Wang, H. J., and H. P. Chen, 2016: Understanding the recent trend of haze pollution in eastern China: roles of climate change. Atmos. Chem. Phys., 16, 4205-4211
In this paper, the variation and trend of haze pollution<br />
in eastern China for winter of 1960–2012 were analyzed.<br />
With the overall increasing number of winter haze days in<br />
this period, the 5 decades were divided into three sub-periods<br />
based on the changes of winter haze days (WHD) in central<br />
North China (30–40 N) and eastern South China (south<br />
of 30 N) for east of 109 E mainland China. Results show<br />
that WHD kept gradually increasing during 1960–1979, remained<br />
stable overall during 1980–1999, and increased fast<br />
during 2000–2012. The author identified the major climate<br />
forcing factors besides total energy consumption. Among all<br />
the possible climate factors, variability of the autumn Arctic<br />
sea ice extent, local precipitation and surface wind during<br />
winter is most influential to the haze pollution change. The<br />
joint effect of fast increase of total energy consumption, rapid<br />
decline of Arctic sea ice extent and reduced precipitation and<br />
surface winds intensified the haze pollution in central North<br />
China after 2000. There is a similar conclusion for haze pollution<br />
in eastern South China after 2000, with the precipitation<br />
effect being smaller and spatially inconsistent.
Feng, Y., and H. P. Chen, 2016: Warming over the North Pacific can intensify snow events in Northeast China. Atmos. Oceanic Sci. Lett., 9(2), 122-128
The variation of winter snowfall intensity over Northeast China and its relationship with the autumn<br />
North Pacific SST are investigated for the period 1960–2012. An upward trend is apparent for the<br />
winter snowfall intensity over Northeast China during the last half-century, coinciding with an<br />
increasing autumn SST over the North Pacific. Their interannual correlation coefficient reaches up to<br />
0.58 for the past five decades, and 0.42 after their trends are removed. Further analyses indicate that<br />
the warming SST during autumn may persist into winter. Correspondingly, large parts of East Asia<br />
and the North Pacific are dominated by an anticyclonic anomaly, which can induce an anomalous<br />
southeasterly over Northeast China, weaken the northerly wind, then warm the surface, increase<br />
the water vapor content and intensify snowfall events. Thus, the autumn North Pacific SST can be<br />
considered as a key predictor for winter snowfall events over Northeast China. Results from leaveone-<br />
out cross-validation and independent validation both show a significant correlation and a<br />
small RMSE between prediction and observation. Therefore, the autumn SST over the North Pacific<br />
is suggested as a potential predictor for winter snowfall intensity in Northeast China.
Wang, H.J., H.P. Chen, and J.P. Liu, 2015: Arctic Sea Ice Decline Intensified Haze Pollution in Eastern China. Atmospheric and Oceanic Science Letters, 8(1), 1-9
Air quality in eastern China has becoming more and more worrying in recent years, and haze is now No.1 air pollution issue. Results in this study show the decreasing Arctic sea ice (ASI) is an important contributor to the recent increased haze days in eastern China. The authors find that the number of winter haze days (WHD) in eastern China is strongly negatively correlated with the preceding autumn ASI during 1979–2012, and about 45%–67% of the WHD interannual to interdecadal variability<br />
can be explained by ASI variability. Following previous studies on the impact of ASI loss on the northern hemisphere climate, the authors’ studies further reveal that the<br />
reduction of autumn ASI leads to positive sea-level pressure anomalies in mid-latitude Eurasia, northward shift of track of cyclone activity in China, and weak Rossby wave<br />
activity in eastern China south of 40N during winter season. These atmospheric circulation changes favor less cyclone activity and more stable atmosphere in eastern<br />
China, leading to more haze days there. Furthermore, the patterns of circulation changes associated with autumn ASI and WHD are in very good agreement over the East Asia, particularly in eastern China. The authors suggest that haze pollution may continue to be a serious issue in the near future as the decline of ASI continues under<br />
Chen, H.P., and J.Q. Sun, 2015: Assessing model performance of climate extremes in China: an intercomparison between CMIP5 and CMIP3. Climatic Change, , DOI 10.1007/s10584-014-1319-5
In this study, we present a brief analysis of the performances of global climate<br />
models participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) in simulating climate extreme events in China and compare the results with those of the previous model generation (CMIP3). The primary focus of this analysis is the climate mean and variability of each extreme index. Results show that the CMIP5 models are generally able to capture the mean climate extremes and trends compared with a new gridded observational dataset. The model spread for some extreme indices is reduced in CMIP5 when compared with CMIP3. Furthermore, the models generally show higher skills in simulating the temperaturebased indices than the precipitation-based indices in terms of means and linear trends. Results from six reanalyses further reveal large uncertainties for these indices and it is difficult to say which reanalysis is better for comparison with the simulations of all indices. Based on the relative errors of the climatology, the model evaluation varies considerably from one index to another. However, some models appear to perform substantially better than the others when the average of all indices is considered for each model, and the median ensembles outperform the individual models in terms of all the extreme indices and their means. Additionally, a relationship is observed between the improved simulation of the climate mean and the improved performance of its variability, although this improvement is limited to particular models.
