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2016 Vol. 33, No. 6

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Estimation and Correction of Model Bias in the NASA/GMAO GEOS5 Data Assimilation System: Sequential Implementation
Banglin ZHANG, Vijay TALLAPRAGADA, Fuzhong WENG, Jason SIPPEL, Zaizhong MA
2016, 33(6): 659-672. doi: 10.1007/ s00376-015-5155-y
This study presents a simplified multivariate bias correction scheme that is sequentially implemented in the GEOS5 data assimilation system and compared against a control experiment without model bias correction. The results show considerable improvement in terms of the mean biases of rawinsonde observation-minus-background (OmB) residuals for observed water vapor, wind and temperature variables. The time series spectral analysis shows whitening of bias-corrected OmB residuals, and mean biases for rawinsonde observation-minus-analysis (OmA) are also improved. Some wind and temperature biases in the control experiment near the equatorial tropopause nearly vanish from the bias-corrected experiment. Despite the analysis improvement, the bias correction scheme has only a moderate impact on forecast skill. Significant interaction is also found among quality-control, satellite observation bias correction, and background bias correction, and the latter positively impacts satellite bias correction.
Ensemble Simulation of Land Evapotranspiration in China Based on a Multi-Forcing and Multi-Model Approach
Jianguo LIU, Binghao JIA, Zhenghui XIE, Chunxiang SHI
2016, 33(6): 673-684. doi: 10.1007/s00376-016-5213-0
In order to reduce the uncertainty of offline land surface model (LSM) simulations of land evapotranspiration (ET), we used ensemble simulations based on three meteorological forcing datasets [Princeton, ITPCAS (Institute of Tibetan Plateau Research, Chinese Academy of Sciences), Qian] and four LSMs (BATS, VIC, CLM3.0 and CLM3.5), to explore the trends and spatiotemporal characteristics of ET, as well as the spatiotemporal pattern of ET in response to climate factors over mainland China during 1982-2007. The results showed that various simulations of each member and their arithmetic mean (Ens_Mean) could capture the spatial distribution and seasonal pattern of ET sufficiently well, where they exhibited more significant spatial and seasonal variation in the ET compared with observation-based ET estimates (Obs_MTE). For the mean annual ET, we found that the BATS forced by Princeton forcing overestimated the annual mean ET compared with Obs_MTE for most of the basins in China, whereas the VIC forced by Princeton forcing showed underestimations. By contrast, the Ens_Mean was closer to Obs_MTE, although the results were underestimated over Southeast China. Furthermore, both the Obs_MTE and Ens_Mean exhibited a significant increasing trend during 1982-98; whereas after 1998, when the last big EI Niño event occurred, the Ens_Mean tended to decrease significantly between 1999 and 2007, although the change was not significant for Obs_MTE. Changes in air temperature and shortwave radiation played key roles in the long-term variation in ET over the humid area of China, but precipitation mainly controlled the long-term variation in ET in arid and semi-arid areas of China.
Comparison of Constant and Time-variant Optimal Forcing Approaches in El Niño Simulations by Using the Zebiak-Cane Model
Ben TIAN, Wansuo DUAN
2016, 33(6): 685-694. doi: 10.1007/s00376-015-5174-8
Model errors offset by constant and time-variant optimal forcing vector approaches (termed COF and OFV, respectively) are analyzed within the framework of El Niño simulations. Applying the COF and OFV approaches to the well-known Zebiak-Cane model, we re-simulate the 1997 and 2004 El Niño events, both of which were poorly degraded by a certain amount of model error when the initial anomalies were generated by coupling the observed wind forcing to an ocean component. It is found that the Zebiak-Cane model with the COF approach roughly reproduced the 1997 El Niño, but the 2004 El Niño simulated by this approach defied an ENSO classification, i.e., it was hardly distinguishable as CP-El Niño or EP-El Niño. In both El Niño simulations, substituting the COF with the OFV improved the fit between the simulations and observations because the OFV better manages the time-variant errors in the model. Furthermore, the OFV approach effectively corrected the modeled El Niño events even when the observational data (and hence the computational time) were reduced. Such a cost-effective offset of model errors suggests a role for the OFV approach in complicated CGCMs.
Simple Metrics for Representing East Asian Winter Monsoon Variability: Urals Blocking and Western Pacific Teleconnection Patterns
Hoffman H. N. CHEUNG, Wen ZHOU
2016, 33(6): 695-705. doi: 10.1007/s00376-015-5204-6
Instead of conventional East Asian winter monsoon indices (EAWMIs), we simply use two large-scale teleconnection patterns to represent long-term variations in the EAWM. First, the Urals blocking pattern index (UBI) is closely related to cold air advection from the high latitudes towards western Siberia, such that it shows an implicit linkage with the Siberian high intensity and the surface air temperature (SAT) variations north of 40°N in the EAWM region. Second, the well-known western Pacific teleconnection index (WPI) is connected with the meridional displacement of the East Asian jet stream and the East Asian trough. This is strongly related to the SAT variations in the coastal area south of 40°N in the EAWM region. The temperature variation in the EAWM region is also represented by the two dominant temperature modes, which are called the northern temperature mode (NTM) and the southern temperature mode (STM). Compared to 19 existing EAWMIs and other well-known teleconnection patterns, the UBI shows the strongest correlation with the NTM, while the WPI shows an equally strong correlation with the STM as four EAWMIs. The UBI-NTM and WPI-STM relationships are robust when the correlation analysis is repeated by (1) the 31-year running correlation and (2) the 8-year high-pass and low-pass filter. Hence, these results are useful for analyzing the large-scale teleconnections of the EAWM and for evaluating this issue in climate models. In particular, more studies should focus on the teleconnection patterns over extratropical Eurasia.
