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2018 Vol. 23, No. 6

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A Study on Urban Heat Island Effect in Beijing Central Business District (CBD)
Zhigang CHENG, Ju LI, Mingyu ZHOU, Junxia DOU, Xiaoyan CAO, Jingjiang ZHANG, Xinyu ZHANG, Xiaodong HE
2018, 23(6): 633-644. doi: 10.3878/j.issn.1006-9585.2017.17036
The diurnal and spatial variations of urban heat island (UHI) and their influence factors in Beijing central business district (CBD) are investigated by using the dense air temperature data measured in this area during 2012-2013. The results illustrate that the CBD air temperature was 0.64℃ higher than the surrounding area. And the UHI in the CBD was stronger in the nighttime and weaker during the daytime, while the "urban cold island" even occurred at the noon. Analysis of seasonal means of the UHI shows that the strongest UHI in the nighttime occurred in the autumn, followed by that in the winter, and the weakest UHI occurred in the spring and summer. During the daytime, the strongest UHI occurred in the summer, followed by that in the winter and the weakest UHI was found in the spring and autumn. Foggy, rainy, and windy days could restrain the UHI compared to the situation in sunny days. Combined with the wavelet analysis results, the reason why the UHI in the autumn was stronger than in the winter is attributed to the high occurrence frequencies of foggy, rainy, and windy days in the winter. Analysis of the UHI spatial variation in the CBD shows that gardens, schools and other public lands could ease the UHI effect. And the standard deviation of spatial variation of the UHI in the CBD in foggy, rainy and windy days was smaller than that in sunny days.
Evaluation of NCAR CESM and CAS ESM Models for the Simulation of Boreal Summer Climate over Eastern Asia: Climatological Mean and Diurnal Cycle of Precipitation
Xingyu LI, Xunqiang BI, He ZHANG
2018, 23(6): 645-656. doi: 10.3878/j.issn.1006-9585.2018.18050
Abstract(1376) PDF (5607KB)(1044)
This study evaluates the performance of two global models, i.e., the NCAR CESM (Community Earth System Model, NCAR) and the CAS ESM (Earth System Model, Chinese Academy of Sciences), for the simulation of boreal summer climate over eastern Asia. Four 19-year (1998-2016) AMIP (Atmospheric Model Intercomparison Project)-type experiments have been conducted using both the NCAR CESM and the CAS ESM with two different horizontal resolutions, and climatological means of 2-m air temperature, precipitation intensity and its diurnal cycle in the boreal summer over eastern Asia are analyzed. The results show that both models can reproduce large-scale features of boreal summer 2-m air temperature and precipitation intensity, although warm biases are found in the simulation of 2-m air temperature and precipitation rate is underestimated. For the diurnal cycle of precipitation, the observed peaks of precipitation occur during the afternoon to early evening over the land and during midnight to early morning over the ocean. Precipitation peaks earlier than observations over the land in the two coarse-resolution experiments, while the nocturnal peak in Sichuan Basin and the morning or early morning peak over some ocean areas cannot be reproduced. Increasing the horizontal resolution can significantly improve the model performance. The two high-resolution experiments distinctly improve the simulation of precipitation peaks over the land and ocean, and can well reproduce the afternoon peak over some islands. Quantitative comparisons between simulations of the two models indicate that the high-resolution CAS ESM experiment can yield the best simulation of precipitation peaks in the entire study region. The simulation of land and sea breezes still needs to be improved. Improvement of physical parameterizations related to precipitation and thermodynamic processes is very important for further improvement of the model performance.
Characteristics of Spatial-Temporal Distribution of the Stratospheric Quasi-Zero Wind Layer in Low-Latitude Regions
Baiqing CHEN, Yi LIU, Liangke LIU, Xinyong SHEN, Yuli ZHANG
2018, 23(6): 657-669. doi: 10.3878/j.issn.1006-9585.2018.17086
In order to find suitable airspace for the stratospheric airship, the characteristics of spatial-temporal distribution of the stratospheric Quasi-Zero Wind Layer (QZWL) in low-latitude regions are analyzed using the EOF (Empirical Orthogonal Function) method and based on the ERA-Interim daily reanalysis data. The QZWL is mainly influenced by the stratospheric seasonal transition and the Quasi-Biennial Oscillation (QBO), and can be classified into two categories. The northern type of QZWL occurs at 20°N from October to April, while the southern type of QZWL occurs all year round during the westerly QBO phase. The southern type of QZWL is located at around 5°N from May to November and merged with the northern type in the remaining time. Monthly statistical modeling studies in different QBO phases indicate that the probability density function of Weibull distribution fits well wind speed data collected at Haikou station and Nansha station. The specific probability distribution of wind speed predicted by the Weibull distribution and cumulative function can be used to determine suitable airspace for the stratospheric airship. As a result, the airspace at the height between 50 hPa and 30 hPa from October to April at Haikou station and at the height between 70 hPa and 50 hPa for the whole year in the westerly QBO phase at Nansha station are suitable for the stratospheric airship to make smooth flight.