Chen, H.P., and J.Q. Sun, 2015: Drought Response to Air Temperature Change over China on the Centennial Scale. Atmospheric and Oceanic Science Letters, 8(3), 113-119
Climate data from the Climatic Research Unit (CRU) for the period 1901–2013 are used to investigate the drought response to air temperature change over China on the centennial scale. Drought is observed to have increased evidently across China, except for some regions in eastern China. This increase is much stronger in northern China compared to southern China, especially in Northwest and North China. These change characteristics of drought are closely associated with air temperature change, with the severe droughts in the major drought episodes of the last century generally coinciding with higher temperatures. The significantly increasing trend of drought in China based on observations only appears when considering the effects of air temperature change, which can explain ~49% of droughts in observations and 30%–65% of droughts in Coupled Model Intercomparison Project Phase 5 (CMIP5) model simulations. Furthermore, the response of drought to air temperature change<br />
generally increases as the drought time scale increases. Furthermore, drought shows relatively high sensitivity in spring and early summer in China on the centennial scale.
Chen, H.P., and J.Q. Sun, 2015: Changes in Drought Characteristics over China Using the Standardized Precipitation Evapotranspiration Index. Journal of Climate, 28, 5430-5447
The Standardized Precipitation Evapotranspiration Index (SPEI) is computed and compared in China using reference evapotranspiration calculated using the Thornthwaite (TH) approach and the Penman-Monteith (PM) equation. The analysis reveals that SPEI_PM outperforms the SPEI_TH with regard to drought monitoring during the period 1961-2012 over China, especially in arid regions of China. Furthermore, the SPEI_PM also performs better with regard to observed variations in soil moisture and streamflow in China. Thus, changes in drought characteristics over China are detected on the basis of variations in the SPEI_PM. The results indicate that droughts over China exhibit pronounced decadal variations over the past 50 years, with more frequent and severe droughts occurring before the 1980s and in the 2000s compared with the 1980s and 1990s. Since the late 1990s, droughts have become more frequent and severe across China, especially in some regions of northern China. Concurrently, consecutive drought events have also increased across China. This suggests that dry conditions in China have been enhanced in recent years. Further analyses illustrate that the temperature and precipitation anomalies exhibit different roles in detecting droughts across China, which is primarily due to the magnitude of their variations and different climate variability. Considering temperature and precipitation perturbations, droughts exhibit relatively larger responses to temperature fluctuations in northern China and relatively larger responses to precipitation anomalies in southern China.
Han, T. T., H. P. Chen, and H. J. Wang, 2015: Recent changes in summer precipitation in Northeast China and the background circulation. International Journal of Climatology, DOI: 10.1002/joc.4280,
This study documents recent changes in the characteristics of summer (July–August–September) precipitation in Northeast China (NEC).Asignificant shift to less precipitation occurred in 1999–2012 as compared with that in 1984–1998. The reduced precipitation in the later period is closely associated with the large-scale anomalous high pressure over East Asia and anomalous descending motion over NEC. Furthermore, the significant reductions in the total cloud cover and moisture content also contribute to the reduced precipitation over NEC. To investigate the possible mechanism for the decadal shift of summer precipitation, a northeast Asian summermonsoon (NEASM) index is defined to describe the monsoon circulation over<br />
NEC. The results indicate that the NEASM has weakened since 1999 and is concurrent with the shift of the Pacific Decadal Oscillation (PDO) to the negative phase. Warming sea surface temperature (SST) in the North Pacific can zonally reduce the land-sea thermal contrast and lead to a weak NEASM. Further investigation indicates that the negative phase of the PDO has significant impacts on the atmospheric circulation associated with the NEASM. Additionally, changes in synchronous Arctic sea ice cover (SIC) also likely induce an anomalous sinking movement and weaken water vapour transport; thus, the summer precipitation over NEC decreases.
Chen, H.P., and H. J. Wang, 2015: Haze Days in North China and the associated atmospheric circulations based on daily visibility data from 1960 to 2012. Journal of Geophysical Research-Atmosphere, 120, DOI:10.1002/2015JD023225
Haze is a severe hazard that greatly influences traffic and daily life with great economic losses and threats to human health. To enhance understanding of the haze occurrences, this study examined the haze variations over North China and their associated atmospheric circulations for the period of 1960-2012 using daily visibility data. Results indicate that the haze events over this region primarily occur in boreal winter of year and mainly in the morning of day. The results of the analysis of the long-term variations indicate that the annual haze days were relatively few in the 1960s but increased steeply in the 1970s and have remained stable to the present. Some differences are obvious among seasons. A stably increasing trend is discernable in summer and autumn, relatively low in the 1960s and the 1990s–2000s and relatively high in the 1970s–1980s in spring and winter. Haze variations in urban regions are quite similar to haze variations in rural regions but with more haze days in urban regions because of the high aerosol emissions. Further analyses indicate that the occurrences of severe haze events in boreal winter generally correlate with the weakened northerly winds and the development of inversion anomalies in the lower troposphere, the weakened East Asian trough in the midtroposphere, and the northward East Asian jet in the high troposphere. All of these factors provide a favorable atmospheric background for the maintenance and development of haze events in this region.http://nzc.iap.ac.cn/uploadfile/2015/0630/20150630081738611.pdf
Li, H. X., H. P. Chen, and H. J. Wang, 2015: Changes in clustered extreme precipitation events in South China and associated atmospheric circulations. Int. J. Climatol., DOI:10.1002/joc.4549,
Previous studies have documented that the summer precipitation over South China (SC) has experienced a prominent inter-decadal increase in 1992/1993, and the possible mechanism has been well revealed. The aim of this study is to investigate the changes in extreme precipitation and clustered extreme precipitation events in recent decades using station observations and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis data. The results indicate that extreme precipitation also experienced a significant inter-decadal increase around 1992/1993. Significant changes can also be found in the associated atmospheric circulations, such as the western North Pacific subtropical high (WNPSH) and the westerly jet stream over East Asia. In addition, the water vapour transport (WVT) related to extreme precipitation differed from mean conditions. For extreme precipitation events, the WVT from the Northwest Pacific and Indian Oceans was much stronger when compared with mean precipitation. When extreme<br />
precipitation events were clustered, the increased WVT mainly originated from the South China Sea and Northwest Pacific Ocean. Further analysis indicates that theWVTincreased from the Indian Ocean but decreased from theNorthwest Pacific after 1992/1993; this finding explains the significant increase in the clustered extreme precipitation events over SC after 1992/1993. In addition, the atmospheric stratification has become more unstable since 1992/1993.
