The WAMME regional model intercomparison study

Results from five regional climate models (RCMs) participating in the West African Monsoon Modeling and Evaluation (WAMME) initiative are analyzed. The RCMs were driven by boundary conditions from National Center for Environmental Prediction reanalysis II data sets and observed sea-surface temperatures (SST) over four May–October seasons, (2000 and 2003–2005). In addition, the simulations were repeated with two of the RCMs, except that lateral boundary conditions were derived from a continuous global climate model (GCM) simulation forced with observed SST data. RCM and GCM simulations of precipitation, surface air temperature and circulation are compared to each other and to observational evidence. Results demonstrate a range of RCM skill in representing the mean summer climate and the timing of monsoon onset. Four of the five models generate positive precipitation biases and all simulate negative surface air temperature biases over broad areas. RCM spatial patterns of June–September mean precipitation over the Sahel achieve spatial correlations with observational analyses of about 0.90, but within two areas south of 10°N the correlations average only about 0.44. The mean spatial correlation coefficient between RCM and observed surface air temperature over West Africa is 0.88. RCMs show a range of skill in simulating seasonal mean zonal wind and meridional moisture advection and two RCMs overestimate moisture convergence over West Africa. The 0.5° computing grid enables three RCMs to detect local minima related to high topography in seasonal mean meridional moisture advection. Sensitivity to lateral boundary conditions differs between the two RCMs for which this was assessed. The benefits of dynamic downscaling the GCM seasonal climate prediction are analyzed and discussed.     

Validating a regional climate model’s downscaling ability for East Asian summer monsoonal interannual variability

Performance of a regional climate model (RCM), WRF, for downscaling East Asian summer season climate is investigated based on 11-summer integrations associated with different climate conditions with reanalysis data as the lateral boundary conditions. It is found that while the RCM is essentially unable to improve large-scale circulation patterns in the upper troposphere for most years, it is able to simulate better lower-level meridional moisture transport in the East Asian summer monsoon. For precipitation downscaling, the RCM produces more realistic magnitude of the interannual variation in most areas of East Asia than that in the reanalysis. Furthermore, the RCM significantly improves the spatial pattern of summer rainfall over dry inland areas and mountainous areas, such as Mongolia and the Tibetan Plateau. Meanwhile, it reduces the wet bias over southeast China. Over Mongolia, however, the performance of precipitation downscaling strongly depends on the year: the WRF is skillful for normal and wet years, but not for dry years, which suggests that land surface processes play an important role in downscaling ability. Over the dry area of North China, the WRF shows the worst performance. Additional sensitivity experiments testing land effects in downscaling suggest the initial soil moisture condition and representation of land surface processes with different schemes are sources of uncertainty for precipitation downscaling. Correction of initial soil moisture using the climatology dataset from GSWP-2 is a useful approach to robustly reducing wet bias in inland areas as well as to improve spatial distribution of precipitation. Despite the improvement on RCM downscaling, regional analyses reveal that accurate simulation of precipitation over East China, where the precipitation pattern is strongly influenced by the activity of the Meiyu/Baiu rainfall band, is difficult. Since the location of the rainfall band is closely associated with both lower-level meridional moisture transport and upper-level circulation structures, it is necessary to have realistic upper-air circulation patterns in the RCM as well as lower-level moisture transport in order to improve the circulation-associated convective rainfall band in East Asia.     

土壤湿度影响中国夏季气候的数值试验

利用"全球土壤湿度计划第2阶段"提供的土壤湿度资料强迫区域气候模式RegCM3,通过数值试验讨论了土壤湿度对东亚夏季气候模拟效果的影响。结果表明,合理考虑土壤湿度的作用,能够提高区域气候模式对中国夏季降水和2 m气温的空间分布型及逐日变化的模拟效果;模拟结果与观测的相关分析显示,降水和2 m气温的年际变化都得到了有效改进,这种改进在气温上尤为明显。不过上述改进具有区域依赖性。数值试验结果表明,气温对土壤湿度的敏感性强于降水,这也从一个侧面说明提高降水模拟效果的难度。总体而言,合理的土壤湿度能够提高区域气候模式对中国夏季气候的模拟能力。因此,合理描述土壤湿度的变化,是提高中国夏季气候预报技巧的潜在途径之一。     

