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Weiwei Lu, Huimin Lei, Wencong Yang, Jingjing Yang, and Dawen Yang

Abstract

Increasing evidence indicates that changes have occurred in heavy precipitation associated with tropical cyclone (TC) and local monsoon (non-TC) systems in the southeastern coastal region of China over recent decades. This leads to the following questions: what are the differences between TC and non-TC flooding, and how do TC and non-TC flooding events change over time? We applied an identification procedure for TC and non-TC floods by linking flooding to rainfall. This method identified TC and non-TC rainfall–flood events by the TC rainfall ratio (percentage of TC rainfall to total rainfall for rainfall–flood events). Our results indicated that 1) the TC rainfall–flood events presented a faster runoff generation process associated with larger flood peaks and rainfall intensities but smaller rainfall volumes, compared to that of non-TC rainfall–flood events, and 2) the magnitude of TC floods exhibited a decreasing trend, similar to the trend in the amount and frequency of TC extreme precipitation. However, the frequency of TC floods did not present obvious changes. In addition, non-TC floods decreased in magnitude and frequency while non-TC extreme precipitation showed an increase. Our results identified significantly different characteristics between TC and non-TC flood events, thus emphasizing the importance of considering different mechanisms of floods to explore the physical drivers of runoff response. Also, our results indicated that significant decreases occurred in the magnitude, but not the frequency, of floods induced by TC from the western North Pacific, which is the most active ocean basin for TC activity, and thus can provide useful information for future studies on the global pattern of TC-induced flooding.

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Fuan Xiao, Dongxiao Wang, and Lei Yang

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In this study, an enhanced footprint of the interdecadal Pacific oscillation (IPO) on the upper-ocean heat content (OHC) in the South China Sea (SCS) since the 1990s is revealed. The negative OHC–IPO correlation is significant (r = −0.71) during 1990–2010 [period 2 (P2)], whereas it is statistically insignificant during 1960–80 [period 1 (P1)]. Analyses show that the scope of the equatorial Pacific wind anomalies is wider during P2 compared with that during P1 due to a larger east–west SST gradient and enhanced tropical warming in the Indian Ocean. When the IPO is negative during P2, a wider scope of the wind stress anomalies associated with the IPO could lead to 1) the southward migration of the North Equatorial Current bifurcation latitude (NECBL) by affecting the wind stress curl over the key region where it is near the climatological NECBL and 2) an increase in the interbasin pressure gradient (sea surface height difference) between the western Pacific and the SCS; these two processes strengthen the Kuroshio and weaken the Luzon Strait transport (LST) or SCS throughflow into the SCS. Also, 3) the equatorial Pacific wind anomalies are wide enough to directly weaken the LST in the SCS through the “island rule.” These three pathways finally change the oceanic gyre in the SCS and increase the OHC. Our results suggest that the scope of the tropical wind stress is the crucial factor when we consider the relationship between the upper ocean thermal conditions in the SCS and the Pacific variability.

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Yan Du, Lei Yang, and Shang-Ping Xie

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In the summer following a strong El Niño, tropical cyclone (TC) number decreases over the Northwest (NW) Pacific despite little change in local sea surface temperature. The authors’ analysis suggests El Niño–induced tropical Indian Ocean (TIO) warming as the cause. The TIO warming forces a warm tropospheric Kelvin wave that propagates into the western Pacific. Inducing surface divergence off the equator, the tropospheric Kelvin wave suppresses convection and induces an anomalous anticyclone over the NW Pacific, both anomalies unfavorable for TCs. The westerly vertical shear associated with the warm Kelvin wave reduces the magnitude of vertical shear in the South China Sea and strengthens it in the NW Pacific, an east–west variation that causes TC activity to increase and decrease in respective regions. These results help improve seasonal TC prediction.

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Xiaodong Liu, Shouguo Ding, Lei Bi, and Ping Yang

Abstract

Nonspherical ice crystal optical properties are of fundamental importance to atmospheric radiative transfer through an ice cloud and the remote sensing of its properties. In practice, the optical properties of individual ice crystals need to be integrated over particle size distributions to derive the bulk optical properties of ice clouds. Given a particle size distribution represented in terms of size bins, the conventional approach uses the microphysical and optical properties of ice crystals at the bin centers as approximations to the bin-averaged values. However, errors are incurred when the size bins are large. To reduce the potential errors, a kernel technique is utilized to calculate the bulk optical properties of ice clouds by computing the bin-averaged values instead of using the bin-center values. Comparisons between the solutions based on the conventional method and the kernel technique for different numbers of size bins from in situ measurements demonstrate that the results computed from the kernel technique are more accurate. The present study illustrates that, for a given size distribution, 40 or more size bins should be used to calculate the bulk optical properties of ice clouds by the conventional method. Although the accuracy of bulk-scattering properties can be improved by using fine bin resolutions in the single-scattering property computation, the advantage of using a precomputed database of scattering kernels allows efficient computation of ice cloud bulk optical properties without losing the accuracy.

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Aizhong Hou, Guangheng Ni, Hanbo Yang, and Zhidong Lei

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A decline of surface wind speed (wind stilling) has been observed in many regions of the world. The greater Beijing metropolitan area in China is taken as an example for analyzing the urbanization impact on wind stilling. This study set up five scenarios with different urbanization ratios and the same atmospheric forces and then simulated wind speed under each scenario using the next-generation Weather Research and Forecasting model. The results suggest that the correspondence between the regional average wind speed ratio of decrease Δu and the ratio of urbanized area δurban (%) fits the relation in summer and in winter. During the period 1961–2008, the ratio of urbanized area over the greater Beijing metropolitan area increased from 1.3% to 11.9%, which is speculated as the cause of a 0.4 m s−1 decline in regional average wind speed and the contributor to about 35% wind stilling.

