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    Accepted:
    Global Monsoon Changes under the Paris Agreement Temperature Goals in CESM1(CAM5)
    Xia QU and Gang HUANG
    DOI: 10.1007/s00376-018-8138-y
    Abstract   ( 21 ) PDF (4225KB) (8)
    Based on experiments with the Community Earth System Model, version 1 (Community Atmosphere Model, version 5) [CESM1(CAM5)], and an observational dataset, we found that CESM1-CAM5 is able to reproduce global monsoon (GM) features, including the patterns of monsoon precipitation and monsoon domains, the magnitude of GM precipitation (GMP, the local summer precipitation), GM area (GMA), and GM percentage (the ratio of the local summer precipitation to annual precipitation). Under the Paris Agreement temperature goals, the GM in CESM1-CAM5 displays the following changes: (1) The GMA is ambiguous under the 1.5°C temperature goal and increases under the 2.0°C temperature goal. The increase mainly results from a change in the monsoon percentage. (2) The GM, land monsoon and ocean monsoon precipitation all significantly increase under both the 1.5°C and 2.0°C goals. The increases are mainly due to the enhancement of humidity and evaporation. (3) The GM percentage and land and ocean monsoons feature little change under the temperature goals. (4) The lengths of the GM, land monsoon and ocean monsoon are significantly prolonged under the temperature goals. The increase in precipitation during the monsoon withdrawal month mainly accounts for the prolonged monsoons. Regarding the differences between the 1.5°C and 2.0°C temperature goals, it is certain that the GMP displays significant discrepancies. In addition, a large-scale enhancement of ascending motion occurs over the southeastern Tibetan Plateau and South China under a warming climate, whereas other monsoon areas experience an overall decline in ascending motion. This leads to an extraordinary wetting over Asian monsoon areas.
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    Accepted:
    Verification and Improvement of the Ability of CFSv2 to Predict the Antarctic Oscillation in Boreal Spring
    Dapeng ZHANG, Yanyan HUANG, Bo SUN, Fei LI, and Huijun WANG
    DOI: 10.1007/s00376-018-8106-6
    Abstract   ( 16 ) PDF (2336KB) (2)
    The boreal spring Antarctic Oscillation (AAO) has a significant impact on the spring and summer climate in China. This study evaluates the capability of the NCEP’s Climate Forecast System, version 2 (CFSv2), in predicting the boreal spring AAO for the period 1983–2015. The results indicate that CFSv2 has poor skill in predicting the spring AAO, failing to predict the zonally symmetric spatial pattern of the AAO, with an insignificant correlation of 0.02 between the predicted and observed AAO Index (AAOI). Considering the interannual increment approach can amplify the prediction signals, we firstly establish a dynamical–statistical model to improve the interannual increment of the AAOI (DY_AAOI), with two predictors of CFSv2-forecasted concurrent spring sea surface temperatures and observed preceding autumn sea ice. This dynamical–statistical model demonstrates good capability in predicting DY_AAOI, with a significant correlation coefficient of 0.58 between the observation and prediction during 1983–2015 in the two-year-out cross-validation. Then, we obtain an improved AAOI by adding the improved DY_AAOI to the preceding observed AAOI. The improved AAOI shows a significant correlation coefficient of 0.45 with the observed AAOI during 1983–2015. Moreover, the unrealistic atmospheric response to March–April–May sea ice in CFSv2 may be the possible cause for the failure of CFSv2 to predict the AAO. This study gives new clues regarding AAO prediction and short-term climate prediction.
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    Accepted:
    Evaluation of Summer Monsoon Clouds over the Tibetan Plateau Simulated in the ACCESS Model Using Satellite Products
    Liang HU, Zhian SUN, Difei DENG, and Greg ROFF
    DOI: 10.1007/s00376-018-7301-9
    Abstract   ( 17 ) PDF (2205KB) (3)
    Cloud distribution characteristics over the Tibetan Plateau in the summer monsoon period simulated by the Australian Community Climate and Earth System Simulator (ACCESS) model are evaluated using COSP [the CFMIP (Cloud Feedback Model Intercomparison Project) Observation Simulator Package]. The results show that the ACCESS model simulates less cumulus cloud at atmospheric middle levels when compared with observations from CALIPSO and CloudSat, but more ice cloud at high levels and drizzle drops at low levels. The model also has seasonal biases after the onset of the summer monsoon in May. While observations show that the prevalent high cloud at 9–10 km in spring shifts downward to 7–9 km, the modeled maximum cloud fractions move upward to 12–15 km. The reason for this model deficiency is investigated by comparing model dynamical and thermodynamical fields with those of ERA-Interim. It is found that the lifting effect of the Tibetan Plateau in the ACCESS model is stronger than in ERA-Interim, which means that the vertical velocity in the ACCESS model is stronger and more water vapor is transported to the upper levels of the atmosphere, resulting in more high-level ice clouds and less middle-level cumulus cloud over the Tibetan Plateau. The modeled radiation fields and precipitation are also evaluated against the relevant satellite observations.