A double-resolution transient RCM climate change simulation experiment for near-coastal eastern zone of the Eastern Mediterranean region

A double-resolution regional experiment on hydrodynamic simulation of climate over the eastern Mediterranean (EM) region was performed using an International Center for Theoretical Physics, Trieste RegCM3 model. The RegCM3 was driven from the lateral boundaries by the data from the ECHAM5/MPI-OM global climate simulation performed at the MPI-M, Hamburg and based on the A1B IPCC scenario of greenhouse gases emission. Two simulation runs for the time period 1960-2060, employing spatial resolutions of 50?km/14?L and 25?km/18?L, are realized. Time variations of the differences in the space distributions of simulated climate parameters are analyzed to evaluate the role of smaller scale effects. Both least-square linear and non-linear trends of several characteristics of the EM climate are evaluated in the study. One of the key findings with regard to linear trends is a notable and statistically significant precipitation drop over the near coastal EM zone during December-February and September-November. Statistically significant positive air temperature trends are projected over the entire EM region during the four seasons. Also projected are increases in air temperature extremes and the relative contribution of convective processes in the Southern Mediterranean coastal zone (ECM) region. A notable sensitivity of projected larger-scale climate change signals to smaller-scale effects is also demonstrated.     

Multiyear hindcast simulations of summer monsoon over South Asia using a nested regional climate model��BCC_RegCM1.0

Multiyear (1983?C2006) hindcast simulation of summer monsoon over South Asia has been carried out using the regional climate model of the Beijing Climate Centre (BCC_RegCM1.0). The regional climate model (hereafter BCC RCM) is nested into the global climate model of the Beijing Climate Centre BCC_CGCM1.0 (here after CGCM). The regional climate model is initialized on 01 May and integrated up to the end of the September for 24?years. Compared to the driving CGCM the BCC RCM reproduces reasonably well the intensity and magnitude of the large-scale features associated with the South Asia summer monsoon such as the upper level anticyclone at 200?hPa, the mid-tropospheric warming over the Tibetan plateau, the surface heat low and the 850?hPa moisture transport from ocean to the land. Both models, i.e., BCC RCM and the driving CGCM overestimates (underestimates) the 850?hPa southwesterly flow over the northern (southern) Arabian Sea. Moreover, both models overestimate the seasonal mean precipitation over much of the South Asia region compared to the observations. However, the precipitation biases are significantly reduced in the BCC RCM simulations. Furthermore, both models simulate reasonably the interannual variability of the summer monsoon over India. The precipitation index simulated by BCC RCM shows significant correlation (0.62) with the observed one. The BCC RCM simulates reasonably well the spatial and temporal variation of the precipitation and surface air temperature compared to the driving CGCM. Further, the temperature biases are significantly reduced (1?C4°C) in the BCC RCM simulations. The simulated vertical structure of the atmosphere show biases above the four sub-regions, however, these biases are significantly reduced in the BCC RCM simulations compared to the driving CGCM. Compared to the driving CGCM, the evolution processes of the onset of summer monsoon, e.g., the meridional temperature gradient and the vertical wind shear are well simulated by the BCC RCM. The 24-year simulations also show that with a little exception the BCC RCM is capable to reproduce the monsoon active and break phases and the intraseasonal precipitation variation over the Indian subcontinent.     

黑龙江省一次伴有龙卷的暖区暴雨成因分析

2014年7月19日夜间黑龙江克山出现雨强超过90 mm的短时强降水,利用常规观测资料、区域站资料、NCEP/NCAR再分析资料等对此次冷锋前部的暖区强降水成因进行分析。结果表明:(1)此次强降水出现在580 dagpm线附近,副高诱发的超低空急流为强降水提供了充沛的水汽和不稳定能量。(2)地面辐合线和地形抬升触发对流。高空急流东移,高空急流出口区左侧和辐散区与低层辐合相耦合促使对流快速发展增强。耦合消失,强降水则快速减弱。(3)低层暖平流明显,尤其地面具有暖锋锋生特征。强降水出现在不稳定层结和上升运动快速增强的阶段。(4)地面～200 hPa辐合层形成深厚的上升运动区,促使对流快速发展。(5)中尺度对流雨带沿地面辐合线生消。降水先出现在暖湿舌前部。随后,强降水产生的冷空气抬升暖湿空气形成冷锋特征的降水,由于强降水和冷空气的正反馈作用,降水持续时间长。冷空气势力最强时,伴随中尺度气旋性环流及0～1 km强垂直风切变有利于龙卷产生。(6)开口状地形的辐合作用、抬升及局地地形导致的中尺度环流风场对暖区降水的形成和维持作用显著。     