Chen, H.P., J.Q. Sun, and X.L. Chen, 2014: Projection and uncertainty analysis of global precipitation-related extremes using CMIP5 models. International Journal of Climatology, 34, 2730-2748
Climate change is expected to influence the occurrence and magnitude of precipitation-related extremes and to increase drought and flood risk. Thus, future changes in dryness and wetness over global land areas are analysed using future climate simulations from the World Climate Research Programme’s (WCRP) Coupled Model Intercomparison Project Phase 5 (CMIP5) under RCP4.5 forcing scenario. Model reproducibility is evaluated first, and it is shown that high performance can be achieved in present-day climate simulations by models, particularly in multi-model ensemble (MME) results. For future climate simulations, the highest reliability regarding changes in precipitation and its related extremes is found over Northern high latitudes, while the lowest confidence levels are mainly localized over the tropics. The projections indicate a high likelihood that there will be a shift to fewer dryness but to more extreme precipitation events or/and flood events in future over Northern high latitudes. Among populated areas, Mediterranean basin is highlighted as displaying<br />
a relatively high reliability of increases in both dryness and wetness indicators, implying increased probabilities of both drought and flood events, despite the fact that there would be less precipitation. In North America and Asian monsoon areas, dryness indictors show no obvious changes, while markedly increases are found in wetness indicators, concurrent with a high model agreement. In contrast, southern Africa, Australia, and the Amazon basin show relatively high reliability regarding increases in dryness, but a low confidence level in wetness. The severity of these changes is not uniform across annual and seasonal scales and is region dependent. Two sources of uncertainty in projections are investigated in this study: internal and inter-model variability. The analysis indicates that internal and inter-model variability are the dominant sources of uncertainty in extreme climate projections, and inter-model variability is much larger and increases with time. Further analysis shows that both sources of uncertainty generally perform lower on annual and global scales than on seasonal and regional ones.
Chen, H.P., and J.Q. Sun, 2014: Changes in climate extreme events in China associated with warming. International Journal of Climatology, , doi:10.1002/joc.4168
The science that humans are the cause of global warming, and that the associated climate change would lead to serious changes in climate extreme events, food production, freshwater resources, biodiversity, human mortality, etc. is unequivocal. After several political negotiations, a 2 ∘C warming has been considered to be the benchmark for such damaging changes. However, an increasing amount of scientific research indicates that higher levels of warming are increasingly likely. What would the world be like if such higher levels of warming occurred? This study aims to provide information for better politically driven mitigation through an investigation of the changes in temperature- and precipitation-based extreme indices using CMIP5 (coupled model intercomparison project phase 5) simulations of a warming of 1, 2, and 3 ∘C in China.Warming simulations show more dramatic effects in China compared with the global average. In general, the results show relatively small change signals in climate extreme events in China at 1 ∘C, larger anomalies at 2 ∘C, and stronger and more extended anomalies at 3 ∘C. Changes in the studied temperature indices indicate thatwarm eventswould bemore frequent and stronger in the future, and that cold events would be reduced and weakened. For changes in the precipitation indices, extreme precipitation generally increases faster than total wet-day precipitation, and China will experience more intensified extreme precipitation events. Furthermore, the risk of flooding is projected to increase, and the dry conditions over northern China are projected to be mitigated. In certain regions, particularly Southwest China, the risks of both drought and flood events would likely increase despite the decreased total precipitation in the future. Uncertainties mainly derived from inter-model and scenario variabilities are attached to these projections, but a high model agreement can be generally observed in the likelihood of these extreme changes.
Chen, H.P., and J.Q. Sun, 2014: Robustness of Precipitation Projections in China: Comparison between CMIP5 and CMIP3 Models. Atmospheric and Oceanic Science Letters, 7(1), 67-73
Three sources of uncertainty in model projections of precipitation change in China for the 21st century were separated and quantified: internal variability, inter-model variability, and scenario uncertainty. Simulations from models involved in the third phase and the fifth phase of the Coupled Model Intercomparison Project<br />
(CMIP3 and CMIP5) were compared to identify improvements in the robustness of projections from the latest generation of models. No significant differences were<br />
found between CMIP3 and CMIP5 in terms of future precipitation projections over China, with the two datasets both showing future increases. The uncertainty can be<br />
attributed firstly to internal variability, and then to both inter-model and internal variability. Quantification analysis revealed that the uncertainty in CMIP5 models has<br />
increased by about 10%–60% with respect to CMIP3, despite significant improvements in the latest generation of models. The increase is mainly due to the increase of internal variability in the initial decades, and then mainly due to the increase of inter-model variability thereafter, especially by the end of this century. The change in scenario uncertainty shows no major role, but makes a negative<br />
contribution to begin with, and then an increase later.
