This paper reviews recent progress in the development of the Beijing Climate Center Climate System Model (BCC_CSM) and its four component models (atmosphere, land surface, ocean, and sea ice). Two recent versions are described: BCC_CSM1.1 with coarse resolution (approximately 2.8125°×2.8125°) and BCC_CSM1.1(m) with moderate resolution (approximately 1.125°×1.125°). Both versions are fully coupled climate-carbon cycle models that simulate the global terrestrial and oceanic carbon cycles and include dynamic vegetation. Both models well simulate the concentration and temporal evolution of atmospheric CO2 during the 20th century with anthropogenic CO2 emissions prescribed. Simulations using these two versions of the BCC_CSM model have been contributed to the Coupled Model Intercomparison Project phase five (CMIP5) in support of the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). These simulations are available for use by both national and international communities for investigating global climate change and for future climate projections.Simulations of the 20th century climate using BCC_CSM1.1 and BCC_CSM1.1(m) are presented and validated, with particular focus on the spatial pattern and seasonal evolution of precipitation and surface air temperature on global and continental scales. Simulations of climate during the last millennium and projections of climate change during the next century are also presented and discussed. Both BCC_CSM1.1 and BCC_CSM1.1(m) perform well when compared with other CMIP5 models. Preliminary analyses indicate that the higher resolution in BCC_CSM1.1(m) improves the simulation of mean climate relative to BCC_CSM1.1, particularly on regional scales.
This paper presents a concise summary of the studies on interdecadal variability of the East Asian winter monsoon (EAWM) from three main perspectives. (1) The EAWM has been significantly affected by global climate change. Winter temperature in China has experienced three stages of variations from the beginning of the 1950s: a cold period (from the beginning of the 1950s to the early or mid 1980s), a warm period (from the early or mid 1980s to the early 2000s), and a hiatus period in recent 10 years (starting from 1998). The strength of the EAWM has also varied in three stages: a stronger winter monsoon period (1950 to 1986/87), a weaker period (1986/87 to 2004/05), and a strengthening period (from 2005). (2) Corresponding to the interdecadal variations of the EAWM, the East Asian atmospheric circulation, winter temperature of China, and the occurrence of cold waves over China have all exhibited coherent interdecadal variability. The upper-level zonal circulation was stronger, the mid-tropospheric trough over East Asia was deeper with stronger downdrafts behind the trough, and the Siberian high was stronger during the cold period than during the warm period. (3) The interdecadal variations of the EAWM seem closely related to major modes of variability in the atmospheric circulation and the Pacific sea surface temperature. When the Northern Hemisphere annular mode/Arctic Oscillation and the Pacific decadal oscillation were in negative (positive) phase, the EAWM was stronger (weaker), leading to colder (warmer) temperatures in China. In addition, the negative (positive) phase of the Atlantic multi decadal oscillation coincided with relatively cold (warm) temperatures and stronger (weaker) EAWMs. It is thus inferred that the interdecadal variations in the ocean may be one of the most important natural factors influencing long-term variability in the EAWM. although global warming may have also played a significant role in weakening the EAWM.
The oceanic and atmospheric conditions and the related climate impacts of the 2015/16 ENSO cycle were analyzed, based on the latest global climate observational data, especially that of China. The results show that this strong El Niño event fully established in spring 2015 and has been rapidly developing into one of the three strongest El Niño episodes in recorded history. Meanwhile, it is also expected to be the longest event recorded, attributable to the stable maintenance of the abnormally warm conditions in the equatorial Pacific Ocean since spring 2014. Owing to the impacts of this strong event, along with climate warming background, the global surface temperature and the surface air temperature over Chinese mainland reached record highs in 2015. Disastrous weather in various places worldwide have occurred in association with this severe El Niño episode, and summer precipitation has reduced significantly in North China, especially over the bend of the Yellow River, central Inner Mongolia, and the coastal areas surrounding Bohai Bay. Serious drought has occurred in some of the above areas. The El Niño episode reached its peak strength during November-December 2015, when a lower-troposphere anomalous anticyclonic circulation prevailed over the Philippines, bringing about abnormal southerlies and substantially increased precipitation in southeastern China. At the same time, a negative phase of the Eurasia-Pacific teleconnection pattern dominated over the mid-high latitudes, which suppressed northerly winds in North China. These two factors together resulted in high concentrations of fine particulate matter (PM2.5) and frequent haze weather in this region. Currently, this strong El Niño is weakening very rapidly, but its impact on climate will continue in the coming months in some regions, especially in China.
