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2013 Vol. 30, No. 6

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Original Article
Remote Effects of Tropical Cyclone Wind Forcing over the Western Pacific on the Eastern Equatorial Ocean
ZHANG Rong-Hua, PEI Yuhua, CHEN Dake
2013, 30(6): 1507-1525. doi: 10.1007/s00376-013-2283-0
An ocean general circulation model (OGCM) is used to demonstrate remote effects of tropical cyclone wind (TCW) forcing in the tropical Pacific. The signature of TCW forcing is explicitly extracted using a locally weighted quadratic least-squares regression (called as LOESS) method from six-hour satellite surface wind data; the extracted TCW component can then be additionally taken into account or not in ocean modeling, allowing isolation of its effects on the ocean in a clean and clear way. In this paper, seasonally varying TCW fields in year 2008 are extracted from satellite data which are prescribed as a repeated annual cycle over the western Pacific regions off the equator (poleward of 10oN/S); two long-term OGCM experiments are performed and compared, one with the TCW forcing part included additionally and the other not. Large, persistent thermal perturbations (cooling in the mixed layer (ML) and warming in the thermocline) are induced locally in the western tropical Pacific, which are seen to spread with the mean ocean circulation pathways around the tropical basin. In particular, a remote ocean response emerges in the eastern equatorial Pacific to the prescribed off-equatorial TCW forcing, characterized by a cooling in the mixed layer and a warming in the thermocline. Heat budget analyses indicate that the vertical mixing is a dominant process responsible for the SST cooling in the eastern equatorial Pacific. Further studies are clearly needed to demonstrate the significance of these results in a coupled ocean-atmosphere modeling context.
Examination of the Quality of GOSAT/CAI Cloud Flag Data over Beijing Using Ground-based Cloud Data
HUO Juan, ZHANG Wenxing, ZENG Xiaoxia, Lü Daren, LIU Yi
2013, 30(6): 1526-1534. doi: 10.1007/s00376-013-2267-0
It has been several years since the Greenhouse Gases Observing Satellite (GOSAT) began to observe the distribution of CO2 and CH4 over the globe from space. Results from Thermal and Near-infrared Sensor for Carbon Observation-Cloud and Aerosol Imager (TANSO-CAI) cloud screening are necessary for the retrieval of CO2 and CH4 gas concentrations for GOSAT TANSO-Fourier Transform Spectrometer (FTS) observations. In this study, TANSO-CAI cloud flag data were compared with ground-based cloud data collected by an all-sky imager (ASI) over Beijing from June 2009 to May 2012 to examine the data quality. The results showed that the CAI has an obvious cloudy tendency bias over Beijing, especially in winter. The main reason might be that heavy aerosols in the sky are incorrectly determined as cloudy pixels by the CAI algorithm. Results also showed that the CAI algorithm sometimes neglects some high thin cirrus cloud over this area.
Kinematic Features of a Bow Echo in Southern China Observed with Doppler Radar
PENG Xindong, ZHANG Renhe, WANG Hongyan
2013, 30(6): 1535-1548. doi: 10.1007/s00376-012-2108-6
A bow echo is a type of mesoscale convective phenomenon that often induces extreme weather and appears with strong reflectivity on radar images. A strong bow echo that developed from a supercell was observed over Foshan City in southern China on 17 April 2011. The intense gusty winds and showers caused huge losses of property and severely affected human lives. This paper presents an analysis of this strong meso--scale convective system based on Doppler radar observations. The isolated bow echo exhibited a horizontal scale of about 80 km in terms of reflectivity above 40 dBZ, and a life span of 8 hours. The system originated from the merging of a couple of weakly organized cells in a shear line, and developed into an arch shape as it moved through the shear zone. Sufficient surface moisture supply ensured the convective instability and development of the bow echo. The low-altitude winds retrieved from single Doppler radar observations showed an obvious rear-inflow jet along the notch area. Different from the conventional definition, no book-end anticyclone was observed throughout the life cycle. Very strong slantwise updrafts and downdrafts were recognizable from the retrieved winds, even though the spatial scale of the bow echo was small. Strong winds and induced damage on the surface are considered to have been caused by the mid-level rear-inflow jet and intense convective downdrafts.
