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    Locating Parent Lightning Strokes of Sprites Observed over a Mesoscale Convective System in Shandong Province, China
    Anjing HUANG, Gaopeng LU, Hongbo ZHANG, Feifan LIU, Yanfeng FAN, Baoyou ZHU, Jing YANG, Zhichao WANG
    DOI: 10.1007/s00376-018-7306-4
    Abstract   ( 23 ) PDF (3479KB) (4)
    In this paper, we report the location results for the parent lightning strokes of more than 30 red sprites observed over an asymmetric mesoscale convective system (MCS) on 30 July 2015 in Shandong Province, China, with a long-baseline lightning location network of very-low-frequency/low-frequency magnetic field sensors. The results show that almost all of these cloud-to-ground (CG) strokes are produced during the mature stage of the MCS, and are predominantly located in the trailing stratiform region, which is similar to analyses of sprite-productive MCSs in North America and Europe. Comparison between the location results for the sprite-producing CG strokes and those provided by the World Wide Lightning Location Network (WWLLN) indicates that the location accuracy of WWLLN for intense CG strokes in Shandong Province is typically within 10 km, which is consistent with the result based on analysis of 2838 sprite-producing CG strokes in the continental United States. Also, we analyze two cases where some minor lightning discharges in the parent flash of sprites can also be located, providing an approach to confine the thundercloud region tapped by the sprite-producing CG strokes.
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    Temporal and Spatial Variations in the Climate Controls of Vegetation Dynamics on the Tibetan Plateau during 1982--2011
    Ting HUA, Xunming WANG
    DOI: 10.1007/s00376-018-7064-3
    Abstract   ( 30 ) PDF (1969KB) (5)
    The ecosystem of the Tibetan Plateau is highly susceptible to climate change. Currently, there is little discussion on the temporal changes in the link between climatic factors and vegetation dynamics in this region under the changing climate. By employing Normalized Difference Vegetation Index data, the Climatic Research Unit temperature and precipitation data, and the in-situ meteorological observations, we report the temporal and spatial variations in the relationships between the vegetation dynamics and climatic factors on the Plateau over the past three decades. The results show that from the early 1980s to the mid-1990s, vegetation dynamics in the central and southeastern part of the Plateau appears to show a closer relationship with precipitation prior to the growing season than that of temperature. From the mid-1990s, the temperature rise seems to be the key climatic factor correlating vegetation growth in this region. The effects of increasing temperature on vegetation are spatially variable across the Plateau: it has negative impacts on vegetation activity in the southwestern and northeastern part of the Plateau, and positive impacts in the central and southeastern Plateau. In the context of global warming, the changing climate condition (increasing precipitation and significant rising temperature) might be the potential contributor to the shift in the climatic controls on vegetation dynamics in the central and southeastern Plateau.
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    Impact of Global Oceanic Warming on Winter Eurasian Climate
    Xin HAO, Shengping HE, Tingting HAN, Huijun WANG
    DOI: 10.1007/s00376-018-7216-5
    Abstract   ( 35 ) PDF (1900KB) (20)
    In the 20th century, Eurasian warming was observed and was closely related to global oceanic warming (the first leading rotated empirical orthogonal function of annual mean sea surface temperature over the period 1901–2004). Here, large-scale patterns of covariability between global oceanic warming and circulation anomalies are investigated based on NCEP–NCAR reanalysis data. In winter, certain dominant features are found, such as a positive pattern of the North Atlantic Oscillation (NAO), low-pressure anomalies over northern Eurasia, and a weakened East Asian trough. Numerical experiments with the CAM3.5, CCM3 and GFDL models are used to explore the contribution of global oceanic warming to the winter Eurasian climate. Results show that a positive NAO anomaly, low-pressure anomalies in northern Eurasia, and a weaker-than-normal East Asian trough are induced by global oceanic warming. Consequently, there are warmer winters in Europe and the northern part of East Asia. However, the Eurasian climate changes differ slightly among the three models. Eddy forcing and convective heating from those models may be the reason for the different responses of Eurasian climate.
