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2017 Vol. 34, No. 7

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Two Ultraviolet Radiation Datasets that Cover China
Hui LIU, Bo HU, Yuesi WANG, Guangren LIU, Liqin TANG, Dongsheng JI, Yongfei BAI, Weikai BAO, Xin CHEN, Yunming CHEN, Weixin DING, Xiaozeng HAN, Fei HE, Hui HUANG, Zhenying HUANG, Xinrong LI, Yan LI, Wenzhao LIU, Luxiang LIN, Zhu OUYANG, Boqiang QIN, Weijun SHEN, Yanjun SHEN, Hongxin SU, Changchun SONG, Bo SUN, Song SUN, Anzhi WANG, Genxu WANG, Huimin WANG, Silong WANG, Youshao WANG, Wenxue WEI, Ping XIE, Zongqiang XIE, Xiaoyuan YAN, Fanjiang ZENG, Fawei ZHANG, Yangjian ZHANG, Yiping ZHANG, Chengyi ZHAO, Wenzhi ZHAO, Xueyong ZHAO, Guoyi ZHOU, Bo ZHU
2017, 34(7): 805-815. doi: 10.1007/s00376-017-6293-1
Ultraviolet (UV) radiation has significant effects on ecosystems, environments, and human health, as well as atmospheric processes and climate change. Two ultraviolet radiation datasets are described in this paper. One contains hourly observations of UV radiation measured at 40 Chinese Ecosystem Research Network stations from 2005 to 2015. CUV3 broadband radiometers were used to observe the UV radiation, with an accuracy of 5%, which meets the World Meteorology Organization's measurement standards. The extremum method was used to control the quality of the measured datasets. The other dataset contains daily cumulative UV radiation estimates that were calculated using an all-sky estimation model combined with a hybrid model. The reconstructed daily UV radiation data span from 1961 to 2014. The mean absolute bias error and root-mean-square error are smaller than 30% at most stations, and most of the mean bias error values are negative, which indicates underestimation of the UV radiation intensity. These datasets can improve our basic knowledge of the spatial and temporal variations in UV radiation. Additionally, these datasets can be used in studies of potential ozone formation and atmospheric oxidation, as well as simulations of ecological processes.
Interdecadal Variations of the South Asian Summer Monsoon Circulation Variability and the Associated Sea Surface Temperatures on Interannual Scales
Ya GAO, Huijun WANG, Dong CHEN
2017, 34(7): 816-832. doi: 10.1007/ s00376-017-6246-8
We investigate the interannual variability of the South Asian summer monsoon (SASM) circulation, which has experienced a significant interdecadal change since 2000. This change is primarily influenced by sea surface temperatures (SSTs) in the tropical Pacific and North Atlantic oceans. During the pre-2000 period examined in this study (1979-99), the SASM is negatively correlated with eastern Pacific SSTs (the canonical ENSO mode) and positively correlated with the negative phase of the North Atlantic SST tripole (NAT). During the post-2000 period (2000-14), the SASM is negatively correlated with central Pacific SSTs and positively correlated with the positive phase of the NAT pattern. The associated Pacific SSTs change from the eastern to central region, leading to the rising (subsiding) branch of the Walker circulation moving westwards to the Maritime Continent in the latter period, which can impact the interannual variability of the SASM through modulating the wind field in the troposphere. In addition to Pacific SSTs, the NAT SSTs can propagate energy from the North Atlantic to the South Asian High (SAH) region through the wave activity flux, and then further impact the SASM via the SAH. Because the SASM is intimately related with precipitation over the Asian region, we briefly discuss the features of the precipitation patterns associated with the SASM during the two periods. The westward shifting Walker circulation leads to the shrinking and weakened anomalous westerlies of the SASM in the lower level, inducing the Maritime Continent rainfall location to move westwards and more moisture to arrive in southern China from the Pacific Ocean in the latter period.
Interdecadal Variability of the Afro-Asian Summer Monsoon System
Yi LI, Yihui DING, Weijing LI
2017, 34(7): 833-846. doi: 10.1007/s00376-017-6247-7
The Afro-Asian summer monsoon is a zonally planetary-scale system, with a large-scale rainbelt covering Africa, South Asia and East Asia on interdecadal timescales both in the past century (1901-2014) and during the last three decades (1979-2014). A recent abrupt change of precipitation occurred in the late 1990s. Since then, the entire rainbelt of the Afro-Asia monsoon system has advanced northwards in a coordinated way. Consistent increases in precipitation over the Huanghe-Huaihe River valley and the Sahel are associated with the teleconnection pattern excited by the warm phase of the Atlantic Multidecadal Oscillation (AMO). A teleconnection wave train, with alternating cyclones/anticyclones, is detected in the upper troposphere. Along the teleconnection path, the configuration of circulation anomalies in North Africa is characterized by coupling of the upper-level anticyclone (divergence) with low-level thermal low pressure (convergence), facilitating the initiation and development of ascending motions in the Sahel. Similarly, in East Asia, a coupled circulation pattern also excites ascending motion in the Huanghe-Huaihe River valley. The synchronous increase in precipitation over the Sahel and Huanghe-Huaihe River valley can be attributed to the co-occurrences and in-phase changes of ascending motion. On the other hand, the warm phase of the AMO results in significant warming in the upper troposphere in North Africa and the northern part of East Asia. Such warming contributes to intensification of the tropical easterly jet through increasing the meridional pressure gradient both at the entrance region (East Asia) and the exit region (Africa). Accordingly, precipitation over the Sahel and Huanghe-Huaihe River valley intensifies, owing to ageostrophic secondary cells. The results of this study provide evidence for a consistent and holistic interdecadal change in the Afro-Asian summer monsoon.