Chen, H.P., and J.Q. Sun, 2014: Sensitivity of Climate Changes to CO2 Emissions in China. Atmospheric and Oceanic Science Letters, 7(5), 422-427
In this study, the authors demonstrate that the Coupled Model Intercomparison Project Phase 5 (CMIP5) models project a robust response in changes of mean and<br />
climate extremes to warming in China. Under a scenario of a 1% CO2 increase per year, surface temperature in China is projected to increase more rapidly than the<br />
global average, and the model ensemble projects more precipitation (2.2%/°C). Responses in changes of climate extremes are generally much stronger than that of climate means. The majority of models project a consistent response, with more warm events but fewer cold events in China due to CO2 warming. For example, the ensemble mean indicates a high positive sensitivity for increasing summer days (12.4%/°C) and tropical nights (26.0%/°C), but a negative sensitivity for decreasing frost days (−4.7%/°C) and ice days (−7.0%/°C). Further analyses indicate that precipitation in China is likely to become more extreme, featuring a high positive sensitivity. The sensitivity is high (2.4%/°C) for heavy precipitation days (> 10 mm d−1) and increases dramatically (5.3%/°C) for very heavy precipitation days (> 20 mm d−1), as well as for precipitation amounts on very wet days (10.8%/°C) and extremely wet days (22.0%/°C). Thus, it is concluded that the more extreme precipitation events generally show higher sensitivity to CO2 warming. Additionally, southern China is projected to experience an increased risk of drought and flood occurrence, while an increased risk of flood but a decreased risk of drought is likely in other regions of China.
Yu, E. T., J. Q. Sun, H. P. Chen, and W. L. Xiang, 2014: Evaluation of a high‑resolution historical simulation over China: climatology and extremes. Climate Dynamics, DOI 10.1007/s00382-014-2452-6,
China faces an increasing challenge in water resources in the coming decades; thus high-confidence climate projection is of particular importance for the country’s future. In this paper, we evaluate the performance of a long high-resolution continuous simulation over China based on multiple observations and the corresponding historical simulation. The simulation is completed using the Weather Research and Forecasting (WRF) model driven by the Model for Interdisciplinary Research on Climate version 5 (MIROC5) in the context of the Coupled Model Intercomparison<br />
Project Phase 5. The results show that both MIROC5 and WRF can capture the distribution and variability of temperature over China, whereas WRF shows improvements, particularly for simulation of regional features. Compared with MIROC5, WRF can reproduce the spatial distribution, annual cycle, probability distribution, and seasonal evolution of the precipitation over mainland China and the subregions with better performance. The trend is of fundamental importance in the future projection estimations, and WRF shows better skill in simulating the annual mean precipitation trend. However, there is overestimation of precipitation<br />
in Southeast China while negative one in the middle latitude of China in WRF simulation, which can be traced back to model bias in atmospheric circulation and water vapor transportation in these regions. Several extreme climate indices<br />
are selected to further assess the model’s performance in simulating climate extremes, WRF can well reproduce the main features with better model skill compared with MIROC5. The better performance of WRF indicates the necessity of the dynamical downscaling technique and the robustness of regional climate simulation in future regional climate projection over China.
Chen, H.P., J.Q. Sun, and X.L. Chen, 2013: Future Changes of Drought and Flood Events in China under a Global Warming Scenario. Atmospheric and Oceanic Science Letters, 6(1), 8-13
This study investigates the impact of global warming on drought/flood patterns in China at the end of the 21st century based on the simulations of 22 global climate models and a regional climate model (RegCM3) under the SRES (Special Report on Emissions Scenarios) A1B scenario. The standardized precipitation index (SPI), which has well performance in monitoring the drought/flood characteristics (in terms of their intensity, duration, and spatial extent) in China, is used in this study. The projected results of 22 coupled models and the RegCM3 simulation are consistent. These models project a decrease in the frequency of droughts in most parts of northern China and a slight increase in the frequency in some parts of southern China. Considering China as a whole, the spatial extents of droughts are projected to be significantly reduced. In contrast, future flood events over most parts of China are projected to occur more frequently with stronger intensity and longer duration than those prevalent currently. Additionally, the spatial extents of flood events are projected to significantly increase.