A long-lived, quasi-stationary mesoscale convective system（MCS） producing extreme rainfall（maximum of 542 mm） over the eastern coastal area of Guangdong Province on 20 May 2015 is analyzed by using high-resolution surface observations, sounding data, and radar measurements. New convective cells are continuously initiated along a mesoscale boundary at the surface, leading to formation and maintenance of the quasi-linear-shaped MCS from about 2000 BT 19 to 1200 BT 20 May. The boundary is originally formed between a cold dome generated by previous convection and southwesterly flow from the ocean carrying higher equivalent potential temperature（θe） air. The boundary is subsequently maintained and reinforced by the contrast between the MCS-generated cold outflow and the oceanic higher-θe air. The cold outflow is weak(wind speed 5 m s-1), which is attributable to the characteristic environmental conditions, i.e., high humidity in the lower troposphere and weak horizontal winds in the middle and lower troposphere. The low speed of the cold outflow is comparable to that of the near surface southerly flow from the ocean, resulting in very slow southward movement of the boundary. The boundary features temperature contrasts of 2–3℃and is roughly 500-m deep. Despite its shallowness, the boundary appears to exert a profound influence on continuous convection initiation because of the very low level of free convection and small convection inhibition of the near surface oceanic air, building several parallel rainbands（of about 50-km length） that move slowly eastward along the MCS and produce about 80% of the total rainfall. Another MCS moves into the area from the northwest and merges with the local MCS at about 1200 BT. The cold outflow subsequently strengthens and the boundary moves more rapidly toward the southeast, leading to end of the event in 3 h.
Satellite measurements are an important source of global observations in support of numerical weather prediction（NWP）.The assimilation of satellite radiances under clear skies has greatly improved NWP forecast scores.However,the application of radiances in cloudy skies remains a significant challenge.In order to better assimilate radiances in cloudy skies,it is very important to detect any clear field-of-view（FOV） accurately and assimilate cloudy radiances appropriately.Research progress on both clear FOV detection methodologies and cloudy radiance assimilation techniques are reviewed in this paper.Overview on approaches being implemented in the operational centers and studied by the satellite data assimilation research community is presented.Challenges and future directions for satellite sounder radiance assimilation in cloudy skies in NWP models are also discussed.
Two homogenized datasets of daily maximum temperature （Tmax）, mean temperature （Tm）, and min- imum temperature （Tmin） series in China have recently been developed. One is CHTM3.0, based on the Multiple Analysis of Series for Homogenization （MASH） method, and includes 753 stations for the period 1960-2013. The other is CHHTD1.0, based on the Relative Homogenization test （RHtest）, and includes 2419 stations over the period 1951-2011. The daily Tmax/Tm/Tmin series at 751 stations, which are in both datasets, are chosen and compared against the raw dataset, with regard to the number of breakpoints, long-term climate trends, and their geographical patterns. The results indicate that some robust break points associated with relocations can be detected, the inhomogeneities are removed by both the MASH and RHtest method, and the data quality is improved in both homogenized datasets. However, the differences between CHTM3.0 and CHHTD1.0 are notable. By and large, in CHHTD1.0, the break points detected are fewer, but the adjustments for inhomogeneities and the resultant changes of linear trend estimates are larger. In contrast, CHTM3.0 provides more reasonable geographical patterns of long-term climate trends over the region. The reasons for the differences between the datasets include： （1） different algorithms for creating reference series for adjusting the candidate seriesmore neighboring stations used in MASH and hence larger-scale regional signals retained; （2） different algorithms for cMculating the adjustmentslarger adjustments in RHtest in general, partly due to the individual local reference information used; and （3） different rules for judging inhomogeneityall detected break points are adjusted in CHTM3.0, based on MASH, while a number of break points detected via RHtest but without supporting metadata are overlooked in CHHTD1.0. The present results suggest that CHTM3.0 is more suitable for analyses of large-scale climate change in China, while CHHTD1.0 contains more original information regarding station temperature records.
