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Xiaqiong Zhou and Bin Wang

Abstract

To understand the mechanisms responsible for the secondary eyewall replacement cycles and associated intensity changes in intense tropical cyclones (TCs), two numerical experiments are conducted in this study with the Weather Research and Forecasting (WRF) model. In the experiments, identical initial conditions and model parameters are utilized except that the concentration of ice particles is enhanced in the sensitivity run. With enhanced ice concentrations, it is found that the secondary eyewall forms at an increased radius, the time required for eyewall replacement is extended, and the intensity fluctuation is relatively large. The enhanced concentrations of ice particles at the upper tropospheric outflow layer produces a noticeable subsidence region (moat) surrounding the primary eyewall. The presence of the moat forces the secondary eyewall to form at a relatively large radius. The axisymmetric equivalent potential temperature budget analysis reveals that the demise of the inner eyewall is primarily due to the interception of the boundary layer inflow supply of entropy by the outer convective ring, whereas the advection of low entropy air from the middle levels to the boundary inflow layers in the moat is not essential. The interception process becomes inefficient when the secondary eyewall is at a large radius; hence, the corresponding eyewall replacement is slow. After the demise of the inner eyewall, the outer eyewall has to maintain a warm core not only in the previous eye, but also in the moat. The presence of low equivalent potential temperature air in the moat results in the significant weakening of storm intensity. The results found here suggest that monitoring the features of the moat and the outer eyewall region can provide a clue for the prediction of TC intensity change associated with eyewall replacement.

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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

The vertical coupling and movement of an adiabatic baroclinic tropical cyclone (TC) are investigated through two numerical experiments in which the TC is affected by either a vertical environmental shear or a differential beta drift. In both cases, the initial response of the symmetric vortex is to tilt in the vertical. In response to the vertical tilt, a three-dimensional asymmetric circulation with a typical radius of 100 km develops within the TC core region. In addition, the wavenumber-one potential vorticity (PV) anomalies develop with positive anomalies downtilt (uptilt) above (below) the maximum PV level in order to maintain a balanced state between the thermal and dynamical fields. On a beta plane, in contrast to the beta gyres, the mesoscale asymmetric circulation is a pair of counterrotating inner gyres centered at the radius of maximum wind. As a result, the resulting three-dimensional mesoscale asymmetric circulation, not the penetration flow, plays an important role in the vertical coupling of adiabatic baroclinic vortices. In both cases, the TC motion is not simply due to the advection of the symmetric PV component by the asymmetric (ventilation) flow. The horizontal advection of the asymmetric PV anomalies by the symmetric cyclonic flow and the vertical PV advection associated with the asymmetric vertical motion also considerably contribute to the TC motion. The latter two processes also play a critical role in the vertical coupling of the baroclinic TC due to the presence of the vertical PV gradient.

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Bin Wang and Qin Zhang

Abstract

The anomalous Philippine Sea anticyclone (PSAC) conveys impacts of El Niño to east Asian climate during the mature and decay of an El Niño (from the winter to ensuing summer). It is shown that the anomalous PSAC forms in fall about one season prior to the peak El Niño; its strength increases with the El Niño intensity and its sign reverses during a La Niña. The PSAC formation concurs with abnormal deepening of the east Asian trough and with increasing number of northward recurvature of tropical storms in the western Pacific. The PSAC establishment is abrupt, coupling with a swing from a wet to dry phase of an intraseasonal oscillation (ISO) and often concurrent with early retreat of the east Asian summer monsoon. The ISO becomes inactive after PSAC establishment.

The development of the PSAC is attributed to combined effects of the remote El Niño forcing, tropical–extratropical interaction, and monsoon–ocean interaction. The developing El Niño induces off-equatorial ascending Rossby wave responses and land surface cooling in northeast Asia; both deepen the east Asian trough in fall and induces vigorous tropical–extratropical exchange of air mass and heat, which enhances the cold air outbreak and initiation of the PSAC. Through exciting descending Rossby waves, the El Niño–induced Indonesian subsidence generates low-level anticyclonic vorticity over south Asia, which is advected by mean monsoon westerly, instigating the anomalous PSAC. The ISO interacting with the underlying ocean plays a critical role in the abrupt establishment of PSAC. The wind–evaporation/entrainment feedback tends to amplify (suppress) ISO before (after) winter northeasterly monsoon commences, suggesting the roles of atmosphereocean interaction and the seasonal march of background winds in changing the Philippine Sea ISO intensity and maintaining PSAC.

