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Xiao Wang and Lin Lin

Abstract

Previous research revealed that if individuals personally experience an unusual weather event as a result of global warming (vs no personal experience), they may hold higher belief certainty that global warming is happening and hence develop more favorable attitudes toward mitigation actions. However, much of the previous research focused on self-reported personal experience and global warming beliefs using cross-sectional surveys; reverse causality is thus possible. Based on weather records and survey data, the present research examined whether actual weather events can influence one’s perceptions of unusual weather and belief certainty. Severe Typhoon Fitow 2013, but not hot summer temperatures, directly predicted the Chinese perceived experience of unusual weather and indirectly predicted their belief certainty and attitudes toward mitigation behavior. However, the effects were relatively small. Possible explanations and implications for environmental education are discussed.

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Ke Wei and Lin Wang

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Water resources are an essential part of the ecosystem in the extremely arid northwestern part of China. Previous studies revealed a dry-to-wet climate change since the late 1980s in this region, which suggested a relief from the drought condition. However, the analysis in this study using the updated data shows that the arid situation has continued and even intensified in the past decade. This is reflected by the fact that the low-level air relative humidity and deep soil relative humidity have decreased in the past decade. Examination of the standardized precipitation evapotranspiration index (SPEI) and self-calibrating Palmer drought severity index (sc-PDSI) indicates that the severity and spatial extent of aridity and drought have increased substantially in northwestern China in the most recent decade. It is shown that the drought intensification in northwestern China is mainly caused by the increase of evaporation that results from the continuous rise in temperature, which will pose a continuous threat to the ecosystem and economic development in this region, especially under the background of global warming.

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Danyang Wang and Yanluan Lin

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Tropical cyclone (TC) wind structure is important for its intensity change and induced damage, but its modulating factors remain to be explored. A heat-engine-based surface wind structure parameter α, reflecting TC’s relative compactness, is introduced and derived based on an entropy budget framework. We found that α is modulated by three key parameters: the thermodynamic efficiency ϵ PI in potential intensity theory, the Carnot efficiency ϵ C of the system, and the degree of irreversibility α irr of the system. A higher α irr contributes to a larger α and a lower heat engine efficiency. An expression linking TC intensity and compactness also emerges under this framework. Idealized simulations of a typical moist TC (CTL), a dry (DRY) TC, and a moist reversible TC (REV; in which hydrometeors do not fall out) evinced that the significantly higher α irr in CTL, due to irreversible entropy productions from precipitation dissipation, water vapor diffusion, and irreversible phase changes, contributes to its much larger compactness compared to DRY and REV. The study illustrates the importance of irreversible entropy production processes in modulating TC surface wind field. Simple estimate suggests that α will increase due to a hypothesized increased α irr with warming because of increased water content. This indicates that TCs will become more compact in a warmer climate.

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Danyang Wang and Yanluan Lin

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The size and structure of tropical cyclones (TCs) are investigated using idealized numerical simulations. Three simulations are conducted: a pure dry TC (DRY), a moist reversible TC (REV) with fallout of hydrometeors in the atmosphere disallowed, and a typical TC (CTL). It was found that the width of the eyewall ascent region and the radius of maximum wind r m are much larger in DRY and REV than those in CTL. This is closely related to the deep inflow layer (~4 km) in DRY and REV associated with a different entropy restoration mechanism under the subsidence region. With the wide ascents, the close link between r m and the outer radius in DRY and REV can be well predicted by the Emanuel and Rotunno (ER11) model. The magnitude of subsidence, mainly controlled by the vertical gradient of entropy in the mid- and upper troposphere, is nearly one order greater in DRY and REV than that in CTL. This study demonstrates that the falling nature of hydrometeors poses a strong constraint on the size and structure of real world TCs via the entropy distribution in the subsidence region. The wide ascent, self-stratification in the outflow, and decently reproduced wind profile in DRY and REV suggest that DRY and REV behave like a prototype of the ER11 model with CTL being an extreme type.

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Lin Wang and Wen Chen

Abstract

The thermal contrast between the Asian continent and the adjacent oceans is the primary aspect of the East Asian winter monsoon (EAWM) that can be well represented in the sea level pressure (SLP) field. Based on this consideration, a new SLP-based index measuring the intensity of the EAWM is proposed by explicitly taking into account both the east–west and the north–south pressure gradients around East Asia. The new index can delineate the EAWM-related circulation anomalies well, including the deepened (shallow) midtropospheric East Asian trough, sharpened and accelerated (widened and decelerated) upper-tropospheric East Asian jet stream, and enhanced (weakened) lower-tropospheric northerly winds in strong (weak) EAWM winters. Compared with previous indices, the new index has a very good performance describing the winter-mean surface air temperature variations over East Asia, especially for the extreme warm or cold winters. The index is strongly correlated with several atmospheric teleconnections including the Arctic Oscillation, the Eurasian pattern, and the North Pacific Oscillation/western Pacific pattern, implying the possible internal dynamics of the EAWM variability. Meanwhile, the index is significantly linked to El Niño–Southern Oscillation (ENSO) and the sea surface temperature (SST) over the tropical Indian Ocean. Moreover, the SST anomalies over the tropical Indian Ocean are more closely related to the index than ENSO as an independent predictor. This adds further knowledge to the prediction potentials of the EAWM apart from ENSO. The predictability of the index is high in the hindcasts of the Centre National de Recherches Météorologiques (CNRM) model from Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER). Hence, it would be a good choice to use this index for the monitoring, prediction, and research of the EAWM.

