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Hao Wang and Eugene S. Takle

Abstract

A neutral boundary layer nonhydrostatic numerical model is used to determine the characteristics of shelterbelt effects on mean wind direction and to study the processing causing wind rotation when air passes through a shelterbelt. The model uses a turbulence scheme that includes prognostic equations for turbulence kinetic energy and a master length scale proposed by Mellor and Yamada. The simulated results are in quantitative agreement with Nord's field measurements. The spatial variation of wind rotation and its dependence on incident angle and shelterbelt porosity is analysed. Dynamic processes of the wind rotation and its interactions with drag force and pressure perturbation are also discussed. It is concluded that shear of wind direction should be considered, along with shear of speed, in determining turbulent fluxes in the vicinity of a shelterbelt.

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Hao Wang and E. S. Takle

Abstract

The authors report results of a numerical model used to simulate wind and turbulence fields for porous, living shelterbelts with seven different cross-sectional shapes. The simulations are consistent with results of Woodruff and Zingg whose wind-tunnel study demonstrated that all shelterbelts with very different shapes have nearly identical reduction of wind and turbulence. The simulations also showed that the pressure-loss (resistance) coefficient for smooth-shaped or streamlined shelterbelts is significantly smaller than that for rectangle-shaped or triangle-shaped shelterbelts with a windward vertical side. However, the shelter effects are not proportional to the pressure-loss coefficient (drag). Analysis of the momentum budget demonstrated that in the near lee and in the far lee, both vertical advection and pressure gradient have opposite roles in the recovery of wind speed. This behavior, combined with differences in permeability, is the likely cause of reduced sensitivity of shelter effects to shelterbelt shape.

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Tingting Han, Huijun Wang, Xin Hao, and Shangfeng Li

Abstract

Northeast China (NEC) has sustained economic losses in recent years because of extreme precipitation events. Despite many efforts, it remains very difficult to predict these extreme events. In this study, we documented the characteristics of extreme precipitation days (EPD) over NEC and established a seasonal prediction model using a year-to-year increment (DY) approach. The results show that most of the EPD over NEC occurred during midsummer, along with large values concentrated over the Greater and Lesser Khingan Mountains and Changbai Mountain. Two variables—the preceding early spring soil moisture DY over central Asia and the sea surface temperature DY in the tropical Atlantic Ocean—were used to construct the statistical model to predict the EPD DY over NEC. These two factors influenced the EPD by modulating the moisture transport over NEC. Cross-validation tests for the period from 1962 to 2016 and independent hindcasts for the period from 1997 to 2016 indicated that the two variables gave good predictions of the EPD over NEC. The observed and predicted year-to-year increments in EPD were well correlated, with a correlation coefficient of 0.65 for the period from 1962 to 2016 in the cross-validation test. In addition, the EPD DY covaried coherently with the midsummer precipitation amount DY over NEC, and those two predictors also gave good predictions for the midsummer precipitation amount over NEC. The correlation coefficient is 0.68 between the observed and predicted year-to-year increment in the amount of midsummer precipitation from 1962 to 2016 in a cross-validation test.

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Hao-Yan Liu, Yuqing Wang, and Jian-Feng Gu

Abstract

This study investigates the intensity change of binary tropical cyclones (TCs) in idealized cloud-resolving simulations. Four simulations of binary interaction between two initially identical mature TCs of about 70 m s−1 with initial separation distance varying from 480 to 840 km are conducted in a quiescent f-plane environment. Results show that two identical TCs finally merge if their initial separation distance is within 600 km. The binary TCs presents two weakening stages (stages 1 and 3) with a quasi-steady evolution (stage 2) in between. Such intensity change of one TC is correlated with the upper-layer vertical wind shear (VWS) associated with the upper-level anticyclone (ULA) of the other TC. The potential temperature budget shows that eddy radial advection of potential temperature induced by large upper-layer VWS contributes to the weakening of the upper-level warm core and thereby the weakening of binary TCs in stage 1. In stage 2, the upper-layer VWS first weakens and then restrengthens with relatively weak magnitude, leading to a quasi-steady intensity evolution. In stage 3, due to the increasing upper-layer VWS, the nonmerging binary TCs weaken again until their separation distance exceeds the local Rossby radius of deformation of the ULA (about 1600 km), which can serve as a dynamical critical distance within which direct interaction can occur between two TCs. In the merging cases, the binary TCs weaken prior to merging because highly asymmetric structure develops as a result of strong horizontal deformation of the inner core. However, the merged system intensifies shortly after merging.

