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Rui Sun, Xueqin Zhang, Yang Sun, Du Zheng, and Klaus Fraedrich

Abstract

Runoff estimation and its response to climate change in ungauged or poorly gauged basins based on hydrological models are frontier research issues of the hydrological cycle. For the Kadongjia River watershed (KRW), a poorly gauged watershed located in southern Tibet, China, the Soil and Water Assessment Tool (SWAT) was adapted to model streamflow and its responses to climate change. The average annual streamflow was simulated to be roughly 124.6 mm with relatively small interannual variation during 1974–2010. The seasonal distribution of streamflow was uneven with a maximum in summer and a minimum in winter. Snowmelt, which was mainly produced in April–May, accounted for 4.0% of annual streamflow. Correlations and regression analysis between the interannual variations of major climatic and hydrological variables indicated that precipitation (temperature) had positive (negative) influence on the annual streamflow variation. For the interannual streamflow variations, warmer temperature was slightly more important than the variation of winter precipitation. Comparing streamflow changes in the current years (1980–99) with the future (2030–49), streamflow variations were more sensitive to changing climate in winter and spring than in the other two seasons. Model improvement is expected to enhance the simulation efficiency of SWAT and the analyses of hydrological responses to climatic change in KRW, thus supporting the model's credibility for hydrological cycle research in alpine regions.

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Zhiqi Zhang, Xuguang Sun, and Xiu-Qun Yang

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East Asian summer monsoon precipitation (EASMP) features complicated interdecadal variability with multiple time periods and spatial patterns. Using century-long datasets of HadISST, CRU precipitation, and the ECMWF twentieth-century reanalysis (ERA-20C), this study examines the joint influence of three oceanic interdecadal signals [i.e., Pacific decadal oscillation (PDO), Atlantic multidecadal oscillation (AMO), and Indian Ocean Basin mode (IOBM)] on the EASMP, which, however, is found not to be simply a linear combination of their individual effects. When PDO and AMO are out of phase, the same-sign SST anomalies occur in the North Pacific and North Atlantic, and a zonally orientated teleconnection wave train appears across the Eurasian mid-to-high latitudes, propagating from the North Atlantic to northern East Asia along the Asian westerly jet waveguide. Correspondingly, the interdecadal precipitation anomalies are characterized by a meridional tripole mode over eastern China. When PDO and AMO are in phase, with opposite sign SST anomalies in the North Pacific and North Atlantic, the sandwich pattern of anomalous stationary Rossby wavenumber tends to reduce the effect of the waveguide in the eastern Mediterranean region, and the teleconnection wave train from the North Atlantic travels only to western central Asia along a great circle route, causing Indian summer monsoon precipitation (ISMP) anomalies. The ISMP anomalies, in turn, interact with the teleconnection wave train induced by the PDO and AMO, leading to a meridional dipole mode of interdecadal precipitation anomalies over eastern China. Through the impact on the ISMP, the IOBM exerts significantly linear modulation on the combined impacts of PDO and AMO, especially over northern East Asia.

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Qiang Fu, W. B. Sun, and Ping Yang

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This paper examines a number of commonly used methods for the calculation of the scattering and absorption properties of nonspherical ice crystals at thermal infrared wavelengths. It is found that, for randomly oriented nonspherical particles, Mie theory using equivalent ice spheres tends to overestimate the absorption efficiency while the anomalous diffraction theory (ADT) and the geometric optics method (GOM) tend to underestimate it. The absorption efficiency is not sensitive to the particle shape when the size parameter is large.

Herein a composite scheme is used that is valid for nonspherical particles with a wide range of size parameters. This scheme is a composite of Mie theory, GOM, and ADT to fit the single-scattering properties of hexagonal particles derived from the GOM for large size parameters and the finite-difference time domain technique for small size parameters. Applying this composite technique, errors in the broadband emissivity of cirrus clouds associated with conventional approaches are examined. It is shown that, when the projected area is preserved, Mie results overestimate the emissivity of cirrus clouds while, when the volume is preserved, Mie results underestimate the emissivity. Mie theory yields the best results when both projected area and volume are preserved (the relative errors are less than 10%). It is also shown that the ADT underestimates cirrus cloud emissivity. In some cases, the relative errors can be as large as 30%. The errors in the GOM are also significant and are largely a result of nonspherical particles with size parameters smaller than 40.

