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2014 Vol. 31, No. 5

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Mesoscale Predictability of Moist Baroclinic Waves: Variable and Scale-dependent Error Growth
BEI Naifang, Fuqing ZHANG
2014, 31(5): 995-1008. doi: 10.1007/s00376-014-3191-7
This study seeks to quantify the predictability of different forecast variables at various scales through spectral analysis of the difference between perturbed and unperturbed cloud-permitting simulations of idealized moist baroclinic waves amplifying in a conditionally unstable atmosphere. The error growth of a forecast variable is found to be strongly associated with its reference-state (unperturbed) power spectrum and slope, which differ significantly from variable to variable. The shallower the reference state spectrum, the more spectral energy resides at smaller scales, and thus the less predictable the variable since the error grows faster at smaller scales before it saturates. In general, the variables with more small-scale components (such as vertical velocity) are less predictable, and vice versa (such as pressure). In higher-resolution simulations in which more rigorous small-scale instabilities become better resolved, the error grows faster at smaller scales and spreads to larger scales more quickly before the error saturates at those small scales during the first few hours of the forecast. Based on the reference power spectrum, an index on the degree of lack (or loss) of predictability (LPI) is further defined to quantify the predictive time scale of each forecast variable. Future studies are needed to investigate the scale- and variable-dependent predictability under different background reference flows, including real case studies through ensemble experiments.
Sensitivity of ENSO Variability to Pacific Freshwater Flux Adjustment in the Community Earth System Model
KANG Xianbiao, HUANG Ronghui, WANG Zhanggui, ZHANG Rong-Hua
2014, 31(5): 1009-1021. doi: 10.1007/s00376-014-3232-2
The effects of freshwater flux (FWF) on modulating ENSO have been of great interest in recent years. Large FWF bias is evident in Coupled General Circulation Models (CGCMs), especially over the tropical Pacific where large precipitation bias exists due to the so-called double ITCZ problem. By applying an empirical correction to FWF over the tropical Pacific, the sensitivity of ENSO variability is investigated using the new version (version 1.0) of the NCAR's Community Earth System Model (CESM1.0), which tends to overestimate the interannual variability of ENSO accompanied by large FWF into the ocean. In response to a small adjustment of FWF, interannual variability in CESM1.0 is reduced significantly, with the amplitude of FWF being reduced due to the applied adjustment part whose sign is always opposite to that of the original FWF field. Furthermore, it is illustrated that the interannual variability of precipitation weakens as a response to the reduced interannual variability of SST. Process analysis indicates that the interannual variability of SST is damped through a reduced FWF-salt-density-mixing-SST feedback, and also through a reduced SST-wind-thermocline feedback. These results highlight the importance of FWF in modulating ENSO, and thus should be adequately taken into account to improve the simulation of FWF in order to reduce the bias of ENSO simulations by CESM.
Charge Structure of a Summer Thunderstorm in North China: Simulation Using a Regional Atmospheric Model System
LIU Dongxia, QIE Xiushu, PENG Liang, LI Wanli
2014, 31(5): 1022-1034. doi: 10.1007/s00376-014-3078-7
Electrification and simple discharge schemes are coupled into a 3D Regional Atmospheric Model System (RAMS) as microphysical parameterizations, in accordance with electrical experiment results. The dynamics, microphysics, and electrification components are fully integrated into the RAMS model, and the inductive and non-inductive electrification mechanisms are considered in the charging process. The results indicate that the thunderstorm mainly had a normal tripole charge structure. The simulated charge structure and lightning frequency are basically consistent with observations of the lightning radiation source distribution. The non-inductive charging mechanism contributed to the electrification during the whole lifetime of the thunderstorm, while the inductive electrification mechanism played a significant role in the development period and the mature stage when the electric field reached a large value. The charge structure in the convective region and the rearward region are analyzed, showing that the charge density in the convective region was double that in the rearward region.