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    Accepted:
    Seesaw Pattern of Rainfall Anomalies between the Tropical Western North Pacific and Central Southern China during Late Summer
    Xinyu LI and Riyu LU
    DOI: 10.1007/s00376-018-8130-6
    Abstract   ( 79 ) PDF (1300KB) (16)
    It is well known that suppressed convection in the tropical western North Pacific (WNP) induces an anticyclonic anomaly, and this anticyclonic anomaly results in more rainfall along the East Asian rain band through more water vapor transport during summer, as well as early and middle summer. However, the present results indicate that during late summer (from mid-August to the beginning of September), the anomalous anticyclone leads to more rainfall over central southern China (CSC), a region quite different from preceding periods. The uniqueness of late summer is found to be related to the dramatic change in climatological monsoon flows: southerlies over southern China during early and middle summer but easterlies during late summer. Therefore, the anomalous anticyclone, which shows a southerly anomaly over southern China, enhances monsoonal southerlies and induces more rainfall along the rain band during early and middle summer. During late summer, however, the anomalous anticyclone reflects a complicated change in monsoon flows: it changes the path, rather than the intensity, of monsoon flows. Specifically, during late summers of suppressed convection in the tropical WNP, southerlies dominate from the South China Sea to southern China, and during late summers of enhanced convection, northeasterlies dominate from the East China Sea to southern China, causing more and less rainfall in CSC, respectively.
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    Accepted:
    Tides and Turbulent Mixing in the North of Taiwan Island
    Xiangzhou SONG, Dexing WU and Xiaohui XIE
    DOI: 10.1007/s00376-018-8098-2
    Abstract   ( 89 ) PDF (2391KB) (4)
    Microstructure and hydrological profiles were collected along two cross-shelf sections from the deep slope to the shallow water in the north of Taiwan Island in the summer of 2006. While the tidal currents on the shelf were dominated by the barotropic tide with the current ellipse stretched across the shelf, significant internal tides were observed on the slope. The depth-mean turbulent kinetic energy (TKE) dissipation rate on the shelf was 10−6 W kg−1, corresponding to a diapycnal diffusivity of 10−2 m2 s−1. The depth-mean TKE dissipation rate on the slope was 1 × 10−7 W kg−1, with diapycnal diffusivity of 3.4 × 10−4 m2 s−1. The shear instability associated with internal tides largely contributed to the TKE dissipation rate on the slope from the surface to 150 m, while the enhanced turbulence on the shelf was dominated by tidal or residual current dissipations caused by friction in the thick bottom boundary layer (BBL). In the BBL, the Ekman currents associated with the northeastward Taiwan Warm Current were identified, showing a near-bottom velocity spiral, which agreed well with the analytical bottom Ekman solution.
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    Accepted:
    Predictability of South China Sea Summer Monsoon Onset
    Gill M. MARTIN, Amulya CHEVUTURI, Ruth E. COMER, Nick J. DUNSTONE, Adam A. SCAIFE, and Daquan ZHANG
    DOI: 10.1007/s00376-018-8100-z
    Abstract   ( 6 ) PDF (1120KB) (1)
    Predicting monsoon onset is crucial for agriculture and socioeconomic planning in countries where millions rely on the timely arrival of monsoon rains for their livelihoods. In this study we demonstrate useful skill in predicting year-to-year variations in South China Sea summer monsoon onset at up to a three-month lead time using the GloSea5 seasonal forecasting system. The main source of predictability comes from skillful prediction of Pacific sea surface temperatures associated with El Niño and La Niña. The South China Sea summer monsoon onset is a known indicator of the broadscale seasonal transition that represents the first stage of the onset of the Asian summer monsoon as a whole. Subsequent development of rainfall across East Asia is influenced by subseasonal variability and synoptic events that reduce predictability, but interannual variability in the broadscale monsoon onset for East Asian summer monsoon still provides potentially useful information for users about possible delays or early occurrence of the onset of rainfall over East Asia.
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    Accepted:
    Determining Atmospheric Boundary Layer Height with the Numerical Differentiation Method Using Bending Angle Data from COSMIC
    Shen YAN, Jie XIANG, and Huadong DU
    DOI: 10.1007/s00376-018-7308-2
    Abstract   ( 10 ) PDF (2740KB) (2)
    This paper presents a new method to estimate the height of the atmospheric boundary layer (ABL) by using COSMIC radio occultation bending angle (BA) data. Using the numerical differentiation method combined with the regularization technique, the first derivative of BA profiles is retrieved, and the height at which the first derivative of BA has the global minimum is defined to be the ABL height. To reflect the reliability of estimated ABL heights, the sharpness parameter is introduced, according to the relative minimum of the BA derivative. Then, it is applied to four months of COSMIC BA data (January, April, July, and October in 2008), and the ABL top heights estimated are compared with two kinds of ABL heights from COSMIC products and with the heights determined by the finite difference method upon the refractivity data. For sharp ABL tops (large sharpness parameters), there is little difference between the ABL heights determined by different methods, i.e., the uncertainties are small; whereas, for non-sharp ABL tops (small sharpness parameters), big differences exist in the ABL heights obtained by different methods, which means large uncertainties for different methods. In addition, the new method can detect thin ABLs and provide a reference ABL height in the cases eliminated by other methods. Thus, the application of the numerical differentiation method combined with the regularization technique to COSMIC BA data is an appropriate choice and has further application value.