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    Assimilation of Sea Surface Temperature in a Global Hybrid Coordinate Ocean Model
    Yueliang CHEN, Changxiang YAN, Jiang ZHU
    DOI: 10.1007/s00376-018-7284-6
    Abstract   ( 24 ) PDF (2784KB) (2)
    The Hybrid Coordinate Ocean Model (HYCOM) uses different vertical coordinate choices in different regions. In HYCOM, the prognostic variables include not only the seawater temperature, salinity and current fields, but also the layer thickness. All prognostic variables are usually adjusted in the assimilation when multivariate data assimilation methods are used to assimilate sea surface temperature (SST). This paper investigates the effects of SST assimilation in a global HYCOM model using the Ensemble Optimal Interpolation multivariate assimilation method. Three assimilation experiments are conducted from 2006--08. In the first experiment, all model variables are adjusted during the assimilation process. In the other two experiments, the temperature alone is adjusted in the entire water column and in the mixed layer. For comparison, a control experiment without assimilation is also conducted. The three assimilation experiments yield notable SST improvements over the results of the control experiment. Additionally, the experiments in which all variables are adjusted and the temperature alone in all model layers is adjusted, produce significant negative effects on the subsurface temperature. Also, they yield negative effects on the subsurface salinity because it is associated with temperature and layer thickness. The experiment adjusting the temperature alone in the mixed layer yields positive effects and outperforms the other experiments. The heat content in the upper 300 m and 300--700 m layers further suggests that it yields the best performance among the experiments.
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    Subseasonal Change in the Seesaw Pattern of Precipitation between the Yangtze River Basin and the Tropical Western North Pacific during Summer
    Xinyu LI, Riyu LU
    DOI: 10.1007/s00376-018-7304-6
    Abstract   ( 8 ) PDF (2328KB) (0)
    There is a well-known seesaw pattern of precipitation between the tropical western North Pacific (WNP) and the Yangtze River basin (YRB) during summer. This study identified that this out-of-phase relationship experiences a subseasonal change; that is, the relationship is strong during early summer but much weaker during mid-summer. We investigated the large-scale circulation anomalies responsible for the YRB rainfall anomalies on the subseasonal timescale. It was found that the YRB rainfall is mainly affected by the tropical circulation anomalies during early summer, i.e., the anticyclonic or cyclonic anomaly over the subtropical WNP associated with the precipitation anomalies over the tropical WNP. During mid-summer, the YRB rainfall is mainly affected by the extratropical circulation anomalies in both the lower and upper troposphere. In the lower troposphere, the northeasterly anomaly north of the YRB favors heavier rainfall over the YRB by intensifying the meridional gradient of the equivalent potential temperature over the YRB. In the upper troposphere, the meridional displacement of the Asian westerly jet and the zonally oriented teleconnection pattern along the jet also affect the YRB rainfall. The subseasonal change in the WNP--YRB precipitation relationship illustrated by this study has important implications for the subseasonal-to-seasonal forecasting of the YRB rainfall.
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    A High-Resolution Modeling Study of the 19 June 2002 Convective Initiation Case during IHOP_2002: Localized Forcing by Horizontal Convective Rolls
    Qi-Wei WANG, Ming XUE
    DOI: 10.1007/s00376-018-7218-3
    Abstract   ( 51 ) PDF (6337KB) (31)
    The initiation processes of one of the initial convective cells near and on the east side of a dryline on 19 June 2002 during the IHOP_2002 field experiment in the central United States is analyzed in detail based on a high-resolution numerical simulation. Prominent horizontal convective rolls and associated near-surface moisture convergence bands [called roll convergence bands (RCBs) here] develop within the convective boundary layer (CBL) due to surface heating, in the hours leading to convective initiation (CI). The RCBs east of the dryline are advected toward the primary dryline convergence boundary (PDCB) by the southerly moist flow as the CBL deepens with time. Backward trajectories of air parcels forming the initial precipitating updraft of the convective cell are found to primarily originate at about 1--1.5 km above ground, within the upper portion of the shallower CBL earlier on. The representative air parcel is found to follow and stay on top of a surface RCB as the RCB moves toward the PDCB, but the RCB forcing alone is not enough to initiate convection. As this RCB gets close to the PDCB, it moves into a zone of mesoscale convergence and a deeper CBL that exhibits an upward moisture bulge associated with the PDCB. The combined upward forcing of the RCB and the mesoscale PDCB convergence quickly lifts the representative air parcel above its level of free convection to initiate convection. A conceptual model summarizing the CI processes is proposed.
