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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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Bin Wang and Sun-Seon Lee

Abstract

Eastward propagation is an essential characteristic of the Madden–Julian oscillation (MJO). Yet, simulation of MJO propagation in general circulation models (GCMs) remains a major challenge and understanding the causes of propagation remains controversial. The present study explores why the GCMs have diverse performances in MJO simulation by diagnosis of 24 GCM simulations. An intrinsic linkage is found between MJO propagation and the zonal structural asymmetry with respect to the MJO convective center. The observed and realistically simulated MJO eastward propagations are characterized by stronger Kelvin easterly waves than Rossby westerly waves in the lower troposphere, which is opposite to the Gill pattern where Rossby westerly waves are 2 times stronger than Kelvin easterly waves. The GCMs simulating stronger Rossby westerly waves tend to show a stationary MJO. MJO propagation performances are robustly correlated with the quality of simulated zonal asymmetries in the 850-hPa equatorial zonal winds, 700-hPa diabatic heating, 1000–700-hPa equivalent potential temperature, and convective instability. The models simulating realistic MJO propagation are exemplified by an eastward propagation of boundary layer moisture convergence (BLMC) that leads precipitation propagation by about 5 days. The BLMC stimulates MJO eastward propagation by preconditioning and predestabilizing the atmosphere, and by generating lower-tropospheric heating and available potential energy to the east of precipitation center. The MJO structural asymmetry is generated by the three-way interaction among convective heating, moisture, and equatorial wave and boundary layer dynamics. In GCMs, differing convective heating representation could produce different MJO structural asymmetry, and thus different propagations. Diagnosis of structural asymmetry may help revealing the models’ deficiency in representing the complex three-way interaction processes, which involves various parameterized processes.

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Bin Wang and Ja-Yeon Moon

Abstract

Modulation of tropical cyclone (TC) genesis by the Madden–Julian oscillation (MJO) has been quantitatively diagnosed by using a climatological genesis potential index (GPI). Analysis of TC genesis during November–April of 1979–2014 indicates the most effective factors controlling intraseasonal TC genesis are 850-hPa relative vorticity weighted by the Coriolis parameter f ζ r850 and 500-hPa vertical motion ω 500. The total vertical wind shear and maximum potential intensity are unimportant, and the role of 600-hPa relative humidity is greatly represented by ω 500. The MJO modulates TC genesis primarily through changing low-level vorticity induced by its Rossby wave gyres and meridional shears of equatorial zonal winds. A new intraseasonal GPI (ISGPI) is proposed to quantify the MJO’s modulation of TC genesis. The ISGPI significantly improves representation of intraseasonal variation of TC genesis in the tropics and in each subregion of the southern Indian Ocean, Australian monsoon, and South Pacific. In the hot spots of the Southern Hemisphere TC genesis zone, the probability of TC genesis can differ by a factor of 5–19 as a result of MJO modulation. The results suggest that the large-scale factors controlling TC genesis may vary with different time scales, and the climatological GPI may not be quite applicable for diagnoses of climate variability and future change of TC genesis potential. To simulate realistic impacts of the MJO on TC genesis, general circulation models must reproduce not only realistic eastward propagation but also the MJO low-level circulation structure. Application of the new ISGPI may have a large potential to improve dynamical subseasonal prediction of TC genesis.

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Tim Li, Bin Wang, and Lu Wang

Abstract

In a recent paper, Stuecker et al. applied a “combination mode” (C-mode) theory to explain the formation of the anomalous western North Pacific anticyclone (WNPAC) during El Niño events. The C-mode, arising from interaction between the annual cycle and ENSO, is an Indo-Pacific basin mode with two “near annual” time scales (roughly 10 and 15 months, respectively). This comment discusses to what extent the C-mode can explain the WNPAC dynamics. The major findings are the following: 1) spectral analysis of the Indo-Pacific circulation anomaly fields indicates that the 10-month mode is not observed and the 15-month mode is only seen in the western North Pacific (WNP), where its spectral peak is statistically insignificant; 2) the 15-month mode (with a period of 13–19 months) accounts for only a small portion (13%) of the observed sea level pressure anomaly in the WNP; and 3) the C-mode evolution does not capture the observed timing of the WNPAC onset in the northern fall of El Niño developing year. In addition it is shown, based on observational analyses and numerical experiments, that local atmosphere–ocean interaction plays an important role in formation of the anomalous anticyclonic center over the Philippine Sea.