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    Accepted:
    Warm-Season Diurnal Variations of Total, Stratiform, Convective, and Extreme Hourly Precipitation over Central and Eastern China
    Yongguang ZHENG, Yanduo GONG, Jiong CHEN, and Fuyou TIAN
    DOI: 10.1007/s00376-018-7307-3
    Abstract   ( 117 ) PDF (3796KB) (20)
    Diurnal variations in amount, frequency and intensity of warm-season hourly precipitation (HP) at seven levels, which are defined as HP ≥ 0.1, 0.5, 1, 5, 10, 20 and 50 mm, are revealed based on no less than 30 years of hourly rain-gauge observations at national stations over central and eastern China (CEC). This study investigates the variations, relationships, differences and similarities of total, stratiform, convective and extreme HP over the entire CEC and various subregions. Results indicate that the variations in the amount and frequency of HP at the seven levels over the entire CEC all display a bimodal feature. For various regions, the variations of total HP mostly feature two peaks, while convective HP mainly occurs in the late afternoon and determines the diurnal variation of total HP intensity. On the basis of the primary peak time periods of HP frequency at all levels over different subregions, the variations can be classified into three main categories: late-afternoon primary peak, nocturnal primary peak, and time-shifting primary peak. However, the variations over some coastal regions like the Liaodong Peninsula, the Shandong Peninsula, and the coastal regions of Guangdong, distinctly differ from those over their corresponding larger regions. Overall, the normalized diurnal variation amplitude increases with the increasing HP intensity; convective precipitation can be represented by HP ≥ 10 mm; and the intensity of HP ≥ 50 mm is slightly larger during the nighttime than during the daytime over the entire CEC. In northern China, diurnal variation in HP ≥ 5 mm can represent well that in convective precipitation.
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    Accepted:
    The Relationship between Deterministic and Ensemble Mean Forecast Errors Revealed by Global and Local Attractor Radii
    Jie FENG, Jianping LI, Jing ZHANG, Deqiang LIU, and Ruiqiang DING
    DOI: 10.1007/s00376-018-8123-5
    Abstract   ( 17 ) PDF (928KB) (4)
    It has been demonstrated that ensemble mean forecasts, in the context of the sample mean, have higher forecasting skill than deterministic (or single) forecasts. However, few studies have focused on quantifying the relationship between their forecast errors, especially in individual prediction cases. Clarification of the characteristics of deterministic and ensemble mean forecasts from the perspective of attractors of dynamical systems has also rarely been involved. In this paper, two attractor statistics—namely, the global and local attractor radii (GAR and LAR, respectively)—are applied to reveal the relationship between deterministic and ensemble mean forecast errors. The practical forecast experiments are implemented in a perfect model scenario with the Lorenz96 model as the numerical results for verification. The sample mean errors of deterministic and ensemble mean forecasts can be expressed by GAR and LAR, respectively, and their ratio is found to approach with lead time. Meanwhile, the LAR can provide the expected ratio of the ensemble mean and deterministic forecast errors in individual cases.
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    Accepted:
    Impact of the Assimilation Frequency of Radar Data with the ARPS 3DVar and Cloud Analysis System on Forecasts of a Squall Line in Southern China
    Yujie PAN and Mingjun WANG
    DOI: 10.1007/s00376-018-8087-5
    Abstract   ( 15 ) PDF (3019KB) (3)
    Assimilation configurations have significant impacts on analysis results and subsequent forecasts. A squall line system that occurred on 23 April 2007 over southern China was used to investigate the impacts of the data assimilation frequency of radar data on analyses and forecasts. A three-dimensional variational system was used to assimilate radial velocity data, and a cloud analysis system was used for reflectivity assimilation with a 2-h assimilation window covering the initial stage of the squall line. Two operators of radar reflectivity for cloud analyses corresponding to single- and double-moment schemes were used. In this study, we examined the sensitivity of assimilation frequency using 10-, 20-, 30-, and 60-min assimilation intervals. The results showed that analysis fields were not consistent with model dynamics and microphysics in general; thus, model states, including dynamic and microphysical variables, required approximately 20 min to reach a new balance after data assimilation in all experiments. Moreover, a 20-min data assimilation interval generally produced better forecasts for both single- and double-moment schemes in terms of equitable threat and bias scores. We conclude that a higher data assimilation frequency can produce a more intense cold pool and rear inflow jets but does not necessarily lead to a better forecast.
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