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Botao Zhou, Zunya Wang, Bo Sun, and Xin Hao

Abstract

Analyses of observation data from 1961 to 2014 by using the empirical orthogonal function (EOF) method indicate that the primary mode (a monosign pattern) of heavy snowfall over northern China in winter shows evident variations from a negative polarity to a positive polarity in the mid-1990s. Associated with this decadal change, the southward displacement of the polar front jet stream and northward shift of the subtropical jet stream in the upper troposphere are apparent. Accordingly, a negative height anomaly dominates the region from Lake Balkhash to Lake Baikal and a positive height anomaly occupies the midlatitudes of the North Pacific in the middle troposphere. Such anomalous patterns in the middle and high troposphere correspond approximately to the northern mode of the East Asian winter monsoon (EAWM) and may favor the interaction of cold air with moist airflows over northern China, which helps increase local heavy snowfall. Further investigation shows that the shift in the Atlantic multidecadal oscillation (AMO) from a cold phase to a warm phase in the 1990s may also play a role, through its linkage to the above atmospheric circulations with the aid of a downstream propagation of wave train that emanates from the Atlantic Ocean.

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Hao-Yan Liu, Yuqing Wang, Jing Xu, and Yihong Duan

Abstract

This study extends an earlier dynamical initialization (DI) scheme for tropical cyclones (TCs) to situations under the influence of terrain. When any terrain lower than 1 km exists between 150 and 450 km from the TC center, topographic variables are defined and a filtering algorithm is used to remove noise due to the presence of terrain before the vortex separation is conducted. When any terrain higher than 1 km exists between 150 and 300 km from the TC center, or the TC center is within 150 km of land, a semi-idealized integration without the terrain is conducted to spin up an axisymmetric TC vortex before the inclusion of the terrain and the merging of the TC vortex with the large-scale analysis field. In addition, a procedure for the vortex size/intensity adjustment is introduced to reduce the initial errors before the forecast run. Two sets of hindcasts, one without (CTRL run) and one with the new DI scheme (DI run), are conducted for nine TCs affected by terrain over the western North Pacific in 2015. Results show that the new DI scheme largely reduces the initial position and intensity errors. The 72-h position errors and the intensity errors up to the 36-h forecasts are smaller in DI runs than in CTRL runs and smaller than those from the HWRF forecasts for the same TCs as well. The new DI scheme is also shown to produce the TC inner-core structure and rainbands more consistent with satellite and radar observations.

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Hao Hu, Yihong Duan, Yuqing Wang, and Xinghai Zhang

Abstract

The diurnal variation of rainfall over China associated with landfalling tropical cyclones (TCs) is investigated using hourly rain gauge observations obtained from 2425 conventional meteorological stations in China. Records between 12 h prior to landfall and 12 h after landfall of 450 landfalling TCs in China from 1957 to 2014 are selected as samples. The harmonic analysis shows an obvious diurnal signal in TC rainfall with a rain-rate peak in the early morning and a minimum in the afternoon. The diurnal cycle in the outer region (between 400- and 900-km radii from the storm center) is found to be larger than in the core region (within 400 km of the storm center). This could be attributed to the effect of land on the inner core of the storms as the diurnal cycle is distinct in the core region well before landfall. As the result of this diurnal cycle, TCs making landfall at night tend to have cumulative precipitation, defined as the precipitation cumulated from the time at landfall to 12 h after landfall, about 30% larger than those making landfall around noon or afternoon. Moreover, the radial propagation of the diurnal cycle in TC rain rate, which has been a controversial phenomenon in some previous studies with remote sensing observations, was not present in this study that is based on rain gauge observations. Results also show that the diurnal signal has little dependence on the storm intensity 12 h prior to landfall.