Assessment of Future Drought in Southwest China Based on CMIP5 Multimodel Projections
2014, 31(5): 1035-1050. doi: 10.1007/s00376-014-3223-3
In the last decade, a series of severe and extensive droughts have swept across Southwest China, resulting in tremendous economic losses, deaths, and disruption to society. Consequently, this study is motivated by the paramount importance of assessing future changes in drought in Southwest China. Precipitation is likely to decrease over most parts of Southwest China around the beginning of the century, followed by widespread precipitation increases; the increase in potential evapotranspiration (PET), due to the joint effects of increased temperature and surface net radiation and decreased relative humidity, will overwhelm the whole region throughout the entire 21st century. In comparative terms, the enhancement of PET will outweigh that of precipitation, particularly under Representative Concentration Pathway (RCP) 8.5, resulting in intensified drought. Generally, the drying tendency will be in the southeast portion, whereas the mountainous region in the northwest will become increasingly wetter owing to abundant precipitation increases. Droughts classified as moderate/severe according to historical standards will become the norm in the 2080s under RCP4.5/RCP8.5. Future drought changes will manifest different characteristics depending on the time scale: the magnitude of change at a time scale of 48 months is nearly twice as great as that at 3 months. Furthermore, we will see that not only will incidences of severe and extreme drought increase dramatically in the future, but extremely wet events will also become more probable.
Relationships between Interannual and Intraseasonal Variations of the Asian-Western Pacific Summer Monsoon Hindcasted by BCC_CSM1.1(m)
LIU Xiangwen, WU Tongwen, YANG Song, LI Qiaoping, CHENG Yanjie, LIANG Xiaoyun, FANG Yongjie, JIE Weihua, NIE Suping
2014, 31(5): 1051-1064. doi: 10.1007/s00376-014-3192-6
Using hindcasts of the Beijing Climate Center Climate System Model, the relationships between interannual variability (IAV) and intraseasonal variability (ISV) of the Asian-western Pacific summer monsoon are diagnosed. Predictions show reasonable skill with respect to some basic characteristics of the ISV and IAV of the western North Pacific summer monsoon (WNPSM) and the Indian summer monsoon (ISM). However, the links between the seasonally averaged ISV (SAISV) and seasonal mean of ISM are overestimated by the model. This deficiency may be partially attributable to the overestimated frequency of long breaks and underestimated frequency of long active spells of ISV in normal ISM years, although the model is capable of capturing the impact of ISV on the seasonal mean by its shift in the probability of phases. Furthermore, the interannual relationships of seasonal mean, SAISV, and seasonally averaged long-wave variability (SALWV; i.e., the part with periods longer than the intraseasonal scale) of the WNPSM and ISM with SST and low-level circulation are examined. The observed seasonal mean, SAISV, and SALWV show similar correlation patterns with SST and atmospheric circulation, but with different details. However, the model presents these correlation distributions with unrealistically small differences among different scales, and it somewhat overestimates the teleconnection between monsoon and tropical central-eastern Pacific SST for the ISM, but underestimates it for the WNPSM, the latter of which is partially related to the too-rapid decrease in the impact of El Nio-Southern Oscillation with forecast time in the model.
A Generalized Frontogenesis Function and Its Application
YANG Shuai, GAO Shouting, LU Chungu
2014, 31(5): 1065-1078. doi: 10.1007/s00376-014-3228-y
With the definition of generalized potential temperature, a new generalized frontogenesis function, which is expressed as the Lagrangian change rate of the magnitude of the horizontal generalized potential temperature gradient, is derived. Such a frontogenesis function is more appropriate for a real moist atmosphere because it can reflect frontogenesis processes, in which the atmosphere in a frontal zone is typically characterized by neither completely dry nor uniform saturation. Furthermore, by derivation, the expression of generalized frontogenesis function includes both temperature and humidity gradients, which is different from and superior to the traditional frontogenesis function in moist processes, which also uses equivalent potential temperature. Diagnostic studies of real cases are performed and show that the generalized frontogenesis function in non-uniformly saturated moist atmosphere indeed provides a useful tool for frontogenesis, compared to using the traditional frontogenesis function. The new frontogenesis function can be used in situations involving either a strong temperature or moisture gradient and is closely correlated with precipitation.
