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Miao Y.,Peking University | Liu S.,Peking University | Chen B.,Peking University | Zhang B.,Peking University | And 3 more authors.
Advances in Atmospheric Sciences | Year: 2013

The airflow and dispersion of a pollutant in a complex urban area of Beijing, China, were numerically examined by coupling a Computational Fluid Dynamics (CFD) model with a mesoscale weather model. The models used were Open Source Field Operation and Manipulation (OpenFOAM) software package and Weather Research and Forecasting (WRF) model. OpenFOAM was firstly validated against wind-tunnel experiment data. Then, the WRF model was integrated for 42 h starting from 0800 LST 08 September 2009, and the coupled model was used to compute the flow fields at 1000 LST and 1400 LST 09 September 2009. During the WRF-simulated period, a high pressure system was dominant over the Beijing area. The WRF-simulated local circulations were characterized by mountain valley winds, which matched well with observations. Results from the coupled model simulation demonstrated that the airflows around actual buildings were quite different from the ambient wind on the boundary provided by the WRF model, and the pollutant dispersion pattern was complicated under the influence of buildings. A higher concentration level of the pollutant near the surface was found in both the step-down and step-up notches, but the reason for this higher level in each configurations was different: in the former, it was caused by weaker vertical flow, while in the latter it was caused by a downward-shifted vortex. Overall, the results of this study suggest that the coupled WRF-OpenFOAM model is an important tool that can be used for studying and predicting urban flow and dispersions in densely built-up areas. © 2013 Chinese National Committee for International Association of Meteorology and Atmospheric Sciences, Institute of Atmospheric Physics, Science Press and Springer-Verlag Berlin Heidelberg. Source

Yin S.,Beijing Normal University | Yin S.,Beijing Climate Center | Li W.,Beijing Climate Center | Chen D.,Gothenburg University | And 2 more authors.
Advances in Atmospheric Sciences | Year: 2011

The present study examined the diurnal variations of summer precipitation in the Beijing area by using subdaily precipitation and wind observations. A combined effect of topography and urbanization on the characteristics of diurnal variations was suggested. It was shown that stations located in the plain area exhibited typical night rain peaks, whereas those in the mountainous area exhibited clear afternoon peaks of precipitation diurnal variations. The precipitation peaks were associated with wind fields around the Beijing area, which were found to be highly modulated by mountain-valley circulation and urban-country circulation. The lower-tropospheric wind exhibited a clear diurnal shift in its direction from north at 0800 LST to south at 2000 LST, which reflected mountain-valley circulation. The transitions from valley to mountain wind and the opposite generally happened after sunset and sunrise, respectively, and both occurred earlier for the stations located closer to mountains. By comparing the diurnal variations of precipitation at stations in a northeast suburb, an urban area, and a southwest suburb, it was revealed that the northeast suburb group had the highest normalized rainfall frequency, but the southwest group had the lowest from late afternoon to late evening. On the contrary, in the early morning from about 0200 to 1000 LST, the southwest group and urban group had the highest normalized rainfall frequency. This pattern might originate from the combined effects of mountain-valley topography and urbanization. © 2011 Chinese National Committee for International Association of Meteorology and Atmospheric Sciences, Institute of Atmospheric Physics, Science Press and Springer-Verlag Berlin Heidelberg. Source

Wang J.,Beijing Regional Climate Center | He L.,Tianjin Climate Center | Zhang X.,Harbin Meteorological Bureau
Dili Xuebao/Acta Geographica Sinica | Year: 2015