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Sicheng He, Jing Yang, Qing Bao, Lei Wang, and Bin Wang

Abstract

Realistic reproduction of historical extreme precipitation has been challenging for both reanalysis and global climate model (GCM) simulations. This work assessed the fidelities of the combined gridded observational datasets, reanalysis datasets, and GCMs [CMIP5 and the Chinese Academy of Sciences Flexible Global Ocean–Atmospheric Land System Model–Finite-Volume Atmospheric Model, version 2 (FGOALS-f2)] in representing extreme precipitation over East China. The assessment used 552 stations’ rain gauge data as ground truth and focused on the probability distribution function of daily precipitation and spatial structure of extreme precipitation days. The TRMM observation displays similar rainfall intensity–frequency distributions as the stations. However, three combined gridded observational datasets, four reanalysis datasets, and most of the CMIP5 models cannot capture extreme precipitation exceeding 150 mm day−1, and all underestimate extreme precipitation frequency. The observed spatial distribution of extreme precipitation exhibits two maximum centers, located over the lower-middle reach of Yangtze River basin and the deep South China region, respectively. Combined gridded observations and JRA-55 capture these two centers, but ERA-Interim, MERRA, and CFSR and almost all CMIP5 models fail to capture them. The percentage of extreme rainfall in the total rainfall amount is generally underestimated by 25%–75% in all CMIP5 models. Higher-resolution models tend to have better performance, and physical parameterization may be crucial for simulating correct extreme precipitation. The performances are significantly improved in the newly released FGOALS-f2 as a result of increased resolution and a more realistic simulation of moisture and heating profiles. This work pinpoints the common biases in the combined gridded observational datasets and reanalysis datasets and helps to improve models’ simulation of extreme precipitation, which is critically important for reliable projection of future changes in extreme precipitation.

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Lei Song, Lin Wang, Wen Chen, and Yang Zhang

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The East Asian trough (EAT) is a distinct component of the boreal winter circulation whose strength corresponds to the amplitude of the Northern Hemispheric stationary waves. In this study, the mechanism and climatic impacts of the intraseasonal variations of the EAT’s strength are investigated through composite analysis and dynamical diagnostics. The significant roles played by the low-frequency Rossby wave (RW) and synoptic transient eddy (TE) are revealed. Before the peaks of strong EAT events, an upper-tropospheric RW train propagates across northern Eurasia and interacts with preexisting surface cold anomalies over central Siberia. This pattern intensifies the Siberian high and causes RW convergence toward the EAT, leading to 30% of the EAT’s amplification directly via the RW-induced feedback forcing. Meanwhile, RW weakens the background baroclinicity and reduces TE activities near the entrance region of the North Pacific storm track. The TE-induced feedback forcing leads to another 30% of the EAT’s amplification. The evolution and dynamical processes of the weak EAT events generally resemble those of the strong events with opposite signs. These results are consistent with the knowledge on the mechanism of the strong and weak EAT events regarding the role of RWs with additional quantitative description and provide new insights regarding the role of TEs. Variations of the EAT’s strength exert significant climatic impacts on East Asia and its downstream region. Near-surface air temperature is below (above) normal over East Asia during the growth and peak stages of the strong (weak) EAT events and above (below) normal over North America afterward.

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Yang Xiao, Zhiguo Cao, Wen Zhuo, Liang Ye, and Lei Zhu

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In this paper, a novel Multiview CLOUD (mCLOUD) visual feature extraction mechanism is proposed for the task of categorizing clouds based on ground-based images. To completely characterize the different types of clouds, mCLOUD first extracts the raw visual descriptors from the views of texture, structure, and color simultaneously, in a densely sampled way—specifically, the scale invariant feature transform (SIFT), the census transform histogram (CENTRIST), and the statistical color features are extracted, respectively. To obtain a more descriptive cloud representation, the feature encoding of the raw descriptors is realized by using the Fisher vector. This is followed by the feature aggregation procedure. A linear support vector machine (SVM) is employed as the classifier to yield the final cloud image categorization result. The experiments on a challenging cloud dataset termed the six-class Huazhong University of Science and Technology (HUST) cloud demonstrate that mCLOUD consistently outperforms the state-of-the-art cloud classification approaches by large margins (at least 6.9%) under all the different experimental settings. It has also been verified that, compared to the single view, the multiview cloud representation generally enhances the performance.

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Lei Yang, Xin Wang, Ke Huang, and Dongxiao Wang
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Yeli Yuan, Lei Han, Feng Hua, Shuwen Zhang, Fangli Qiao, Yongzeng Yang, and Changshui Xia

Abstract

Some basic statistics for wave breaking have been derived based on the statistical model of real sea waves. The analytic expressions of breaking entrainment depth and surface whitecap coverage involved with both sea wave characteristics and surface wind velocity have been derived on the basis of the whitecap formation model. The concept of the upper envelope for all the whitecap coverage data versus wind speed has been proposed, and it is assumed to correspond to the whitecap coverage in the case of the infinite wind duration and fetch to determine the model constants. The analytic expressions of breaking entrainment depth and whitecap coverage have been compared with the observations in several ways, and consistently favorable agreement can be found for most observations.

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