This commentary is based on a series of recent lectures on aerosol-monsoon interactions I gave at the Beijing Normal University in August 2015. A main theme of the lectures is on a new paradigm of ＂An Aerosol-Monsoon-Climate-System＂, which posits that aerosol, like rainfall, cloud, and wind, is an integral component of the monsoon climate system, influencing monsoon weather and climate on M1 timescales. Here, salient issues discussed in my lectures and my personal perspective regarding interactions between atmospheric dynamics and aerosols from both natural and anthropogenic sources are summarized. My hope is that under this new paradigm, we can break down traditional disciplinary barriers, advance a deeper understanding of weather and climate in monsoon regions, as well as entrain a new generation of geoscientists to strive for a sustainable future for one of the most complex and challenging human-natural climate sub-system of the earth.
An overview of Chinese contribution to Coupled Model Intercomparison Project-Phase 5 (CMIP5) is presented. The performances of five Chinese Climate/Earth System Models that participated in the CMIP5 project are assessed in the context of climate mean states, seasonal cycle, intraseasonal oscillation, interannual variability, interdecadal variability, global monsoon, Asian-Australian monsoon, 20th-century historical climate simulation, climate change projection, and climate sensitivity. Both the strengths and weaknesses of the models are evaluated. The models generally show reasonable performances in simulating sea surface temperature (SST) mean state, seasonal cycle, spatial patterns of Madden-Julian oscillation (MJO) amplitude and tropical cyclone Genesis Potential Index (GPI), global monsoon precipitation pattern, El Niño-Southern Oscillation (ENSO), and Pacific Decadal Oscillation (PDO) related SST anomalies. However, the performances of the models in simulating the time periods, amplitude, and phase locking of ENSO, PDO time periods, GPI magnitude, MJO propagation, magnitude of SST seasonal cycle, northwestern Pacific monsoon and North American monsoon domains, as well as the skill of large-scale Asian monsoon precipitation need to be improved. The model performances in simulating the time evolution and spatial pattern of the 20th-century global warming and the future change under representative concentration pathways projection are compared to the multimodel ensemble of CMIP5 models. The model discrepancies in terms of climate sensitivity are also discussed.
Emissions of mineral dust and its mixing with anthropogenic air pollutants affect both regional and global climates. Our fieldwork in late spring 2007（April 25-June 15） measured the physical and optical properties of dust storms mixed with local air pollutants at a rural site about 48 km southeast of central Lanzhou. Levels of air pollutants and aerosol optical properties were observed during the experiment, with concentrations of NOx（6.8 ± 3.3 ppb, average ± standard deviation）, CO（694 ± 486 ppb）, SO2（6.2 ±10 ppb）, O3（50.7 ± 13.1 ppb）, and PM10（172 ± 180 μg m-3）, and aerosol scattering coefficient（164 ±89 Mm-1; 1 Mm = 106m） and absorption coefficient（11.7 ± 6.6 Mm-1）, all much lower than the values observed during air pollution episodes in urban areas. During a major dust storm, the mass concentration of PM10 reached 4072 μg m-3, approximately 21-fold higher than in non-dust storm periods. The mixing ratios of trace gases declined noticeably after a cold front passed through. The observed CO/SO2 and CO/NOx ratios during air pollution episodes were 4.2-18.3 and 13.7-80.5, respectively, compared with the corresponding ratios of 38.1-255.7 and 18.0-245.9 during non-pollution periods. Our investigations suggest that dust storms have a significant influence on air quality in areas far from their source, and this large-scale transport of dust and air pollutants produces major uncertainties in the quantification of the global effects of emissions over Northwest China.