Chen, H.P., and J.Q. Sun, 2013: How Large Precipitation Changes over Global Monsoon Regions by CMIP5 Models?. Atmospheric and Oceanic Science Letters, 6(5), 306-311
Future changes in precipitation over global monsoon domains and their adjacent dry regions are investigated using present-day climate simulations (1986–2005) and future climate simulations under the Representative Concentration Pathways (RCP4.5) scenario by the Coupled Model Intercomparison Project Phase 5 (CMIP5)<br />
models. In the present-day climate simulations, high reproducibility of the extents of global monsoon domains and dry regions is observed from the multi-model ensemble<br />
(MME) result; the associated local summer precipitation variation and its interannual variability are also successfully reproduced. In the future, the global monsoon domains are projected to be expanded, while the dry regions are expected to initially increase and then decrease. The summer precipitation and its variability show significant increases over most global monsoon domains and obvious decreases over their adjacent dry regions. These results indicate that currently wet regions will become<br />
wetter and dry areas will be dryer under global warming conditions. Further analysis indicates that changes in summer precipitation over global monsoon and dry regions<br />
can be interpreted as moisture convergence changes associated with changes in horizontal moisture transport.
Chen, H.P., and J.Q. Sun, 2013: Projected change in East Asian summer monsoon precipitation under RCP scenario. Meteorology and Atmospheric Physics, 121, 55-77
Future changes in East Asian summer monsoon precipitation climatology, frequency, and intensity are analyzed using historical climate simulations and future climate simulations under the RCP4.5 scenario using the World Climate Research Programme’s (WCRP) Coupled Model Intercomparison Project 5 (CMIP5) multi-model<br />
dataset. The model reproducibility is evaluated, and well performance in the present-day climate simulation can be obtained by most of the studied models. However, underestimation is obvious over the East Asian region for precipitation<br />
climatology and precipitation intensity, and overestimation is observed for precipitation frequency. The overestimation of precipitation frequency is mainly due to the large positive bias of the light precipitation (precipitation < 10 mm/day) days, and the underestimation of precipitation intensity is mainly caused by the negative bias<br />
of the intense precipitation (precipitation > 10 mm/day) intensity. For the future climate simulations, simple multimodel ensemble (MME) averages using all of the models<br />
show increases in precipitation and its intensity over almost all of East Asia, while the precipitation frequency is projected to decrease over eastern China and around Japan and increase in other regions. When the weighted MME is considered, no large difference can be observed compared with the simple MME. For the MME using the six best models that have good performance in simulating the present-day climate, the future climate changes over East Asia are very similar to those predicted using all of the models. Further analysis shows that the frequency and intensity of intense precipitation events are also projected to significantly increase over East Asia. Increases in precipitation frequency and intensity are the main contributors to increases in precipitation, and the contribution of frequency increases (contributed by 40.8 % in the near future and by 58.9 % by the end of the twenty-first century) is<br />
much larger than that of intensity increases (contributed by 29.9 % in the near future and by 30.1 % by the end of the twenty-first century). This finding also implies an increased risk of intense precipitation events over the East Asian region under global warming scenario. These results regarding future climate simulations show much greater reliability than those using CMIP3 simulations.
Chen, H.P., 2013: Projected change in extreme rainfall events in China by the end of the 21st century using CMIP5 models. Chinese Science Bulletin, 58(12), 1462-1472
Projection of future climate changes and their regional impact is critical for long-term planning at the national and regional levels aimed at adaptation and mitigation. This study assesses the future changes in precipitation in China and the associated atmospheric circulation patterns using the Couple Model Intercomparison Project 5 Phase (CMIP5) simulations under the RCP4.5 and RCP8.5 scenarios. The results consistently indicate that the annual precipitation in China is projected to significantly increase at the end of the 21st century compared to the present-day levels. The number of days and the intensity of medium rain, large rain and heavy rain are obviously increased, while the number of trace rain days is projected to decrease over the entire area of China. Further analysis indicates that the significant increase of annual precipitation in Northwest China is primarily due to the increase of light rain and the increases in North and Northeast China are primarily due to the increase of medium rain. In the region of southern China, the increases of large rain and heavy rain play an important role in the increase of annual precipitation, while light rain events play a negative role. Analysis of the changes in atmospheric circulation indicates that the East Asian summer monsoon circulation is projected to be considerably stronger, and the local atmospheric stratification is projected to be more unstable, all of which provide a background benefit for the increase of precipitation and extreme rainfall events in China under global warmingscenarios.
Chen, H.P., J.Q. Sun, and H.J. Wang , 2012: A Statistical Downscaling Model for Forecasting Summer Rainfall in China from DEMETER Hindcast Datasets. Weather and Forecasting, 27, 608-628
A new statistical downscaling (SD) scheme is proposed to predict summertime multisite rainfall measurements in China. The potential predictors are multiple large-scale variables from operational dynamical model output.Akey step in this SD scheme is finding optimal predictors that have the closest and most stable relationship with rainfall at a given station. By doing so, the most robust signals from the large-scale circulation can be statistically projected onto local rainfall, which can significantly improve forecast skill in predicting the summer rainfall at the stations. This downscaling prediction is performed separately for each simulation with a leave-one-out cross-validation approach and an independent sample validation framework.<br />
The prediction skill scores exhibited at temporal correlation, anomaly correlation coefficient, and root-meansquare error consistently demonstrate that dynamical model prediction skill is significantly improved under the SD scheme, especially in the multimodel ensemble strategy. Therefore, this SD scheme has the potential to improve the operational skill when forecasting rainfall based on the coupled models.