Autumn Snow Cover Variability over Northern Eurasia and Roles of Atmospheric Circulation
Kunhui YE, Renguang WU
2017, 34(7): 847-858. doi: 10.1007/s00376-017-6287-z
This study analyzes the variability of northern Eurasian snow cover (SC) in autumn and the impacts of atmospheric circulation changes. The region of large SC variability displays a southward shift from September to November, following the seasonal progression of the transition zones of surface air temperature (SAT). The dominant pattern of SC variability in September and October features a zonal distribution, and that in November displays an obvious west-east contrast. Surface air cooling and snowfall increase are two factors for larger SC. The relative contribution of SAT and snowfall changes to SC, however, varies with the region and depends upon the season. The downward longwave radiation and atmospheric heat advection play important roles in SAT changes. Anomalous convergence of water vapor flux contributes to enhanced snowfall. The changes in downward longwave radiation are associated with those in atmospheric water content and column thickness. Changes in snowfall and the transport of atmospheric moisture determine the atmospheric moisture content in September and October, and the snowfall appears to be a main factor for atmospheric moisture change in November. These results indicate that atmospheric circulation changes play an important role in snow variability over northern Eurasia in autumn. Overall, the coupling between autumn Eurasian snow and atmospheric circulation may not be driven by external forcing.
Cloud Radiative Effects and Changes Simulated by the Coupled Model Intercomparison Project Phase 5 Models
Sun-Hee SHIN, Ok-Yeon KIM, Dongmin KIM, Myong-In LEE
2017, 34(7): 859-876. doi: 10.1007/s00376-017-6089-3
Using 32 CMIP5 (Coupled Model Intercomparison Project Phase 5) models, this study examines the veracity in the simulation of cloud amount and their radiative effects (CREs) in the historical run driven by observed external radiative forcing for 1850-2005, and their future changes in the RCP (Representative Concentration Pathway) 4.5 scenario runs for 2006-2100. Validation metrics for the historical run are designed to examine the accuracy in the representation of spatial patterns for climatological mean, and annual and interannual variations of clouds and CREs. The models show large spread in the simulation of cloud amounts, specifically in the low cloud amount. The observed relationship between cloud amount and the controlling large-scale environment are also reproduced diversely by various models. Based on the validation metrics, four models——ACCESS1.0, ACCESS1.3, HadGEM2-CC, and HadGEM2-ES——are selected as best models, and the average of the four models performs more skillfully than the multimodel ensemble average. All models project global-mean SST warming at the increase of the greenhouse gases, but the magnitude varies across the simulations between 1 and 2 K, which is largely attributable to the difference in the change of cloud amount and distribution. The models that simulate more SST warming show a greater increase in the net CRE due to reduced low cloud and increased incoming shortwave radiation, particularly over the regions of marine boundary layer in the subtropics. Selected best-performing models project a significant reduction in global-mean cloud amount of about -0.99% K-1 and net radiative warming of 0.46 W m-2 K-1, suggesting a role of positive feedback to global warming.
Comparison of Aerosol Effects on Simulated Spring and Summer Hailstorm Clouds
Huiling YANG, Hui XIAO, Chunwei GUO, Guang WEN, Qi TANG, Yue SUN
2017, 34(7): 877-893. doi: 10.1007/s00376-017-6138-y
Numerical simulations are carried out to investigate the effect of cloud condensation nuclei (CCN) concentrations on microphysical processes and precipitation characteristics of hailstorms. Two hailstorm cases are simulated, a spring case and a summer case, in a semiarid region of northern China, with the Regional Atmospheric Modeling System. The results are used to investigate the differences and similarities of the CCN effects between spring and summer hailstorms. The similarities are: (2) The total hydrometeor mixing ratio decreases, while the total ice-phase mixing ratio enhances, with increasing CCN concentration; (2) Enhancement of the CCN concentration results in the production of a greater amount of small-sized hydrometeor particles, but a lessening of large-sized hydrometeor particles; (3) As the CCN concentration increases, the supercooled cloud water and rainwater make a lesser contribution to hail, while the ice-phase hydrometeors take on active roles in the growth of hail; (4) When the CCN concentration increases, the amount of total precipitation lessens, while the role played by liquid-phase rainfall in the amount of total precipitation reduces, relatively, compared to that of ice-phase precipitation. The differences between the two storms include: (2) An increase in the CCN concentration tends to reduce pristine ice mixing ratios in the spring case but enhance them in the summer case; (2) Ice-phase hydrometeor particles contribute more to hail growth in the spring case, while liquid water contributes more in the summer case; (3) An increase in the CCN concentration has different effects on surface hail precipitation in different seasons.
