Search Results

You are looking at 1 - 10 of 111 items for

  • Author or Editor: Wei Zhang x
  • All content x
Clear All Modify Search
Junhong Wei and Fuqing Zhang

Abstract

A series of cloud-permitting simulations with the Weather Research and Forecast model (WRF) are performed to study the characteristics and source mechanisms of mesoscale gravity waves in moist baroclinic jet–front systems with varying degrees of convective instability. These idealized experiments are initialized with the same baroclinic jet but with different initial moisture content, which produce different life cycles of moist baroclinic waves, to investigate the relative roles of moist processes and baroclinicity in the generation and propagation of mesoscale gravity waves. The dry experiment with no moisture or convection simulates gravity waves that are consistent with past modeling studies. An experiment with a small amount of moisture produces similar baroclinic life cycles to the dry experiment but with the introduction of weak convective instability. Subsequent initiation of convection, although weak, may considerably amplify the gravity waves that are propagating away from the upper-level jet exit region crossing the ridge to the jet entrance region. The weak convection also generates a new wave mode of shorter-scale wave packets that are believed to interact with, strengthen, and modify the dry gravity wave modes. Further increase of the moisture content (up to 5 times) leads to strong convective instability and vigorous moist convection. Besides a faster-growing moist baroclinic wave, the convectively generated gravity waves emerge much earlier, are more prevalent, and are larger in amplitude; they are fully coupled with, and hardly separable from, the dry gravity wave modes under the complex background moist baroclinic waves.

Full access
Wei Zhong and Da-Lin Zhang

Abstract

In this study, the linearized, f-plane, shallow-water equations are discretized into a matrix eigenvalue problem to examine the full spectrum of free waves on barotropic (monopolar and hollow) vortices. A typical wave spectrum for weak vortices shows a continuous range between zero and an advective frequency associated with vortex Rossby waves (VRWs) and two discrete ranges at both sides associated with inertio-gravity waves (IGWs). However, when the vortex intensity reaches a critical value, higher-frequency waves will be “red shifted” into the continuous spectrum, while low-frequency waves will be “violet shifted” into the discrete spectrum, leading to the emergence of mixed vortex Rossby–inertio-gravity waves (VRIGWs).

Results show significant (little) radial wavelike structures of perturbation variables for IGWs (VRWs) with greater (much smaller) divergence than vorticity and the hybrid IGW–VRW radial structures with equal amplitudes of vorticity and divergence for mixed VRIGWs. In addition, VRWs only occur within a critical radius at which the perturbation azimuthal velocity is discontinuous. As the azimuthal wavenumber increases, lower-frequency waves tend to exhibit more mixed-wave characteristics, whereas higher-frequency waves will be more of the IGW type. Two-dimensional wave solutions show rapid outward energy dispersion of IGWs and slower dispersion of VRWs and mixed VRIGWs in the core region. These solutions are shown to resemble the previous analytical solutions, except for certain structural differences caused by the critical radius. It is concluded that mixed VRIGWs should be common in the eyewall and spiral rainbands of intense tropical cyclones. Some different wave behaviors associated with the monopolar and hollow vortices are also discussed.

Full access
Yi Zhang and Wei-Chyung Wang

Abstract

Two 100-yr equilibrium simulations from the NCAR Community Climate Model coupled to a nondynamic slab ocean are used to investigate the activity of northern winter extratropical cyclones and anticyclones under a greenhouse warming scenario. The first simulation uses the 1990 observed CO2, CH4, N2O, CFC-11, and CFC-12 concentrations, and the second adopts the year 2050 concentrations according to the Intergovernmental Panel on Climate Change business-as-usual scenario. Variables that describe the characteristic properties of the cyclone-scale eddies, such as surface cyclone and anticyclone frequency and the bandpassed root-mean-square of 500-hPa geopotential height, along with the Eady growth rate maximum, form a framework for the analysis of the cyclone and anticyclone activity.

Objective criteria are developed for identifying cyclone and anticyclone occurrences based on the 1000-hPa geopotential height and vorticity fields and tested using ECMWF analyses. The potential changes of the eddy activity under the greenhouse warming climate are then examined. Results indicate that the activity of cyclone-scale eddies decreases under the greenhouse warming scenario. This is not only reflected in the surface cyclone and anticyclone frequency and in the bandpassed rms of 500-hPa geopotential height, but is also discerned from the Eady growth rate maximum. Based on the analysis, three different physical mechanisms responsible for the decreased eddy activity are discussed: 1) a decrease of the extratropical meridional temperature gradient from the surface to the midtroposphere, 2) a reduction in the land–sea thermal contrast in the east coastal regions of the Asian and North American continents, and 3) an increase in the eddy meridional latent heat fluxes. Uncertainties in the results related to the limitations of the model and the model equilibrium simulations are discussed.