Revisiting forest impact on atmospheric water vapor transport and precipitation

Using a robust global precipitation database, we analyze coast-to-interior seasonal precipitation distributions over the world’s major forest regions. We find that the active functioning of boreal forests in summer is associated with an intense ocean-to-land moisture transport, which declines in winter when forest functioning is minimal. This seasonal switch manifests itself as a change in the exponential scale length of precipitation distribution, which exceeds 15?×?10³ km in summer but decreases to (3–4)?×?10³ km in winter. In equatorial rainforests, which are photosynthetically active throughout the year, annual precipitation remains approximately constant, while the coefficient of variation of monthly precipitation significantly declines toward the continent interior. Precipitation over forest during the periods of active forest functioning is always higher than over the adjacent ocean. Such precipitation patterns support the biotic pump concept according to which forest cover drives the ocean-to-land atmospheric moisture transport on a continental scale.     

亚洲中部干旱气候研究综述与机理分析

综述了亚洲中部干旱气候若干研究进展,并对亚洲中纬度干旱区形成机理进行了深入分析。依据大尺度气候分类法将全球陆地划分为季风、地中海及西风带气候区,分别对应于年内季节循环中多雨—高温的同位相、反位相及不相关等类型水—热配置特征。亚洲中部干旱区南北部分属地中海和西风带气候区,其不同水—热位相配置又对应于不同的水汽通量及其散度等特征。研究揭示了欧亚大陆中部干旱区形成于降水不足且季节性水—热配置不当,起因于大气环流平均槽脊季节变化引起的大气动力—热力配置,其根源是海陆热力差异及大地形对大气环流强迫的结果。本文还讨论了季风区与非季风区之间平均气流水汽散度的季节性互动,以及行星尺度上不同气候区之间平均气流与瞬变涡动水汽散度之间的配置等问题。     

Climate change projections of the North American Regional Climate Change Assessment Program (NARCCAP)

We investigate major results of the NARCCAP multiple regional climate model (RCM) experiments driven by multiple global climate models (GCMs) regarding climate change for seasonal temperature and precipitation over North America. We focus on two major questions: How do the RCM simulated climate changes differ from those of the parent GCMs and thus affect our perception of climate change over North America, and how important are the relative contributions of RCMs and GCMs to the uncertainty (variance explained) for different seasons and variables? The RCMs tend to produce stronger climate changes for precipitation: larger increases in the northern part of the domain in winter and greater decreases across a swath of the central part in summer, compared to the four GCMs driving the regional models as well as to the full set of CMIP3 GCM results. We pose some possible process-level mechanisms for the difference in intensity of change, particularly for summer. Detailed process-level studies will be necessary to establish mechanisms and credibility of these results. The GCMs explain more variance for winter temperature and the RCMs for summer temperature. The same is true for precipitation patterns. Thus, we recommend that future RCM-GCM experiments over this region include a balanced number of GCMs and RCMs.     

