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Fei Liu and Bin Wang

Abstract

The eastward-propagating tropical low-frequency disturbances, such as the moist Kelvin waves or the Madden–Julian oscillation (MJO), are often observed to experience convective enhancement when meeting with the westward-propagating 2-day waves. A scale interaction (SI) model is built to understand the nature of the interaction between the 2-day waves and moist Kelvin waves or MJO. In this model, the convective complex of moist Kelvin waves modulates the strength and location of the 2-day waves, which feed back through the upscale eddy transfer. An ageostrophic model describing the 2-day waves is first solved, and the resultant westward-propagating, backward-tilted disturbances are consistent with the observed 2-day waves. An explicit representation of eddy momentum transfer (EMT), eddy heating transfer (EHT), and eddy moisture transfer (EQT) arising from the 2-day waves is then formulated. The SI model shows that the 2-day waves in front of moist Kelvin waves produce an EMT accelerating the low-frequency easterly in the lower troposphere, an EHT cooling down the middle troposphere, and an EQT moistening the middle troposphere. These three transfer terms have comparable magnitude. Although the negative EHT tends to damp the moist Kelvin waves, both the EMT and EQT provide instability sources for the moist Kelvin waves. The 2-day waves also slow down the moist Kelvin waves, mainly through the advective effects of the EMT. So the unstable moist Kelvin waves may exhibit convective enhancement when meeting with the 2-day waves. The theoretical results presented here point to the need to further observe the multiscale structures within the moist Kelvin waves and the MJO.

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Fei Liu and Bin Wang

Abstract

This work is an extension and improvement of the minimal Madden–Julian oscillation (MJO) “skeleton” model developed by Majda and Stechmann, which can capture some important features of the MJO—slow eastward propagation, quadrupole-vortex structure, and independence of frequency on wavelength—but is unable to produce unstable growth and selection of eastward-propagating planetary waves. With the addition of planetary boundary layer frictional moisture convergence, these deficiencies can be remedied. The frictional boundary layer “selects” the planetary-scale eastward propagation as the most unstable mode, but the dynamics remains confined to atmospheric processes only. Here the authors study the role of air–sea interaction by implementing an oceanic mixed-layer (ML) model of Wang and Xie into the MJO skeleton model. In this new air–sea coupled skeleton model, the features of the original skeleton model remain; additionally, the air–sea interaction under mean westerly winds is shown to produce a strong instability that selectively destabilizes the eastward-propagating planetary-scale waves. Although the cloud–shortwave radiation–sea surface temperature (CRS) feedback destabilizes both eastward and westward modes, the air–sea feedback associated with the evaporation and oceanic entrainment favors planetary-scale eastward modes. Over the Western Hemisphere where easterly background winds prevail, the evaporation and entrainment feedbacks yield damped modes, indicating that longitudinal variation of the mean surface winds plays an important role in regulation of the MJO intensity in addition to the longitudinal variation of the mean sea surface temperature or mean moist static stability. This theoretical analysis suggests that accurate simulation of the climatological mean state is critical for capturing the realistic air–sea interaction and thus the MJO.

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Fei Liu and Bin Wang

Abstract

The Madden–Julian oscillation (MJO) is a multiscale system. A skeleton model, developed by Majda and Stechmann, can capture some of planetary-scale aspects of observed features such as slow eastward propagation, nondispersive behavior, and quadrupole-vortex structure. However, the Majda–Stechmann model cannot explain the source of instability and the preferred planetary scale of the MJO. Since the MJO major convection region is leaded by its planetary boundary layer (PBL) moisture convergence, here a frictional skeleton model is built by implementing a slab PBL into the neutral skeleton model. As a skeleton model allowing the scale interaction, this model is only valid for large-scale waves. This study shows that the PBL frictional convergence provides a strong instability source for the long eastward modes, although it also destabilizes very short westward modes. For the long waves (wavenumber less than 5), the PBL Ekman pumping moistens the low troposphere to the east of the MJO convective envelope, and sets up favorable moist conditions to destabilize the MJO and favor only eastward modes. Sensitivity experiments show that a weak PBL friction will enhance the instability slightly. The sea surface temperature (SST) with a maximum at the equator also prefers the long eastward modes. These theoretical analysis results encourage further observations on the PBL regulation of mesosynoptic-scale motions, and exploration of the interaction between PBL and multiscale motions, associated with the MJO to improve the MJO simulation in general circulation models (GCMs).

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Renguang Wu and Bin Wang

Abstract

Climatological summer monsoon onset over the South China Sea (SCS) and the western North Pacific (WNP) (defined as the region of 10°–20°N, 120°–160°E) displays three distinct stages. Around mid-May, monsoon rain commences in the SCS and the Philippines. In early to mid-June, the monsoon rain extends to the southwestern Philippine Sea. After mid-July, the rainy season starts in the northeastern part of the WNP. The onset anomaly, however, displays an in-phase interannual variation across the entire WNP domain. The standard deviation of the onset date increases eastward from 3 pentads in the SCS to 5 pentads in the northeastern part of the domain. The large onset variability in the WNP is mainly attributed to large year-to-year changes of the seasonal cycle. The role of the intraseasonal oscillation is secondary but important especially in the SCS region. The El Niño–Southern Oscillation (ENSO)-related thermal contrast between the WNP and the equatorial central Pacific modulates significantly the seasonal migration of the monsoon trough, the subtropical high, and the convection zone over the WNP during late spring–early summer in the ENSO decay phase. Thus, ENSO plays a dominant role in the interannual variation of the WNP summer monsoon onset.