Earth System Model FGOALS-s2: Coupling a Dynamic Global Vegetation and Terrestrial Carbon Model with the Physical Climate System Model
WANG Jun, BAO Qing, Ning ZENG, LIU Yimin, WU Guoxiong, JI Duoying
2013, 30(6): 1549-1559. doi: 10.1007/s00376-013-2169-1
Earth System Models (ESMs) are fundamental tools for understanding climate-carbon feedback. An ESM version of the Flexible Global Ocean-Atmosphere-Land System model (FGOALS) was recently developed within the IPCC AR5 Coupled Model Intercomparison Project Phase 5 (CMIP5) modeling framework, and we describe the development of this model through the coupling of a dynamic global vegetation and terrestrial carbon model with FGOALS-s2. The performance of the coupled model is evaluated as follows. The simulated global total terrestrial gross primary production (GPP) is 124.4 PgC yr-1 and net primary production (NPP) is 50.9 PgC yr-1. The entire terrestrial carbon pools contain about 2009.9 PgC, comprising 628.2 PgC and 1381.6 PgC in vegetation and soil pools, respectively. Spatially, in the tropics, the seasonal cycle of NPP and net ecosystem production (NEP) exhibits a dipole mode across the equator due to migration of the monsoon rainbelt, while the seasonal cycle is not so significant in Leaf Area Index (LAI). In the subtropics, especially in the East Asian monsoon region, the seasonal cycle is obvious due to changes in temperature and precipitation from boreal winter to summer. Vegetation productivity in the northern mid-high latitudes is too low, possibly due to low soil moisture there. On the interannual timescale, the terrestrial ecosystem shows a strong response to ENSO. The model-simulated Nio3.4 index and total terrestrial NEP are both characterized by a broad spectral peak in the range of 27 years. Further analysis indicates their correlation coefficient reaches 0.7 when NEP lags the ElNio3.4 index for about 12 months.
Spring Indian Ocean-Western Pacific SST Contrast and the East Asian Summer Rainfall Anomaly
CAO Jie, LU Riyu, HU Jinming, WANG Hai
2013, 30(6): 1560-1568. doi: 10.1007/s00376-013-2298-6
After studying the relationship between SST in the tropical Indian Ocean (TIO), tropical western Pacific (TWP), and tropical eastern Pacific (TEP) and East Asian summer rainfall (EASR), using data provided by NOAA/OAR/ESRL PSD and the National Climate Center of China for the period 1979-2008, an index, SSTDI, was defined to describe the SST difference between the TIO and TWP. In comparison with the winter ENSO, the spring SST contrast between the TIO and TWP was found to be more significantly associated with summer rainfall in East Asia, especially along the EASR band and in Northeast China. This spring SST contrast can persist into summer, resulting in a more significant meridional teleconnection pattern of lower-tropospheric circulation anomalies over the western North Pacific and East Asia. These circulation anomalies are dynamically consistent with the summer rainfall anomaly along the EASR band. When the SSTDI is higher (lower) than normal, the EASR over the Yangtze River valley, Korea, and central and southern Japan is heavier (less) than normal. The present results suggest that this spring SST contrast can be used as a new and better predictor of EASR anomalies.
The Impact of Different Vertical Diffusion Schemes in a Three-Dimensional Oil Spill Model in the Bohai Sea
LI Yan, ZHU Jiang, WANG Hui
2013, 30(6): 1569-1586. doi: 10.1007/s00376-012-2201-x
Vertical transport is critical to the movement of oil spills in seawater. Breaking waves play an important role by developing a well-defined mixing layer in the upper part of the water column. A three-dimensional (3-D) Lagrangian random walk oil spill model was used here to study the influence of sea surface waves on the vertical turbulence movement of oil particles. Three vertical diffusion schemes were utilized in the model to compare their impact on oil dispersion and transportation. The first scheme calculated the vertical eddy viscosity semi-empirically. In the second scheme, the vertical diffusion coefficient was obtained directly from an Eulerian hydrodynamic model (Princeton Ocean Model, POM2k) while considering wave-caused turbulence. The third scheme was formulated by solving the Langevin equation. The trajectories, percentages of oil particles intruding into water, and the vertical distribution structures of oil particles were analyzed for a series of numerical experiments with different wind magnitudes. The results showed that the different vertical diffusion schemes could generate different horizontal trajectories and spatial distributions of oil spills on the sea surface. The vertical diffusion schemes caused different water-intruding and resurfacing oil particle behaviors, leading to different horizontal transport of oil particles at the surface and subsurface of the ocean. The vertical diffusion schemes were also applied to a realistic oil spill simulation, and these results were compared to satellite observations. All three schemes yielded acceptable results, and those of the third scheme most closely simulated the observed data.