Influence of Internal Decadal Variability on the Summer Rainfall in Eastern China as Simulated by CCSM4
Yali ZHU, Tao WANG, Jiehua MA
2016, 33(6): 706-714. doi: 10.1007/s00376-016-5269-x
The combined impact of the Pacific Decadal Oscillation (PDO) and Atlantic Multidecadal Oscillation (AMO) on the summer rainfall in eastern China was investigated using CCSM4. The strongest signals occur with the combination of a positive PDO and a negative AMO ( +PDO-AMO), as well as a negative PDO and a positive AMO ( -PDO+AMO). For the +PDO-AMO set, significant positive rainfall anomalies occur over the lower reaches of the Yangtze River valley (YR), when the East Asian summer monsoon becomes weaker, while the East Asian westerly jet stream becomes stronger, and ascending motion over the YR becomes enhanced due to the jet-related secondary circulation. Contrary anomalies occur over East Asia for the -PDO+AMO set. The influence of these two combinations of PDO and AMO on the summer rainfall in eastern China can also be observed in the two interdecadal rainfall changes in eastern China in the late 1970s and late 1990s.
A Double-Index Method to Classify Kuroshio Intrusion Paths in the Luzon Strait
Zhida HUANG, Hailong LIU, Jianyu HU, Pengfei LIN
2016, 33(6): 715-729. doi: 10.1007/s00376-015-5171-y
A double index (DI), which is made up of two sub-indices, is proposed to describe the spatial patterns of the Kuroshio intrusion and mesoscale eddies west to the Luzon Strait, based on satellite altimeter data. The area-integrated negative and positive geostrophic vorticities are defined as the Kuroshio warm eddy index (KWI) and the Kuroshio cold eddy index (KCI), respectively. Three typical spatial patterns are identified by the DI: the Kuroshio warm eddy path (KWEP), the Kuroshio cold eddy path (KCEP), and the leaking path. The primary features of the DI and three patterns are further investigated and compared with previous indices. The effects of the integrated area and the algorithm of the integration are investigated in detail. In general, the DI can overcome the problem of previously used indices in which the positive and negative geostrophic vorticities cancel each other out. Thus, the proportions of missing and misjudged events are greatly reduced using the DI. The DI, as compared with previously used indices, can better distinguish the paths of the Kuroshio intrusion and can be used for further research.
Impact of Spectral Nudging on the Downscaling of Tropical Cyclones in Regional Climate Simulations
Suk-Jin CHOI, Dong-Kyou LEE
2016, 33(6): 730-742. doi: 10.1007/s00376-016-5061-y
This study investigated the simulations of three months of seasonal tropical cyclone (TC) activity over the western North Pacific using the Advanced Research WRF Model. In the control experiment (CTL), the TC frequency was considerably overestimated. Additionally, the tracks of some TCs tended to have larger radii of curvature and were shifted eastward. The large-scale environments of westerly monsoon flows and subtropical Pacific highs were unreasonably simulated. The overestimated frequency of TC formation was attributed to a strengthened westerly wind field in the southern quadrants of the TC center. In comparison with the experiment with the spectral nudging method, the strengthened wind speed was mainly modulated by large-scale flow that was greater than approximately 1000 km in the model domain. The spurious formation and undesirable tracks of TCs in the CTL were considerably improved by reproducing realistic large-scale atmospheric monsoon circulation with substantial adjustment between large-scale flow in the model domain and large-scale boundary forcing modified by the spectral nudging method. The realistic monsoon circulation took a vital role in simulating realistic TCs. It revealed that, in the downscaling from large-scale fields for regional climate simulations, scale interaction between model-generated regional features and forced large-scale fields should be considered, and spectral nudging is a desirable method in the downscaling method.
