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2015 Vol. 32, No. 4

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Changes of Air-sea Coupling in the North Atlantic over the 20th Century
YANG Yun, WU Lixin
2015, 32(4): 445-456. doi: 10.1007/s00376-014-4090-7
Changes of air-sea coupling in the North Atlantic Ocean over the 20th century are investigated using reanalysis data, climate model simulations, and observational data. It is found that the ocean-to-atmosphere feedback over the North Atlantic is significantly intensified in the second half of the 20th century. This coupled feedback is characterized by the association between the summer North Atlantic Horseshoe (NAH) SST anomalies and the following winter North Atlantic Oscillation (NAO). The intensification is likely associated with the enhancement of the North Atlantic storm tracks as well as the NAH SST anomalies. Our study also reveals that most IPCC AR4 climate models fail to capture the observed NAO/NAH coupled feedback.
Attributing Analysis on the Model Bias in Surface Temperature in the Climate System Model FGOALS-s2 through a Process-Based Decomposition Method
YANG Yang, REN Rongcai, Ming CAI, RAO Jian
2015, 32(4): 457-469. doi: 10.1007/s00376-014-4061-z
This study uses the coupled atmosphere-surface climate feedback-response analysis method (CFRAM) to analyze the surface temperature biases in the Flexible Global Ocean-Atmosphere-Land System model, spectral version 2 (FGOALS-s2) in January and July. The process-based decomposition of the surface temperature biases, defined as the difference between the model and ERA-Interim during 1979-2005, enables us to attribute the model surface temperature biases to individual radiative processes including ozone, water vapor, cloud, and surface albedo; and non-radiative processes including surface sensible and latent heat fluxes, and dynamic processes at the surface and in the atmosphere. The results show that significant model surface temperature biases are almost globally present, are generally larger over land than over oceans, and are relatively larger in summer than in winter. Relative to the model biases in non-radiative processes, which tend to dominate the surface temperature biases in most parts of the world, biases in radiative processes are much smaller, except in the sub-polar Antarctic region where the cold biases from the much overestimated surface albedo are compensated for by the warm biases from non-radiative processes. The larger biases in non-radiative processes mainly lie in surface heat fluxes and in surface dynamics, which are twice as large in the Southern Hemisphere as in the Northern Hemisphere and always tend to compensate for each other. In particular, the upward/downward heat fluxes are systematically underestimated/overestimated in most parts of the world, and are mainly compensated for by surface dynamic processes including the increased heat storage in deep oceans across the globe.
Synergistic Use of AIRS and MODIS for Dust Top Height Retrieval over Land
YAO Zhigang, Jun LI, ZHAO Zengliang
2015, 32(4): 470-476. doi: 10.1007/s00376-014-4046-y
It is nontrivial to extract the dust top height (DTH) accurately from passive instruments over land due to the complexity of the surface conditions. The Moderate Resolution Imaging Spectroradiometer (MODIS) deep blue (DB) algorithm can be used to infer the aerosol optical depth (AOD) over high-reflective surfaces. The Atmospheric Infrared Sounder (AIRS) can simultaneously obtain the DTH and optical depth information. This study focuses on the synergistic use of AIRS observations and MODIS DB results for improving the DTH by using a stable relationship between the AIRS infrared and MODIS DB AODs. A one-dimensional variational (1DVAR) algorithm is applied to extract the DTH from AIRS. Simulation experiments indicate that when the uncertainty of the dust optical depth decreases from 50% to 20%, the improvement of the DTH retrieval accuracy from AIRS reaches 200 m for most of the assumed dust conditions. For two cases over the Taklimakan Desert, the results are compared against Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) measurements. The results confirm that the MODIS DB product could help extract the DTH over land from AIRS.
Comparison of Column-Integrated Aerosol Optical and Physical Properties in an Urban and Suburban Site on the North China Plain
FAN Xuehua, XIA Xiang'ao, CHEN Hongbin
2015, 32(4): 477-486. doi: 10.1007/s00376-014-4097-0
The column-integrated optical properties of aerosol in Beijing and Xianghe, two AErosol RObotic NETwork (AERONET) sites situated on the North China Plain (NCP), are investigated based on Cimel sunphotometer measurements from October 2004 to June 2012. The outstanding feature found is that the seasonal medians of aerosol optical depth (AOD) at the two stations are in good agreement. The correlation coefficients and the absolute differences between AOD at the two stations are larger than 0.84 and less than 0.05, respectively. Good agreement in AOD at these two sites (one urban and the other suburban; 70 km apart) indicates that aerosol pollution in the Greater Beijing area is regional in nature. However, we find significant differences in the absorption ngstrm Exponent (AAE), the real and imaginary part of the refractive index, and thereby the single scattering albedo (SSA), and the difference is seasonally dependent. The feature is found to be more prominent in fall when the fine-mode fraction (FMF) and fine-mode effective radius are significantly different at the two stations, besides the parameters mentioned above. The SSA in Beijing at four wavelengths shows lower values as compared to those in Xianghe, although the difference is not significant in some cases. Significant differences in AAE and fine-mode effective radius indicate that there are differences in aerosol physical and chemical properties in urban and suburban regions on the NCP.