Sun, J.Q., and H.P. Chen, 2012: A statistical downscaling scheme to improve global precipitation forecasting. Meteorology and Atmospheric Physics, 117, 87-102
Based on hindcasts obtained from the ‘‘Development of a European Multimodel Ensemble system for seasonal to inTERannual prediction’’ (DEMETER) project,<br />
this study proposes a statistical downscaling (SD) scheme suitable for global precipitation forecasting. The key idea of this SD scheme is to select the optimal predictors that are best forecast by coupled general circulation models (CGCMs) and that have the most stable relationships with observed precipitation. Developing the prediction model and further making predictions using these predictors can extract useful information from the CGCMs. Cross-validation and independent sample tests indicate that this SD scheme can significantly improve the prediction capability of CGCMs during the boreal summer (June–August), even over polar regions. The predicted and observed precipitations are significantly correlated, and the root-mean -square-error of the SD scheme-predicted precipitation is largely decreased compared with the raw CGCM predictions. An inter-model comparison shows that the multimodel<br />
ensemble provides the best prediction performance. This study suggests that combining a multi-model ensemble with the SD scheme can improve the prediction skill for precipitation globally, which is valuable for current operational precipitation prediction.
Chen, H.P., J.Q. Sun, X.L. Chen, and W. Zhou , 2012: CGCM projections of heavy rainfall events in China. International Journal of Climatology, 32, 441-450
This paper discusses projections of heavy rainfall events in China during the 21st century based on daily precipitation data from the Fourth Assessment Report’s (AR4) Coupled General Circulation Models (CGCM). Results show that all three experimental scenarios (scenarios A2, A1B, and B1) project consistent changes in frequency and intensity of heavy rainfall at the end of 21st century. In the regions of Northeast China and North China, there are no significant changes in frequency but there are remarkable increases in intensity of heavy rainfall, indicating that enhanced intensity is the main contributor to increased ratios of heavy rainfall to total annual precipitation in these regions. In regions of the lower reaches of Yangtze River and South China, increases in the amount of heavy rainfall are closely associated with<br />
increased frequency and increased intensity. Projected frequencies of heavy rainfall at the end of 21st century increase by 30.9 - 56.6% in the Yangtze River and 35.9 - 50.2% in South China compared to the period of 1980–1999, and projected intensities increase by 1.0 - 5.7% and 2.8 - 6.3%, respectively. Additionally, the ratios of heavy rainfall to total annual precipitation increase by 2.3 - 5.4% in the Yangtze River and 1.8 - 3.8% in South China. The significant increases of heavy rainfall ratios indicate that as the climate warms, heavy rainfall events are expected to increase at rates that are much faster than increases in total precipitation amounts, indicating that China will experience increased amounts of flooding. These results are substantially consistent among the three IPCC (Intergovernmental Panel on Climate Change) scenarios.<br />
The increased probability of heavy rainfall events in China is closely connected with increased transportation of water vapour from the Arabian Sea and the South China Sea. Additionally, atmosphere stratification has become increasingly unstable, which has provided a favorable background for the initiation of heavy rainfall at the end of the 21st century.
Wang, H.J., and H.P. Chen, 2012: Climate control for southeastern China moisture and precipitation: Indian or East Asian monsoon?. Journal of Geophysical Research, 117, D12109, doi:10.1029/2012JD017734
In this study, the water vapor sources for the precipitation processes in southeastern<br />
China (SECN) during 1981–2010 were investigated using atmospheric reanalysis data. We also studied the factors influencing the summer atmospheric moisture over SECN. These two issues are all closely related to the climate signals recorded in stalagmites recovered from caves in SECN. Result supports that the atmospheric water vapor over SECN during the whole summer time is primarily transported from the Indian Ocean. However, the vertically integrated water vapor content throughout the year in SECN has two main sources: the Indian Ocean and the tropical western Pacific. In addition, the water vapor transport for the precipitation processes in SECN has complex vertical structure. At approximately 700 hPa to 500 hPa, part of the water vapor for the precipitation in SECN comes from the Arab-Caspian region. Finally, the water vapor content over SECN is regulated primarily by both the Indian and East Asian monsoons. Further analysis indicated that the variability of the East Asian summer monsoon is substantially regulated by the western Pacific subtropical high, the Eurasia–Atlantic thermal conditions, as well as the large-scale Eurasia-Atlantic atmospheric circulation. Therefore, the SECN Cave proxies can record the signals from faraway middle and high latitude Eurasia-Atlantic climate, besides the regional East Asian monsoon and remote Indian monsoon.
Chen, H.P., J.Q. Sun, and K. Fan, 2012: Decadal Features of Heavy Rainfall Events in Eastern China. Acta Meteor. Sinica, 26(3), 289-303
Based on daily precipitation data, the spatial-temporal features of heavy rainfall events (HREs) during 1960–2009 are investigated. The results indicate that the HREs experienced strong decadal variability in the past 50 years, and the decadal features varied across regions. More HRE days are observed in the 1960s, 1980s, and 1990s over Northeast China (NEC); in the 1960s, 1970s, and 1990s over North China (NC); in the early 1960s, 1980s, and 2000s over the Huaihe River basin (HR); in the 1970s–1990s over the mid-lower reaches of the Yangtze River valley (YR); and in the 1970s and 1990s over South China (SC). These decadal changes of HRE days in eastern China are closely associated with the decadal variations of water content and<br />
stratification stability of the local atmosphere. The intensity of HREs in each sub-region is also characterized by strong decadal variability. The HRE intensity and frequency co-vary on the long-term trend, and show consistent variability over NEC, NC, and YR, but inconsistent variability over SC and HR. Further analysis of the relationships between the annual rainfall and HRE frequency as well as intensity indicates that the HRE frequency is the major contributor to the total rainfall variability in eastern China, while the HRE intensity shows only relative weak contribution.