区域和全球模式的嵌套技术 及其长期积分试验

将区域模式嵌入澳大利亚CSIRO (Commonwealth Scientific and Industrial Research Organization）的全球模式中,并将其应用于区域模式的长期气候积分试验。模拟结果表明,当区域与全球模式嵌套时,边界吸收问题十分重要,由区域模式得到的高分辨率大尺度环流形式在边界上必须与全球模式提供的强迫一致,同时区域模式必须给出基于模式内部物理过程产生的高分辨信息。因此,在嵌套过程中,必须仔细考虑缓冲区的设置,使大尺度强迫与中尺度特征充分混合,既保持区域模式内外的一致性,又使区域内部中尺度强迫物理过程得到充分发展。将区域模式与澳大利亚CSIRO的9层21波三角形截断谱模式嵌套后,完成了连续3年的区域气候模式积分。模拟结果表明,由于区域模式较好地刻划了区域尺度的地形、下垫面和海岸线分布等的细节特征,模拟的区域气候特征比全球模式有较大的改进,尤其是对季风降水的模拟,区域模式明显改进了全球模式的模拟结果。     

Resolution effects on regional climate model simulations of seasonal precipitation over Europe

We analyze a set of nine regional climate model simulations for the period 1961–2000 performed at 25 and 50 km horizontal grid spacing over a European domain in order to determine the effects of horizontal resolution on the simulation of precipitation. All of the models represent the seasonal mean spatial patterns and amount of precipitation fairly well. Most models exhibit a tendency to over-predict precipitation, resulting in a domain-average total bias for the ensemble mean of about 20% in winter (DJF) and less than 10% in summer (JJA) at both resolutions, although this bias could be artificially enhanced by the lack of a gauge correction in the observations. A majority of the models show increased precipitation at 25 km relative to 50 km over the oceans and inland seas in DJF, JJA, and ANN (annual average), although the response is strongest during JJA. The ratio of convective precipitation to total precipitation decreases over land for most models at 25 km. In addition, there is an increase in interannual variability in many of the models at 25 km grid spacing. Comparison with gridded observations indicates that a majority of models show improved skill in simulating both the spatial pattern and temporal evolution of precipitation at 25 km compared to 50 km during the summer months, but not in winter or on an annual mean basis. Model skill at higher resolution in simulating the spatial and temporal character of seasonal precipitation is found especially for Great Britain. This geographic dependence of the increased skill suggests that observed data of sufficient density are necessary to capture fine-scale climate signals. As climate models increase their horizontal resolution, it is thus a key priority to produce high quality fine scale observations for model evaluation.     

Sources of systematic errors in the climatology of a regional climate model over Europe

 Two ten-year simulations made with a European regional climate model (RCM) are compared. They are driven by the same observed sea surface temperatures but use different lateral boundary forcing. For one simulation, RCM AMIP, this forcing is obtained from a standard integration of a global general circulation model (GCM AMIP), whereas for the other simulation, RCM ASSIM, it is derived from a time series of operational analyses. The archive of analysis fields (surface pressure plus winds and temperatures on various pressure levels) is not sufficiently comprehensive to provide directly the full set of driving fields required for the RCM (in particular, no moisture fields are present), so these are obtained via a GCM integration, GCM ASSIM, in which the model is continuously relaxed towards the analysis fields using a data assimilation technique. Errors in RCM AMIP can arise either from the internal RCM physics or from errors in the lateral boundary forcing inherited from GCM AMIP. Errors in RCM ASSIM can arise from the internal RCM physics or the boundary moisture forcing but not from the driving circulation. Although previous studies have considered RCM integrations driven either by output from standard GCM integrations or operational analyses, our study is the first to compare parallel integrations of each type. This allows the total systematic error in an RCM integration driven by standard GCM output to be partitioned into components arising from the driving circulation and the internal RCM physics. These components indicate the scope for reducing regional simulation biases by improving either the driving GCM or the RCM itself. The results relate mainly to elements of surface climate likely to be influenced by both the driving circulation and regional physical processes operating in the RCM. For cloud cover, errors are found to arise largely from the internal RCM physics. Values are too low despite a positive relative humidity bias, indicating shortcomings in the parametrisation scheme used to calculate cloud cover. In summer, surface temperature and precipitation errors are also explained principally by regional processes. For example excessive solar heating leads to anomalously high surface temperatures over southern Europe and excessive drying of the soil reduces precipitation in the south eastern sector of the domain. The lateral boundary forcing reduces precipitation in the south eastern sector of the domain. The lateral boundary forcing also exerts some influence, mainly via a tropospheric cold bias which partially offsets the warming over southern Europe and also increases precipitation. In other seasons the lateral boundary forcing and the regional physics both contribute significantly to the errors in surface temperature and precipitation. In winter the boundary forcing (apart from moisture) is responsible for about 60% of the total error variance for temperature and about 40% for precipitation, due to the cold bias and circulation errors such as a southward shift in the storm track. The remaining errors arise from the regional physics, although for precipitation an excessive supply of moisture from the lateral boundaries also contributes. The skill of the mesoscale component of the surface temperature and precipitation distributions exceeds previous estimates, due to more realistic observed climatology. The mesoscale patterns are very similar in the two RCM simulations indicating that errors in the simulation of fine scale detail arise mainly from inadequate representations of local forcings rather than errors in the large-scale circulation. Circulation errors in RCM AMIP (e.g. cold bias, southward shift of storm track) are also present in GCM AMIP, but are largely absent in RCM ASSIM except in summer. This confirms evidence from previous work that the key to reducing most circulation errors in the RCM lies in improving the driving GCM. Regional processes only make a major contribution to circulation errors in summer, when reduced advection from the boundaries allows errors in surface temperature to be transmitted more effectively into the troposphere. Finally, we find evidence of error balances in the GCM which act to minimise biases in important climatological variables. This reflects tuning of the model physics at GCM resolution. In order to achieve simultaneous optimisation of the RCM and GCM at widely differing resolutions it may be necessary to introduce explicit scale dependences into some aspects of the physics. Received: 17 September 1997/Accepted: 10 March 1998     