Future Changes and Uncertainties in Temperature and Precipitation over China Based on CMIP5 Models
TIAN Di, GUO Yan*, DONG Wenjie
2015, 32(4): 487-496. doi: 10.1007/s00376-014-4102-7
Climate changes in future 21st century China and their uncertainties are evaluated based on 22 climate models from the Coupled Model Intercomparison Project Phase 5 (CMIP5). By 2081-2100, the annual mean surface air temperature (SAT) is predicted to increase by 1.3 C 0.7C, 2.6C 0.8C and 5.2C 1.2under the Representative Concentration Pathway (RCP) scenarios RCP2.6, RCP4.5 and RCP8.5, relative to 1986-2005, respectively. The future change in SAT averaged over China increases the most in autumn/winter and the least in spring, while the uncertainty shows little seasonal variation. Spatially, the annual and seasonal mean SAT both show a homogeneous warming pattern across China, with a warming rate increasing from southeastern China to the Tibetan Plateau and northern China, invariant with time and emissions scenario. The associated uncertainty in SAT decreases from northern to southern China. Meanwhile, by 2081-2100, the annual mean precipitation increases by 5% 5%, 8% 6% and 12% 8% under RCP2.6, RCP4.5 and RCP8.5, respectively. The national average precipitation anomaly percentage, largest in spring and smallest in winter, and its uncertainty, largest in winter and smallest in autumn, show visible seasonal variations. Although at a low confidence level, a homogeneous wetting pattern is projected across China on the annual mean scale, with a larger increasing percentage in northern China and a weak drying in southern China in the early 21st century. The associated uncertainty is also generally larger in northern China and smaller in southwestern China. In addition, both SAT and precipitation usually show larger seasonal variability on the sub-regional scale compared with the national average.
Improving Multi-model Ensemble Probabilistic Prediction of Yangtze River Valley Summer Rainfall
LI Fang, LIN Zhongda
2015, 32(4): 497-504. doi: 10.1007/s00376-014-4073-8
Seasonal prediction of summer rainfall over the Yangtze River valley (YRV) is valuable for agricultural and industrial production and freshwater resource management in China, but remains a major challenge. Earlier multi-model ensemble (MME) prediction schemes for summer rainfall over China focus on single-value prediction, which cannot provide the necessary uncertainty information, while commonly-used ensemble schemes for probability density function (PDF) prediction are not adapted to YRV summer rainfall prediction. In the present study, an MME PDF prediction scheme is proposed based on the ENSEMBLES hindcasts. It is similar to the earlier Bayesian ensemble prediction scheme, but with optimization of ensemble members and a revision of the variance modeling of the likelihood function. The optimized ensemble members are regressed YRV summer rainfall with factors selected from model outputs of synchronous 500-hPa geopotential height as predictors. The revised variance modeling of the likelihood function is a simple linear regression with ensemble spread as the predictor. The cross-validation skill of 1960-2002 YRV summer rainfall prediction shows that the new scheme produces a skillful PDF prediction, and is much better-calibrated, sharper, and more accurate than the earlier Bayesian ensemble and raw ensemble.