Full access
Wei Wei, Renhe Zhang, Min Wen, and Song Yang

Abstract

Diagnostic analyses are performed to investigate the relationship between the upper-level Asian westerly jet stream (AWJS) and the associated rainfall pattern over the AWJS region in boreal summer on interannual time scales. Results show an out-of-phase rainfall variation over central Asia (CA) and north China (NC), which is closely related to the southeast–northwest (SE–NW) shift of the AWJS. The physical mechanisms on the relationship between the AWJS and the rainfall pattern are revealed by exploring the effects of the Indian summer monsoon (ISM) and the South Asian high (SAH). It is found that the SE–NW shifts of the AWJS and SAH associated with the ISM lead to the anomalous circulations over the midlatitudes in the AWJS region and cause rainfall anomalies over CA and NC. A weak ISM results in a southeastward shift of SAH, which is responsible for a southeastward shift of the AWJS. The anomalous atmospheric circulation associated with the southeastward located SAH produces anomalous updrafts (downdrafts) over the western (eastern) AWJS region, resulting in increased rainfall over CA and decreased rainfall over NC. It is proposed that the upper-level system SAH plays a crucial role in the teleconnection among the summer rainfall over the midlatitude AWJS region and ISM region.

Full access
Da-Lin Zhang and Wei-Zhong Zheng

Abstract

Although most of the planetary boundary layer (PBL) parameterizations have demonstrated the capability to reproduce many meteorological phenomena in the lowest few kilometers, little attention has been paid to the prediction of the diurnal cycles of surface wind speed (V SFC) in relation to surface temperature (T SFC). In this study, the performance of five widely used PBL parameterizations in reproducing the diurnal cycles of V SFC and T SFC is evaluated using the 3-day mesoscale simulations of summertime weak-gradient flows over the central United States where little organized convection and topographical forcing were present. The time series of area-averaged V SFC and T SFC, as well as the vertical wind and thermal profiles from the five sensitivity simulations, are compared with hourly surface observations and other available data. The hourly surface observations reveal that the diurnal cycles of V SFC are in phase (but surface wind directions are 5–6 h out of phase) with those of T SFC. It is shown that both V SFC and T SFC are very sensitive to the PBL parameterizations, given the identical conditions for all of the other model parameters. It is found that all five of the PBL schemes can reproduce the diurnal phases of T SFC (and wind directions), albeit with different amplitudes. However, all of the schemes underestimate the strength of V SFC during the daytime, and most of them overestimate it at night. Moreover, some PBL schemes produce pronounced phase errors in V SFC or substantially weak V SFC all of the time, despite their well-simulated diurnal cycle of T SFC. The results indicate that a perfect simulation of the diurnal T SFC cycle (and the thermal structures above) does not guarantee the reproduction of the diurnal cycles of V SFC. The final outcome would depend on how various physical processes, such as the vertical turbulent exchanges of the mass and momentum under different stability conditions, are parameterized. Because the upper portion of the PBL flow is often nearly opposite in phase to V SFC under weak-gradient conditions, the results have significant implications for the predictability of diurnal precipitation and the studies of air quality, wind energy, and other environmental problems.

Full access
Qing Yan, Ting Wei, and Zhongshi Zhang

Abstract

Simulations of past warm climate provide an opportunity to better understand how the climate system may respond to increased greenhouse gas emissions. Using the ~25-km-resolution Community Atmosphere Model, version 4 (CAM4), we examine climate change over China in the Late Pliocene warm period (3.264–3.025 Ma) and further explore the influences of different sea surface temperature (SST) forcings and model horizontal resolutions. Initial evaluation shows that the high-resolution CAM4 performs well in capturing the climatological distribution of present-day temperature, precipitation, and low-level monsoon circulations over China. Based on the standard Pliocene Research, Interpretation and Synoptic Mapping (version 4; PRISM4) boundary conditions, CAM4 predicts an increase of annual mean temperature by ~0.5°C over China in the Late Pliocene relative to the preindustrial era, with the greatest warming in northwest China but cooling in southwest China. Enhanced annual mean precipitation is observed in the Late Pliocene over most of China except for northwest China where precipitation is decreased. The East Asian summer (winter) monsoon is intensified (weakened) in the Late Pliocene as suggested by geological evidence, which is attributed to the enhanced (reduced) land–sea thermal contrast. The East Asian monsoon domain exhibits a northwestward expansion in the Late Pliocene, especially over the Tibetan Plateau. Additionally, our results indicate that the modeled climate change is sensitive to the Late Pliocene SST forcings and model resolution. Particularly, different SST forcings [PRISM4-based vs Pliocene Model Intercomparison Project (PlioMIP)-based SSTs] affect the modeled phase change of summer monsoon and the associated precipitation change, while model resolution (~25 vs 400 km) mainly impacts precipitation change.