Potential Deformation and Its Application to the Diagnosis of Heavy Precipitation in Mesoscale Convective Systems
Na LI, Lingkun RAN, Linna ZHANG, Shouting GAO
2017, 34(7): 894-908. doi: 10.1007/s00376-017-6282-4
Many observational studies have shown that deformation, like vertical vorticity and divergence, is closely related to the occurrence and distribution of strong precipitation. In this paper, to involve deformation in precipitation diagnosis, a new parameter called potential deformation (PD) is derived and then applied to precipitation detection within a simulated mesoscale convective system (MCS). It is shown that PD includes both stretching deformation and shearing deformation and shares similar characteristics with deformation insofar as it does not change with the rotating coordinate. Diagnosis of the simulated MCS reveals that PD performs well in tracing the MCS' precipitation. In terms of their distributional pattern, the large-value areas of PD are similar to the precipitation in the different development stages of the MCS. A detailed analysis of the physical processes contained within the PD shows that it can reflect the three-dimensional moisture variation, vertical wind shear and wind deformation within the MCS. These structures are usually a comprehensive reflection of the characteristics of the surface cold pool, rear inflow jet, downward cold air flow and upward warm moist flow within the precipitating convective cells. For this reason, the PD shows much stronger anomalies in the precipitating atmosphere than the non-precipitating atmosphere, which implies considerable potential for its application in detecting heavy precipitation within MCSs.
A Modified Algorithm for Identifying and Tracking Extratropical Cyclones
Chuhan LU
2017, 34(7): 909-924. doi: 10.1007/s00376-017-6231-2
In this study, a modified identification and tracking algorithm for extratropical cyclones is developed. This identification scheme is based on triangular-mesh contouring techniques combined with a connected-component labeling method in order to detect the outer boundaries and spatial domain characteristics of individual cyclones. A new tracking method allowing for the identification of cyclone merging and splitting events, as well as short-lived windstorms, is developed to reduce the uncertainty in the tracking of extratropical cyclones. I also show that this method excludes the tracks of open systems that would have been unnecessarily detected using conventional NCP methods. The climatological features of the distribution of cyclone frequencies are substantially larger over the traditional storm track regions compared to those seen in previous studies. Interestingly, a significant increase in the cyclone density in the Arctic occurs during all four seasons (up to 19% in summer) compared to that seen with a latitude-longitude gridded mesh analysis. I develop two new regional intensity indices (depth and vorticity) based on the cyclonic domain to better quantify the cyclonic activity in the Arctic region, and find that the interannual variabilities in these two indices are highly consistent. The results of this analysis may shed light on high-latitude cyclonic behavior studies via the newly detected 2D cyclone atlas derived from this cyclonic-domain-based algorithm.
Statistically Extrapolated Nowcasting of Summertime Precipitation over the Eastern Alps
Min CHEN, Benedikt BICA, Lukas TÜCHLER, Alexander KANN, Yong WANG
2017, 34(7): 925-938. doi: 10.1007/s00376-017-6185-4
This paper presents a new multiple linear regression (MLR) approach to updating the hourly, extrapolated precipitation forecasts generated by the INCA (Integrated Nowcasting through Comprehensive Analysis) system for the Eastern Alps. The generalized form of the model approximates the updated precipitation forecast as a linear response to combinations of predictors selected through a backward elimination algorithm from a pool of predictors. The predictors comprise the raw output of the extrapolated precipitation forecast, the latest radar observations, the convective analysis, and the precipitation analysis. For every MLR model, bias and distribution correction procedures are designed to further correct the systematic regression errors. Applications of the MLR models to a verification dataset containing two months of qualified samples, and to one-month gridded data, are performed and evaluated. Generally, MLR yields slight, but definite, improvements in the intensity accuracy of forecasts during the late evening to morning period, and significantly improves the forecasts for large thresholds. The structure-amplitude-location scores, used to evaluate the performance of the MLR approach, based on its simulation of morphological features, indicate that MLR typically reduces the overestimation of amplitudes and generates similar horizontal structures in precipitation patterns and slightly degraded location forecasts, when compared with the extrapolated nowcasting.