Climatic Changes in the Twenty-Four Solar Terms Based on Temperature Observations Back to 1873
Cheng QIAN, Zhongwei YAN, Lijuan CAO, Zhen LI
2018, 23(6): 670-682. doi: 10.3878/j.issn.1006-9585.2018.18044
Abstract(1441) PDF (1693KB)(1337)
Twenty-four Solar Terms (24STs) have been widely used for guiding human activities in China over more than 2000 years. However, the implication of the conventional 24STs has been changing under global warming. The climatic 24STs proposed recently impose a time-varying characteristic on the conventional 24STs, thus they can better serve as guidance for people under current situation. Previous studies only focused on linear trend of the 24STs since 1960. In this study, climatic changes in the 24STs back to 1873 are analyzed based on homogenized daily temperature series at Beijing station for the period 1940-2017 and at Shanghai station for the period 1873-2017. The results show that the annual mean temperature as well as temperatures of the 24STs at Beijing station for the period 1941-2016 and at Shanghai station for the period 1874-2016 all show warming trends, thus resulting in advancing trends during the warming stage (around spring) and delaying trends during the cooling stage (around autumn) in the timings of the climatic Solar Terms in the seasonal cycle. Most of these trends are statistically significant. The occurrence of extreme cold days shows a significant decreasing trend at both stations of Beijing and Shanghai, whereas the occurrence of extreme hot days at Shanghai station shows a significant increasing trend. In addition to the long-term trend, there exists apparent multi-decadal variability with a period of 60-80 years in both the occurrence of extreme hot days and summer temperature at Shanghai station, which is correlated with the Atlantic Multi-decadal Oscillation. These results can provide an important scientific base for climate change adaptation and benefit the understanding of modern climatic warming in China from a perspective of fine evolution of the seasonal cycle.
Impacts of Human Activities on Land Surface Water and Energy—A Case Study in Weishui River Watershed
Shuang LIU, Zhenghui XIE, Yujin ZENG, Bin LIU, Junqiang GAO, Ruichao LI
2018, 23(6): 683-701. doi: 10.3878/j.issn.1006-9585.2018.17107
A typical watershed in the subtropical monsoon region was selected for a case study. A land surface hydrological model for Weishui River Basin was established by coupling the schemes of crop growth, empirical reservoir irrigation and groundwater lateral flow into with a land surface model. First, the model with a constant land cover dataset was used to quantify the impacts of groundwater lateral flow, crop cultivation, groundwater exploitation and reservoir irrigation on land surface water and energy. The land cover change and its effects on the watershed were then investigated using several remote-sensing images and modeling studies. The results show that:1) The groundwater lateral flow makes the simulation more reasonable with lower depth in downstream areas (lower than four meters) and deeper table in mountainous areas (deeper than several decameters); the crop model yields a larger leaf area index than the fixed one, and thus increases the transpiration in cropland, which consumes more water in the watershed and lessens the latent heat flux; irrigation then offsets the water loss by utilizing the surface water and increases the latent heat flux. In addition, it is found that the effect of groundwater pumping is not significant. 2) The land cover changed a lot from 1990 to 2012 with small changes during the 1990-2000 period and large changes during the 2001-2012 period. From 2001 to 2012, the area of cropland first decreased, then increased, and then decreased again (the trend of woodland is opposite); the changes of vegetation types that belong to the same land use type are significant, which leads to large differences in land surface modeling.