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Xingbao Wang and Da-Lin Zhang

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Because of the lack of three-dimensional (3D) high-resolution data and the existence of highly nonelliptic flows, few studies have been conducted to investigate the inner-core quasi-balanced characteristics of hurricanes. In this study, a potential vorticity (PV) inversion system is developed, which includes the nonconservative processes of friction, diabatic heating, and water loading. It requires hurricane flows to be statically and inertially stable but allows for the presence of small negative PV. To facilitate the PV inversion with the nonlinear balance (NLB) equation, hurricane flows are decomposed into an axisymmetric, gradient-balanced reference state and asymmetric perturbations. Meanwhile, the nonellipticity of the NLB equation is circumvented by multiplying a small parameter ε and combining it with the PV equation, which effectively reduces the influence of anticyclonic vorticity. A quasi-balanced ω equation in pseudoheight coordinates is derived, which includes the effects of friction and diabatic heating as well as differential vorticity advection and the Laplacians of thermal advection by both nondivergent and divergent winds.

This quasi-balanced PV–ω inversion system is tested with an explicit simulation of Hurricane Andrew (1992) with the finest grid size of 6 km. It is shown that (a) the PV–ω inversion system could recover almost all typical features in a hurricane, and (b) a sizeable portion of the 3D hurricane flows are quasi-balanced, such as the intense rotational winds, organized eyewall updrafts and subsidence in the eye, cyclonic inflow in the boundary layer, and upper-level anticyclonic outflow. It is found, however, that the boundary layer cyclonic inflow and upper-level anticyclonic outflow also contain significant unbalanced components. In particular, a low-level outflow jet near the top of the boundary layer is found to be highly unbalanced (and supergradient). These findings are supported by both locally calculated momentum budgets and globally inverted winds. The results indicate that this PV inversion system could be utilized as a tool to separate the unbalanced from quasi-balanced flows for studies of balanced dynamics and propagating inertial gravity waves in hurricane vortices.

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Dashan Wang, Xianwei Wang, Lin Liu, Dagang Wang, and Zhenzhong Zeng

Abstract

Urban areas demonstrate great influence on precipitation, yet the spatial clustering features of precipitation is still unclear over urban areas. This study quantitatively examines the spatial clustering of precipitation intensity in 130 urban-affected regions over mainland China during 2008-2015 using a high-resolution merged precipitation product. Results show that the spatial heterogeneity patterns display diverse distribution and vary with precipitation intensity and urban sizes. Extreme and heavy precipitation has higher spatial heterogeneity than light precipitation over the urban-affected regions of grade 1 cities, and their mean Moran’s I are 0.49, 0.47 and 0.37 for the intensity percentiles of ≥95%, 75-95% and <75%, respectively. The urban signatures in the spatial clustering of precipitation extremes are observed in 37 cities (28%), mainly occurring in the Haihe River Basin, the Yangtze River Basin and the Pearl River Basin. The spatial clustering patterns of precipitation extremes are affected by the local dominant synoptic conditions, such as the heavy storms of convective precipitation in Beijing (Moran’s I =0.47) and the cold frontal system in the Pearl River Delta (Moran’s I =0.78), resulting in large regional variability. The role of urban environments for the spatial clustering is more evident in wetter conditions (e.g., RH >75% over Beijing and RH >85% over the Pearl River Delta) and warmer conditions (T >25°C over Beijing and T >28°C over the Pearl River Delta). This study highlights the urban modification on the spatial clustering of some precipitation extremes, and calls for precautions and adaptation strategies to mitigate the adverse effect of the highly clustered extreme rainfall events.

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Y. J. Lin, T. C. Wang, and J. H. Lin

Abstract

Some dynamic and thermodynamic properties of a convective cell within a squall line that occurred on 6 June 1979 were studied based on dual-Doppler observations. The domain under investigation had a horizontal dimension of 27 km × 27 km with 12 levels in the vertical. The grid spacing used was 1 km. Vertical velocities were computed from the anelastic continuity equation by integrating downward with variational adjustment. Fields of deviation perturbation pressure, density and virtual temperature were recovered from a three-dimensional wind field using the thermodynamic retrieval method. These retrieved fields were then subjected to internal consistency checks to determine the level of confidence.