Mid-twenty-first century warm season climate change in the Central United States. Part I: regional and global model predictions

A regional climate model (RCM) constrained by future anomalies averaged from atmosphere–ocean general circulation model (AOGCM) simulations is used to generate mid-twenty-first century climate change predictions at 30-km resolution over the central U.S. The predictions are compared with those from 15 AOGCM and 7 RCM dynamic downscaling simulations to identify common climate change signals. There is strong agreement among the multi-model ensemble in predicting wetter conditions in April and May over the northern Great Plains and drier conditions over the southern Great Plains in June through August for the mid-twenty-first century. Projected changes in extreme daily precipitation are statistically significant over only a limited portion of the central U.S. in the RCM constrained with future anomalies. Projected changes in monthly mean 2-m air temperature are generally consistent across the AOGCM ensemble average, North American Regional Climate Change Assessment Program RCM ensemble average, and RCM constrained with future anomalies, which produce a maximum increase in August of 2.4–2.9 K over the northern and southern Great Plains and Midwest. Changes in extremes in daily 2-m air temperature from the RCM downscaled with anomalies are statistically significant over nearly the entire Great Plains and Midwest and indicate a positive shift in the warm tail of the daily 2-m temperature distribution that is larger than the positive shift in the cold tail.     

Multi-decadal scenario simulation over Korea using a one-way double-nested regional climate model system. Part 1: recent climate simulation (1971–2000)

We present an analysis of a high resolution multi-decadal simulation of recent climate (1971–2000) over the Korean Peninsula with a regional climate model (RegCM3) using a one-way double-nested system. Mean climate state as well as frequency and intensity of extreme climate events are investigated at various temporal and spatial scales, with focus on surface air temperature and precipitation. The mother intermediate resolution model domain encompasses the eastern regions of Asia at 60 km grid spacing while the high resolution nested domain covers the Korean Peninsula at 20 km grid spacing. The simulation spans the 30-year period of January 1971 through December 2000, and initial and lateral boundary conditions for the mother domain are provided from ECHO-G fields based on the IPCC SRES B2 scenario. The model shows a good performance in reproducing the climatological and regional characteristics of surface variables, although some persistent biases are present. Main results are as follows: (1) The RegCM3 successfully simulates the fine-scale structure of the temperature field due to topographic forcing but it shows a systematic cold bias mostly due to an underestimate of maximum temperature. (2) The frequency distribution of simulated daily mean temperature agrees well with the observed seasonal and spatial patterns. In the summer season, however, daily variability is underestimated. (3) The RegCM3 simulation adequately captures the seasonal evolution of precipitation associated to the East Asia monsoon. In particular, the simulated winter precipitation is remarkably good, clearly showing typical precipitation patterns that occur on the northwestern areas of Japan during the winter monsoon. Although summer precipitation is underestimated, area-averaged time series of precipitation over Korea show that the RegCM3 agrees better with observations than ECHO-G both in terms of seasonal evolution and precipitation amounts. (4) Heavy rainfall phenomena exceeding 300 mm/day are simulated only at the high resolution of the double nested domain. (5) The model shows a tendency to overestimate the number of precipitation days and to underestimate the precipitation intensities. (6) A CSEOF analysis reveals that the model captures the strength of the annual cycle and the surface warming trend throughout the simulated period.     

