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2015 Vol. 32, No. 12

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Use of Incremental Analysis Updates in 4D-Var Data Assimilation
Banglin ZHANG, Vijay TALLAPRAGADA, Fuzhong WENG, Jason SIPPEL, Zaizhong MA
2015, 32(12): 1575-1582. doi: 10.1007/s00376-015-5041-7
The four-dimensional variational (4D-Var) data assimilation systems used in most operational and research centers use initial condition increments as control variables and adjust initial increments to find optimal analysis solutions. This approach may sometimes create discontinuities in analysis fields and produce undesirable spin ups and spin downs. This study explores using incremental analysis updates (IAU) in 4D-Var to reduce the analysis discontinuities. IAU-based 4D-Var has almost the same mathematical formula as conventional 4D-Var if the initial condition increments are replaced with time-integrated increments as control variables. The IAU technique was implemented in the NASA/GSFC 4D-Var prototype and compared against a control run without IAU. The results showed that the initial precipitation spikes were removed and that other discontinuities were also reduced, especially for the analysis of surface temperature.
Extended Range (10-30 Days) Heavy Rain Forecasting Study Based on a Nonlinear Cross-Prediction Error Model
XIA Zhiye, CHEN Hongbin, XU Lisheng, WANG Yongqian
2015, 32(12): 1583-1591. doi: 10.1007/s00376-015-4252-2
Extended range (10-30 d) heavy rain forecasting is difficult but performs an important function in disaster prevention and mitigation. In this paper, a nonlinear cross prediction error (NCPE) algorithm that combines nonlinear dynamics and statistical methods is proposed. The method is based on phase space reconstruction of chaotic single-variable time series of precipitable water and is tested in 100 global cases of heavy rain. First, nonlinear relative dynamic error for local attractor pairs is calculated at different stages of the heavy rain process, after which the local change characteristics of the attractors are analyzed. Second, the eigen-peak is defined as a prediction indicator based on an error threshold of about 1.5, and is then used to analyze the forecasting validity period. The results reveal that the prediction indicator features regarded as eigen-peaks for heavy rain extreme weather are all reflected consistently, without failure, based on the NCPE model; the prediction validity periods for 1-2 d, 3-9 d and 10-30 d are 4, 22 and 74 cases, respectively, without false alarm or omission. The NCPE model developed allows accurate forecasting of heavy rain over an extended range of 10-30 d and has the potential to be used to explore the mechanisms involved in the development of heavy rain according to a segmentation scale. This novel method provides new insights into extended range forecasting and atmospheric predictability, and also allows the creation of multi-variable chaotic extreme weather prediction models based on high spatiotemporal resolution data.
Ten-Year Climatological Features and Air Origin of Midlatitude Double Tropopauses
WU Xue and LÜ Daren
2015, 32(12): 1592-1602. doi: 10.1007/s00376-015-5036-4
The 10-year climatological features related to midlatitude double tropopause events (DTs) are examined using ERA-Interim data from 2003 to 2012. The analysis is based on tropopauses defined by lapse rate. Results show that DTs are permanent or semi-permanent in the midlatitudes, and high DT frequency bands move poleward in winter and equatorward in summer, which is consistent with the seasonal movement of the subtropical jet. Based on our statistics, the second tropopause is found at about 100 hPa in the subtropics and at slightly lower altitudes in sub-polar regions. The thickness between the first and second tropopause is smaller in the subtropics and increases with latitude. Next, the origin of air sandwiched between the first and second tropopause of DTs is studied with a revised version of the UK Universities Global Atmospheric Modelling Programme Offline Trajectory Code (Version 3) diabatic trajectory model. The results show that, in the lower or middle troposphere, air is transported into the DTs from lower latitudes, mainly in the tropics. The dominant source regions are mainly areas of deep convection and steep orography, e.g., the western Pacific and Himalayan Mountains, and they show strong seasonality following the seasonal shift of these strong upwelling regions.