The Roles of Different Mechanisms Related to the Tide-induced Fronts in the Yellow Sea in Summer
REN Shihe, XIE Jiping, ZHU Jiang*
2014, 31(5): 1079-1089. doi: 10.1007/s00376-014-3236-y
In summer, the Yellow Sea Cold Water Mass (YSCWM) is a stable water mass of low temperature lying at the bottom of the central Yellow Sea (YS). It is fringed by some typical tidal fronts, which separate deep, stratified water on the offshore side from the well-mixed, shallow water on the inshore side. Three striking frontsSubei Bank Front (SBF), Shandong Peninsula Front (SPF), and Mokpo Front (MKF; a front off the southwestern tip of the Korean Peninsula)have been identified by various studies from both satellite observations and model results. Tide plays an important role in the formation and maintenance of these fronts. However, it is still a matter of debate as to the roles these two kinds of mechanisms of upwelling and tidal mixing play, and how importance they are in the maintenance processes of the above three fronts. Basing a nested high-resolution model HYCOM (the Hybrid Coordinate Ocean Model), this study focuses on the different mechanisms of tidal effects on the thermal fronts in the YS in summertime. Through comparative experiments with and without tidal forcing, the results indicate that the MKF is mainly driven by tide-induced upwelling. For the SPF, tidal mixing is the dominant factor, when lower cold water is stirred upwards along the sloping topography of the western YS. Meanwhile, the combined effect of upwelling and tidal mixing is the main cause of the formation of the SBF. Diagnostic analysis of thermal balance shows that horizontal nonlinear advection induced by strong tidal currents also contributes to the thermal balance of frontal areas.
Effect of Tropical Cyclone Intensity and Instability on the Evolution of Spiral Bands
HUANG Hong, JIANG Yongqiang, CHEN Zhongyi, LUO Jian, WANG Xuezhong
2014, 31(5): 1090-1100. doi: 10.1007/s00376-014-3108-5
The evolution of spiral-band-like structures triggered by asymmetric heating in three tropical-cyclone-like vortices of different intensities is examined using the Three-Dimensional Vortex Perturbation Analyzer and Simulator (3DVPAS) model. To simulate the spiral bands, asymmetric thermal perturbations are imposed on the radius of maximum wind (RMW) of vortices, which can be considered as the location near the eyewall of real tropical cyclones (TCs). All the three vortices experience a hydrostatic adjustment after the introduction of thermal asymmetries. It takes more time for weaker and stable vortices to finish such a process. The spiral-band-like structures, especially those distant from the vortex centers, form and evolve accompanying this process. In the quasi-balance state, the spiral bands are gradually concentrated to the inner core, the wave behavior of which resembles the features of classic vortex Rossby (VR) waves. The unstable vortices regain nonhydrostatic features after the quasi-balance stage. The spiral bands further from the vortex center, similar to distant spiral bands in real TCs, form and maintain more easily in the moderate basic-state vortex, satisfying the conditions of barotropic instability. The widest radial extent and longest-lived distant bands always exist in weak and stable vortices. This study represents an attempt to determine the role of TC intensity and stability in the formation and evolution of spiral bands via hydrostatic balance adjustment, and provides some valuable insights into the formation of distant spiral rainbands.
Carbon Dioxide Concentration and Flux in an Urban Residential Area in Seoul, Korea
Moon-Soo PARK, Seung Jin JOO, Soon-Ung PARK
2014, 31(5): 1101-1112. doi: 10.1007/s00376-013-3168-y
The carbon dioxide (CO2) concentrations and fluxes measured at a height of 17.5 m above the ground by a sonic anemometer and an open-path gas analyzer at an urban residential site in Seoul, Korea from February 2011 to January 2012 were analyzed. The annual mean CO2 concentration was found to be 750 mg m-3, with a maximum monthly mean concentration of 827 mg m-3 in January and a minimum value of 679 mg m-3 in August. Meanwhile, the annual mean CO2 flux was found to be 0.45 mg m-2 s-1, with a maximum monthly mean flux of 0.91 mg m-2 s-1 in January and a minimum value of 0.19 mgm-2 s-1 in June. The hourly mean CO2 concentration was found to show a significant diurnal variation; a maximum at 0700-0900 LST and a minimum at 14001600 LST, with a large diurnal range in winter and a small one in summer, mainly caused by diurnal changes in mixing height, CO2 flux, and surface complexity. The hourly mean CO2 flux was also found to show a significant diurnal variation, but it showed two maxima at 07000900 LST and 2100-2400 LST, and two minima at 11001500 LST and 03000500 LST, mainly caused by a diurnal pattern in CO2 emissions and sinks from road traffic, domestic heating and cooking by liquefied natural gas use, and the different horizontal distribution of CO2 sources and sinks near the site. Differential advection with respect to wind direction was also found to be a cause of diurnal variations in both the CO2 concentration and flux.