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Omer Lutfi Sen, Yuqing Wang, and Bin Wang

Abstract

This paper describes a study that investigates the local and remote effects of Indochina deforestation on the east Asian summer monsoon. During the summer months, the peninsula is subject to strong monsoonal flow whose downstream rainfall is of vital importance for China, where a significant fraction of the world's population lives. It is, therefore, extremely important to investigate how the landscape change affects this monsoonal flow, and whether this effect is strong enough to cause notable changes in the regional rainfall. For this reason, a modeling experiment with the International Pacific Research Center regional climate model was carried out. Ensemble simulations with the current vegetation cover in the peninsula and its reforested scenario were performed for the period from 20 April to 31 August 1998. The results of the experiment indicate that deforestation in the peninsula has not only local, but also far-reaching effects on the east Asian summer monsoon. Locally, the effect could be described as increases in wind speed and temperature, and as a decrease in water vapor mixing ratio from the surface up to about 850 mb. Furthermore, the deforestation tends to enhance the rising motions, and, hence, tends to reduce surface pressure and geopotential height up to about 850 mb over the deforested area. The local landscape changes tend to increase rainfall on the downwind side and decrease it on the upwind side. Far-reaching effects in summer include a weakening of the monsoonal flow over east China, near the Tibetan Plateau, and a strengthening over the neighboring seas to the east. These changes yield sandwich-type drier and wetter bands that are elongated along the main flow path of the east Asian summer monsoon. A comparison of the modeled changes with the observed rainfall trends suggests that the deforestation in the Indochina Peninsula could be one of the major factors causing changes in the climate of the region.

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Yuqing Wang, Omer L. Sen, and Bin Wang

Abstract

East Asia is a region with complex topography, land surface conditions, coastlines, and with large contribution from mesoscale phenomena, such as the mei-yu/baiu frontal systems and tropical storms. To study the regional climate in such a region, a highly resolved regional climate model (IPRC-RegCM) has been recently developed at the International Pacific Research Center (IPRC). The distinct features of this model include the direct feedback of cumulus detrained cloud ice and cloud water into the grid-resolved quantities; the effect of cloud buoyancy on turbulence production with mixed-ice phase clouds; an explicit coupling between the cloud microphysics and radiation via cloud properties; an explicit coupling between land surface and radiation via surface albedo, direct and diffuse radiation fluxes; and the effect of frictionally generated dissipative heating.

The model is documented in detail and the performance of the model is demonstrated by its simulation of the 1998 severe flooding event over China, the worst one since 1955. With the use of the objective analysis of the European Centre for Medium-Range Weather Forecasts (ECMWF), which is available at 12-h intervals with a resolution of 2.5° × 2.5°, as both the initial and lateral boundary conditions, the model was integrated from 26 April to 31 August 1998 with a resolution of 0.5° × 0.5° covering the area 5°–45°N, 90°–140°E. The model simulated realistically not only the temporal evolution of the area-averaged precipitation and the monthly mean precipitation spatial pattern but also the daily precipitation intensity distribution. The model reproduced the monsoon circulations, in particular, two episodes of the intraseasonal oscillation events that are believed to be closely related to the unusual double mei-yu periods over the Yangtze River basin in 1998.

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Chao Wang, Bin Wang, and Liguang Wu

ABSTRACT

It has been a common practice to predict total tropical cyclone (TC) genesis frequency over the entire western North Pacific (WNP). Here we show that TC genesis (TCG) exhibits distinct regional variability and sources of predictability. Therefore, we divide the WNP into four quadrants with 140°E and 17°N being dividing lines plus the South China Sea (SCS) to predict five subregional TCG frequencies as well as the entire WNP TCG frequency. Besides the well-known ENSO-induced seesaw relationship between the TCGs in the southeast and northwest quadrants, we found that 1) an enhanced TCG in the northeast WNP is associated with a pronounced anomalous cyclonic circulation, which is maintained through its interaction with the underlying sea surface temperature (SST) anomalies; 2) an active TCG in the southwest WNP is accompanied by a zonally elongated positive vorticity anomaly and SST warming over the equatorial eastern Pacific; and 3) the SCS TCG is influenced by the upper-level South Asia high through modulating large-scale environmental parameters. Physically meaningful predictors are identified and a set of empirical prediction models for TCG frequency is established for each subregion. Both the cross-validated reforecast for 1965–2000 and independent forecast for 2001–16 show significant temporal correlation skills. Moreover, the sum of the predicted TCG frequency in five subregions yields a basinwide TCG frequency prediction with a temporal correlation skill of 0.76 for the independent forecast period of 2001–16. The results indicate its potential utility to improve the TC forecasting in the WNP.

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Zhuo Wang, C-P. Chang, and Bin Wang

Abstract

The impacts of El Niño and La Niña on the U.S. climate during northern summer are analyzed separately. Composite analyses reveal that a continental-scale anomalous high dominates over most of North America during La Niña events and leads to hot and dry summers over the central United States. However, the impacts of El Niño over North America are weaker and more variable.