Evaluation of CMIP5 Climate Models in Simulating 1979-2005 Oceanic Latent Heat Flux over the Pacific
CAO Ning, REN Baohua, ZHENG Jianqiu
2015, 32(12): 1603-1616. doi: 10.1007/s00376-015-5016-8
The climatological mean state, seasonal variation and long-term upward trend of 1979-2005 latent heat flux (LHF) in historical runs of 14 coupled general circulation models from CMIP5 (Coupled Model Intercomparison Project Phase 5) are evaluated against OAFlux (Objectively Analyzed air-sea Fluxes) data. Inter-model diversity of these models in simulating the annual mean climatological LHF is discussed. Results show that the models can capture the climatological LHF fairly well, but the amplitudes are generally overestimated. Model-simulated seasonal variations of LHF match well with observations with overestimated amplitudes. The possible origins of these biases are wind speed biases in the CMIP5 models. Inter-model diversity analysis shows that the overall stronger or weaker LHF over the tropical and subtropical Pacific region, and the meridional variability of LHF, are the two most notable diversities of the CMIP5 models. Regression analysis indicates that the inter-model diversity may come from the diversity of simulated SST and near-surface atmospheric specific humidity. Comparing the observed long-term upward trend, the trends of LHF and wind speed are largely underestimated, while trends of SST and air specific humidity are grossly overestimated, which may be the origins of the model biases in reproducing the trend of LHF.
Effect of Methane Emission Increases in East Asia on Atmospheric Circulation and Ozone
SHANG Lin, LIU Yi, TIAN Wenshou, ZHANG Yuli
2015, 32(12): 1617-1627. doi: 10.1007/s00376-015-5028-4
We used a fully coupled chemistry-climate model (version 3 of the Whole Atmosphere Community Climate Model, WACCM3) to investigate the effect of methane (CH4) emission increases, especially in East Asia and North America, on atmospheric temperature, circulation and ozone (O3). We show that CH4 emission increases strengthen westerly winds in the Northern Hemisphere midlatitudes, accelerate the Brewer-Dobson (BD) circulation, and cause an increase in the mass flux across the tropopause. However, the BD circulation in the tropics between 10°S and 10°N at 100 hPa weakens as CH4 emissions increase in East Asia and strengthens when CH4 emissions increase in North America. When CH4 emissions are increased by 50% in East Asia and 15% globally, the stratospheric temperature cools by up to 0.15 K, and the stratospheric O3 increases by 45 ppbv and 60 ppbv, respectively. A 50% increase of CH4 emissions in North America (with an amplitude of stratospheric O3 increases by 60 ppbv) has a greater influence on the stratospheric O3 than the same CH4 emissions increase in East Asia. CH4 emission increases in East Asia and North America reduce the concentration of tropospheric hydroxyl radicals (4% and 2%, respectively) and increase the concentration of mid-tropospheric O3 (5% and 4%, respectively) in the Northern Hemisphere midlatitudes. When CH4 emissions increase in East Asia, the increase in the tropospheric O3 concentration is largest in August. When CH4 emissions increase in North America, the increase in the O3 concentration is largest in July in the mid-troposphere, and in April in the upper troposphere.
Comparison of Cloud Properties between CloudSat Retrievals and Airplane Measurements in Mixed-Phase Cloud Layers of Weak Convective and Stratus Clouds
QIU Yujun, Thomas CHOULARTON, Jonathan CROSIER, Zixia LIU
2015, 32(12): 1628-1638. doi: 10.1007/s00376-015-4287-4
Cloud microphysical properties including liquid and ice particle number concentration (NC), liquid water content (LWC), ice water content (IWC) and effective radius (RE) were retrieved from CloudSat data for a weakly convective and a widespread stratus cloud. Within the mixed-phase cloud layers, liquid-phase fractions needed to be assumed in the data retrieval process, and one existing linear (p1) and two exponential (p2 and p3) functions, which estimate the liquid-phase fraction as a function of subfreezing temperature (from -20°C to 0°C), were tested. The retrieved NC, LWC, IWC and RE using p1 were on average larger than airplane measurements in the same cloud layer. Function p2 performed better than p1 or p3 in retrieving the NCs of cloud droplets in the convective cloud, while function p1 performed better in the stratus cloud. Function p3 performed better in LWC estimation in both convective and stratus clouds. The REs of cloud droplets calculated using the retrieved cloud droplet NC and LWC were closer to the values of in situ observations than those retrieved directly using the p1 function. The retrieved NCs of ice particles in both convective and stratus clouds, on the assumption of liquid-phase fraction during the retrieval of liquid droplet NCs, were closer to those of airplane observations than on the assumption of function p1.