The Impacts of Two Types of El Nio on Global Ozone Variations in the Last Three Decades
XIE Fei, LI Jianping, TIAN Wenshou, ZHANG Jiankai, SHU Jianchuan
2014, 31(5): 1113-1126. doi: 10.1007/s00376-013-3166-0
The effects of El Nio Modoki events on global ozone concentrations are investigated from 1980 to 2010 El Nio Modoki events cause a stronger Brewer-Dobson (BD) circulation which can transports more ozone-poor air from the troposphere to stratosphere, leading to a decrease of ozone in the lower-middle stratosphere from 90S to 90N. These changes in ozone concentrations reduce stratospheric column ozone. The reduction in stratospheric column ozone during El Nio Modoki events is more pronounced over the tropical eastern Pacific than over other tropical areas because transport of ozone-poor air from middle-high latitudes in both hemispheres to low latitudes is the strongest between 60W and 120W. Because of the decrease in stratospheric column ozone during El Nio Modoki events more UV radiation reaches the tropical troposphere leading to significant increases in tropospheric column ozone An empirical orthogonal function (EOF) analysis of the time series from 1980 to 2010 of stratospheric and tropospheric ozone monthly anomalies reveals that: El Nio Modoki events are associated with the primary EOF modes of both time series. We also found that El Nio Modoki events can affect global ozone more significantly than canonical El Nio events. These results imply that El Nio Modoki is a key contributor to variations in global ozone from 1980 to 2010.
Mapping Near-surface Air Temperature, Pressure, Relative Humidity and Wind Speed over Mainland China with High Spatiotemporal Resolution
LI Tao, ZHENG Xiaogu, DAI Yongjiu, YANG Chi, CHEN Zhuoqi, ZHANG Shupeng, WU Guocan, WANG Zhonglei, HUANG Chengcheng, SHEN Yan, LIAO Rongwei
2014, 31(5): 1127-1135. doi: 10.1007/s00376-014-3190-8
As part of a joint effort to construct an atmospheric forcing dataset for mainland China with high spatiotemporal resolution, a new approach is proposed to construct gridded near-surface temperature, relative humidity, wind speed and surface pressure with a resolution of 1 km 1 km. The approach comprises two steps: (2) fit a partial thin-plate smoothing spline with orography and reanalysis data as explanatory variables to ground-based observations for estimating a trend surface; (3) apply a simple kriging procedure to the residual for trend surface correction. The proposed approach is applied to observations collected at approximately 700 stations over mainland China. The generated forcing fields are compared with the corresponding components of the National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis dataset and the Princeton meteorological forcing dataset. The comparison shows that, both within the station network and within the resolutions of the two gridded datasets, the interpolation errors of the proposed approach are markedly smaller than the two gridded datasets.
An Introduction to the Integrated Climate Model of the Center for Monsoon System Research and Its Simulated Influence of El Nio on East Asian-Western North Pacific Climate
HUANG Ping, WANG Pengfei, HU Kaiming, HUANG Gang, ZHANG Zhihua, LIU Yong, YAN Bangliang
2014, 31(5): 1136-1146. doi: 10.1007/s00376-014-3233-1
This study introduces a new global climate modelthe Integrated Climate Model (ICM)developed for the seasonal prediction of East Asian-western North Pacific (EA-WNP) climate by the Center for Monsoon System Research at the Institute of Atmospheric Physics (CMSR, IAP), Chinese Academy of Sciences. ICM integrates ECHAM5 and NEMO2.3 as its atmospheric and oceanic components, respectively, using OASIS3 as the coupler. The simulation skill of ICM is evaluated here, including the simulated climatology, interannual variation, and the influence of El Nio as one of the most important factors on EA-WNP climate. ICM successfully reproduces the distribution of sea surface temperature (SST) and precipitation without climate shift, the seasonal cycle of equatorial Pacific SST, and the precipitation and circulation of East Asian summer monsoon. The most prominent biases of ICM are the excessive cold tongue and unrealistic westward phase propagation of equatorial Pacific SST. The main interannual variation of the tropical Pacific SST and EA-WNP climateEl Nio and the East Asia-Pacific Patternare also well simulated in ICM, with realistic spatial pattern and period. The simulated El Nio has significant impact on EA-WNP climate, as in other models. The assessment shows ICM should be a reliable model for the seasonal prediction of EA-WNP climate.