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    Possible Sources of Forecast Errors Generated by the Global/Regional Assimilation and Prediction System for Landfalling Tropical Cyclones. Part II: Model Uncertainty
    Feifan ZHOU, Wansuo DUAN, He ZHANG, Munehiko YAMAGUCHI
    DOI: 10.1007/s00376-018-7095-9
    Abstract   ( 7 ) PDF (4080KB) (1)
    This paper investigates the possible sources of errors associated with tropical cyclone (TC) tracks forecasted using the Global/Regional Assimilation and Prediction System (GRAPES). In Part I, it is shown that the model error of GRAPES may be the main cause of poor forecasts of landfalling TCs. Thus, a further examination of the model error is the focus of Part II. Considering model error as a type of forcing, the model error can be represented by the combination of good forecasts and bad forecasts. Results show that there are systematic model errors. The model error of the geopotential height component has periodic features, with a period of 24 h and a global pattern of wavenumber 2 from west to east located between 60°S and 60°N. This periodic model error presents similar features as the atmospheric semidiurnal tide, which reflect signals from tropical diabatic heating, indicating that the parameter errors related to the tropical diabatic heating may be the source of the periodic model error. The above model errors are subtracted from the forecast equation and a series of new forecasts are made. The average forecasting capability using the rectified model is improved compared to simply improving the initial conditions of the original GRAPES model. This confirms the strong impact of the periodic model error on landfalling TC track forecasts. Besides, if the model error used to rectify the model is obtained from an examination of additional TCs, the forecasting capabilities of the corresponding rectified model will be improved.
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    Investigating the Initial Errors that Cause Predictability Barriers for Indian Ocean Dipole Events Using CMIP5 Model Outputs
    Rong FENG, Wansuo DUAN
    DOI: 10.1007/s00376-018-7214-7
    Abstract   ( 25 ) PDF (3450KB) (9)
    By analyzing the outputs of the pre-industrial control runs of four models within phase 5 of the Coupled Model Intercomparison Project, the effects of initial sea temperature errors on the predictability of Indian Ocean Dipole events were identified. The initial errors cause a significant winter predictability barrier (WPB) or summer predictability barrier (SPB). The WPB is closely related with the initial errors in the tropical Indian Ocean, where two types of WPB-related initial errors display opposite patterns and a west–east dipole. In contrast, the occurrence of the SPB is mainly caused by initial errors in the tropical Pacific Ocean, where two types of SPB-related initial errors exhibit opposite patterns, with one pole in the subsurface western Pacific Ocean and the other in the upper eastern Pacific Ocean. Both of the WPB-related initial errors grow the fastest in winter, because the coupled system is at its weakest, and finally cause a significant WPB. The SPB-related initial errors develop into a La Niña–like mode in the Pacific Ocean. The negative SST errors in the Pacific Ocean induce westerly wind anomalies in the Indian Ocean by modulating the Walker circulation in the tropical oceans. The westerly wind anomalies first cool the sea surface water in the eastern Indian Ocean. When the climatological wind direction reverses in summer, the wind anomalies in turn warm the sea surface water, finally causing a significant SPB. Therefore, in addition to the spatial patterns of the initial errors, the climatological conditions also play an important role in causing a significant predictability barrier.
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