Evaluation of Spring Persistent Rainfall over East Asia in CMIP3/CMIP5 AGCM Simulations
ZHANG Jie, Laurent LI, ZHOU Tianjun, XIN Xiaoge
2013, 30(6): 1587-1600. doi: 10.1007/s00376-013-2139-7
The progress made from Phase 3 to Phase 5 of the Coupled Model Intercomparison Project (CMIP3 to CMIP5) in simulating spring persistent rainfall (SPR) over East Asia was examined from the outputs of nine atmospheric general circulation models (AGCMs). The majority of the models overestimated the precipitation over the SPR domain, with the mean latitude of the SPR belt shifting to the north. The overestimation was about 1mm d-1 in the CMIP3 ensemble, and the northward displacement was about 3, while in the CMIP5 ensemble the overestimation was suppressed to 0.7 mm d-1 and the northward shift decreased to 2.5. The SPR features a northeast-southwest extended rain belt with a slope of 0.4N/E. The CMIP5 ensemble yielded a smaller slope (0.2N/E), whereas the CMIP3 ensemble featured an unrealistic zonally-distributed slope. The CMIP5 models also showed better skill in simulating the interannual variability of SPR. Previous studies have suggested that the zonal land-sea thermal contrast and sensible heat flux over the southeastern Tibetan Plateau are important for the existence of SPR. These two thermal factors were captured well in the CMIP5 ensemble, but underestimated in the CMIP3 ensemble. The variability of zonal land-sea thermal contrast is positively correlated with the rainfall amount over the main SPR center, but it was found that an overestimated thermal contrast between East Asia and South China Sea is a common problem in most of the CMIP3 and CMIP5 models. Simulation of the meridional thermal contrast is therefore important for the future improvement of current AGCMs.
Nonstationary Time Series Prediction by Incorporating External Forces
WANG Geli, YANG Peicai, ZHOU Xiuji
2013, 30(6): 1601-1607. doi: 10.1007/s00376-013-2134-z
Almost all climate time series have some degree of nonstationarity due to external forces of the observed system. Therefore, these external forces should be taken into account when reconstructing the climate dynamics. This paper presents a novel technique in predicting nonstationary time series. The main difference of this new technique from some previous methods is that it incorporates the driving forces in the prediction model. To appraise its effectiveness, three prediction experiments were carried out using the data generated from some known classical dynamical models and a climate model with multiple external forces. Experimental results indicate that this technique is able to improve the prediction skill effectively.
Spatial Modeling of the Highest Daily Maximum Temperature in Korea via Max-stable Processes
Youngsaeng LEE, Sanghoo YOON, Md. Sharwar MURSHED, Maeng-Ki KIM, ChunHo CHO, Hee-Jeong BAEK, Jeong-Soo PARK
2013, 30(6): 1608-1620. doi: 10.1007/s00376-013-2216-y
This paper examines the annual highest daily maximum temperature (DMT) in Korea by using data from 56 weather stations and employing spatial extreme modeling. Our approach is based on max-stable processes (MSP) with Schlather's characterization. We divide the country into four regions for a better model fit and identify the best model for each region. We show that regional MSP modeling is more suitable than MSP modeling for the entire region and the pointwise generalized extreme value distribution approach. The advantage of spatial extreme modeling is that more precise and robust return levels and some indices of the highest temperatures can be obtained for observation stations and for locations with no observed data, and so help to determine the effects and assessment of vulnerability as well as to downscale extreme events.