Simulation and Causes of Eastern Antarctica Surface Cooling Related to Ozone Depletion during Austral Summer in FGOALS-s2
YANG Jing, BAO Qing, JI Duoying, GONG Daoyi, MAO Rui, ZHANG Ziyin, Seong-Joong KIM
2014, 31(5): 1147-1156. doi: 10.1007/s00376-014-3144-1
Two parallel sets of numerical experiments (an ozone-hole simulation and a non-ozone-hole simulation) were performed to investigate the effect of ozone depletion on surface temperature change using the second spectral version of the Flexible Global Ocean-Atmosphere-Land System model (FGOALS-s2), focusing on the eastern Antarctica (EA) continent in austral summer. First, we evaluated the ability of the model to simulate the EA surface cooling, and found the model can successfully reproduce the cooling trend of the EA surface, as well as the circulation change circling the South Pole in the past 30 years. Second, we compared the two experiments and discovered that the ozone depletion causes the cooling trend and strengthens the circumpolar westerly flow. We further investigated the causes of the EA surface cooling associated with the ozone hole and found two major contributors. The first is the ozone-hole direct radiation effect (DRE) upon the surface that happens because the decrease of the downward longwave (LW) radiation overcomes the increase of the downward shortwave (SW) radiation under clear sky. The second is the cloud radiation effect (CRE) induced by ozone depletion, which happens because the decreased downward SW radiation overcomes the increased downward LW radiation in the case of increased cloud. Although the CRE is theoretically opposite to the DRE, their final net effect makes comparable contributions to the EA surface cooling. Compared with the surface radiation budget, the surface heat flux budgets have a much smaller contribution. We additionally note that the CRE is basically ascribed to the circulation change.
Precipitation Chemistry and Corresponding Transport Patterns of Influencing Air Masses at Huangshan Mountain in East China
SHI ChunE, DENG Xueliang, YANG Yuanjian, WU Biwen,
2014, 31(5): 1157-1166. doi: 10.1007/s00376-014-3189-1
One hundred and ten samples of rainwater were collected for chemical analysis at the summit of Huangshan Mountain, a high-altitude site in East China, from July 2010 to June 2011. The volume-weighted-mean (VWM) pH for the whole sampling period was 5.03. SO42- and Ca2+ were the most abundant anion and cation, respectively. The ionic concentrations varied monthly with the highest concentrations in winter/spring and the lowest in summer. Evident inter-correlations were found among most ions, indicating the common sources for some species and fully mixing characteristics of the alpine precipitation chemistry. The VWM ratio of [SO42-]/[NO3-] was 2.54, suggesting the acidity of rainwater comes from both nitric and sulfuric acids. Compared with contemporary observations at other alpine continental sites in China, the precipitation at Huangshan Mountain was the least polluted, with the lowest ionic concentrations. Trajectories to Huangshan Mountain on rainy days could be classified into six groups. The rainwater with influencing air masses originating in Mongolia was the most polluted with limited effect. The emissions of Jiangxi, Anhui, Zhejiang and Jiangsu provinces had a strong influence on the overall rain chemistry at Huangshan Mountain. The rainwater with influencing air masses from Inner Mongolia was heavily polluted by anthropogenic pollutants.