Atmospheric aerosols influence the earth’s radiative balance directly through scattering and absorbing solar radiation, and indirectly through affecting cloud properties. An understanding of aerosol optical properties is fundamental to studies of aerosol effects on climate. Although many such studies have been undertaken, large uncertainties in describing aerosol optical characteristics remain, especially regarding the absorption properties of different aerosols. Aerosol radiative effects are considered as either positive or negative perturbations to the radiation balance, and they include direct, indirect (albedo effect and cloud lifetime effect), and semi-direct effects. The total direct effect of anthropogenic aerosols is negative (cooling), although some components may contribute a positive effect (warming). Both the albedo effect and cloud lifetime effect cool the atmosphere by increasing cloud optical depth and cloud cover, respectively. Absorbing aerosols, such as carbonaceous aerosols and dust, exert a positive forcing at the top of atmosphere and a negative forcing at the surface, and they can directly warm the atmosphere. Internally mixed black carbon aerosols produce a stronger warming effect than externally mixed black carbon particles do. The semidirect effect of absorbing aerosols could amplify this warming effect. Based on observational (ground- and satellite-based) and simulation studies, this paper reviews current progress in research regarding the optical properties and radiative effects of aerosols and also discusses several important issues to be addressed in future studies.
Climate data of mean monthly temperature and total monthly precipitation compiled from different sources in northern Patagonia were interpolated to 20-km resolution grids over the period 1997-2010. This northern Patagonian climate grid （NPCG） improves upon previous gridded products in terms of its spatial resolution and number of contributing stations, since it incorporates 218 and 114 precipitation and temper- ature records, respectively. A geostatistical method using surface elevation from a Digital Elevation Model （DEM） as the ancillary variable was used to interpolate station data into even spaced points. The maps provided by NPCG are consistent with the broad spatial and temporal patterns of the northern Patagonian climate, showing a comprehensive representation of the latitudinal and altitudinal gradients in temperature and precipitation, as well as their related patterns of seasonality and continentality. We compared the per- formance of NPCG and various other datasets available to the climate community for northern Patagonia. The grids used for the comparison included those of the Global Precipitation Climatology Project, ERA- Interim, Climate Research Unit （University of East Anglia）, and University of Delaware. Based on three statistics that quantitatively assess the spatial coherence of gridded data against available observations （bias, MAE, and RMSE）, NPCG outperforms other global grids. NPCG represents a useful tool for understand- ing climate variability in northern Patagonia and a valuable input for regional models of hydrological and ecological processes. Its resolution is optimal for validating data from the general circulation models and working with raster data derived from remote sensing, such as vegetation indices.
Dust aerosol effects on the properties of cirrus and altocumulus cloud in Northwest China were studied for the period March-May 2007 by using the satellite data of Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations （CALIPSO）, Aqua, and CloudSat. Dusty clouds were defined as those mixed with dust aerosols or existing in dust aerosol conditions, while pure clouds were those in a dust-free environment. For dusty altocumulus clouds, the mean values of cloud optical depth （OPD）, cloud liquid water path （LWP）, cloud ice water path （IWP）, cloud effective particle radius （Re）, and cloud effective particle diameter （Dr） were 6.40, 40.23 g m-2, 100.70 g m-2, 8.76 μm, and 40.72 μm, respectively. For pure altocumulus clouds, the corresponding mean values were 9.28, 76.70 g m-2, 128.75 g m-2, 14.03 μm, and 48.92 μm, respectively. These results show a significant decrease of OPD, LWP, IWP, Re, and De of approximately 31%, 48%, 22%, 38%, and 17% because of the effects of dust aerosols. Moreover, the effects of dust aerosols on liquid-phase altocumulus clouds were greater than on ice-phase altocumulus clouds. Regarding dusty cirrus clouds, the mean values of OPD, IWP, and De were 5.11, 137.53 g in 2, and 60.44 μm, respectively. In contrast, the mean values were 6.69, 156.17 g m-2, and 66.63 μm, respectively, for pure cirrus clouds, with a 24% decrease in OPD, a 12% decrease in IWP, and a 9% decrease in De. These results indicate that dust aerosols can significantly change cloud properties, leading to a reduction of OPD, LWP, and effective particle size for both altocumulus and cirrus clouds in Northwest China.