A Global Ocean Reanalysis Product in the China Ocean Reanalysis (CORA) Project
HAN Guijun, FU Hongli, ZHANG Xuefeng, LI Wei, WU Xinrong, WANG Xidong, ZHANG Lianxin
2013, 30(6): 1621-1631. doi: 10.1007/s00376-013-2198-9
The first version of a global ocean reanalysis over multiple decades (1979-2008) has been completed by the National Marine Data and Information Service within the China Ocean Reanalysis (CORA) project. The global ocean model employed is based upon the ocean general circulation model of the Massachusetts Institute of Technology. A sequential data assimilation scheme within the framework of 3D variational (3DVar) analysis, called multi-grid 3DVar, is implemented in 3D space for retrieving multiple-scale observational information. Assimilated oceanic observations include sea level anomalies (SLAs) from multi-altimeters, sea surface temperatures (SSTs) from remote sensing satellites, and (i)n-situ temperature/salinity profiles. Evaluation showed that compared to the model simulation, the annual mean heat content of the global reanalysis is significantly approaching that of World Ocean Atlas 2009 (WOA09) data. The quality of the global temperature climatology was found to be comparable with the product of Simple Ocean Data Assimilation (SODA), and the major ENSO events were reconstructed. The global and Atlantic meridional overturning circulations showed some similarity as SODA, although significant differences were found to exist. The analysis of temperature and salinity in the current version has relatively larger errors at high latitudes and improvements are ongoing in an updated version. CORA was found to provide a simulation of the subsurface current in the equatorial Pacific with a correlation coefficient beyond about 0.6 compared with the Tropical Atmosphere Ocean (TAO) mooring data. The mean difference of SLAs between altimetry data and CORA was less than 0.1 m in most years.
Aerosol Spectra and New Particle Formation Observed in Various Seasons in Nanjing
ZHU Bin, WANG Honglei, SHEN Lijuan, KANG Hanqing, YU Xingna
2013, 30(6): 1632-1644. doi: 10.1007/s00376-013-2202-4
The aerosol number spectrum and gas pollutants were measured and the new particle formation (NPF) events were discussed in Nanjing. The results showed that the size distributions of aerosol number concentrations exhibited distinct seasonal variations, implying the relations of particle sizes and their sources and sinks. The number concentrations of particles in the nuclei mode (10-30 nm), Aitken mode (30-100 nm), accumulation mode (100-1000 nm) and coarse mode (1 m) varied in the order of summer spring autumn, summer autumn spring, autumn summer spring, and spring autumn summer, respectively. The diurnal variation of total aerosol number concentrations showed three peaks in all observed periods, which corresponded to two rush hours and the photochemistry period at noon. In general, the NPF in summer occurred under the conditions of east winds and dominant air masses originating from marine areas with high relative humidity (50%-70%) and strong solar radiations (400-700 W m-2). In spring, the NPF were generally accompanied by low relative humidity (14%-30%) and strong solar radiations (400-600 W m-2). The new particle growth rates (GR) were higher in the summertime in the range of 10-16 nm h-1. In spring, the GR were 6.8-8.3 nm h-1. Under polluted air conditions, NPF events were seldom captured in autumn in Nanjing. During NPF periods, positive correlations between 10-30 nm particles and O3 were detected, particularly in spring, indicating that NPF can be attributed to photochemical reactions.
Evaluation of the Twentieth Century Reanalysis Dataset in Describing East Asian Winter Monsoon Variability
ZHANG Ziyin, GUO Wenli, GONG Daoyi, Seong-Joong KIM
2013, 30(6): 1645-1652. doi: 10.1007/s00376-012-2226-1
The Twentieth Century Reanalysis (20thCR) dataset released in 2010 covers the period 1871-2010 and is one of the longest reanalysis datasets available worldwide. Using ERA-40, ERA-Interim and NCEP-NCAR reanalysis data, as well as HadSLP2 data and meteorological temperature records over eastern China, the performances of 20thCR in reproducing the spatial patterns and temporal variability of the East Asian winter monsoon (EAWM) are examined. Results indicate that 20thCR data: (1) can accurately reproduce the most typical configuration patterns of all sub-factors involved in the EAWM system, albeit with some differences in the main circulation fields over East Asia in comparison to ERA-40 reanalysis data; (2) is reliable and stable in describing the temporal variability of EAWM since the 1930s; and (3) can describe the high-frequency variability of EAWM better than the low-frequency fluctuations, especially in the early period. In conclusion, caution should be taken when using 20thCR data to study interdecadal variabilities or long-term trends of the EAWM, especially prior to the 1930s.