The general circulation model results suggest that during El Niño events, the warm SST anomalies in the equatorial eastern-central Pacific play a major role in generation of large-scale upper-level convergence and descent anomalies over the WNP. Meanwhile, the cold SST anomalies in the WNP induce lower-level anticyclonic wind anomalies and reduce convective instability. Both the remote and local SST forcing are important for delaying the seasonal movement of the monsoon trough and the western Pacific subtropical high and hence the onset of the monsoon rain. In the La Niña case, the local warm SST anomalies in the WNP are more important than the cold SST anomalies in the equatorial eastern-central Pacific in the generation of lower-level cyclonic wind anomalies and enhancement of convective instability.

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Zhiling Xie and Bin Wang

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Multiple bias-corrected top-quality reanalysis datasets, gauge-based observations, and selected satellite products are synthetically employed to revisit the climatology and variability of the summer atmospheric heat sources over the Tibetan Plateau (TP). Verification-based selection and ensemble-mean methods are utilized to combine various datasets. Different from previous works, this study pays special attention to estimating the total heat source (TH) and its components over the data-void western plateau (70°–85°E), including the surface sensible heat (SH), latent heat released by precipitation (LH), and net radiation flux (RD). Consistent with previous studies, the climatology of summer SH (LH) typically increases (decreases) from southeast to northwest. Generally, LH dominates TH over most of the TP. A notable new finding is a minimum TH area over the high-altitude region of the northwestern TP, where the Karakoram mountain range is located. We find that during the period of 1984–2006, TH shows insignificant trends over the eastern and central TP, whereas it exhibits an evident increasing trend over the western TP that is attributed to the rising tendency of LH before 1996 and to that of RD after 1996. The year-to-year variation of TH over the central–eastern TP is highly correlated with that of LH, but that is not the case over the western TP. It is also worth noting that the variations of TH in each summer month are not significantly correlated with each other, and hence study of the interannual variation of the TP heat sources should consider the remarkable subseasonal variations.

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Bin Wang and Tianming Li

Abstract

Tropical boundary-layer flows interact with the free tropospheric circulation and underlying sea surface temperature, playing a critical role in coupling collective effects of cumulus heating with equatorial dynamics. In this paper a unified theoretical framework is developed in which convective interaction with large-scale circulation includes three mechanisms: convection–wave convergence (CWC) feedback, evaporation–wind (EW) feedback, and convection–frictional convergence (CFC) feedback. We examine the dynamic instability resulting from the convective interaction with circulation, in particular the role of CFC feedback mechanism.

CFC feedback results in an unstable mode that has distinctive characteristics from those occurring from CWC feedback or EW feedback in the absence of mean flow. The instability generated by CFC feedback is of low frequency with a typical growth rate on an order of 10−6 s−1. The unstable mode is a multiscale wave packet; a global-scale circulation couples with a large-scale (several thousand kilometers) convective complex. The complex is organized by boundary-layer convergence and may consist of a few synoptic-scale precipitation cells. The heating released in the complex in turn couples the moist Kelvin wave and the Rossby wave with the gravest meridional structure, forming a dispersive system. The energy propagates slower than the individual cells within the wave packet. A transient boundary layer is shown to favor planetary-scale instability due to the fractionally created phase shift between the maximum vertical motion and the heating associated with boundary-layer convergence.

The implications of the theory to the basic dynamics of tropical intraseasonal oscillation are discussed.

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Kazuyoshi Kikuchi and Bin Wang

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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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Hualan Rui and Bin Wang

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The development and dynamical structure of intraseasonal low-frequency convection anomalies in the equatorial region are investigated using 10 years (1975–85) of outgoing longwave radiation (OLR) and 7 years (1979–85) of 200 and 850 mb wind data.

The composite OLR anomalies for 36 cases show a four-stage development process: initiation over equatorial Africa, rapid intensification when passing through the Indian Ocean, mature evolution characterized by a weakening in the maritime continent and redevelopment over the western Pacific, and dissipation near the date line in moderate events or emanation from the equator toward North America and southeastern Pacific in strong events.

A noticeable feature in vertical structure is that the 850 mb convergence leads convection and midtropospheric upward motion by about 30 degrees longitude in both developing and mature phases. Equatorial upper- (lower-) level easterly (westerly) anomalies and associated twin anomalous anticyclonic (cyclonic) circulation anomalies couple with equatorial convection anomalies. The wind anomalies, however, generally lag convection anomalies in development and early mature phases, but nearly overlap in late mature phase and slightly lead the convection anomalies in dissipation phase. The upper-level twin cyclonic cells associated with the westerly anomalies in front of the convection travel across eastern Pacific after the convection ceases in the central Pacific, while the low-level wind anomalies die out east of the date line.

The implications of the findings in relation to theoretical hypotheses on low-frequency motion are discussed.

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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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Xiaqiong Zhou and Bin Wang

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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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