Using daily snowfall observations (1961-2012) of 227 meteorological observation stations which are treated by a series of climatic statistical methods, analysis is performed on the temporal and spatial characteristics of winter snowfall in the agri-pasture transitional zone of North China and its relations with circulation factors. The result shows that the high value center of snowfall is located in the northeast of Inner Mongolia in early winter, and then it moves to the southern part of North China at the end of winter and the beginning of spring. The periods of the 1960s and 1970s witnessed more snowfalls at all levels than normal, with high value centers moving eastward from West Inner Mongolia (in the 1960s) to most parts of Hebei and Shanxi provinces (in the 1970s). Since the 2000s, heavy snowfalls across North China have been the most significant in the north of Shanxi and Hebei provinces, with Hulunbuir coming next. Regarding variations of snowfall frequencies, there is a decline in the frequency of heavy snowfalls in different regions, with the most significant decrease occurring in Hebei province and the south of Shanxi province (named as the VI region in this paper). During the period with fewer snowfalls in North China (in the 1980s and 1990s), vapor transport was weak, moving from northwest to southeast; whereas during the years of strong vapor transport, the water transport in the past decade moved from southeast to northwest. The inter-decadal snowfall has a negative correlation with the air temperature and Arctic Oscillation (AO) index, whereas the snowfall at moderate and high levels is positively correlated with the air temperature and AO index in high latitude areas like the Greater Hinggan and Taihang Mountains in northeastern Inner Mongolia. © 2015, Science Press. All right reserved. Source

Wang J.,Beijing Regional Climate Center | Zou L.,China Meteorological Administration Training Center | Zhao C.,Liaoning Climate Center | Lou D.,Qiqihar Meteorological Bureau
Journal of Geographical Sciences | Year: 2011

Based on 1961-2005 observed winter precipitation data in Northeast China, the temporal and spatial variations of snow concentration degree (SCD) and snow concentration period (SCP), together with the circulation characteristics when there is a higher SCD, are computed and analyzed. Results show that SCD in Northeast China presents a yearly rising tendency and SCP decreases obviously. In terms of decadal variation, there is a 12-year periodic variation in PCP, and since the mid-1970s there has been an 8-year short periodic variation. As to spatial variation, SCD in winter of Northeast China has increased gradually from the eastern part to the western, and the minimum value of SCD occurs in the east of Jilin Province, while the high value center is observed in the central part of the province. For the whole Northeast China, the variation tendencies are consistent in the eastern and central parts, where SCD presents a rising tendency and SCP shows a decreasing tendency. SCD in the southwestern and northern parts has a slight rising tendency, with SCD in the southwestern part having the slightest increasing tendency, and SCP in the northern part showing the slightest decreasing tendency. When a high SCD value is observed, the whole region is controlled by the East Asian deep trough at 500 hPa, and the trough becomes deeper in the western part, while a high pressure, which is easily formed and intensified in the eastern part, makes the East Asian deep trough move eastward slowly. Upper-level jet stream and low-level jet stream co-exist, and the former is stronger and takes more of a southwestward position than the latter. The high value zone of water vapor transport over the Pacific is intensified obviously, and the extent also increases. Northeast China is influenced by the water vapor transported to the northwest along the north of the high value center. © 2011 Science China Press and Springer-Verlag Berlin Heidelberg. Source

Guo W.-L.,Beijing Regional Climate Center | Shi H.-B.,Beijing Regional Climate Center | Ma J.-J.,Beijing Regional Climate Center | Zhang Y.-J.,Beijing Regional Climate Center | And 3 more authors.
Advances in Climate Change Research | Year: 2013

The spatial and temporal variations of some important near-surface climate parameters and extreme climate events in North China during 1961-2010 are analyzed by using 94 meteorological stations' data in the study area. Results show that the annual mean surface air temperature in North China increased at the rate of 0.36°C per decade, higher than the national average in the same period. Increasing was particularly significant since the mid-1980s, with maximum increase in the middle and northeastern parts of Inner Mongolia. Increasing rate of the annual mean minimum temperature is much higher than that of the maximum temperature, which results in the decrease of the annual mean diurnal temperature range. Noticeable decrease is also observed in the frequency of cold wave. Annual precipitation shows a slight decreasing trend, with more pronounced decrease in southern Shanxi and eastern Hebei provinces, which is mainly represented as decreasing in contribution rates of rainstorm and heavy storm in flood-season (May to September). During 1961-2010, North China is characterized by a noticeable reduction in annual extreme precipitation, and an increase in high-temperature days over most parts, as well as more frequent droughts. There are remarkable reductions in annual sunshine duration and mean wind speed, associated with the most significant reduction of mean wind speed in midwestern and eastern parts of Inner Mongolia. Meanwhile, North China has experienced a noticeable decrease/increase in annual mean sanddust/haze days during the study period. However, there is no significant trend in fog days, except a pronounced decrease since the 1990s. Source

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