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Ziyan Li, Shengzhi Huang, Shuai Zhou, Guoyong Leng, Dengfeng Liu, Qiang Huang, Hao Wang, Zhiming Han, and Hao Liang

Abstract

An understanding of the propagation process from meteorological to hydrological drought contributes to accurate prediction hydrological drought. However, the comprehensive influence of direct human activities involved in drought propagation is not well understood. In this study, an identification framework for drought propagation time was constructed to quantify the effects of direct human activities (i.e., reservoir storage, irrigation, industrial, domestic and agricultural water consumption) on drought propagation. Subsequently, the effects of meteorological and underlying surface factors on the drought propagation process were clarified based on random forest method, and the driving effect of teleconnection factors was investigated from top to bottom. The Wei River Basin (WRB), the largest tributary of the Yellow River Basin, was selected as the case study. Results disclosed that the propagation time from meteorological to hydrological drought was short in summer (approximately 2 months) and autumn (approximately 3 months), while long in spring (approximately 3–5 months) and winter (approximately 3–8 months), exhibiting noticeable spatial variability. In a changing environment, the propagation time generally showed a decreasing trend in spring and winter, while increasing propagation time was observed in summer and autumn. The dynamic drought propagation time of each season was all jointly controlled by the different extent variation of meteorological and underlying surface conditions, and the basic flow is all relatively significant throughout the period. Direct human activities had an effect on the seasonal dynamics of drought propagation, especially during the winter of the non-flood season, which alleviated the severity of winter hydrological drought to some extent, thus delaying the transmission of meteorological signals to hydrological systems. Sunspots, the dominant direct teleconnection driving force in the WRB, could indirectly affect the local precipitation and base flow in spring, autumn, and winter and interferes with the drought propagation process. This study sheds new insights into the attribution of drought propagation dynamics in a changing environment.

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Zhaosheng Wang, Mei Huang, Rong Wang, Shaoqiang Wang, Xiaodong Liu, Xiaoning Xie, Zhengjia Liu, He Gong, and Man Hao

Abstract

Vertically integrated atmospheric water vapor (VIWV) over the Indo-Pacific warm pool (IPWP) indirectly affects terrestrial vegetation growth (TVG) patterns through atmospheric water vapor transmission. However, their linkages and mechanisms are poorly understood. This study intends to understand the contributions of VIWVIPWP to TVG and the mechanisms by which VIWVIPWP impacts TVG. Combining monthly SST, VIWV, and NDVI data from 1982 to 2015, the linkage between VIWVIPWP and NDVI is investigated during April–June (AMJ). A strong correlation between VIWVIPWP and NDVI suggests that VIWVIPWP is an important factor affecting TVG. A composite analysis of VIWVIPWP anomalies and their relation to NDVI patterns shows that VIWVIPWP positively influences the NDVI of 68.1% of global green land during high-VIWVIPWP years but negatively influences 74.7% in low years. Corresponding to these results, during high-VIWVIPWP years, the warm and humid terrestrial climate conditions improved TVG by 9% and 2% in the Northern and Southern Hemispheres, respectively, but cold and dry conditions inhibited TVG for both hemispheres during the low years. Additionally, strong spatial correlations between VIWVIPWP and precipitation imply that VIWVIPWP affects the spatial–temporal pattern of precipitation. There is a stronger interaction between the Pacific north–south ridge and the two land troughs during high-VIWVIPWP years than during low-VIWVIPWP years. The zonally averaged wind at 850 hPa and VIWV results indicate that, during high-VIWVIPWP years, the enhanced wind from the ocean brings more atmospheric water vapor to land, increasing the probability of precipitation and resulting in moist climate conditions that promote AMJ vegetation growth. In brief, VIWVIPWP indirectly induces vegetation growth by affecting the distributions of terrestrial VIWV and precipitation.

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Jiapei Ma, Hongyi Li, Jian Wang, Xiaohua Hao, Donghang Shao, and Huajin Lei

Abstract

Gridded precipitation data are very important for hydrological and meteorological studies. However, gridded precipitation can exhibit significant statistical bias that needs to be corrected before application, especially in regions where high wind speeds, frequent snowfall, and sparse observation networks can induce significant uncertainties in the final gridded datasets. In this paper, we present a method for the production of gridded precipitation on the Tibetan Plateau (TP). This method reduces the statistical distribution error by correcting for wind-induced undercatch and optimizing the interpolation method. A gridded precipitation product constructed by this method was compared with previous products on the TP. The results show that undercatch correction is necessary for station data, which can reduce the distributional error by 30% at most. A thin-plate splines interpolation algorithm considering altitude as a covariate is helpful to reduce the statistical distributional error in general. Our method effectively inhibits the smoothing effect in gridded precipitation, and compared to previous products, results in a higher mean value, larger 98th percentile, and greater temporal variance. This study can help to improve the quality of gridded precipitation over the TP.

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