The interannual variations of intensity of the Madden-Julian Oscillation（MJO） during boreal winter are investigated by using the observed outgoing longwave radiation（OLR） and the reanalysis data of ECMWF and NCEP.The standard deviation of 20-80-day filtered OLR anomaly is used to measure the MJO intensity.The dominant spatial structure of the interannual variability is revealed by an EOF analysis of the MJO intensity field.It is found that the leading mode is associated with eastern Pacific type ENSO,whereas the second mode is related to central Pacific type ENSO.A simple atmospheric model is used to investigate the relative roles of background moisture and wind changes in affecting the overall strength of MJO.The numerical experiments indicate that the background moisture effect is dominant while the background wind change has a minor effect.
Three atmospheric boundary layer （ABL） schemes and two land surface models that are used in the Weather Research and Forecasting （WRF） model, version 3.4.1, were evaluated with numerical simulations by using data from the north coast of France （Dunkerque）. The ABL schemes YSU （Yonsei University）, ACM2 （Asymmetric Convective Model version 2）, and MYJ （Mellor-Yamada-Janjic） were combined with two land surface models, Noah and RUC （Rapid Update Cycle）, in order to determine the performances under sea-breeze conditions. Particular attention is given in the determination of the thermal internal boundary layer （TIBL）, which is very important in air pollution scenarios. The other physics parameterizations used in the model were consistent for all simulations. The predictions of the sea-breeze dynamics output from the WRF model were compared with observations taken from sonic detection and ranging, light detection and ranging systems and a meteorological surface station to verify that the model had reasonable accuracy in predicting the behavior of local circulations. The temporal comparisons of the vertical and horizontal wind speeds and wind directions predicted by the WRF model showed that all runs detected the passage of the sea-breeze front. However, except for the combination of MYJ and Noah, all runs had a time delay compared with the frontal passage measured by the instruments. The proposed study shows that the synoptic wind attenuated the intensity and penetration of the sea breeze. This provided changes in the vertical mixing in a short period of time and on soil temperature that could not be detected by the WRF model simulations with the computational grid used. Additionally, among the tested schemes, the combination of the local- closure MYJ scheme with the land surface Noah scheme was able to produce the most accurate ABL height compared with observations, and it was also able to capture the TIBL.
The aim of this study is to compare the impacts of climate change on the potential productivity and potential productivity gaps of sunflower （Helianthus annuus）, potato （Solanurn tuberosurn）, and spring wheat （Triticumaestivurn Linn） in the agro-pastoral ecotone （APE） of North China. A crop growth dynamics statistical method was used to calculate the potential productivity affected by light, temperature, precipitation, and soil fertility. The growing season average temperature increased by 0.47, 0.48, and 0.52℃ per decade （p 〈 0.05） for sunflower, potato, and spring wheat, respectively, from 1981 to 2010. Meanwhile, the growing season solar radiation showed a decreasing trend （p 〈 0.05） and the growing season precipitation changed non-significantly across APE. The light-temperature potential productivity increased by 4.48% per decade for sunflower but decreased by 1.58% and 0.59% per decade for potato and spring wheat. The climate soil potential productivity reached only 31.20%, 27.79%, and 20.62% of the light-emperature potential produc- tivity for sunflower, potato, and spring wheat, respectively. The gaps between the light-temperature and climate-soil potential productivity increased by 6.41%, 0.97%, and 1.29% per decade for sunflower, potato, and spring wheat, respectively. The increasing suitability of the climate for sunflower suggested that the sown area of sunflower should be increased compared with potato and spring wheat in APE under future climate warming.
