to retain accurate representation of the deformed interfaces among parcels. The tracer mass carried by parcels is mapped onto the regular latitude–longitude mesh by a first-order conservative remapping algorithm that can handle concave polygons. Several standard test cases have been carried out to show the effectiveness of the new scheme.

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Justin D. Ventham and Bin Wang

Abstract

NCEP–NCAR reanalysis data are used to identify large-scale environmental flow patterns around western North Pacific tropical storms with the goal of finding a signal for those most favorable for rapid intensification, based on the hypothesis that aspects of the horizontal flow influence tropical cyclone intensification at an early stage of development. Based on the finding that intensification rate is a strong function of initial intensity (Joint Typhoon Warning Center best track), very rapid, rapid, and slow 24-h intensification periods from a weak tropical storm stage (35 kt) are defined. By using composite analysis and scalar EOF analysis of the zonal wind around these subsets, a form of the lower-level (850 mb) combined monsoon confluence–shearline pattern is found to occur dominantly for the very rapid cases. Based on the strength of the signal, it may provide a new rapid intensification predictor for operational use. At 200 mb the importance of the location of the tropical storm under a region of flow splitting into the midlatitude westerlies to the north and the subequatorial trough to the south is identified as a common criterion for the onset of rapid intensification. Cases in which interactions with upper-level troughs occurred, prior to and during slow and rapid intensification, are studied and strong similarities to prior Atlantic studies are found.

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Ying Zhao, Bin Wang, and Juanjuan Liu

Abstract

In this study, a new data assimilation system based on a dimension-reduced projection (DRP) technique was developed for the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5) modeling system. As an initial step to test the newly developed system, observing system simulation experiments (OSSEs) were conducted using a simulated sea level pressure (SLP) field as “observations” and assimilation experiments using a specified SLP field to evaluate the effects of the new DRP–four-dimensional variational data assimilation (4DVar) method, initialization, and simulation of a tropical storm—Typhoon Bilis (2006) over the western North Pacific. In the OSSEs, the “nature” run, which was assumed to represent the “true” atmosphere, was simulated by the MM5 model, which was initialized with the 1.0° × 1.0° NCEP final global tropospheric analyses and integrated for 120 h. The simulated SLP field was then used as the observations in the data assimilation. It is shown that the MM5 DRP–4DVar system can successfully assimilate the (simulated) model output (used as observations) because the OSSEs resulted in improved storm-track forecasts. In addition, compared with an experiment that assimilated the SLP data fixed at the end of a 6-h assimilation window, the experiment that assimilated the SLP data every 3 min in a 30-min assimilation window further improved the typhoon-track forecasts, especially in terms of the initial vortex location and landfall location. Finally, the assimilation experiments with a specified SLP field have demonstrated the effectiveness of the new method.

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