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Hui Wu

Abstract

Pressure anomaly set by the open ocean affects the dynamic topography and associated circulation over the continental shelf, which is explored here on a linearized β-plane arrested topographic wave framework that considers the variation in Coriolis parameter with latitude. It was found that on a meridional shelf, a nondimensional parameter Peβ, termed the β Péclet number, signifies the characteristics of open ocean–shelf interaction. The PeβD β/α is determined by the ratio of long-wave-limit planetary to topographic Rossby wave speeds, i.e., the β drift D β, and the linear Ekman number α. On the western boundary shelf, due to the westward planetary Rossby wave, open ocean pressure propagates shoreward as Peβ > 1, and shelf circulation peaks where Peβ drops to 1. At this location, the planetary β effect is balanced by the bottom friction. The Peβ = 1 must occur either on the shelf or on the coastal wall when Peβ > 1 is observed at the shelf edge. On the eastern boundary shelf, however, Peβ < 0, the pressure anomaly is removed from the shelf, and hence the inductive circulation decays rapidly from the shelf edge. This β effect is robust on gently sloping meridional shelves. For zonal shelves, the planetary β increases the effective bottom slope on the northern boundary shelf but decreases it on the southern one, in a sense of potential vorticity conservation. However, this effect could be less significant in reality, given the complex dynamics involved. The above mechanism can explain the dynamics driving the Taiwan Warm Current in the East China Sea and its bifurcation around 28°N.

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Mu Mu, V. Vladimirov, and Yong-Hui Wu

Abstract

This paper is concerned with nonlinear symmetric stability problems. For the moist, adiabatic (saturated) system, the authors utilize the ECM (energy–Casimir method) to establish nonlinear stability criteria, which extends the previous work from the dry atmosphere to the moist case and demonstrates the complexity related to the moist symmetric instability problem.

For the nonhydrostatic, Boussinesq equations on an f plane with the northward component of the earth rotation f = 2Ω cosϕ, which has been utilized to show the importance of f term in the mesoscale linear symmetric instability problem, both ECM and the ELM (energy–Lagrange method) are employed to study the “zonal” and“meridional” nonlinear symmetric stability problems. In both cases, the nonlinear stability of the basic states are obtained if the potential vorticity and the vertical component of absolute vorticity of the basic state are positive (for f > 0). In the zonal case, the potential vorticity depends upon f explicitly, and this shows the influence of the f term to the nonlinear symmetric stability. In the meridional case the potential vorticity is independent of the f term, which implies that the f term plays no role in the nonlinear symmetric stability.

The upper bounds on the disturbance field to the nonlinearly stable basic state are established, which consists of its initial value multiplied by an amplification factor independent of time. The ELM proposed by Xu is simplifed to be more concise and understandable.

The applicable capacity of ECM and ELM is investigated. Both methods are applicable to the symmetric nonlinear stability problem of dry atmosphere, but only ECM has application to the moist problem.

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Hui Wang, Chun-Chieh Wu, and Yuqing Wang

Abstract

The secondary eyewall formation (SEF) in an idealized simulation of a tropical cyclone (TC) is examined from the perspective of both the balanced and unbalanced dynamics and through the tangential wind (Vt) budget analysis. It is found that the expansion of the azimuthal-mean Vt above the boundary layer occurs prior to the development of radial moisture convergence in the boundary layer. The Vt expansion results primarily from the inward angular momentum transport by the mid- to lower-tropospheric inflow induced by both convective and stratiform heating in the spiral rainbands. In response to the Vt broadening is the development of radial inflow convergence and the supergradient flow near the top of the inflow boundary layer. Results from the Vt budget analysis show that the combined effect of the mean advection and the surface friction is to spin down Vt in the boundary layer, while the eddy processes (eddy radial and vertical advection) contribute positively to the spinup of Vt in the SEF region in the boundary layer. Therefore, eddies play an important role in the spinup of Vt in the boundary layer during SEF. The balanced Sawyer–Eliassen solution can well capture the secondary circulation in the full-physics model simulation. The radial inflow diagnosed from the Sawyer–Eliassen equation is shown to spin up Vt and maintain the vortex above the boundary layer. However, the axisymmetric balanced dynamics cannot explain the spinup of Vt in the boundary layer, which results mainly from the eddy processes.