Development of a Meteorological and Hydrological Coupling Index for Droughts and Floods along the Yangtze River Valley of China
NIU Tao, WANG Jizhi, YANG Yuanqin, LIU Hongli, CHEN Miao, LIU Jiyan
2013, 30(6): 1653-1662. doi: 10.1007/s00376-013-2303-0
To comprehensively investigate characteristics of summer droughts and floods in the Yangtze River valley, a meteorological and hydrological coupling index (MHCI) was developed using meteorological and hydrological data. The results indicate that: (1) in representing drought/flood information for the Yangtze River valley, the MHCI can reflect composite features of precipitation and hydrological observations; (2) comprehensive analysis of the interannual phase difference of the precipitation and hydrological indices is important to recognize and predict annual drought/flood events along the valley; the hydrological index contributes more strongly to nonlinear and continuity features that indicate transition from long-term drought to flood conditions; (3) time series of the MHCI from 1960-2009 are very effective and sensitive in reflecting annual drought/flood characteristics, i.e. there is more rainfall or typical flooding in the valley when the MHCI is positive, and vice versa; and (4) verification of the MHCI indicates that there is significant correlation between precipitation and hydrologic responses in the valley during summer; the correlation coefficient was found to reach 0.82, exceeding the 0.001 significance level.
Simulating Urban Flow and Dispersion in Beijing by Coupling a CFD Model with the WRF Model
MIAO Yucong, LIU Shuhua, CHEN Bicheng, ZHANG Bihui, WANG Shu, LI Shuyan
2013, 30(6): 1663-1678. doi: 10.1007/s00376-013-2234-9
The airflow and dispersion of a pollutant in a complex urban area of Beijing, China, were numerically examined by coupling a Computational Fluid Dynamics (CFD) model with a mesoscale weather model. The models used were Open Source Field Operation and Manipulation (OpenFOAM) software package and Weather Research and Forecasting (WRF) model. OpenFOAM was firstly validated against wind-tunnel experiment data. Then, the WRF model was integrated for 42 h starting from 0800 LST 08 September 2009, and the coupled model was used to compute the flow fields at 1000 LST and 1400 LST 09 September 2009. During the WRF-simulated period, a high pressure system was dominant over the Beijing area. The WRF-simulated local circulations were characterized by mountain valley winds, which matched well with observations. Results from the coupled model simulation demonstrated that the airflows around actual buildings were quite different from the ambient wind on the boundary provided by the WRF model, and the pollutant dispersion pattern was complicated under the influence of buildings. A higher concentration level of the pollutant near the surface was found in both the step-down and step-up notches, but the reason for this higher level in each configurations was different: in the former, it was caused by weaker vertical flow, while in the latter it was caused by a downward-shifted vortex. Overall, the results of this study suggest that the coupled WRF-OpenFOAM model is an important tool that can be used for studying and predicting urban flow and dispersions in densely built-up areas.
Diurnal Cycles of Precipitation over Subtropical China in IPCC AR5 AMIP Simulations
YUAN Weihua
2013, 30(6): 1679-1694. doi: 10.1007/s00376-013-2250-9
Atmospheric Intercomparison Project simulations of the summertime diurnal cycle of precipitation and low-level winds over subtropical China by Intergovernmental Panel on Climate Change Fifth Assessment Report models were evaluated. By analyzing the diurnal variation of convective and stratiform components, results confirmed that major biases in rainfall diurnal cycles over subtropical China are due to convection parameterization and further pointed to the diurnal variation of convective rainfall being closely related to the closure of the convective scheme. All models captured the early-morning peak of total rainfall over the East China Sea, but most models had problems in simulating diurnal rainfall variations over land areas of subtropical China. When total rainfall was divided into stratiform and convective rainfall, all models successfully simulated the diurnal variation of stratiform rainfall with a maximum in the early morning. The models, overestimating noon-time (nocturnal) total rainfall over land, generally simulated too much convective rainfall, which peaked close to noon (midnight), sharing some similarities in the closures of their deep convection schemes. The better performance of the Meteorological Research Institute atmosphere-ocean coupled global climate model version 3 (MRI-CGCM3) is attributed to the well captured ratio of the two kinds of rainfall, but not diurnal variations of the two components. Therefore, a proper ratio of convective and stratiform rainfall to total rainfall is also important to improve simulated diurnal rainfall variation.