The Boreal Summer Intraseasonal Oscillation Simulated by Four Chinese AGCMs Participating in the CMIP5 Project
ZHAO Chongbo, ZHOU Tianjun, SONG Lianchun, REN Hongli
2014, 31(5): 1167-1180. doi: 10.1007/s00376-014-3211-7
The performances of four Chinese AGCMs participating in the Coupled Model Intercomparison Project Phase 5 (CMIP5) in the simulation of the boreal summer intraseasonal oscillation (BSISO) are assessed. The authors focus on the major characteristics of BSISO: the intensity, significant period, and propagation. The results show that the four AGCMs can reproduce boreal summer intraseasonal signals of precipitation; however their limitations are also evident. Compared with the Climate Prediction Center Merged Analysis of Precipitation (CMAP) data, the models underestimate the strength of the intraseasonal oscillation (ISO) over the eastern equatorial Indian Ocean (IO) during the boreal summer (May to October), but overestimate the intraseasonal variability over the western Pacific (WP). In the model results, the westward propagation dominates, whereas the eastward propagation dominates in the CMAP data. The northward propagation in these models is tilted southwest-northeast, which is also different from the CMAP result. Thus, there is not a northeast-southwest tilted rain belt revolution off the equator during the BSISO's eastward journey in the models. The biases of the BSISO are consistent with the summer mean state, especially the vertical shear. Analysis also shows that there is a positive feedback between the intraseasonal precipitation and the summer mean precipitation. The positive feedback processes may amplify the models' biases in the BSISO simulation.
On the Blocking Flow Patterns in the Euro-Atlantic Sector: A Simple Model Study
LUO Dehai, YAO Yao
2014, 31(5): 1181-1196. doi: 10.1007/s00376-014-3197-1
The flow patterns of Euro-Atlantic blocking events in winter are investigated by dividing the sector into three sub-regions: 6030W (Greenland region); 20W30E [eastern Atlantic-Europe (EAE) region]; and 5090E (Ural region). It is shown that blocking events in winter are extremely frequent in the three sub-regions. Composite 500-mb geopotential height fields for intense and long-lived blocking events demonstrate that the blocking fields over Greenland and Ural regions exhibit southwest-northeast (SW-NE) and southeast-northwest (SE-NW) oriented dipole-type patterns, respectively, while the composite field over the EAE region exhibits an -type pattern. The type of composite blocking pattern seems to be related to the position of the blocking region relative to the positive center of the climatological stationary wave (CSW) anomaly existing near 10W. The physical cause of why there are different composite blocking types in the three sub-regions is identified using a nonlinear multiscale interaction model. It is found that when the blocking event is in almost the same position as the positive CSW anomaly, the planetary-scale field can exhibit an -type pattern due to the enhanced positive CSW anomaly. Nevertheless, a SW-NE (SE-NW) oriented dipole-type block can occur due to the reduced positive CSW anomaly as it is farther in the west (east) of the positive CSW anomaly. The total fields of blocking in the three regions may exhibit a meandering flow comprised of several isolated anticyclonic and cyclonic vortices, which resembles the Berggren-Bolin-Rossby meandering jet type.
Parameterizations of Surface Radiation in the Semiarid Grasslands of Inner Mongolia under Clear-Sky Conditions Using MODIS Data
PENG Lichun, LI Wanbiao
2014, 31(5): 1197-1208. doi: 10.1007/s00376-014-3215-3
A set of improved and efficient radiation parameterization schemes for surface radiation balance components under clear-sky conditions was developed by using general surface measurements and MODIS data. The set of schemes was then adapted for regions similar to the present study sites under different grazing intensities and varying degrees of drought in the semiarid grasslands of Inner Mongolia. Specifically, we mainly improved two schemes for estimating downward shortwave and longwave radiation at the surface, which could be applied to regions with certain degrees of drought. The validation datasets were from ground-based observations at various grazing sites during the growing season (May to September) of different drought years, 2005 and 2006. Through comparisons of parameterized versus measured radiation values, the increased or modified factors in the original schemes demonstrated improved estimation accuracy, and the rationalities of input parameters and variables were analyzed. The regional instantaneous net radiation estimations had root-mean-square errors of less than 30 W m-2 compared with ground measurements at the sites during the study period. The statistical results showed the improved schemes are suitable for estimating surface net radiation in regional semiarid areas during the growing season. Analyses of the sensitivity of the schemes to corresponding variables were conducted to ascertain the major error sources of the schemes and potential variables for improving the performance of the schemes in agreement with observations.