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Xuejuan Ren, Xiu-Qun Yang, and Xuguang Sun

Abstract

This study examines the relationship between the zonal oscillation of the western Pacific subtropical high (WPSH) and underneath sea surface temperature (SST) variation on a subseasonal time scale, associated with the persistent heavy rainfall (PHR) events over the middle and lower reaches of the Yangtze River valley (MLYRV) in China. A total of 76 PHR events and 45 break events in the summers of 1979–2011 are first identified over the MLYRV and divided into early and late summer groups. During the PHR events over the MLYRV for both groups, the WPSH stretches more westward, accompanied by the positive anomalies of the 500-hPa geopotential height field over East Asia and its coastal region south of 30°N and the subseasonal warmer SSTs beneath the WPSH western edge. The time-lagged composites suggest that the WPSH western edge exhibits westward-then-eastward migration on a subseasonal time scale for the PHR events. The zonal changes of the WPSH and anomalous circulation and SST anomaly (SSTA) signals for break events is almost the mirror image of that for the PHR events for the early summer group. Accompanied by the WPSH westward extension, the increased incident solar radiation and decreased latent heat flux over the coastal region of East Asia contribute to the positive SSTAs beneath the western part of the WPSH. The positive SSTAs construct a convective instability that provides an adverse condition for maintaining the anticyclonic anomalies in the mid–lower levels. The persistent SST warming is also favorable to the transition of low-level circulation from anticyclonic to cyclonic anomalies over the coastal region. As a result, the WPSH withdraws eastward after the peak of the rainfall events over the MLYRV.

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Huang Yang, Lantao Sun, and Gang Chen

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Previous studies have suggested that Southern Hemisphere (SH) summertime trends in the atmospheric circulation in the second half of the twentieth century are mainly driven by stratospheric ozone depletion in spring. Here, the authors show that the pattern and timing of observed trends, characterized by downward propagation of signals, can be approximately captured in an idealized atmospheric global circulation model (AGCM) by imposing ozone depletion–like radiative cooling.

It is further shown that the synoptic eddies dominantly contribute to the transient tropospheric response to polar stratospheric cooling. The authors examine three possible mechanisms on the downward influence of polar stratospheric cooling. The polar stratospheric cooling affects tropospheric synoptic eddies via (i) the direct influences on the lower-stratospheric synoptic eddies, (ii) the planetary wave–induced residual circulation, and (iii) the planetary eddy–synoptic eddy nonlinear interaction. It is argued that the planetary wave–induced residual circulation is not the dominant mechanism and that the planetary eddies and further nonlinear interaction with synoptic eddies are more likely the key to the downward influence of the ozone depletion–like cooling.

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Xinlin Yang, Jianhua Sun, and Wanli Li

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The cloud-to-ground (CG) lightning data being detected by the China Lightning Detection Network between 2010 and 2013 are employed to gain insight into the spatial and temporal distribution of CG lightning in China. There are clear interannual and seasonal variations of CG lightning activity. The mean total CG and positive CG (PCG) flashes in 2010–13 are approximately 6.44 million and 0.42 million, respectively, and the mean percentage of PCG (PPCG) is 6.6%. CG and PCG flashes predominately occur during summer, with August being the peak month for CG and June for PCG. PPCG in the cold season is considerably greater than in the warm season; its maximum of 56.2% is in January and the minimum value of 4.0% is found in August. The centers of maximum mean annual CG density are scattered throughout southern China, the Sichuan basin, and the south of Jiangsu Province. The CG density in the high elevations and arid regions of western China is less than that in the low elevations and coastal regions of southeastern China. In addition, daily CG density and CG lightning days in southeastern China are greater than in northwestern China, but PPCG in western China is apparently greater than that in eastern China. Areas experiencing more than 30 CG lightning days per year are primarily south of 30°N, with 10–30 lightning days per year in northern and northeastern China, and approximately 10–20 lightning days per year over the central Tibetan Plateau.