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Kazuyoshi Kikuchi and Bin Wang

Abstract

Meteorological and geophysical phenomena involve multiple-scale processes. Here the spatiotemporal wavelet transform (STWT) is applied to detect significant, nonstationary, wave propagation signals from a time–space domain. One of the major advantages of the STWT is the capability to localize the wave properties in both space and time, which facilitates the study of interactions among multiple-scale disturbances by providing relevant information about energy concentration at a given time and space. The global wavelet spectrum (scalogram) of the STWT, which gives an integrated view of the spectrum as wavenumber and frequency, provides a lucid picture of the spectral power distribution that is consistent with the result obtained from the Fourier-based space–time power spectrum. The STWT has also the capability of reconstruction and thus can be used as a spatiotemporal wave filter.

The STWT analysis is applied to analyze the multiscale structure of the Madden–Julian oscillation (MJO) studied by Nakazawa. All types of convectively coupled equatorial waves were identified. The analysis results reveal the structural differences between the MJO and Kelvin waves and their different relationships with the embedded westward propagating inertio-gravity (WIG) waves: for the Kelvin wave the enhanced activity of the WIG waves coincides with the most active convective area, whereas for the MJO the enhanced WIG waves occur to the east of the MJO convective center. In addition, the WIG waves in the MJO have shorter wavelengths and periods, but those in the Kelvin waves have longer wavelengths and periods. This difference may hold a key to understanding the propagation speed difference between the MJO and Kelvin waves. The possible “upscale feedback” of the WIG waves on the MJO and Kelvin waves is also discussed.

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Bin Wang and Xiaofan Li

Abstract

Tropical cyclone propagation (the beta drift) is driven by a secondary circulation associated with axially asymmetric gyres (beta gyres) in the vicinity of the cyclone center. In the presence of the beta effect, the environmental flow may interact with the symmetric circulation and beta gyres of the cyclone, affecting the development of the gyres and thereby the cyclone propagation. An energetics analysis is carried out to elucidate the development mechanism of the beta gyres and to explain variations in propagation speed of a barotropic cyclone embedded in a meridionally varying zonal flow on a beta plane. Two types of zonal flows are considered: one with a constant meridional shear resembling those in the vicinity of a subtropical ridge or a monsoon trough, and the other with a constant relative vorticity gradient as in the vicinity of an easterly (westerly) jet.

Zonal flow with a constant meridional shear changes the generation rate of the gyre kinetic energy through an exchange of energy directly with the gyres. The momentum flux associated with gyres acting on the meridional shear of zonal flow accounts for this energy conversion process. Zonal flow with an anticyclonic (cyclonic) shear feeds (extracts) kinetic energy to (from) the gyres. The magnitude of this energy conversion is proportional to the strength of the meridional shear and the gyre intensity. As a result, the gyres are stronger and the beta drift is faster near a subtropical ridge (anticyclonic shear) than within a monsoon trough (cyclonic shear).

Zonal flow with a constant relative vorticity gradient affects gyre intensity via two processes that have opposing effects. A southward vorticity gradient, on the one hand, weakens the gyres by reducing the energy conversion rate from symmetric circulation to gyres; on the other hand, it enhances the gyres by indirectly feeding energy to the symmetric circulation, whose strengthening in turn accelerates the energy conversion from symmetric circulation to gyres. The effect of the second process tends to eventually become dominant.

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Xiouhua Fu and Bin Wang

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This study assesses the impact of stratiform rainfall (i.e., large-scale rainfall) in the development and maintenance of the Madden–Julian oscillation (MJO) in a contemporary general circulation model: ECHAM4 AGCM and its coupled version. To examine how the model MJO would change as the stratiform proportion (the ratio of the stratiform versus total rainfall) varies, a suite of sensitivity experiments has been carried out under a weather forecast setting and with three 20-yr free integrations. In these experiments, the detrainment rates of deep/shallow convections that function as a water supply to stratiform clouds were modified, which results in significant changes of stratiform rainfall.