Climate change in the northeastern US: regional climate model validation and climate change projections

A high resolution regional climate model (RCM) is used to simulate climate of the recent past and to project future climate change across the northeastern US. Different types of uncertainties in climate simulations are examined by driving the RCM with different boundary data, applying different emissions scenarios, and running an ensemble of simulations with different initial conditions. Empirical orthogonal functions analysis and K-means clustering analysis are applied to divide the northeastern US region into four climatologically different zones based on the surface air temperature (SAT) and precipitation variability. The RCM simulations tend to overestimate SAT, especially over the northern part of the domain in winter and over the western part in summer. Statistically significant increases in seasonal SAT under both higher and lower emissions scenarios over the whole RCM domain suggest the robustness of future warming. Most parts of the northeastern US region will experience increasing winter precipitation and decreasing summer precipitation, though the changes are not statistically significant. The greater magnitude of the projected temperature increase by the end of the twenty-first century under the higher emissions scenario emphasizes the essential role of emissions choices in determining the potential future climate change.     

Analysis on the Long-Lasting Freezing Rain and Snowstorm Event at the Beginning of 2008

In this paper,main characteristics of the long-lasting freezing rain and snowstorm event in southern China at the beginning of 2008,features of the related atmospheric circulation and the causes thereof are analyzed.During the event,patterns of the atmospheric circulation stayed stable; the polar vortex located in the northern part of the Eastern Hemisphere was strong with little movement; the cold front from the polar region and the active warm air mass from the tropical ocean confronted each other for a long time; the blocking high to the west of Baikal remained strong and steady; the trough over central nd western Asia maintained its position for quite long with a group of little troughs splitting from it frequently; the dominant wind at 700 hPa was southwesterly while shears and vortexes at 850 hPa developed continually,providing the necessary low-level convergence for subsequent precipitation.Meanwhile,in the mid troposphere,eddies were generated over the Tibetan Plateau and positive vorticity disturbances in the Sichuan Basin propagated eastward to the coastal regions of eastern China.The western Pacific subtropical high was intensive with westward and northward migrations.The subtropical frontal zone was puissant and the north-south temperature gradient was large.Quasi-stationary fronts over South China and the Yunnan-Guizhou Plateau remained stable.Warm air masses over the tropical ocean were active,so was the trough in the southern branch of the westerlies over the Bay of Bengal.There were four episodes associated with this event.The first one was featured with the interaction of strong cold and warm air,while the other three with the quasi-stationary fronts over South China and the Yunnan-Guizhou Plateau as well as vigorous penetration of cold air from the north.The existence of the inversion layer and the thick melting layer were one of the main reasons for the long-lasting freezing rains.The main reason for the snowstorms was that the positive vorticity over the Sichuan Basin propagated eastward to the coastal regions of eastern China.Abundant water vapor and intense updraft also favored the heavy snows.     

High-resolution simulations of West African climate using regional climate model (RegCM3) with different lateral boundary conditions

To downscale climate change scenarios, long-term regional climatologies employing global model forcing are needed for West Africa. As a first step, this work examines present-day integrations (1981–2000) with a regional climate model (RCM) over West Africa nested in both reanalysis data and output from a coupled atmospheric–ocean general circulation model (AOGCM). Precipitation and temperature from both simulations are compared to the Climate Research Unit observations. Their spatial distributions are shown to be realistic. Annual cycles are considerably correlated. Simulations are also evaluated with respect to the driving large-scale fields. RCM offers some improvements compared to the AOGCM driving field. Evaluation of seasonal precipitation biases reveals that RCM dry biases are highest on June–August around mountains. They are associated to cold biases in temperature which, in turn, are connected to wet biases in precipitation outside orographic zones. Biases brought through AOGCM forcing are relatively low. Despite these errors, the simulations produce encouraging results and show the ability of the AOGCM to drive the RCM for future projections.     