On the Relationship between the Winter Eurasian Teleconnection Pattern and the Following Summer Precipitation over China
Junhu ZHAO, Liu YANG, Bohui GU, Jie YANG, Guolin FENG
2016, 33(6): 743-752. doi: 10.1007/s00376-015-5195-3
The Eurasian teleconnection pattern (EU) is an important low-frequency pattern with well-known impacts on climate anomalies in Eurasia. The difference of low-level v-winds in several regions in the Eurasian mid-high latitudes is defined as the EU index (EUI V). In this study, the relationship between the winter EUI V and precipitation in the following summer over China is investigated. Results show that there is a significant positive (negative) correlation between the winter EUI V and the following summer precipitation over North China (the Yangtze River-Huaihe River basins). Meanwhile, an interdecadal variability exists in the interannual relationship, and the correlation has become significantly enhanced since the early 1980s. Thus, the proposed EUI V may have implications for the prediction of summer precipitation anomalies over China. In positive winter EUI V years, three cyclonic circulation anomalies are observed——over the Ural Mountains, the Okhotsk Sea, and the subtropical western North Pacific. That is, the Ural blocking and Okhotsk blocking are inactive, zonal circulation prevails in the mid-high latitudes, and the western Pacific subtropical high tends to be weaker and locates to the north of its normal position in the following summer. This leads to above-normal moisture penetrating into the northern part of East China, and significant positive (negative) precipitation anomalies over North China (the Yangtze River-Huaihe River basins), and vice versa. Further examination shows that the SST anomalies over the Northwest Pacific and subtropical central North Pacific may both contribute to the formation of EUI V-related circulation anomalies over the western North Pacific.
Impact of Anthropogenic Aerosols on Summer Precipitation in the Beijing-Tianjin-Hebei Urban Agglomeration in China: Regional Climate Modeling Using WRF-Chem
Jun WANG, Jinming FENG, Qizhong WU, Zhongwei YAN
2016, 33(6): 753-766. doi: 10.1007/s00376-015-5103-x
The WRF model with chemistry (WRF-Chem) was employed to simulate the impacts of anthropogenic aerosols on summer precipitation over the Beijing-Tianjin-Hebei urban agglomeration in China. With the aid of a high-resolution gridded inventory of anthropogenic emissions of trace gases and aerosols, we conducted relatively long-term regional simulations, considering direct, semi-direct and indirect effects of the aerosols. Comparing the results of sensitivity experiments with and without emissions, it was found that anthropogenic aerosols tended to enhance summer precipitation over the metropolitan areas. Domain-averaged rainfall was increased throughout the day, except for the time around noon. Aerosols shifted the precipitation probability distribution from light or moderate to extreme rain. Further analysis showed that the anthropogenic aerosol radiative forcing had a cooling effect at the land surface, but a warming effect in the atmosphere. However, enhanced convective strength and updrafts accompanied by water vapor increases and cyclone-like wind shear anomalies were found in the urban areas. These responses may originate from cloud microphysical effects of aerosols on convection, which were identified as the primary cause for the summer rainfall enhancement.
The Congo Basin Zonal Overturning Circulation
2016, 33(6): 767-782. doi: 10.1007/s00376-015-5190-8
The Gulf of Guinea in the equatorial Atlantic is characterized by the presence of strong subsidence at certain times of the year. This subsidence appears in June and becomes well established from July to September. Since much of the West African monsoon flow originates over the Gulf, Guinean subsidence is important for determining moisture sources for the monsoon. Using reanalysis products, I contribute to a physical understanding of what causes this seasonal subsidence, and how it relates to precipitation distributions across West Africa. There is a seasonal zonal overturning circulation above the Congo basin and the Gulf of Guinea in the ERA-Interim, ERA-40, NCEP2, and MERRA reanalyses. The up-branch is located in the Congo basin around 20°E. Mid-tropospheric easterly flows constitute the returning-branch and sinking over the Gulf of Guinea forms the down-branch, which diverges at 2°W near the surface, with winds to the east flowing eastward to complete the circulation. This circulation is driven by surface temperature differences between the eastern Gulf and Congo basin. Land temperatures remain almost uniform, around 298 K, throughout a year, but the Guinean temperatures cool rapidly from 294 K in May to about 290 K in August. These temperature changes increase the ocean/land temperature contrast, up to 8 K, and drive the circulation. I hypothesize that when the overturning circulation is anomalously strong, the northward moisture transport and Sahelian precipitation are also strong. This hypothesis is supported by ERA-Interim and PERSIANN-CDR (Precipitation Estimation from Remotely Sensed Information using Artificial Neural Networks-Climate Data Record) data.
Numerical Simulation of the Impact of Urban Non-uniformity on Precipitation
Yuqiang SONG, Hongnian LIU, Xueyuan WANG, Ning ZHANG, Jianning SUN
2016, 33(6): 783-793. doi: 10.1007/s00376-016-5042-1
To evaluate the influence of urban non-uniformity on precipitation, the area of a city was divided into three categories (commercial, high-density residential, and low-density residential) according to the building density data from Landsat satellites. Numerical simulations of three corresponding scenarios (urban non-uniformity, urban uniformity, and non-urban) were performed in Nanjing using the WRF model. The results demonstrate that the existence of the city results in more precipitation, and that urban heterogeneity enhances this phenomenon. For the urban non-uniformity, uniformity, and non-urban experiments, the mean cumulative summer precipitation was 423.09 mm, 407.40 mm, and 389.67 mm, respectively. Urban non-uniformity has a significant effect on the amount of heavy rainfall in summer. The cumulative precipitation from heavy rain in the summer for the three numerical experiments was 278.2 mm, 250.6 mm, and 236.5 mm, respectively. In the non-uniformity experiments, the amount of precipitation between 1500 and 2200 (LST) increased significantly. Furthermore, the adoption of urban non-uniformity into the WRF model could improve the numerical simulation of summer rain and its daily variation.