Black carbon (BC) is the most effective insoluble light-absorbing particulate (ILAP), which can strongly absorb solar radiation at visible wavelengths. Once BC is deposited in snow via dry or wet process, even a small amount of BC could significantly decrease snow albedo, enhance absorption of solar radiation, accelerate snow melting, and cause climate feedback. BC is considered the second most important component next to CO2 in terms of global warming. Similarly, mineral dust (MD) is another type of ILAP. So far, little attention has been paid to quantitative measurements of BC and MD deposition on snow surface in the midlatitudes of East Asia, especially over northern China. In this paper, we focus on reviewing several experiments performed for collecting and measuring scavenging BC and MD in the high Asian glaciers over the mount’ range (such as the Himalayas) and in seasonal snow over northern China. Results from the surveyed literature indicate that the absorption of ILAP in seasonal snow is dominated by MD in the Qilian Mount’s and by local soil dust in the Inner Mongolian region close to dust sources. The detection of BC in snow and ice cores using modern techniques has a large bias and uncert’ty when the snow sample is mixed with MD. Evidence also indicates that the reduction of snow albedo by BC and MD perturbations can significantly increase the net surface solar radiation, cause surface air temperature to rise, reduce snow accumulation, and accelerate snow melting.
This paper summarizes the recent progress in studies of the diurnal variation of precipitation over contiguous China. The main results are as follows. (1) The rainfall diurnal variation over contiguous China presents distinct regional features. In summer, precipitation peaks in the late afternoon over the southern inland China and northeastern China, while it peaks around midnight over southwestern China. In the upper and middle reaches of Yangtze River valley, precipitation occurs mostly in the early morning. Summer precipitation over the central eastern China (most regions of the Tibetan Plateau) has two diurnal peaks, i.e., one in the early morning (midnight) and the other in the late afternoon. (2) The rainfall diurnal variation experiences obvious seasonal and sub-seasonal evolutions. In cold seasons, the regional contrast of rainfall diurnal peaks decreases, with an early morning maximum over most of the southern China. Over the central eastern China, diurnal monsoon rainfall shows sub-seasonal variations with the movement of summer monsoon systems. The rainfall peak mainly occurs in the early morning (late afternoon) during the active (break) monsoon period. (3) Cloud properties and occurrence time of rainfall diurnal peaks are different for long- and short-duration rainfall events. Long-duration rainfall events are dominated by stratiform precipitation, with the maximum surface rain rate and the highest profile occurring in the late night to early morning, while short-duration rainfall events are more related to convective precipitation, with the maximum surface rain rate and the highest profile occurring between the late afternoon and early night. (4) The rainfall diurnal variation is influenced by multi-scale mountain-valley and land-sea breezes as well as large-scale atmospheric circulation, and involves complicated formation and evolution of cloud and rainfall systems. The diurnal cycle of winds in the lower troposphere also contributes to the regional differences in the rainfall diurnal variation. (5) Evaluation of the model performance shows that the present numerical models are weak in simulating the rainfall diurnal variation over contiguous China. The simulations are not significantly improved by increasing the model horizontal resolution alone. The key is to reduce the uncertainty in physical parameterizations related to the rainfall processes.