Impact of UV-A Radiation on Erythemal UV and UV-index Estimation over Korea
Sang Seo PARK, Yun Gon LEE, Jung Hyun KIM
2015, 32(12): 1639-1646. doi: 10.1007/s00376-015-4231-7
Because total UV (TUV) in the UV-A region is 100 times higher than in the UV-B region, UV-A is a considerable component when calculating erythemal UV (EUV) and UV-index. The ratio of EUV to TUV in the UV-A value [EUV(A)/TUV(A)] is investigated to convert the EUV(A) from TUV(A) for broadband observation. The representative value of EUV(A)/TUV(A), from the simulation study, is 6.9× 10-4, changing from 6.1× 10-4 to 7.0× 10-4 as aerosol optical depth, total ozone and solar zenith angle change. By adopting the observational data of EUV(B) and TUV(A) from UV-biometer measurements at Yonsei University [(37.57°N, 126.95°E), 84 m above sea level], the EUV irradiance increases to 15% of EUV(B) due to the consideration of EUV(A) from the data of TUV(A) observation. Compared to the total EUV observed from the Brewer spectrophotometer at the same site, the EUV(B) from the UV-biometer observes only 95% of total EUV, and its underestimation is caused by neglecting the effect of UV-A. However, the sum of EUV(B) and EUV(A) [EUV(A+B)] from two UV-biometers is 10% larger than the EUV from the Brewer spectrophotometer because of the spectral overlap effect in the range 320-340 nm. The correction factor for the overlap effect adjusts 8% of total EUV.
Air-Sea Coupling Enhances the East Asian Winter Climate Response to the Atlantic Multidecadal Oscillation
ZHOU Xiaomin, LI Shuanglin, LUO Feifei, GAO Yongqi, Tore FUREVIK
2015, 32(12): 1647-1659. doi: 10.1007/s00376-015-5030-x
A simple air-sea coupled model, the atmospheric general circulation model (AGCM) of the National Centers for Environmental Prediction coupled to a mixed-layer slab ocean model, is employed to investigate the impact of air-sea coupling on the signals of the Atlantic Multidecadal Oscillation (AMO). A regional coupling strategy is applied, in which coupling is switched off in the extratropical North Atlantic Ocean but switched on in the open oceans elsewhere. The coupled model is forced with warm-phase AMO SST anomalies, and the modeled responses are compared with those from parallel uncoupled AGCM experiments with the same SST forcing. The results suggest that the regionally coupled responses not only resemble the AGCM simulation, but also have a stronger intensity. In comparison, the coupled responses bear greater similarity to the observational composite anomaly. Thus, air-sea coupling enhances the responses of the East Asian winter climate to the AMO. To determine the mechanism responsible for the coupling amplification, an additional set of AGCM experiments, forced with the AMO-induced tropical SST anomalies, is conducted. The SST anomalies are extracted from the simulated AMO-induced SST response in the regionally coupled model. The results suggest that the SST anomalies contribute to the coupling amplification. Thus, tropical air-sea coupling feedback tends to enhance the responses of the East Asian winter climate to the AMO.