Response of the North Pacific Storm Track to Systematic Variation in Kuroshio Extension and Its Energy Transformation Mechanism
Fengliang HU, Liqiong WANG, Ruiting ZUO, Chao WANG
2018, 23(6): 702-714. doi: 10.3878/j.issn.1006-9585.2018.17151
The first and third modes, and the second and fourth modes of the empirical orthogonal function (EOF) decomposition of the zonal sea surface temperatures (SST) perturbations (ZSSTP) in the Northwest Pacific are in good synchronous correlations, respectively. By synthetizing the typical phases of the two separately, the typical SST modes of the Kuroshio Extension (KE) in contraction and elongation states can be obtained. The influences of the variability of KE on the North Pacific storm track and the main mechanisms in different energy conversion processes are discussed based on the CESM1.2.0 model simulations, which are initialized with SST forcing fields in contraction and elongation modes and with climatological SST, respectively. It is found that under the KE's contraction mode, the intensity of the North Pacific storm track basically enhances, but the vortex activities weaken to the south of the center; under the KE's elongation mode, the intensity of the storm track decreases to the west of the center but the vortex activities increase to the east of the center. Diagnostic analysis of energy conversion shows that the barotropic energy conversion process makes little contribution to the change of the eddy kinetic energy (EKE). Near the center of the storm track, its major effect is to consume EKE. Under the contraction mode of the KE, the EKE consumption in the barotropic energy conversion process weakens, while it enhances under the elongation state of KE. The above differences are mainly due to different deformations of the transient eddies under different SST anomalies. The baroclinic potential energy release is one order of magnitude higher than the barotropic energy conversion. It is completed through the interaction between the meridional temperature gradients of the base flow and the meridional vortex heat transport. The atmospheric baroclinicity (meridional temperature gradient) plays a key role in this process, while the spatial distribution of the atmospheric baroclinic anomalies, the meridional temperature gradient anomalies of base flows, the baroclinic effective potential energy release anomalies and the storm tracks anomalies all have a good correspondence. This process may also be the main physical process for SST anomalies of the KE to affect the North Pacific storm track. Eddy effective potential energy needs to convert to EKE to produce transient vortex motions. The magnitude of the release of the eddy effective potential energy is approximately the same as that of the baroclinic effective potential energy, but the value is relatively small. This process is accomplished by the warm air rising and cold air falling. The changes in the negative correlation between perturbed vertical velocity and perturbed temperature also agree well with the variations of the conversion from the eddy effective potential energy to the eddy kinetic energy.
Climatological Characteristics and Long-Term Trend of Relative Humidity in Wuhan
Kanzhuo SUONAN, Guoyu REN, Wenqian JIA, Xiubao SUN
2018, 23(6): 715-724. doi: 10.3878/j.issn.1006-9585.2017.17122
Abstract(1904) PDF (2747KB)(1112)
Based on the homogeneity test and correction of the relative humidity series, climatological characteristics and long-term trend of relative humidity in the urban area of Wuhan are analyzed. The results showed that:(1) The non-homogeneity deviation caused by relocated stations in the relative humidity series is prodigious, and the deviation caused by relocation of Wuhan station in 2010 is 8%. (2) Annual average relative humidity in the urban area of Wuhan remains high, and the values vary between 75%-85% with the highest value occurring at Wuhan station. (3) The relative humidity gradually increases from January to June, and then begins to decrease slowly after reaching its peak value in June. Monthly differences are pretty small with the values within 0.1%-2.5%. Seasonal characteristics of relative humidity in the urban area of Wuhan show that the RH is the highest in the summer and lowest in the winter during the whole year. (4) From 1961 to 2015, the average relative humidity in the urban area of Wuhan urban area exhibited a long-term decreasing trend, and the linear trend was accelerated after the 1990s. (5) In the past 55 years, time series of average relative humidity in the spring, autumn, and winter all show declining trends in varied degrees, of which the trend in the spring is more obvious, and a significant rising trend is found in the summer. However, the annual and seasonal means are showing a significant downward trend after 1991.
Spatial and Temporal Distribution of CO2 and Its Spectrum Characteristic in Beijing Urban Area
Yue DUAN, Xueling CHENG, Wei HUA, Jianan XU, Donglei FENG
2018, 23(6): 725-736. doi: 10.3878/j.issn.1006-9585.2018.17132
Using the turbulence data measured by the open path eddy covariance system deployed at the Beijing 325-m meteorological tower, the concentration and flux and spectrum of CO2 at seven different height levels are calculated for the period from December 2014 to November 2015. Analysis of the results indicate that the CO2 concentration displays diurnal variation with double peaks except in the winter. Human activities must be considered in the winter since they reduce the diurnal variation of CO2 concentration and lead to a relatively flat pattern. At all observational heights, the CO2 concentration and flux show obvious seasonal variations with the maximum values appearing in the winter and the minimum values occurring at the end of the spring and summer. The CO2 concentration decreases with height as a whole. Beijing is a carbon dioxide source. Daily changes in the CO2 flux are not as obvious as daily changes in the CO2 concentration. The CO2 flux is negative below 47 m and positive above 47 m. The CO2 flux decreases with height below 140 m and increases with height above 140 m. According to the analysis of spatial and temporal distribution of CO2, it is found that the CO2 concentration and flux in urban boundary layer are strongly affected by surface carbon emission source, underlying surface vegetation, atmospheric stability, environment temperature and weather process and so on. The results of turbulence spectra in the present study are very close to the results of Kaimal, i.e., the normalized velocity spectrum and the CO2 spectrum have a slope of -2/3 in the inertial subregion and they both have relationships with the stability parameter (Z/L) in the low frequency zone. This indicates that the turbulence spectra of the city with complex topography are not quite different from those with flat topography.