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Jinghua Chen, Xiaoqing Wu, Yan Yin, Qian Huang, and Hui Xiao

Abstract

Constrained by ERA-Interim, a cloud-resolving model is employed to characterize cloud systems over the Tibetan Plateau (TP) and east China. The authors focus on analyzing the role of different physical processes on cloud macro- and microscale properties of the cloud systems, especially convective cloud systems between east China and the TP. It is found that convective clouds over the TP are thinner than over east China. This difference is also reflected in the albedo at the top of the atmosphere, where smaller albedos are found for the clouds over the TP. Furthermore, the lifetimes of the deep cloud systems over the TP are shorter than over east China. For the entire simulated period, the latent heat released by phase transitions contributes the most to the total heating and moisture budget, followed by eddy transport over all regions. In addition, radiative heating also plays a nonnegligible role in the total heating effects over the TP. These results also suggest that the influence of ice phase processes is more important over the TP than east China, especially during deep convective periods. Affected by strong surface heat flux, the cloud-top height of convective clouds over the TP exhibits a diurnal cycle, leading to a diurnal cycle of rainfall.

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Jinghua Chen, Xiaoqing Wu, Yan Yin, and Hui Xiao

Abstract

In this study, the summer clouds and precipitation over eastern China and the Tibetan Plateau (TP) are examined by analyzing the satellite observations and the apparent heat source Q 1 and moisture sink Q 2 computed from the NCEP–NCAR reanalysis. The vertically integrated [Q 1] and [Q 2] and precipitation have similar interannual variations in eastern China, revealing the important contribution from the condensation process. This relationship is weakened in east TP (ETP) because of the contribution of the surface sensible heat flux. In west TP (WTP), [Q 1] is negatively correlated with precipitation because the surface sensible heat flux can be sharply weakened by the decrease of ground–air temperature difference due to rainfall. High clouds and deep convection are closely related with [Q 1] and [Q 2] over eastern China and ETP, while middle clouds and nimbostratus are responsible for the condensation over WTP. During the rainy summer, more convective rains and stronger upward motion appear in eastern China. Greater Q 1 and Q 2 and stronger upward motion present over ETP, while weaker Q 1 and upward motion are observed over WTP in the rainy summer when compared to the dry summer. The cloud-water path over eastern China positively correlates with [Q 1] and [Q 2] over ETP. The deep convection over eastern China also positively correlates with the convection over ETP. These correlations suggest that moisture due to the evaporation of cloud water in anvil clouds detrained from the deep convection over ETP can be transported downstream and benefit the development of convection over eastern China.

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Xianhua Wu, Zhe Xu, Hui Liu, Ji Guo, and Lei Zhou

Abstract

To investigate the general principle of the impact of tropical cyclones on employment, explore the reason for the divergence among existing research conclusions, and put forward some suggestions for post-disaster reconstruction, this paper employs meta-regression analysis to study the impacts of tropical cyclones on the quantity of labor employed and employee remuneration from four aspects: industry dimension, time dimension, income dimension, and tropical cyclone intensity. The results are as follows: 1) Tropical cyclones create an impact on the intensity of changes in employment remuneration in the primary industry, and the impact in the secondary industry is greater than that in the tertiary industry. 2) In the short term, the impact of tropical cyclones on employment is negative and the impact intensity is strong, whereas in the medium and long terms, the impact is positive and the intensity of impact decreases. 3) Although tropical cyclones increase the quantity of labor employed from low-income groups, they decrease their employment remuneration. In addition, the impact of disasters on the number of employed high-income groups is relatively small compared to that of low-income groups. 4) A higher category of tropical cyclone results in a greater positive impact on the employment of labor force. Accordingly, the following suggestions are made: 1) The government should issue corresponding policies to provide “temporary disaster subsidies” for disaster-stricken low-income groups. 2) Insurance companies should introduce commercial insurance concerning “post-disaster employment” for employers to purchase before any disaster occurs so as to offer disaster-stricken employees compensation.

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Xianhua Wu, Lei Zhou, Ji Guo, and Hui Liu

Abstract

What impacts do typhoons have on local labor markets? Few empirical researches have been conducted in China. By collecting the data of 23 quarters (3-month intervals) of Guangdong province from 2009 to 2014 and using the generalized method of moments (GMM), this paper analyzes the impacts of typhoons on labor markets from the perspectives of general effect, regional effect, intensity effect, and time effect. In addition, a comparative analysis is carried out between this study and similar studies of developed countries. The results show that 1) massive typhoons resulted in a 12.5% increase in employment but did not have a significant impact on Guangdong’s per capita employee remuneration, and 2) there are periodic features to typhoons’ impacts on employment. Typhoons influence employment in a four-quarter cycle. In the quarter affected by a typhoon, the first quarter, the number of employees increased by 17.4%. The quantity of labor employed in the subsequent two quarters shows no significant change. In the last quarter, the number of employed people decreases by 17.0%, which returns to predisaster levels. Additionally, 3) the results of this study are different from those of studies involving developed countries, which may be caused by the distinctiveness of China’s labor market. Finally, conclusions and corresponding suggestions are presented.