Analyses of Extreme Climate Events over China Based on CMIP5 Historical and Future Simulations
YANG Shili, FENG Jinming, DONG Wenjie, CHOU Jieming
2014, 31(5): 1209-1220. doi: 10.1007/s00376-014-3119-2
Based on observations and 12 simulations from Coupled Model Intercomparison Project Phase 5 (CMIP5) models, climatic extremes and their changes over China in the past and under the future scenarios of three Representative Concentration Pathways (RCPs) are analyzed. In observations, frost days (FD) and low-temperature threshold days (TN10P) show a decreasing trend, and summer days (SU), high-temperature threshold days (TX90P), heavy precipitation days (R20), and the contribution of heavy precipitation days (P95T) show an increasing trend. Most models are able to simulate the main characteristics of most extreme indices. In particular, the mean FD and TX90P are reproduced the best, and the basic trends of FD, TN10P, SU and TX90P are represented. For the FD and SU indexes, most models show good ability in capturing the spatial differences between the mean state of the periods 1986-2005 and 1961-80; however, for other indices, the simulation abilities for spatial disparity are less satisfactory and need to be improved. Under the high emissions scenario of RCP8.5, the century-scale linear changes of the multi-model ensemble (MME) for FD, SU, TN10P, TX90P, R20 and P95T are46.9, 46.0, -27.1, 175.4, and 2.9 days, and 9.9%, respectively; and the spatial change scope for each index is consistent with the emissions intensity. Due to the complexities of physical process parameterizations and the limitation of forcing data, great uncertainty still exists with respect to the simulation of climatic extremes.
Effects of Additional HONO Sources on Visibility over the North China Plain
LI Ying, AN Junling, Ismail GULTEPE
2014, 31(5): 1221-1232. doi: 10.1007/s00376-014-4019-1
The objective of the present study was to better understand the impacts of the additional sources of nitrous acid (HONO) on visibility, which is an aspect not considered in current air quality models. Simulations of HONO contributions to visibility over the North China Plain (NCP) during August 2007 using the fully coupled Weather Research and Forecasting/Chemistry (WRF/Chem) model were performed, including three additional HONO sources: (2) the reaction of photo-excited nitrogen dioxide (NO2*) with water vapor; (3) the NO2 heterogeneous reaction on aerosol surfaces; and (4) HONO emissions. The model generally reproduced the spatial patterns and diurnal variations of visibility over the NCP well. When the additional HONO sources were included in the simulations, the visibility was occasionally decreased by 20%30% (34 km) in local urban areas of the NCP. Monthly-mean concentrations of NO3-, NH4+, SO42- and PM2.5 were increased by 20%52% (311 g m-3), 10%38%, 6%10%, and 6%11% (917 g m-3), respectively; and in urban areas, monthly-mean accumulation-mode number concentrations (AMNC) and surface concentrations of aerosols were enhanced by 15%20% and 10%20%, respectively. Overall, the results suggest that increases in concentrations of PM2.5, its hydrophilic components, and AMNC, are key factors for visibility degradation. A proposed conceptual model for the impacts of additional HONO sources on visibility also suggests that visibility estimation should consider the heterogeneous reaction on aerosol surfaces and the enhanced atmospheric oxidation capacity due to additional HONO sources, especially in areas with high mass concentrations of NOx and aerosols.
High-Resolution Modeling Study of the Kuroshio Path Variations South of Japan
LI Rui, ZHANG Zuowei, WU Lixin
2014, 31(5): 1233-1244. doi: 10.1007/s00376-014-3230-4
A high-resolution ocean general circulation model (OGCM) is used to investigate the Kuroshio path variations south of Japan. The model reproduces many important features of the Kuroshio system including its interannual bimodal variability south of Japan. A decreasing trend of the spatial averaged relative vorticity is detected when the Kuroshio takes the non-large meander (NLM) path, and during the transition period from the NLM to the large meander (LM), a sudden release of velocity shear corresponds well to the weakening of the Shikoku recirculation gyre (SRG), which plays a key role in modulating the Kuroshio path variations. Analysis of eddy energetics indicates that baroclinic instability is mainly responsible for the formation of the LM. In addition, further analysis shows that the strength of the SRG could be largely influenced by the baroclinic Rossby wave adjustment process, forced by the wind stress curl anomalies in the North Pacific basin, based on the model investigation. It is suggested that the cyclonic disturbances might account for the weakening of the SRG, and act as a remote trigger for the baroclinic instability of the Kuroshio south of Japan.