Contrasting Characteristics of the Surface Energy Balance between the Urban and Rural Areas of Beijing
WANG Linlin, GAO Zhiqiu, MIAO Shiguang, GUO Xiaofeng, SUN Ting, Maofeng LIU, Dan LI
2015, 32(4): 505-514. doi: 10.1007/s00376-014-3222-4
A direct comparison of urban and rural surface energy balances, as well as a variety of other variables including incoming shortwave/longwave radiation and aerosol optical depth, is conducted for the Beijing metropolitan area. The results indicate that, overall, the urban area receives a smaller amount of incoming shortwave radiation but a larger amount of incoming longwave radiation. However, comparisons in the aerosol optical depth and cloud fraction at the two locations suggest that neither aerosol optical depth nor cloud fraction alone can explain the difference in the incoming shortwave radiation. The urban-rural differences in the incoming longwave radiation are unlikely to be caused by the presence of more abundant greenhouse gases over the urban area, as suggested by some previous studies, given that water vapor is the most dominant greenhouse gas and precipitable water is found to be less in urban areas. The higher incoming longwave radiation observed over the urban area is mostly likely due to the higher temperatures of the ambient air. The urban area is also found to always produce higher sensible heat fluxes and lower latent heat fluxes in the growing season. Furthermore, the urban area is associated with a larger amount of available energy (the sum of sensible and latent heat fluxes) than the rural area, except in May and October when evapotranspiration in the rural area significantly exceeds that in the urban area. This study provides observational evidence of urban-rural contrasts in relevant energy-balance components that plausibly arise from urban-rural differences in atmospheric and land-surface conditions.
Statistical Analysis of Thunderstorms on the Eastern Tibetan Plateau Based on Modified Thunderstorm Indices
YOU Wei, ZANG Zengliang, PAN Xiaobin, ZHANG Lifeng, LI Yi
2015, 32(4): 515-527. doi: 10.1007/s00376-014-4039-x
The Tibetan Plateau, with an average altitude above 4000 m, is the highest and largest plateau in the world. The frequency of thunderstorms in this region is extremely high. Many indices are used in operational forecasting to assess the stability of the atmosphere and predict the probability of severe thunderstorm development. One of the disadvantages of many of these indices is that they are mainly based on observations from plains. However, considering the Plateau's high elevation, most convective parameters cannot be applied directly, or their application is ineffective. The pre-convective environment on thunderstorm days in this region is investigated based on sounding data obtained throughout a five-year period (2006-10). Thunderstorms occur over the Tibetan Plateau under conditions that differ strikingly from those in plains. On this basis, stability indices, such as the Showalter index (including SI and SI CCL), and the K index are improved to better assess the thunderstorm environments on the Plateau. Verification parameters, such as the true-skill statistic (TSS) and Heidke skill score (HSS), are adopted to evaluate the optimal thresholds and relative forecast skill for each modified index. Lastly, the modified indices are verified with a two-year independent dataset (2011-12), showing satisfactory results for the modified indices. For determining whether or not a thunderstorm day is likely to occur, we recommend the modified SI CCL index.
Downstream Development of Baroclinic Waves in the Midlatitude Jet Induced by Extratropical Transition: A Case Study
2015, 32(4): 528-540. doi: 10.1007/s00376-014-3263-8
This study uses eddy kinetic energy analysis and a targeting method to investigate how an extratropical transition (ET) event induced downstream development (the modification of the midlatitude flow downstream of the ET system) in the midlatitude jet environment. The downstream development showed distinct characteristics of coupling development and being boundary-trapped. Eddies (potential disturbances) first developed at the upper levels, and these triggered lower-level eddy development, with all eddies decaying away from the tropopause and the surface. Thereafter, a lower-level eddy caught up with the upper-level eddy ahead of it, and they coupled to form a cyclone extending through the whole troposphere. Vertical ageostrophic geopotential flux may be a crucial dynamic factor throughout the eddy's lower-level growth, boundary-trapping, and coupling development. Together with barotropic conversion, the ageostrophic geopotential fluxes that were transported from Hurricane Fabian (2003) to the midlatitudes by the outflow led to downstream ridge development in the upper-level jet. The strong downstream advection of eddy kinetic energy in the exit region of the jet streak triggered downstream trough development. The well-known ridge-trough couplet thus formed. The vertical ageostrophic fluxes that were transported downward from the developed upper-level systems converged near the surface and resulted in lower-level eddy growth. Baroclinic conversion was negligible near the boundaries, while it was the main source of eddy kinetic energy at mid-levels. In the upper-level jet, potential energy was converted to the mean kinetic energy of the jet, which in turn was converted to eddy kinetic energy through barotropic conversion.