Full access
Xi Hu, Xiujuan Zhang, and Jiuchang Wei

Abstract

Hazard warning is vital in disaster management. The rapid development of social media allows warning producers and receivers to exchange warning messages effectively and sufficiently. This study investigates the factors that influence public attention to natural hazard warning information on social media. Drawing from the protective action decision model and framing theory, this study classifies antecedents into three groups, namely, hazard information, publisher’s/reader’s characteristics, and frame setting. To test the hypotheses empirically, we select Sina Weibo, the leading social network in China, as the research context. From this platform, 3452 warning messages issued by authorities in the target area are collected. We code each message based on its attributes that are related to our study for linear regression analyses. Results show that all the factors related to publisher’s/reader’s characteristics exert significant effects on public attention. However, the affected range indicated by a warning message and the formality of the message’s language are not significantly related to public attention to the message.

Full access
Jie Cao, Wei-kang Zhang, and Yun Tao

Abstract

This paper describes the study of the relationship between the thermal configuration of the Bay of Bengal (BOB)–Tibetan Plateau (TP) region and the precipitation anomaly in Yunnan, a province in China, in May using ERA-Interim data and precipitation data for May from 125 meteorological stations across Yunnan for 1979–2014. Results from the analysis indicate that the interannual variability of May precipitation in Yunnan is significantly modulated by the BOB–TP thermal configuration. Model runs with a linear baroclinic model suggest physical consistency. The thermal conditions over the BOB mainly impact the May precipitation anomaly in Yunnan via changes in water vapor transport from the eastern BOB northeastward to southwestern Yunnan. The second factor influencing precipitation anomalies relates to the characteristics and variability of cold air transport from the TP to northeastern Yunnan. When the BOB (the TP) is occupied by positive (negative) diabatic heating, a thermal gradient with a warmer (colder) center over the BOB and a colder (warmer) center over the TP is established, and more-than-normal (less than normal) precipitation in Yunnan will occur in May. This relationship can persist from April to the following May to some extent; therefore, the BOB–TP thermal configuration in April could be used to forecast May precipitation in Yunnan.

Full access
Si Gao, Zhifan Chen, and Wei Zhang

Abstract

This study examines the impacts of tropical North Atlantic (TNA) sea surface temperature anomaly (SSTA) on western North Pacific (WNP) landfalling tropical cyclones (TCs). The authors find that TNA SSTA has significant negative correlations with the frequency of TCs making landfall in China, Vietnam, the Korean Peninsula and Japan, and the entirety of East Asia. TNA SSTA influences the frequency of TC landfalls in these regions by regulating TC genesis location and frequency associated with modulated environmental conditions. During cold TNA SST years, larger low-level relative vorticity and weaker vertical wind shear lead to more TC formations over the South China Sea (SCS) and western Philippine Sea (WPS), and larger low-level relative vorticity, higher midlevel relative humidity, and weaker vertical wind shear result in more TC formations over the eastern part of WNP (EWNP). More TCs forming over different regions are important for more TC landfalls in Vietnam (mainly forming over the SCS and WPS), south China (predominantly forming over the SCS), Taiwan (mostly forming over the WPS), and the Korean Peninsula and Japan (forming over the WPS and EWNP). Tracks of these landfalling TCs basically follow the mean steering flow in spite of different directions of steering flow anomalies in the vicinity. The modulation of large-scale environments by TNA SSTA may be through two possible pathways proposed in previous studies: the Indian Ocean relaying effect and the subtropical eastern Pacific relaying effect. The results of this study suggest that TNA SSTA is a potential predictor for the frequency of TCs making landfall in China, Vietnam, the Korean Peninsula and Japan, and the entirety of East Asia.

Full access
Jun Zhang, Wei Zheng, and Matthew J. Menne

Abstract

In this paper, the authors present a Bayes factor model for detecting undocumented artificial discontinuities in a network of temperature series. First, they generate multiple difference series for each station with the pairwise comparison approach. Next, they treat the detection problem as a Bayesian model selection problem and use Bayes factors to calculate the posterior probabilities of the discontinuities and estimate their locations in time and space. The model can be applied to large climate networks and realistic temperature series with missing data. The effectiveness of the model is illustrated with two realistic large-scale simulations and four sensitivity analyses. Results from applying the algorithm to observed monthly temperature data from the conterminous United States are also briefly discussed in the context of what is currently known about the nature of biases in the U.S. surface temperature record.

Full access