Wang, H.J., J.Q. Sun, H.P. Chen, et al., 2012: Extreme Climate in China: Facts, Simulation and Projection. Meteorologische Zeitschrift, 21(3), 279-304
In this paper, studies on extreme climate in China including extreme temperature and precipitation, dust weather activity, tropical cyclone activity, intense snowfall and cold surge activity, floods, and droughts are reviewed based on the peer-reviewed publications in recent decades. The review is focused first onthe climatological features, variability, and trends in the past half century and then on simulations and<br />
projections based on global and regional climate models. As the annual mean surface air temperature (SAT) increased throughout China, heat wave intensity and frequency overall increased in the past half century, with a large rate after the 1980s. The daily or yearly minimum SAT increased more significantly than the mean or maximum SAT. The long-term change in precipitation is predominantly characterized by the so-called<br />
southern flood and northern drought pattern in eastern China and by the overall increase over Northwest China. The interdecadal variation of monsoon, represented by the monsoon weakening in the end of 1970s, is largely responsible for this change in mean precipitation. Precipitation-related extreme events (e.g., heavy rainfall and intense snowfall) have become more frequent and intense generally over China in the recent years, with large spatial features. Dust weather activity, however, has become less frequent over northern China in the recent years, as result of weakened cold surge activity, reinforced precipitation, and improved vegetation condition. State-of-the-art climate models are capable of reproducing some features of the mean climate and extreme climate events. However, discrepancies among models in simulating and projecting the mean and extreme climate are also demonstrated by many recent studies. Regional models with higher resolutions often perform better than global models. To predict and project climate variations and extremes, many new<br />
approaches and schemes based on dynamical models, statistical methods, or their combinations have been developed, resulting in improved skills.With the improvements of climate model capability and resolution as well as our understanding of regional climate variability and extremes, these new approaches and techniques<br />
are expected to further improve the prediction and projection on regional climate variability and extremes over China in the future.
Fan, K., Y. Liu, and H.P. Chen, 2012: Improving the Prediction of the East Asian Summer Monsoon: New Approaches. Weather and Forecasting, 27, 1017-1030
East Asian summer monsoon (EASM) prediction is difficult because of the summer monsoon’s weak and unstable linkage with El Nino–Southern Oscillation (ENSO) interdecadal variability and its complicated association with high-latitude processes. Two statistical prediction schemes were developed to include the interannual increment approach to improve the seasonal prediction of the EASM’s strength. The schemes were applied to three models [i.e., the Centre National de Recherches Meteorologiques (CNRM), the Met Office (UKMO), and the European Centre for Medium-Range Weather Forecasts (ECMWF)] and the Multimodel Ensemble (MME) from the Development of a European Multimodel Ensemble System for Seasonal-to-Interannual Prediction (DEMETER) results for 1961–2001. The inability of the three dynamical models to reproduce the weakened East Asian monsoon at the end of the 1970s leads to low prediction ability for the interannual variability of the EASM. Therefore, the interannual increment prediction approach was applied to overcome this issue. Scheme I contained the EASM in the form of year-to-year increments as<br />
a predictor that is derived from the direct outputs of the models. Scheme II contained two predictors: both the EASM and also the western North Pacific circulation in the form of year-to-year increments. Both the crossvalidation test and the independent hindcast experiments showed that the two prediction schemes have a much better prediction ability for the EASM than does the original scheme. This study provides an efficient approach for predicting the EASM.
陈活泼，孙建奇，范可, 2012: 新疆夏季降水年代际转型的归因分析. 地球物理学报, 55（6）, 1844-1851
陈活泼，孙建奇，陈晓丽, 2012: 我国夏季降水及相关大气环流场未来变化的 预估及不确定性分析. 气候与环境研究, 17(2), 171-183
利用政府间气候变化专门委员会第四次评估报告(IPCC AR4)的15个耦合气候模式在不同排放情景下的模拟结果,对我国夏季降水及相关大气环流场的未来时空变化特征与模式之间的不确定性作了研究.结果表明,在全球变暖背景下,我国夏季降水表现出较强的局地特征.其中,我国东部和高原地区的降水在21世纪表现出明显的增加趋势,而且这种趋势随着变暖的加剧而增强,同时模式模拟结果之间的一致性也更好,表明这一结果的可信度较高.在全球变暖背景下,我国新疆南部地区表现为持续的降水减少趋势,而我国西南地区夏季降水的变化则呈现出先减少(21世纪初)后增加的特征,不同模式对降水这些局地特征的模拟也都表现出较好的一致性.其他地区夏季降水在21世纪的变化不大,同时模式模拟的一致性也较差.多模式模拟的我国未来百年夏季降水的这些变化特征在温室气体高,中,低不同排放情景下基本一致,A2情景预估结果变化最大,A1B次之,B1相对最小.<br />
Sun, J.Q., and H.P. Chen, 2011: Predictability of western North Pacific typhoon activity and its factors using DEMETER coupled models. Chinese Science Bulletin, 56, 3474-3479
Climate prediction using a coupled model with a one-tier scheme is an important research direction. In this study, based on 1974–2001 hindcasts obtained from the “Development of a European Multimodel Ensemble system for seasonal to inTERannual prediction” (DEMETER) project, the capability of coupled general circulation models (CGCMs) to predict six climatic factors that have a close relationship with the western North Pacific typhoon activity is investigated over summer (June–October). Results indicate that all six DEMETER CGCMs well predict the six factors. Using the statistical relationship between these six factors and the typhoon frequency, the ability of the CGCMs to predict typhoon frequency is further explored. It is found that the six CGCMs also well predict the variability in typhoon frequency. Comparison analysis shows that the prediction skill of the statistical<br />
downscaling method is much better than that of the raw CGCMs. In addition, the six-model ensemble has the best prediction performance. This study suggests that combining a multi-model ensemble and statistical downscaling greatly improves the CGCM prediction skill, and will be an important research direction for typhoon prediction.