Further study on the effect of buffer zone size on regional climate modeling

In this study, the effect of buffer zone size on the performance of a regional climate model (RCM) was investigated. In contrast to carrying out simulations where the buffer zone expands inward, numerical experiments were performed with the buffer zone expanded outward, which ensured an objective comparison on the effect of buffer zone sizes. Case simulations on the abnormal 1998 East Asian summer monsoon demonstrated that an RCM’s performance depended on the outward expansion of the buffer zone. Moreover, a broader buffer zone is able to improve large-scale circulation simulation in the upper troposphere; however, a narrower buffer zone shows better performance in the middle and lower troposphere. Numerical experiments show that the performance of RCMs in simulating the seasonal evolution of rainbands over eastern China is not completely consistent with that of the western Pacific subtropical high (WPSH). A possible reason why the configuration with the buffer zone expanding inward can improve the extension/withdrawal of the summer monsoon and the associated precipitation evolution over eastern China is discussed.     

Winter precipitation isotope slopes of the contiguous USA and their relationship to the Pacific/North American (PNA) pattern

This study compares the synoptic-dynamic relationship between two phases of the Pacific/North American (PNA) pattern and winter precipitation isotopes at 73 sites across the contiguous USA. We use the spatial pattern of isotope slope—the rate of changes in precipitation isotope ratios with distance—to identify features in the seasonal precipitation isotope fields related to climatic patterns, PNA positive and PNA negative. Our results show relationships between zones of high isotope slopes and the spatial position of the polar jet stream and juxtaposition of air masses associated with the PNA pattern. During a positive PNA winter, zones of high isotope slope in the eastern USA shift southward. This change is coincident with a southward displacement of the polar jet stream in this region, which leads to a greater frequency of polar air masses and ¹⁸O-depleted isotope values of precipitation in the region. In the western USA, zones of high slope shift eastward during the positive PNA winter, associated with more frequent penetration of tropical air masses that bring ¹⁸O-enriched precipitation to the region. Differences in δ¹⁸O/temperature relationships between the PNA-positive and -negative winters and contrasting δ¹⁸O/temperature behaviors in the eastern and western USA provide support for the role of variation in moisture source and transport as a control on the isotopic patterns. These findings highlight the importance of synoptic climate driven by PNA pattern in determining the spatial patterns of precipitation isotopes and provide constraints on paleo-water isotope interpretation and modern isotope hydrological processes.     

Future changes in summer precipitation in regional climate simulations over the Korean Peninsula forced by multi-RCP scenarios of HadGEM2-AO

In this study, the regional climate of the Korean Peninsula (KP) was dynamically downscaled using a high-resolution regional climate model (RCM) forced by multi- representative concentration pathways (RCP) scenarios of HadGEM2-AO, and changes in summer precipitation were investigated. Through the evaluation of the present climate, the RCM reasonably reproduced long-term climatology of summer precipitation over the KP, and captured the sub-seasonal evolution of Changma rain-band. In future projections, all RCP experiments using different RCP radiative forcings (i.e., RCP2.6, RCP4.5, RCP6.0, and RCP8.5 runs) simulated an increased summer precipitation over the KP. However, there were some differences in changing rates of summer precipitation among the RCP experiments. Future increases in summer precipitation were affected by future changes in moisture convergence and surface evaporation. Changing ranges in moisture convergences among RCP experiments were significantly larger than those in surface evaporation. This indicates that the uncertainty of changes in summer precipitation is related to the projection of the monsoon circulation, which determines the moisture convergence field through horizontal advection. Changes in the sub-seasonal evolution of Changma rain-band were inconsistent among RCP experiments. However, all experiments showed that Changma rain-band was enhanced during late June to early July, but it was weakened after mid-July due to the expansion of the western North Pacific subtropical high. These results indicate that precipitation intensity related to Changma rain-band will be increased, but its duration will be reduced in the future.