With the Zebiak-Cane model, the present study investigates the role of model errors represented by the nonlinear forcing singular vector (NFSV) in the "spring predictability barrier" (SPB) phenomenon in ENSO prediction. The NFSV-related model errors are found to have the largest negative effect on the uncertainties of El Nino prediction and they can be classified into two types: the first is featured with a zonal dipolar pattern of SST anomalies (SSTA), with the western poles centered in the equatorial central-western Pacific exhibiting positive anomalies and the eastern poles in the equatorial eastern Pacific exhibiting negative anomalies; and the second is characterized by a pattern almost opposite to the first type. The first type of error tends to have the worst effects on El Nino growth-phase predictions, whereas the latter often yields the largest negative effects on decaying-phase predictions. The evolution of prediction errors caused by NFSV-related errors exhibits prominent seasonality, with the fastest error growth in spring and/or summer; hence, these errors result in a significant SPB related to El Nino events. The linear counterpart of NFSVs, the (linear) forcing singular vector (FSV), induces a less significant SPB because it contains smaller prediction errors. Random errors cannot generate an SPB for El Nino events. These results show that the occurrence of an SPB is related to the spatial patterns of tendency errors. The NFSV tendency errors cause the most significant SPB for El Nino events. In addition, NFSVs often concentrate these large value errors in a few areas within the equatorial eastern and central-western Pacific, which likely represent those areas sensitive to El Nino predictions associated with model errors. Meanwhile, these areas are also exactly consistent with the sensitive areas related to initial errors determined by previous studies. This implies that additional observations in the sensitive areas would not only improve the accuracy of the initial field but also promote the reduction of model errors to greatly improve ENSO forecasts.
In previous statistical forecast models, prediction of summer precipitation along the Yangtze River valley and in North China relies heavily on its close relationships with the western Pacific subtropical high (WPSH), the blocking high in higher latitudes, and the East Asian summer monsoon (EASM). These relationships were stable before the 1990s but have changed remarkably in the recent two decades. Before the 1990s, precipitation along the Yangtze River had a significant positive correlation with the intensity of the WPSH, but the correlation weakened rapidly after 1990, and the correlation between summer rainfall in North China and the WPSH also changed from weak negative to significantly positive. The changed relationships present a big challenge to the application of traditional statistical seasonal prediction models. Our study indicates that the change could be attributed to expansion of the WPSH after around 1990. Owing to global warming, increased sea surface temperatures in the western Pacific rendered the WPSH stronger and further westward. Under this condition, more moisture was transported from southern to northern China, leading to divergence and reduced (increased) rainfall over the Yangtze River (North China). On the other hand, when the WPSH was weaker, it stayed close to its climatological position (rather than more eastward), and the circulations showed an asymmetrical feature between the stronger and weaker WPSH cases owing to the decadal enhancement of the WPSH. Composite analysis reveals that the maximum difference in the moisture transport before and after 1990 appeared over the western Pacific. This asymmetric influence is possibly the reason why the previous relationships between monsoon circulations and summer rainfall have now changed.
Urbanization has a substantial effect on urban meteorology.It can alter the atmospheric diffusion capability in urban areas and therefore affect pollutant concentrations.To study the effects of Hangzhou’s urban development in most recent decade on its urban meteorological characteristics and pollutant diffusion,90 weather cases were simulated,covering 9 weather types,with the Nanjing University City Air Quality Prediction System and high-resolution surface-type data and urban construction data for 2000 and 2010.The results show that the most recent decade of urban development in Hangzhou substantially affected its urban meteorology.Specifically,the average urban wind speed decreased by 1.1 m s-1;the average intensity of the heat island increased by 0.5℃;and the average urban relative humidity decreased by 9.7%.Based on one case for each of the nine weather types,the impact of urbanization on air pollution diffusion was investigated,revealing that the changes in the meteorological environment decreased the urban atmosphere’s diffusion capability,and therefore increased urban pollutant concentrations.For instance,the urban nitrogen oxides concentration increased by 2.1 μg m-3 on average;the fine particulate matter（diameter of 2.5 μm or less;PM2.5） pollution concentration increased by 2.3 μg m-3 on average;in highly urbanized areas,the PM2.5concentration increased by 30 μg m-3 and average visibility decreased by 0.2 km,with a maximum decrease of 1 km;the average number of daily hours of haze increased by 0.46 h;and the haze height lifted by 100-300 m.The ＂self-cleaning time＂ of pollutants increased by an average of 1.5 h.