A linear barotropic model is used to explore the maintenance of the anomalous patterns. Various forcing terms derived from observations via a single-level vorticity budget analysis are used to drive the model. When the barotropic model is driven by the total forcing (Rossby wave source plus transient eddy forcing plus nonlinear interactions), the model simulations resemble the observed patterns, and a strong and extensive anticyclone is reproduced in the La Niña simulation. The model responses to the individual forcing terms suggested that the vorticity stretching term ( fD) and the transient eddy forcing contribute most to the responses over North America. The stretching term ( fD) excites a low in the El Niño simulation and a high in the La Niña simulation over North America. However, the transient eddy forcing favors an anomalous high over North America in both El Niño and La Niña simulations, such that it weakens the El Niño pattern and strengthens the La Niña pattern.

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Bo Yang, Yuqing Wang, and Bin Wang

Abstract

In a quiescent environment on an f plane, the internal dynamic processes of a tropical cyclone (TC) can generate axially asymmetric circulations (asymmetries) in its inner-core region. The present study investigates how these inner-core asymmetries affect TC intensity. For this purpose, a three-dimensional (3D) TC model and its axisymmetric (2D) version were used. Both have identical model vertical structure and use the same set of parameters and the same initial conditions. The differences between the two model runs are considered to be due to mainly the effects of the TC asymmetries. The results show that the presence of asymmetries in the 3D run reduces the TC final intensity by about 15% compared with the 2D run, suggesting that the TC asymmetry is a limiting factor to the potential intensity (PI).

In the 2D run without asymmetries, the convective heating in the eyewall generates an annular tower of high potential vorticity (PV) with relatively low PV in the eye. The eyewall tilts outward with height significantly. Underneath the tilted eyewall the downdrafts induced by evaporation of rain and melting of snow and graupel make the subcloud-layer inflow dry and cool, which lowers the boundary layer equivalent potential temperature (θe), thus increasing the entropy difference between the air and sea in the vicinity of the radius of maximum wind (RMW). The increased air–sea entropy deficit leads to more energy input into TC from the underlying ocean and thus a greater final intensity. On the other hand, in the 3D run, the model-resolved asymmetric eddies, which are characterized by the vortex Rossby waves in the mid-lower troposphere, play important roles in modifying the symmetric structure of the TC. Potential vorticity and θe budgets indicate that significant inward PV mixing from the eyewall into the eye results in a less-tilted eyewall, which in turn limits the drying and cooling effects of downdrafts in the subcloud layer and reduces the air–sea entropy deficit under the eyewall, thereby reducing the TC intensity. The angular momentum budget analysis shows that the asymmetric eddies tend to reduce the strength of the primary circulation in the vicinity of the RMW. This eddy contribution to the azimuthal mean angular momentum budget is larger than the parameterized horizontal diffusion contribution in the 3D run, suggesting an overall diffusive effect of the asymmetric eddies on the symmetric circulation.

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Bin Wang, Qinghua Ding, and P. V. Joseph

Abstract

The onset of the Indian summer monsoon (ISM) over the southern tip of the Indian peninsula [also known as monsoon onset over Kerala (MOK)] has been considered the beginning of India’s rainy season. The Indian Meteorological Department (IMD) makes an official prediction of ISM onset every year using a subjective method. Based on an analysis of the past 60-yr (1948–2007) record, the authors show that the onset date can be objectively determined by the beginning of the sustained 850-hPa zonal wind averaged over the southern Arabian Sea (SAS) from 5° to 15°N, and from 40° to 80°E. The rapid establishment of a steady SAS westerly is in excellent agreement with the abrupt commencement of the rainy season over the southern tip of the Indian peninsula. In 90% of the years analyzed, this simple and objective index has excellent agreement with the onset dates that are subjectively defined by the IMD. There are only 3 yr of the past 60 yr during which the two onset dates differ by more than 10 days, and none of them perfectly reflects the MOK.

A prominent onset precursor on the biweekly time scale is the westward extension of the convection center from the equatorial eastern Indian Ocean toward the southeast Arabian Sea. On the intraseasonal time scale, the onset tends to be led by northeastward propagation of an intraseasonal convective anomaly from the western equatorial Indian Ocean. The objective determination of the onset based on the SAS low-level westerly is a characteristic representation of the complex process of the ISM onset. Given its objectiveness and its representation of the large-scale circulation, the proposed new onset definition provides a useful metric for verifying numerical model performance in simulating and predicting the ISM onset and for studying predictability of interannual variations of the onset.

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