A Modeling Analysis of Rainfall and Water Cycle by the Cloud-resolving WRF Model over the Western North Pacific
GAO Wenhua, SUI Chung-Hsiung
2013, 30(6): 1695-1711. doi: 10.1007/s00376-013-2288-8
Simulated regional precipitation, especially extreme precipitation events, and the regional hydrologic budgets over the western North Pacific region during the period from May to June 2008 were investigated with the high-resolution (4-km grid spacing) Weather Research and Forecast (WRF v3.2.1) model with explicit cloud microphysics. The model initial and boundary conditions were derived from the National Centers for Environmental Prediction/Department of Energy (NCEP/DOE) Reanalysis 2 data. The model precipitation results were evaluated against the Tropical Rainfall Measuring Mission (TRMM) Multisatellite Precipitation Analysis 3B42 product. The results show that the WRF simulations can reasonably reproduce the spatial distributions of daily mean precipitation and rainy days. However, the simulated frequency distributions of rainy days showed an overestimation of light precipitation, an underestimation of moderate to heavy precipitation, but a good representation of extreme precipitation. The downscaling approach was able to add value to the very heavy precipitation over the ocean since the convective processes are resolved by the high-resolution cloud-resolving model. Moreover, the water vapor budget analysis indicates that heavy precipitation is contributed mostly by the stronger moisture convergence; whereas, in less convective periods, the precipitation is more influenced by the surface evaporation. The simulated water vapor budgets imply the importance in the tropical monsoon region of cloud microphysics that affects the precipitation, atmospheric latent heating and, subsequently, the large-scale circulation.
Recent Trends in Winter Temperature Extremes in Eastern China and their Relationship with the Arctic Oscillation and ENSO
CHEN Shangfeng, CHEN Wen, WEI Ke
2013, 30(6): 1712-1724. doi: 10.1007/s00376-013-2296-8
Interannual variations in the number of winter extreme warm and cold days over eastern China (EC) and their relationship with the Arctic Oscillation (AO) and El Nio-Southern Oscillation (ENSO) were investigated using an updated temperature dataset comprising 542 Chinese stations during the period 1961-2011. Results showed that the number of winter extreme warm (cold) days across EC experienced a significant increase (decrease) around the mid-1980s, which could be attributed to interdecadal variation of the East Asian Winter Monsoon (EAWM). Probability distribution functions (PDFs) of winter temperature extremes in different phases of the AO and ENSO were estimated based on Generalized Extreme Value Distribution theory. Correlation analysis and the PDF technique consistently demonstrated that interannual variation of winter extreme cold days in the northern part of EC (NEC) is closely linked to the AO, while it is most strongly related to the ENSO in the southern part (SEC). However, the number of winter extreme warm days across EC has little correlation with both AO and ENSO. Furthermore, results indicated that, whether before or after the mid-1980s shift, a significant connection existed between winter extreme cold days in NEC and the AO. However, a significant connection between winter extreme cold days in SEC and the ENSO was only found after the mid-1980s shift. These results highlight the different roles of the AO and ENSO in influencing winter temperature extremes in different parts of EC and in different periods, thus providing important clues for improving short-term climate prediction for winter temperature extremes.
Impact of Dust Aerosol on Glacial-Interglacial Climate
LIU Yuzhi, SHI Guangyu, XIE Yongkun
2013, 30(6): 1725-1731. doi: 10.1007/s00376-013-2289-7
The temperature anomaly and dust concentrations recorded from central Antarctic ice core records display a strong negative correlation. The dust concentration recorded from an ice core in central Antarctica is 5070 times higher during glacial periods than interglacial periods. This study investigated the impact of dust aerosol on glacial-interglacial climate, using a zonal energy balance model and dust concentration data from an Antarctica ice core. Two important effects of dust, the direct radiative effect and dust-albedo feedback, were considered. On the one hand, the direct radiative effect of dust significantly cooled the climate during the glacial period, with cooling during the last glacial maximum being as much as 2.05oC in Antarctica. On the other hand, dust deposition onto the ice decreased the surface albedo over Antarctica, leading to increased absorption of solar radiation, inducing a positive feedback that warmed the region by as much as about 0.9oC during the glacial period. However, cooling by the direct dust effect was found to be the controlling effect for the glacial climate and may be the major influence on the strong negative correlation between temperature and dust concentration during glacial periods.
