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2021 Vol. 38, No. 10

News & Views
China’s EarthLab—Forefront of Earth System Simulation Research
Zhaoyang CHAI, He ZHANG, Mulan ZHANG, Xiao TANG, Weipeng ZHENG, Jiang ZHU, Guangqing ZHOU, Junji CAO, Qingcun ZENG
2021, 38(10): 1611-1620. doi: 10.1007/s00376-021-1175-y
This article introduces “EarthLab”, a major new Earth system numerical simulation facility developed in China. EarthLab is a numerical simulation system for a physical climate system, an environmental system, an ecological system, a solid earth system, and a space weather system as a whole with a high-performance scientific computing platform. EarthLab consists of five key elements—namely: a global earth numerical simulation system, a regional high-precision simulation system, a supercomputing support and management system, a database, data assimilation and visualization system, and a high-performance computing system for earth sciences. EarthLab helps to study the atmosphere, hydrosphere, cryosphere, lithosphere, and biosphere, as well as their interactions, to improve the accuracy of predictions by integrating simulations and observations, and to provide a scientific foundation for major issues such as national disaster prevention and mitigation. The construction and operation of EarthLab will involve close cooperation with joint contributions and shared benefits.
On the Importance of High-Resolution in Large-Scale Ocean Models
Eric P. CHASSIGNET, Xiaobiao XU
2021, 38(10): 1621-1634. doi: 10.1007/s00376-021-0385-7
Eddying global ocean models are now routinely used for ocean prediction, and the value-added of a better representation of the observed ocean variability and western boundary currents at that resolution is currently being evaluated in climate models. This overview article begins with a brief summary of the impact on ocean model biases of resolving eddies in several global ocean–sea ice numerical simulations. Then, a series of North and Equatorial Atlantic configurations are used to show that an increase of the horizontal resolution from eddy-resolving to submesoscale-enabled together with the inclusion of high-resolution bathymetry and tides significantly improve the models’ abilities to represent the observed ocean variability and western boundary currents. However, the computational cost of these simulations is extremely large, and for these simulations to become routine, close collaborations with computer scientists are essential to ensure that numerical codes can take full advantage of the latest computing architecture.
Original Paper
Projected Increase in Probability of East Asian Heavy Rainy Summer in the 21st Century by CMIP5 Models
Yuanhai FU, Riyu LU, Dong GUO
2021, 38(10): 1635-1650. doi: 10.1007/s00376-021-0347-0
In the summer of 1998, heavy rainfall persisted throughout the summer and resulted in a severe prolonged flooding event over East Asia. Will a similar rainy summer happen again? To date, many studies have investigated projected changes in the seasonality or daily extreme precipitation events over East Asia; however, few studies have focused on the changes in extreme summer-averaged East Asian rainfall. This type of summer is referred to as a “heavy rainy summer (HRS)” in this study, and an investigation of future changes in its probability is performed by analyzing CMIP5 model outputs in historical climate simulation (HIST) and under RCP4.5 and RCP8.5.  All models project increased probabilities of HRS by a factor of two to three. The projected East Asian summer rainfall (EASR) (EASRRCPs−EASRHIST) in both climatology and HRS is expected to intensify significantly. The increased EASR could be attributed to significantly intensified water vapor transport (WVT) originating from the tropical Indian Ocean (TIO) and the eastern subtropical North Pacific (SNP), which is a result of the thermodynamic component. The WVT from the TIO would supply more moisture for EASR because of its stronger intensity and faster rate of increase. Meanwhile, the EASR anomaly in HRS relative to climatology (EASRHRS−EASRCLM) would increase by approximately 11%–33%. In HIST, the associated WVT anomaly, caused only by the dynamic component, converges moisture from adjacent land and ocean. However, under the RCPs, the WVT anomaly from the TIO, resulted from the thermodynamic component, would appear and increase by a factor of three to be comparable to the WVT anomaly from the eastern SNP. The latter would result from the dynamic component but increase by only half.