Spatial and Temporal Distributions of Warm Cloud Properties in Eastern China and Its Adjacent Ocean and Their Relationships with Meteorological Conditions
Hailing JIA, Xiaoyan MA, Feilin XIONG
2018, 23(6): 737-748. doi: 10.3878/j.issn.1006-9585.2018.17134
Spatial distribution and seasonal variation of cloud properties in eastern China and its adjacent ocean are analyzed based on MODerate resolution Imaging Spectroradiometer (MODIS) level three daily cloud products from 2003 to 2016. The relationships of cloud properties with dynamic and thermodynamic conditions and water vapor condition are investigated by combining MODIS and ERA-Interim reanalysis data over the same period. The high cloud fraction (CF) area is found over eastern China in the summer and over southern China and eastern ocean in the winter. The cloud droplet effective radius (CER) decreases from eastern ocean to inland region in both the summer and winter. Compared to that in the summer, the spatial distribution pattern of liquid water path (LWP) in the winter is greatly inhomogeneous with a dividing line between high and low values located near 30°N, which corresponds to the distribution of water vapor. It is found that CF exhibits the same seasonal variation over the land and ocean, i.e., low in summer and high in winter, but the magnitude is larger in eastern China land area than in the adjacent ocean. On the contrary, the CER and LWP are greater over the land than over its adjacent ocean with lower values in the winter and higher values in the summer. In general, CF has an obvious positive correlation with precipitable water vapor (PWV) and low troposphere stability (LTS) and a negative correlation with 850-hPa vertical velocity (ω850hPa), which indicates that abundant moisture and ascending motion in lower levels are favorable for the formation of clouds while the stable atmospheric stratification suppresses the growth of clouds. This explains why cloud fraction in the middle and lower levels is high. Both the CER and LWP show negative correlations with LTS and ω850hPa. However, they display low sensitivity to variations of PWV, which suggests that the CER and LWP are mainly affected by dynamic and thermodynamic lifting instead of the atmospheric moisture condition.
Impacts of Two Types of El Niño Events on Winter Precipitation in Southwest China
Wei TAO, Quanliang CHEN
2018, 23(6): 749-757. doi: 10.3878/j.issn.1006-9585.2018.17165
The present study analyzes different effects of EP El Niño and CP El Niño on winter precipitation in Southwest China using monthly precipitation data collected at 93 stations in southwestern region of China and the NCEP/NCAR monthly reanalysis data. The results show that in the winters of EP El Niño years and the two winters (i.e., 1994/1995 and 2002/2003) of CP El Niño years, the southwestern region of China was affected by abnormal easterly winds with water vapor supply coming from the Bay of Bengal and the South China Sea. At the same time, obvious convergence and ascending motion occurred in Southwest China and the updraft was abnormally strong, leading to severe convection and higher than normal winter precipitation in Southwest China. However, in the winters of CP El Niño years (i.e., 1968/1969, 1977/1978, 2004/2005, and 2009/2010), Southwest China was affected by abnormal northwesterly winds, and the water vapor supply from the Bay of Bengal and the South China Sea decreased. Meanwhile, significant divergence and descending motion developed in Southwest China, which suppressed the rising of airmass and weakened convection, leading to the decrease in precipitation in the winter in Southwest China.
A Numerical Study of Aerosol Effects on Microphysical Process and Electrification in Thunderstorms
Jun LIU, Yongbo TAN, Zheng SHI, Mengyi WANG, Mengying YU, Tianxue ZHENG
2018, 23(6): 758-768. doi: 10.3878/j.issn.1006-9585.2018.18047
A two-dimensional (2-D) cumulus model with electrification and lightning processes is implemented to investigate aerosol effects on microphysical process, electrification, and charge structure in thunderstorms. The 2-D cumulus model coupled with a droplet freezing module is used to simulate a SEET (Studies of Electrical Evolution in Thunderstorms) case. The results show that the number concentration of cloud droplets increases and its scale decreases with increasing aerosol concentration, but the content of raindrops reduces. Ice crystals grow rapidly in low temperature region, which is attributed to droplets freezing, and the number concentration of ice crystals increases but the scale decreases. The number concentration of graupels increases first and then decreases sharply because ice crystals are difficult to grow into graupels with larger scale when the aerosol concentration exceeds 1000 cm-3. In addition, the charge structure of thunderstorms is not affected by aerosol concentration, but aerosols have significant effect on the strength of electrification, i.e., when the aerosol concentration is lower, more ice crystals and graupels collision enhances the non-inductive charging process and increases the charge density as the aerosol concentration increases; however, the non-inductive charging rate starts to decrease when the aerosol concentration exceeds 1000 cm-3 due to the appearance of small ice crystals and little graupels, and the charge density is reduced.