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Li L.,Shenzhen National Climate Observatory | Fang X.,Beijing Municipal Climate Center | Zhang L.,Shenzhen National Climate Observatory
Huanjing Kexue Xuebao/Acta Scientiae Circumstantiae

Based on the computational fluid dynamics (CFD) method, the influences of air temperature stratification and ground heating on the flow field and the diffusion process in street canyon are studied. Aiming at two kinds of street canyons with depth-width ratios of 0.5 and 1.0, respectively, 18 numerical sensitivity tests are performed. The results of the numerical tests show that the ground heating plays a more important role than the air temperature stratification in the pollutant diffusion process in street canyon. The analysis shows that ground heating is able to remarkably promote the diffusion capability in the street canyon. When ground heating exists, the variations of the flow structure, the air exchange rate and the turbulence intensity generally tend to be favorable in removing the pollutant in the street canyon. Even under the condition of stable stratification, thermal circulation induced by ground heating is also able to transport and disperse the pollutant out of the street canyon, and consequently leads to the dropping of the pollutant concentration near ground. Source

Xing P.,Tsinghua University | Xing P.,Beijing Municipal Climate Center | Chen X.,Tsinghua University | Chen X.,Joint Center for Global Change Studies | And 8 more authors.

Large-scale climate history of the past millennium reconstructed solely from tree-ring data is prone to underestimate the amplitude of low-frequency variability. In this paper, we aimed at solving this problem by utilizing a novel method termed "MDVM", which was a combination of the ensemble empirical mode decomposition (EEMD) and variance matching techniques. We compiled a set of 211 tree-ring records from the extratropical Northern Hemisphere (30-90°N) in an effort to develop a new reconstruction of the annual mean temperature by the MDVM method. Among these dataset, a number of 126 records were screened out to reconstruct temperature variability longer than decadal scale for the period 850-2000 AD. The MDVM reconstruction depicted significant low-frequency variability in the past millennium with evident Medieval Warm Period (MWP) over the interval 950-1150 AD and pronounced Little Ice Age (LIA) cumulating in 1450-1850 AD. In the context of 1150-year reconstruction, the accelerating warming in 20th century was likely unprecedented, and the coldest decades appeared in the 1640s, 1600s and 1580s, whereas the warmest decades occurred in the 1990s, 1940s and 1930s. Additionally, the MDVM reconstruction covaried broadly with changes in natural radiative forcing, and especially showed distinct footprints of multiple volcanic eruptions in the last millennium. Comparisons of our results with previous reconstructions and model simulations showed the efficiency of the MDVM method on capturing low-frequency variability, particularly much colder signals of the LIA relative to the reference period. Our results demonstrated that the MDVM method has advantages in studying large-scale and low-frequency climate signals using pure tree-ring data. © 2016 Xing et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Source

Zhang Y.-Z.,Beijing Normal University | Zhang Y.-Z.,Institute of Urban Meteorology | Miao S.-G.,Institute of Urban Meteorology | Dai Y.-J.,Beijing Normal University | Liu Y.-H.,Beijing Municipal Climate Center
Chinese Journal of Geophysics (Acta Geophysica Sinica)

Weather Research and Forecasting (WRF) modeling system coupled with Noah land surface model and single layer urban canopy model was improved and optimized. Performances of the modeling system before and after optmization were examined, and diurnal variation of features of urban boundary layer was analyzed, through the numerical study of clear day during 6th and 7th Aug, 2010 over Beijing area. Furthermore, the impacts of urban underlying surface in Beijing and Tianjin on sea breeze were studied through two groups of model sensitivity tests using optimized modeling system. The results show that the optimization can evidently improve the performance of modeling system in the case study in the article. The modeling system can simulate the features of diurnal variation of summer boundary layer in Beijing generally well. Accurate geography data, such as land use and land cover classification information, play a very important role in improving the accuracy of model results. Beijing city and Tianjin city have obvious impacts on the evolution of sea breeze. As the sea breeze penetrating inland, the cities can pose an obstacle to the landward advance of sea breeze current, strengthen the horizontal convergence of wind and vertical updraft. The urban underlying surface in Beijing can also produce faster penetration with stronger intensity of sea breeze before it reaching the city, and increase the lifecycle and inland penetration distance of sea breeze after it passing over the city. Source

Li L.,Shenzhen National Climate Observatory | Zhang L.-J.,Shenzhen National Climate Observatory | Zhang N.,Nanjing University | Hu F.,CAS Institute of Atmospheric Physics | And 3 more authors.
Wind and Structures, An International Journal

A meteorological model, RAMS, and a commercial computational fluid dynamics (CFD) model, FLUENT are combined as a one-way off-line nested modeling system, namely, RAMS/FLUENT system. The system is experimentally applied in the wind simulation over a complex terrain, with which numerical simulations of wind field over Foyeding weather station located in the northwest mountainous area of Beijing metropolis are performed. The results show that the method of combining a meteorological model and a CFD model as a modeling system is reasonable. In RAMS/FLUENT system, more realistic boundary conditions are provided for FLUENT rather than idealized vertical wind profiles, and the finite volume method (FVM) of FLUENT ensures the capability of the modeling system on describing complex terrain in the simulation. Thus, RAMS/FLUENT can provide fine-scale realistic wind data over complex terrains. Source

Xu Y.,Nanjing University of Information Science and Technology | Xu Y.,Beijing Institute of Urban Meteorology | Liu Y.,Beijing Municipal Climate Center
Geographical Research

Urban heat island (UHI) effect is an important impact factor of the regional climate and ecological environment. How to observe and analyse the spatial distribution of UHI has become an important issue of urban environmental research. In this paper, the near-surface air temperature of Beijing was derived based on the Landsat/TM satellite imagery on 26 July 2011 to study the near-surface UHI. A statistical model at 195-m window size was established to estimate the air temperature, using land surface temperature, normalized difference vegetation index, altitude, and surface albedo as independent variables. The mean absolute error (MAE) of the model was 0.87°C, and the R2 was 0.66, indicating that the method can be used to effectively estimate the air temperature. The air temperature distribution obtained from remote sensing revealed that the UHI effect in Beijing was very significant and showed a concentrated pattern. The heat island intensity was stronger in the southern part than in the northern part of the city. In addition, the relationship between the air temperature and impervious surfaces was analysed. The air temperature increased with increasing impervious surface coverage, and the rate of change depended on the impervious surface coverage. When the impervious surface coverage was below 40 per cent, the temperature increased rapidly with increasing impervious surface coverage, and when the impervious surface coverage was above 40 per cent, the temperature increased slowly. This study provides a new approach to monitor near-surface UHI and reveals its relationship with impervious surface, providing a scientific reference for urban planning and environmental assessment. © 2014 Institute of Australian Geographers. Source

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