Our findings demonstrate that thermodynamic retrieval is feasible when random errors inherent in the radial wind components are minimized by proper smoothing. Errors in the computation of vertical velocity can be substantially reduced when a variational approach is used with the anelastic continuity equation applied to the vertically integrated horizontal mass divergence as an integral constraint. Results show that the gust front (GF) is primarily responsible for vigorous convection in the storm. Distinct features of strong wind shear, pressure change and temperature contrast are evident across the GF. The derived pressure and temperature perturbations are closely related to the updraft–downdraft structure. In particular, high pressure forms on the upshear side of an updraft with low pressure on the downshear side. The orientation of maximum pressure gradient across an updraft is in the direction of the environmental shear vector. Strong perturbation temperature gradients occur in the vicinity of an updraft with warning on its upwind side and cooling on its downwind side. The appearance of a downdraft in the immediate vicinity of an updraft is of importance in affecting the magnitude and distribution of pressure and temperature perturbations within the storm.

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Yuh-Lang Lin and Ting-An Wang

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Four regimes are identified for two-dimensional, unstructured, nonrotating, continuously stratified, hydrostatic, uniform Boussinesq flow over an isolated mountain ridge: (I) flow with neither wave breaking aloft nor upstream blocking (F≥1.12, where F = U/ NH; U and N are upstream basic flow speed and Brunt-Väisälä frequency, respectively; and h is the mountain height), (II) flow with wave breaking aloft in the absence of upstream blocking (0.9 < F≤1. 12), (III) flow with both wave breaking and upstream blocking, but where wave breaking occurs first (0.6≤F≤0.9), and (IV) flow with both wave breaking and upstream blocking, but where blocking occurs first (0.3≤F≤0.6). In regime I, neither wave breaking nor upstream blocking occurs, but columnar disturbance does exist. The basic flow structure resembles either linear or weakly nonlinear mountain waves. It is found that the columnar disturbance is independent of the wave breaking aloft. In regime II, an internal jump forms at the downstream edge of the wave-breaking region, propagates downstream, and then becomes quasi-stationary. The region of wave breaking also extends downward toward the lee slope. After the internal jump travels farther downstream, a stationary mountain wave becomes established in the vicinity of the mountain above the dividing streamline, which is induced by wave breaking. A high-drag state is predicted in this flow regime. In addition, a vertically propagating hydrostatic gravity wave is generated by the propagating jump and travels with it. Along the lee slope, a strong downslope wind develops. Static and Kelvin-Helmholtz instabilities may occur locally in the region of wave breaking. The critical F for wave breaking is about 1.12, which agrees well with the value 1.18 found by Miles and Huppert. This study also found that the flow responses in this flow regime, as well as in the other regimes, are similar for constant F.

In regime III, the downstream internal jump propagates downstream in the early stage, retrogresses in the direction against the basic flow once blocking occurs, and then becomes quasi-stationary. The retrogression of the downstream jump may be caused by the modification of the upstream boundary conditions. The layer depth of blocked fluid is independent of F and h/a, where a is the mountain half-width. In regime IV, the internal jump quickly becomes stationary over the lee slope once it forms. It is found that the presence of wave breaking aloft is not necessary for upstream blocking to occur. A vertically propagating gravity wave is generated by the upstream reversed flow and travels with it. The speed of the upstream reversed flow is proportional to h/a. The surface drag increases abruptly from regime I to II, while it decreases gradually from regime II (III) to III (IV). The surface drag is a function of h/a and is a minimum for h/a=0.05 for constant F. The average dividing streamline height generated by wave breaking is roughly 0.85λz (HdN/U=5.34), and the level at which overturning initially occurs is found to be about 4.4 in the high-drag state, where λz and Hd, are the dimensional hydrostatic vertical wavelength and dividing streamline height, respectively. This indicates that the initial wave overturning occurs at the level of the largest gradient of streamline deflection. It is found that nonlinearity tends to accelerate the upslope flow, decelerate the flow near the mountain peak and top of the leeslope, and accelerate the flow near the internal jump.

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Yuanlong Li, Yuqing Wang, and Yanluan Lin

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The dynamics of eyewall contraction of tropical cyclones (TCs) has been revisited in this study based on both three-dimensional and axisymmetric simulations and dynamical diagnostics. Because eyewall contraction is closely related to the contraction of the radius of maximum wind (RMW), its dynamics is thus often studied by examining the RMW tendency in previous studies. Recently, Kieu and Stern et al. proposed two different frameworks to diagnose the RMW tendency but had different conclusions. In this study, the two frameworks are evaluated first based on theoretical analysis and idealized numerical simulations. It is shown that the framework of Kieu is a special case of the earlier framework of Willoughby et al. if the directional derivative is applied. An extension of Stern et al.’s approach not only can reproduce but also can predict the RMW tendency. A budget of the azimuthal-mean tangential wind tendency indicates that the contributions by radial and vertical advections to the RMW tendency vary with height. Namely, radial advection dominates the RMW contraction in the lower boundary layer, and vertical advection favors the RMW contraction in the upper boundary layer and lower troposphere. In addition to the curvature, the increase of the radial gradient of horizontal mixing (including the resolved eddy mixing in three dimensions) near the eyewall prohibits eyewall contraction in the lower boundary layer. Besides, the vertical mixing including surface friction also plays an important role in the cessation of eyewall contraction in the lower boundary layer.

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