Both the forecast experiments and long-term free integrations indicate that only when the model produces a significant proportion (≥30%) of stratiform rainfall can a robust MJO be sustained. When the stratiform rainfall proportion becomes small, the tropical rainfall in the model is dominated by drizzle-like regimes with neither eastward-propagating nor northward-propagating MJO being sustained.

It is found that the latent heat release of stratiform rainfall significantly warms up the upper troposphere. The covariability between the heating and positive temperature anomaly produces eddy available potential energy that sustains the MJO against dissipation and also allows the direct interaction between the precipitation heating and large-scale low-frequency circulations, which is critical to the development and maintenance of the MJO. This finding calls for better representations of stratiform rainfall and its connections with the convective component in GCMs in order to improve their simulations of the MJO.

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Xiouhua Fu and Bin Wang

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This paper reveals major deficiencies of the existing intermediate climate models for tropical surface winds and elaborates the important roles of cloud-longwave radiational forcing and boundary layer thermodynamics in driving the tropical surface winds.

The heat sink associated with the cloud-longwave radiation is demonstrated as an important driving force for boreal summer northeast trades and Indian Ocean southwest monsoons. Over the western North Pacific and Atlantic Oceans, low cloudiness and high sea surface temperature enhance longwave radiation cooling, strengthening subtropical high and associated trades. On the other hand, in the regions of heavy rainfall over South Asia, reduced cloud-longwave radiation cooling enhances monsoon trough and associated southwest monsoons. The boundary layer thermodynamic processes, primarily both the surface heat fluxes and the vertical temperature advection, are shown to be critical for a realistic simulation of the intertropical convergence zone, the equatorial surface winds, and associated divergence field.

To successfully simulate the tropical surface winds, it is essential for intermediate models to adequately describe the feedback of the boundary layer frictional convergence to convective heat source, cloud-longwave radiation forcing, boundary layer temperature gradient forcing, and their interactions. The capability and limitations of the intermediate tropical climate model in reproducing both climatology and interannual variations are discussed.

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Fei Liu and Bin Wang

Abstract

The Indian summer monsoon (ISM) and western North Pacific summer monsoon (WNPSM) are two subsystems of the Asian summer monsoon, and they exhibit different global teleconnection patterns. The enhanced ISM strengthens the South Asian high and Mascarene high, and the WNPSM excites a meridional tripolar wave train in the Northern Hemisphere and affects the Australian high in the Southern Hemisphere. To understand the dynamics behind these global teleconnections, especially the processes responsible for the cross-equatorial teleconnection, an intermediate model, describing a two-level troposphere and a steady planetary boundary layer (PBL), is linearized from the background horizontal wind field. The model results indicate that the ISM heating, located under the strong easterly vertical shear (VS) and close to the westerly jet in the Northern Hemisphere, can excite a barotropic Rossby wave that emanates northwestward and then propagates downstream along the westerly jet. Since the WNPSM heating is far away from the westerly jet over the North Pacific, it only excites a weak Rossby wave train, which cannot explain the meridional tripolar teleconnection associated with the WNPSM. It is found that both the ISM and WNPSM heating excite strong teleconnections in the Southern Hemisphere via an advection mechanism; that is, the background upper-level northerly winds can transport energy across the equator from the Northern Hemisphere summer monsoon to the Southern Hemisphere westerly jet. In addition, the PBL enhances monsoon teleconnections by trapping more energy in the upper troposphere. The elevated maximum monsoon heating also reinforces upper-level perturbations and enhances the teleconnection pattern.

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Liguang Wu and Bin Wang

Abstract

The recently reported increase in the proportion of intense hurricanes is considerably larger than those projected by the maximum potential intensity (MPI) theory and the results of numerical simulation. To reconcile this discrepancy, the authors examined the best-track datasets for the North Atlantic (NA), western North Pacific (WNP), and eastern North Pacific (ENP) basins. It was found that the changes in the tropical cyclone formation locations and prevailing tracks may have contributed to the changes in the proportion of the intense hurricanes over the past 30 yr. The authors suggest that the changes in the formation locations and prevailing tracks have a profound impact on the basinwide tropical cyclone intensity. Thus, how the atmospheric circulation in the tropical cyclone basins responds to the global warming may be a critical factor in understanding the impacts of global warming on tropical cyclone intensity.

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