Impacts of Two Types of El Nio on Atmospheric Circulation in the Southern Hemisphere
SUN Dan, XUE Feng, ZHOU Tianjun
2013, 30(6): 1732-1742. doi: 10.1007/s00376-013-2287-9
Based on NCEP/NCAR (National Centers for Environmental Prediction/National Center for Atmospheric Research) reanalysis data from 1979 to 2010, the impacts of two types of El Nio on atmospheric circulation in the Southern Hemisphere (SH) are analyzed. It is shown that, when a warming event occurs in the equatorial eastern Pacific (EP El Nio), there is a negative sea level pressure (SLP) anomaly in the eastern Pacific and a positive one in the western Pacific. Besides, there exists a negative anomaly between 40oS and 60oS and a positive anomaly to the south of 60oS. When a warming event in the central Pacific (CP El Nio) occurs, there appears a negative SLP anomaly in the central Pacific and a positive SLP anomaly in the eastern and western Pacific, but the SLP anomalies are not so evident in the SH extratropics. In particular, the Pacific-South America (PSA) pattern induced by the CP El Nio is located more northwestward, with a weaker anomaly compared with the EP El Nio. This difference is directly related with the different position of heating centers associated with the two types of El Nio events. Because the SST anomaly associated with CP El Nio is located more westward than that associated with EP El Nio, the related heating center tends to move westward and the response of SH atmospheric circulation to the tropical heating changes accordingly, thus exciting a different position of the PSA pattern. It is also noted that the local meridional cell plays a role in the SH high latitudes during EP El Nio. The anomalous ascending motion due to the enhancement of convection over the eastern Pacific leads to an enhancement of the local Hadley cell and the meridional cell in the middle and high latitudes, which in turn induces an anomalous descending motion and the related positive anomaly of geopotential height over the Amundsen-Bellingshausen Sea.
Two Types of El Nio-related Southern Oscillation and Their Different Impacts on Global Land Precipitation
XU Kang, ZHU Congwen, HE Jinhai
2013, 30(6): 1743-1757. doi: 10.1007/s00376-013-2272-3
The contrast between the eastern and central Pacific (EP- and CP-) El Nio is observed in the different responses of zonal and vertical circulation in the tropics. To measure the different responses of the atmospheric circulation to the two types of El Ni, an eastern and a central Pacific southern oscillation index (EP- and CP-SOI) are defined based on the air-sea coupled relationship between eddy sea level pressure and sea surface temperature. Analyses suggest that while the EP-SOI exhibits variability on an interannual (27-yr) time scale, decadal (1015-yr) variations in the CP-SOI are more dominant; both are strongly coupled with their respective EP- and CP-El Nio patterns. Composite analysis suggests that, during EP-ENSO, the Walker circulation exhibits a dipole structure in the lower-level (850 hPa) and upper-level (200 hPa) velocity potential anomalies and exhibits a signal cell over the Pacific. In the case of CP-ENSO, however, the Walker circulation shows a tripole structure and exhibits double cells over the Pacific. In addition, the two types of ENSO events show opposite impacts on global land precipitation in the boreal winter and spring seasons. For example, seasonal precipitation across mainland China exhibits an opposite relationship with the EP- and CP-ENSO during winter and spring, but the rainfall over the lower reaches of the Yangtze River and South China shows an opposite relationship during the rest of the seasons. Therefore, the different relationships between rainfall and EP- and CP-ENSO should be carefully considered when predicting seasonal rainfall in the East Asian monsoon regions.
Why Does FGOALS-gl Reproduce a Weak Medieval Warm Period But a Reasonable Little Ice Age and 20th Century Warming?
GUO Zhun, ZHOU Tianjun
2013, 30(6): 1758-1770. doi: 10.1007/s00376-013-2227-8
To understand the strengths and limitations of a low-resolution version of Flexible Global Ocean-Atmosphere-Land-Sea-ice (FGOALS-gl) to simulate the climate of the last millennium, the energy balance, climate sensitivity and absorption feedback of the model are analyzed. Simulation of last-millennium climate was carried out by driving the model with natural (solar radiation and volcanic eruptions) and anthropogenic (greenhouse gases and aerosols) forcing agents. The model feedback factors for (model sensitivity to) different forcings were calculated. The results show that the system feedback factor is about 2.5 (W m-2) K-1 in the pre-industrial period, while 1.9 (W m-2) K-1 in the industrial era. Thus, the model's sensitivity to natural forcing is weak, which explains why it reproduces a weak Medieval Warm Period. The relatively reasonable simulation of the Little Ice Age is caused by both the specified radiative forcing and unforced linear cold drift. The model sensitivity in the industrial era is higher than that of the pre-industrial period. A negative net cloud radiative feedback operates during whole-millennial simulation and reduces the model's sensitivity to specified forcing. The negative net cloud radiative forcing feedback under natural forcing in the period prior to 1850 is due to the underestimation (overestimation) of the response of cloudiness (in-cloud water path). In the industrial era, the strong tropospheric temperature response enlarges the effective radius of ice clouds and reduces the fractional ice content within cloud, resulting in a weak negative net cloud feedback in the industrial period. The water vapor feedback in the industrial era is also stronger than that in the pre-industrial period. Both are in favor of higher model sensitivity and thus a reasonable simulation of the 20th century global warming.