Using Quantile Regression to Detect Relationships between Large-scale Predictors and Local Precipitation over Northern China
FAN Lijun, XIONG Zhe
2015, 32(4): 541-552. doi: 10.1007/s00376-014-4058-7
Quantile regression (QR) is proposed to examine the relationships between large-scale atmospheric variables and all parts of the distribution of daily precipitation amount at Beijing Station from 1960 to 2008. QR is also applied to evaluate the relationship between large-scale predictors and extreme precipitation (90th quantile) at 238 stations in northern China. Finally, QR is used to fit observed daily precipitation amounts for wet days at four sample stations. Results show that meridional wind and specific humidity at both 850 hPa and 500 hPa (V850, SH850, V500, and SH500) strongly affect all parts of the Beijing precipitation distribution during the wet season (April-September). Meridional wind, zonal wind, and specific humidity at only 850 hPa (V850, U850, SH850) are significantly related to the precipitation distribution in the dry season (October-March). Impacts of these large-scale predictors on the daily precipitation amount with higher quantile become stronger, whereas their impact on light precipitation is negligible. In addition, SH850 has a strong relationship with wet-season extreme precipitation across the entire region, whereas the impacts of V850, V500, and SH500 are mainly in semi-arid and semi-humid areas. For the dry season, both SH850 and V850 are the major predictors of extreme precipitation in the entire region. Moreover, QR can satisfactorily simulate the daily precipitation amount at each station and for each season, if an optimum distribution family is selected. Therefore, QR is valuable for detecting the relationship between the large-scale predictors and the daily precipitation amount.
Diurnal and Seasonal Variations of CO2 Fluxes and Their Climate Controlling Factors for a Subtropical Forest in Ningxiang
JIA Binghao, XIE Zhenghui, ZENG Yujin, WANG Linying, WANG Yuanyuan, XIE Jinbo, XIE Zhipeng
2015, 32(4): 553-564. doi: 10.1007/s00376-014-4069-4
In this study, the diurnal and seasonal variations of CO2 fluxes in a subtropical mixed evergreen forest in Ningxiang of Hunan Province, part of the East Asian monsoon region, were quantified for the first time. The fluxes were based on eddy covariance measurements from a newly initiated flux tower. The relationship between the CO2 fluxes and climate factors was also analyzed. The results showed that the target ecosystem appeared to be a clear carbon sink in 2013, with integrated net ecosystem CO2 exchange (NEE), ecosystem respiration (RE), and gross ecosystem productivity (GEP) of -428.8, 1534.8 and 1963.6 g C m-2 yr-1, respectively. The net carbon uptake (i.e. the -NEE), RE and GEP showed obvious seasonal variability, and were lower in winter and under drought conditions and higher in the growing season. The minimum NEE occurred on 12 June (-7.4 g C m-2 d-1), due mainly to strong radiation, adequate moisture, and moderate temperature; while a very low net CO2 uptake occurred in August (9 g C m-2 month-1), attributable to extreme summer drought. In addition, the NEE and GEP showed obvious diurnal variability that changed with the seasons. In winter, solar radiation and temperature were the main controlling factors for GEP, while the soil water content and vapor pressure deficit were the controlling factors in summer. Furthermore, the daytime NEE was mainly limited by the water-stress effect under dry and warm atmospheric conditions, rather than by the direct temperature-stress effect.
Statistical Characteristics and Mechanistic Analysis of Suddenly Reversed Tropical Cyclones over the Western North Pacific Ocean
LUO Xia, FEI Jianfang, HUANG Xiaogang, CHENG Xiaoping, DING Juli, HE Yiqiang
2015, 32(4): 565-576. doi: 10.1007/s00376-014-4064-9
Based on best track data of tropical cyclones (TCs) from the Japan Meteorological Agency, the characteristics of suddenly reversed TCs (SRTCs), which have turning angles usually approaching 180°, are statistically analyzed from 1949 to 2011 over the western North Pacific Ocean. The typical large-scale circulation patterns of SRTCs are investigated using reanalysis data and dynamical composite analysis. Results show that turnings mainly occur in low latitudes between 10°N and 20°N, and mainly west of 135°E. The majority of SRTCs reach their peak intensity at, or slightly before, the turning time and subsequently decrease at some variable rate. Specifically, SRTCs are divided into four types, each containing two groups (i.e. eight groups in total) in terms of the moving-direction changes. The moving speed of all SRTC types except the south-north type decreases to its lowest during the 24 h, corresponding to a significant reduction in the primary steering components. According to the analysis of the 13 typical flow patterns found in this study, we suggest that sudden track changes are caused by the reversal steering flow. The original balance of the background flow patterns are broken up by new systems, e.g. binary TCs or dispersion-induced anticyclones. Additionally, sudden track changes are often due to double ridge variations of the subtropical high or weakened/strengthened high pressure in the east and west, respectively.