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Xiangyu Li, W. Rockwell Geyer, Jianrong Zhu, and Hui Wu

Abstract

The roles of straining and dissipation in controlling stratification are derived analytically using a vertical salinity variance method. Stratification is produced by converting horizontal variance to vertical variance via straining, that is, differential advection of horizontal salinity gradients, and stratification is destroyed by the dissipation of vertical variance through turbulent mixing. A numerical model is applied to the Changjiang estuary in order to demonstrate the salinity variance balance and how it reveals the factors controlling stratification. The variance analysis reveals that dissipation reaches its maximum during spring tide in the Changjiang estuary, leading to the lowest stratification. Stratification increases from spring tide to neap tide because of the increasing excess of straining over dissipation. Throughout the spring–neap tidal cycle, straining is almost always larger than dissipation, indicating a net excess of production of vertical variance relative to dissipation. This excess is balanced on average by advection, which exports vertical variance out of the estuarine region into the plume. During neap tide, tidal straining shows a general tendency of destratification during the flood tide and restratification during ebb, consistent with the one-dimensional theory of tidal straining. During spring tide, however, positive straining occurs during flood because of the strong baroclinicity induced by the intensified horizontal salinity gradient. These results indicate that the salinity variance method provides a valuable approach for examining the spatial and temporal variability of stratification in estuaries and coastal environments.

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Min-Hui Lo, Wen-Ying Wu, Lois Iping Tang, Dongryeol Ryu, Mehnaz Rashid, and Ren-Jie Wu

Abstract

One of the critical components in understanding the climate system is the interaction between the land and the atmosphere. Whereas previous studies on land–atmosphere coupling mostly focus on its spatial hotspots, we explore the temporal evolution of land surface coupling strength (LCS) during a large-scale flood event in a semiarid region in northern Australia. The LCS indicates the relationship between soil moisture and latent heat flux, and the spatiotemporal variability in precipitation and soil water strongly affects the variability of LCS. The LCS is usually positive in the semiarid climate, where evapotranspiration (ET) occurs under the soil moisture–limited regime and thus increases with soil moisture. However, our analyses of combined land surface modeling and observational datasets show high temporal variability of LCS in the course of the extreme flood event followed by a drying period. The wet regions transferred the ET regime from the soil moisture–limited to the transition section, weakening the linear growth of ET with soil moisture, which resulted in the decline of LCS. The LCS remained weak until the flood retreated and the soil water approached the prestorm average state. Such temporal variation of the LCS has important implications for realistic parameterization of the land–atmosphere coupling and consequently improving subseasonal to seasonal climate forecast.

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Ting-Chi Wu, Hui Liu, Sharanya J. Majumdar, Christopher S. Velden, and Jeffrey L. Anderson

Abstract

The influence of assimilating enhanced atmospheric motion vectors (AMVs) on mesoscale analyses and forecasts of tropical cyclones (TC) is investigated. AMVs from the geostationary Multifunctional Transport Satellite (MTSAT) are processed by the Cooperative Institute for Meteorological Satellite Studies (CIMSS, University of Wisconsin–Madison) for the duration of Typhoon Sinlaku (2008), which included a rapid intensification phase and a slow, meandering track. The ensemble Kalman filter and the Weather Research and Forecasting Model are utilized within the Data Assimilation Research Testbed. In addition to conventional observations, three different groups of AMVs are assimilated in parallel experiments: CTL, the same dataset assimilated in the NCEP operational analysis; CIMSS(h), hourly datasets processed by CIMSS; and CIMSS(h+RS), the dataset including AMVs from the rapid-scan mode. With an order of magnitude more AMV data assimilated, the CIMSS(h) analyses exhibit a superior track, intensity, and structure to CTL analyses. The corresponding 3-day ensemble forecasts initialized with CIMSS(h) yield smaller track and intensity errors than those initialized with CTL. During the period when rapid-scan AMVs are available, the CIMSS(h+RS) analyses offer additional modifications to the TC and its environment. In contrast to many members in the ensemble forecasts initialized from the CTL and CIMSS(h) analyses that predict an erroneous landfall in China, the CIMSS(h+RS) members capture recurvature, albeit prematurely. The results demonstrate the promise of assimilating enhanced AMV data into regional TC models. Further studies to identify optimal strategies for assimilating integrated full-resolution multivariate data from satellites are under way.

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