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Xinlin Yang, Jianhua Sun, and Yongguang Zheng

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A method using cloud-to-ground lightning was developed to retrieve severe convective wind (SCW) events from significant weather report data over China during the period 2010–14. The results showed that SCW events were a feature of local weather activity, and their distribution showed clear seasonal and diurnal variations. The SCW events mainly occurred over eastern China during the midafternoon in the warm season and rarely occurred over western China. The highest frequency of SCW events was recorded in north China and Guangdong Province. There was also a high frequency of SCW events in the middle and lower reaches of the Yangtze River. The most frequent occurrence of SCW events was in Guangdong Province in spring, while a high frequency of SCW events was observed in both north China and Guangdong Province during the summer months. The peak month for SCW events was July over the whole of China and June in north China. The pattern in Guangdong Province had a bimodal distribution, with the peak months being May and August. The majority of SCW events occurred between 1200 and 2000 local time.

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Qiang Fu, Ping Yang, and W. B. Sun

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An accurate parameterization is presented for the infrared radiative properties of cirrus clouds. For the single-scattering calculations, a composite scheme is developed for randomly oriented hexagonal ice crystals by comparing results from Mie theory, anomalous diffraction theory (ADT), the geometric optics method (GOM), and the finite-difference time domain technique. This scheme employs a linear combination of single-scattering properties from the Mie theory, ADT, and GOM, which is accurate for a wide range of size parameters. Following the approach of Q. Fu, the extinction coefficient, absorption coefficient, and asymmetry factor are parameterized as functions of the cloud ice water content and generalized effective size (D ge). The present parameterization of the single-scattering properties of cirrus clouds is validated by examining the bulk radiative properties for a wide range of atmospheric conditions. Compared with reference results, the typical relative error in emissivity due to the parameterization is ∼2.2%. The accuracy of this parameterization guarantees its reliability in applications to climate models. The present parameterization complements the scheme for the solar radiative properties of cirrus clouds developed by Q. Fu for use in numerical models.

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Ruixin Yang, Jiang Tang, and Donglian Sun

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This study applies a data mining technique called association rule mining to the analysis of intensity changes of North Atlantic tropical cyclones (TCs). The “best track” data from the National Hurricane Center and the Statistical Hurricane Intensity Prediction Scheme databases were stratified into tropical depressions, tropical storms, and category 1–5 hurricanes based on the Saffir–Simpson hurricane scale. After stratification, the seven resulting groups of TCs plus two additional aggregation groups were further separated into intensifying, weakening, and stable TCs. The analysis of the stratified data for preprocessing revealed that faster northward storm motion (the meridional component of storm motion) favors tropical storm intensification but does not favor the intensification of hurricanes. Intensifying tropical storms are more strongly associated with a higher convergence in the upper atmosphere (200-hPa relative eddy momentum flux convergence) than weakening tropical storms, while intensifying hurricanes are more strongly associated with lower convergence values. The mined association rules showed that cofactors usually display higher-intensity prediction power in the stratified TC groups. The data mining results also identified a predictor set with fewer factors but improved probabilities of rapid intensification. This study found that the data mining technique not only sheds light on the roles of multiple-associated physical processes in tropical cyclone development—especially in rapid intensification processes—but also will help improve TC intensity forecasting. This paper provides an outline on how to use this data mining technique and how to overcome low occurrences of mined conditions in order to improve TC intensity forecasting capabilities.

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Mengmiao Yang, Guang J. Zhang, and De-Zheng Sun

Abstract

As key variables in general circulation models, precipitation and moisture in four leading models from CMIP5 (phase 5 of the Coupled Model Intercomparison Project) are analyzed, with a focus on four tropical oceanic regions. It is found that precipitation in these models is overestimated in most areas. However, moisture bias has large intermodel differences. The model biases in precipitation and moisture are further examined in conjunction with large-scale circulation by regime-sorting analysis. Results show that all models consistently overestimate the frequency of occurrence of strong upward motion regimes and peak descending regimes of 500-hPa vertical velocity . In a given regime, models produce too much precipitation compared to observation and reanalysis. But for moisture, their biases differ from model to model and also from level to level. Furthermore, error causes are revealed through decomposing contribution biases into dynamic and thermodynamic components. For precipitation, the contribution errors in strong upward motion regimes are attributed to the overly frequent . In the weak upward motion regime, the biases in the dependence of precipitation on and the probability density function (PDF) make comparable contributions, but often of opposite signs. On the other hand, the biases in column-integrated water vapor contribution are mainly due to errors in the frequency of occurrence of , while thermodynamic components contribute little. These findings suggest that errors in the frequency of occurrence are a significant cause of biases in the precipitation and moisture simulation.

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