A Two-way Stratosphere-Troposphere Coupling of Submonthly Zonal-Mean Circulations in the Arctic
LI Xiaofeng, LI Jianping, Xiangdong ZHANG
2013, 30(6): 1771-1785. doi: 10.1007/s00376-013-2210-4
This paper examines the dominant submonthly variability of zonally symmetrical atmospheric circulation in the Northern Hemisphere (NH) winter within the context of the Northern Annular Mode (NAM), with particular emphasis on interactive stratosphere-troposphere processes. The submonthly variability is identified and measured using a daily NAM index, which concentrates primarily on zonally symmetrical circulation. A schematic lifecycle of submonthly variability is developed that reveals a two-way coupling process between the stratosphere and troposphere in the NH polar region. Specifically, anomalous tropospheric zonal winds in the Atlantic and Pacific sectors of the Arctic propagate upwards to the low stratosphere, disturbing the polar vortex, and resulting in an anomalous stratospheric geopotential height (HGT) that subsequently propagates down into the troposphere and changes the sign of the surface circulations. From the standpoint of planetary-scale wave activities, a feedback loop is also evident when the anomalous planetary-scale waves (with wavenumbers 2 and 3) propagate upwards, which disturbs the anomalous zonally symmetrical flow in the low stratosphere, and induces the anomalous HGT to move poleward in the low stratosphere, and then propagates down into the troposphere. This increases the energy of waves at wavenumbers 2 and 3 in the low troposphere in middle latitudes by enhancing the land-sea contrast of the anomalous HGT field. Thus, this study supports the viewpoint that the downward propagation of stratospheric NAM signals may not originate in the stratosphere.
Case Studies of Sprite-producing and Non-sprite-producing Summer Thunderstorms
YANG Jing, YANG Meirong, LIU Chao, FENG Guili
2013, 30(6): 1786-1808. doi: 10.1007/s00376-013-2120-5
Three summer thunderstorms in the eastern region of China were analyzed in detail using multiple data, including Doppler radar, lightning location network, TRMM (Tropical Rainfall Measuring Mission), MTSAT (Multi-Function Transport Satellite) images, NCEP (National Centers for Environmental Prediction) Reanalysis, and radiosonde. Two of the three storms were sprite-producing and the other was non-sprite-producing. The two sprite-producing storms occurred on 12 August and 2728 July 2007, producing 16 and one sprite, respectively. The non-sprite-producing storm occurred on 2930 July 2007. The major objective of the study was to try to find possible differences between sprite-producing and non-sprite producing storms using the multiple datasets. The results showed that the convection in the 12 August storm was the strongest compared with the other storms, and it produced the largest number of sprites. Precipitation ice, cloud ice and cloud water content in the convective regions in the 12 August storm were larger than in the other two storms, but the opposite was true in the weak convective regions. The storm microphysical properties along lines through parent CG (cloud-to-ground lightning) locations showed no special characteristics related to sprites. The flash rate evolution in the 12 August storm provided additional confirmation that major sprite activity coincides with a rapid decrease in the negative CG flash rate. However, the evolution curve of the CG flash rate was erratic in the sprite-producing storm on 2728 July, which was significantly different from that in the 12 August storm. The average positive CG peak current in sprite-producing storms was larger than that in the non-sprite-producing one.
Effects of Vertical Wind Shear, Radiation and Ice Microphysics on Precipitation Efficiency during a Torrential Rainfall Event in China
ZHOU Yushu
2013, 30(6): 1809-1820. doi: 10.1007/s00376-013-3007-1
The effects of vertical wind shear, radiation and ice microphysics on precipitation efficiency (PE) were investigated through analysis of modeling data of a torrential rainfall event over Jinan, China during July 2007. Vertical wind shear affected PE by changing the kinetic energy conversion between the mean and perturbation circulations. Cloud-radiation interaction impacted upon PE, but the relationship related to cloud radiative effects on PE was not statistically significant. The reduction in deposition processes associated with the removal of ice microphysics suppressed efficiency. The relationships related to effects of vertical wind shear, radiation and ice clouds on PEs defined in cloud and surface rainfall budgets were more statistically significant than that defined in the rain microphysical budget.