The Relationship between Spring Soil Moisture and Summer Hot Extremes over North China
WU Lingyun, ZHANG Jingyong
2015, 32(12): 1660-1668. doi: 10.1007/s00376-015-5003-0
The increase in the occurrence of hot extremes is known to have resulted in serious consequences for human society and ecosystems. However, our ability to seasonally predict hot extremes remains poor, largely due to our limited understanding of slowly evolving earth system components such as soil moisture, and their interactions with climate. In this study, we focus on North China, and investigate the relationship of the spring soil moisture condition to summer hot extremes using soil moisture data from the Global Land Data Assimilation System and observational temperature for the period 1981-2008. It is found that local soil moisture condition in spring is closely linked to summer hot days and heat waves over North China, accounting for 19%-34% of the total variances. Spring soil moisture anomalies can persist to the summer season, and subsequently alter latent and sensible heat fluxes, thus having significant effects on summer hot extremes. Our findings indicate that the spring soil moisture condition can be a useful predictor for summer hot days and heat waves over North China.
Predictability of the Summer East Asian Upper-Tropospheric Westerly Jet in ENSEMBLES Multi-Model Forecasts
LI Chaofan, LIN Zhongda
2015, 32(12): 1669-1682. doi: 10.1007/s00376-015-5057-z
The interannual variation of the East Asian upper-tropospheric westerly jet (EAJ) significantly affects East Asian climate in summer. Identifying its performance in model prediction may provide us another viewpoint, from the perspective of upper-tropospheric circulation, to understand the predictability of summer climate anomalies in East Asia. This study presents a comprehensive assessment of year-to-year variability of the EAJ based on retrospective seasonal forecasts, initiated from 1 May, in the five state-of-the-art coupled models from ENSEMBLES during 1960-2005. It is found that the coupled models show certain capability in describing the interannual meridional displacement of the EAJ, which reflects the models' performance in the first leading empirical orthogonal function (EOF) mode. This capability is mainly shown over the region south of the EAJ axis. Additionally, the models generally capture well the main features of atmospheric circulation and SST anomalies related to the interannual meridional displacement of the EAJ. Further analysis suggests that the predicted warm SST anomalies in the concurrent summer over the tropical eastern Pacific and northern Indian Ocean are the two main sources of the potential prediction skill of the southward shift of the EAJ. In contrast, the models are powerless in describing the variation over the region north of the EAJ axis, associated with the meridional displacement, and interannual intensity change of the EAJ, the second leading EOF mode, meaning it still remains a challenge to better predict the EAJ and, subsequently, summer climate in East Asia, using current coupled models.
A Brief Introduction to BNU-HESM1.0 and Its Earth Surface Temperature Simulations
YANG Shili, DONG Wenjie, CHOU Jieming, FENG Jinming, YAN Xiaodong, WEI Zhigang, YUAN Wenping, GUO Yan, TANG Yanli, HU Jiacong
2015, 32(12): 1683-1688. doi: 10.1007/s00376-015-5050-6
Integrated assessment models and coupled earth system models both have their limitations in understanding the interactions between human activity and the physical earth system. In this paper, a new human-earth system model, BNU-HESM1.0, constructed by combining the economic and climate damage components of the Dynamic Integrated Model of Climate Change and Economy to the BNU-ESM model, is introduced. The ability of BNU-HESM1.0 in simulating the global CO2 concentration and surface temperature is also evaluated. We find that, compared to observation, BNU-HESM1.0 underestimates the global CO2 concentration and its rising trend during 1965-2005, due to the uncertainty in the economic components. However, the surface temperature simulated by BNU-HESM1.0 is much closer to observation, resulting from the overestimates of surface temperature by the original BNU-ESM model. The uncertainty of BNU-ESM falls within the range of present earth system uncertainty, so it is the economic and climate damage component of BNU-HESM1.0 that needs to be improved through further study. However, the main purpose of this paper is to introduce a new approach to investigate the complex relationship between human activity and the earth system. It is hoped that it will inspire further ideas that prove valuable in guiding human activities appropriate for a sustainable future climate.