Sun, J.Q., H.J. Wang, W. Yuan, and H.P. Chen, 2010: Spatial‐temporal features of intense snowfall events in China and their possible change. Journal of Geophysical Research, 115,D16110, doi:10.1029/2009JD013541
The statistical spatial‐temporal features of the intense snowfall event (ISE) in China<br />
are investigated over the period of 1962–2000. The results indicate that eastern China, northern Xinjiang, the eastern Tibetan plateau, and northeastern China are four key regions for the ISE, with more frequency and strong variability. Annual cycle analysis shows the ISE exhibits a unimodal distribution with maximum frequency at winter months for eastern China, a bimodal distribution with maximum frequency at early winter and spring months for northern Xinjiang and northeastern China, and a bimodal distribution with maximum frequency at autumn and spring months for the eastern Tibetan plateau. Linear trend analysis indicates that in the last 39 years, the ISE exhibits a decreasing trend for eastern China and an increasing trend for northern Xinjiang and the eastern Tibetan plateau. The linear trend of the ISE is weak over northeastern China. Based on the simulations of the most recent and comprehensive climate models in the 20th century run, the performance of the current climate models in simulating the Chinese ISE is investigated. The results indicate that, of the 20 models, there are four models that can reasonably reproduce the spatial‐temporal features of the Chinese ISE. Based on these four models’ simulation for the 21st century under A1B and A2 scenarios, the future variability of the Chinese ISE is projected. It is found that global warming will cause the ISE frequency over southern China to decrease, while the ISE over northern China will initially increase and then decrease.
陈晓丽，沈学顺，陈活泼, 2010: 陆面过程对2007年淮河流域强降水数值预报的影响分析 . 热带气象学报, 26(6), 667-679
基于 GRAPES 中尺度数值天气预报模式，研究了不同复杂程度陆面过程和不同土壤初始条件对2007 年中国夏季淮河流域降水数值预报的影响。重点分析了下垫面非均匀性对造成我国夏季强降水的中小尺度对流系统的启动和发展的影响，探讨了中小尺度对流发生发展中陆气相互作用的可能影响。对2007 年7 月8 日发生在淮河流域的一次对流过程采用两组敏感性试验进行研究，结果表明对流启动对陆面过程有很强的敏感性。Noah 陆面模式能合理地模拟出对流的启动，而Slab 陆面模式模拟的对流会延迟1～2 小时，这与Noah 陆面模式能合理描述地表感、潜热通量有关。GLDAS 初始土壤资料能更加合理地描述实际土壤温湿的分布状况，有利于更加准确地模拟出对流降水的启动和分布位置。另外，对2007 年7 月淮河流域的持续性强降水天气过程进行了对比模拟，研究表明：不同复杂程度陆面过程对连续强降水数值预报有明显的影响。使用Noah 陆面模式能提高对强降水预报的能力，而且随着模式分辨率的提高，降水的TS 评分也在提高，Noah 的TS 评分总体上高于Slab。
Chen, H.P., and J.Q. Sun, 2009: How the Best Models Project the Future Precipitation Change in China. Advances in Atmospheric Sciences, 26(4), 773-782
Projected changes in summer precipitation characteristics in China during the 21st century are assessed using the monthly precipitation outputs of the ensemble of three best models under the Special Report on Emissions Scenarios (SRES) A1B, A2, and B1 scenarios. The excellent reproducibility of the models both in spatial and temporal patterns for the precipitation in China makes the projected summer precipitation change more believable for the future 100 years. All the three scenarios experiments indicate a consistent enhancement of summer precipitation in China in the 21st century. However, the projected summer precipitation in China demonstrates large variability between sub-regions. The projected increase in precipitation in South China is significant and persistent, as well as in North China. Meanwhile, in the early period of the 21st century, the region of Northeast China is projected to be much drier than the present. But, this situation changes and the precipitation intensifies later, with a precipitation anomaly increase of 12.4%-20.4% at the end of the 21st century. The region of the Xinjiang Province probably undergoes a drying trend in the future 100 years, and is projected to decrease by 1.7%-3.6% at the end of the 21st century. There is no significant long-term change of the projected summer precipitation in the lower reaches of the Yangtze River valley. A high level of agreement of the ensemble of the regional precipitation change in some parts of China is found across scenarios but smaller changes are projected for the B1 scenario and slightly larger changes for the A2 scenario.