Controls on the Northward Movement of the ITCZ over the South China Sea in Autumn: A Heavy Rain Case Study
Fei WANG, Lifang SHENG, Xiadong AN, Qian LIU, Haixia ZHOU, Yingying ZHANG, Jianping LI
2021, 38(10): 1651-1664. doi: 10.1007/s00376-021-0445-z
The autumn Intertropical Convergence Zone (ITCZ) over the South China Sea (SCS) is typically held south of 10°N by prevailing northeasterly and weakening southwesterly winds. However, the ITCZ can move north, resulting in heavy rainfall in the northern SCS (NSCS). We investigate the mechanisms that drove the northward movement of the ITCZ and led to heavy non-tropical-cyclone rainfall over the NSCS in autumn of 2010. The results show that the rapid northward movement of the ITCZ on 1 and 2 October was caused by the joint influence of the equatorial easterlies (EE), southwesterly winds, and the easterly jet (EJ) in the NSCS. A high pressure center on the east side of Australia, strengthened by the quasi-biweekly oscillation and strong Walker circulation, was responsible for the EE to intensify and reach the SCS. The EE finally turned southeast and together with enhanced southwesterly winds associated with an anticyclone, pushed the ITCZ north. Meanwhile, the continental high moved east, which reduced the area of the EJ in the NSCS and made room for the ITCZ. Further regression analysis showed that the reduced area of the EJ and increased strength of the EE contributed significantly to the northward movement of the ITCZ. The enhancement of the EE preceded the northward movement of the ITCZ by six hours and pushed the ITCZ continually north. As the ITCZ approached 12°N, it not only transported warm moist air but also strengthened the dynamic field by transporting the positive vorticity horizontally and vertically which further contributed to the heavy rainfall.
Changes of Extreme Precipitation and its Associated Mechanisms in Northwest China
Shan LU, Zeyong HU, Haipeng YU, Weiwei FAN, Chunwei FU, Di WU
2021, 38(10): 1665-1681. doi: 10.1007/s00376-021-0409-3
Characterized by scarce water resources and fragile ecosystems, Northwest China (NWC) has experienced a climate shift from warm-dry to warm-wet conditions since the 1980s that has garnered extensive concern in recent years. In this study, the variability in extreme precipitation (EP) during 1961–2016 in different climate zones of NWC and the possible mechanisms for this variation are investigated. The results show that the EP trends significantly increased in most of the westerly zone (WZ) and plateau zone (PZ), while the EP trends did not significantly decrease in the monsoon zone (MZ). The start dates of extreme precipitation (SDEP) and end dates of extreme precipitation (EDEP) advanced and were postponed, respectively, in the WZ and PZ, while the opposite occurred in the MZ. Summer atmospheric circulation, water vapor transport, and atmospheric instability over NWC varied greatly with the interdecadal shift in EP before and after 1986. During 1986–2016, upper-level divergence and lower-level convergence occurred in the MZ and PZ, which strengthened ascending flow. In addition, the summer water vapor and atmospheric instability increased in the WZ and PZ. These characteristics created favorable conditions for increased occurrences of EP in the WZ and PZ in summer. Conversely, the upper-level convergence and lower-level divergence in the MZ strengthened descending flow. Decreases in summer water vapor and atmospheric instability occurred in the MZ after 1986. Hence, the environmental conditions in the MZ may have prevented the occurrence and development of EP in summer during 1986–2016.
All-sky Data Assimilation of MWTS-2 and MWHS-2 in the Met Office Global NWP System.
2021, 38(10): 1682-1694. doi: 10.1007/s00376-021-1071-5
Microwave radiances from passive polar-orbiting radiometers have been, until recently, assimilated in the Met Office global numerical weather prediction system after the scenes significantly affected by atmospheric scattering are discarded. Recent system upgrades have seen the introduction of a scattering-permitting observation operator and the development of a variable observation error using both liquid and ice water paths as proxies of scattering-induced bias. Applied to the Fengyun 3 Microwave Temperature Sounder 2 (MWTS-2) and the Microwave Humidity Sounder 2 (MWHS-2), this methodology increases the data usage by up to 8% at 183 GHz. It also allows for the investigation into the assimilation of MWHS-2 118 GHz channels, sensitive to temperature and lower tropospheric humidity, but whose large sensitivity to ice cloud have prevented their use thus far. While the impact on the forecast is mostly neutral with small but significant short-range improvements, 0.3% in terms of root mean square error, for southern winds and low-level temperature, balanced by 0.2% degradations of short-range northern and tropical low-level temperature, benefits are observed in the background fit of independent instruments used in the system. The lower tropospheric temperature sounding Infrared Atmospheric Sounding Interferometer (IASI) channels see a reduction of the standard deviation in the background departure of up to 1.2%. The Advanced Microwave Sounding Unit A (AMSU-A) stratospheric sounding channels improve by up to 0.5% and the Microwave Humidity Sounder (MHS) humidity sounding channels improve by up to 0.4%.
An Objective Identification Method for Wintertime Cold Fronts in Eurasia
Mengru FENG, Yujing QIN, Chuhan LU
2021, 38(10): 1695-1705. doi: 10.1007/s00376-021-0315-8
The present study identifies wintertime cold fronts in Eurasia from gridded datasets using a new objective two-step identification scheme. The simple and classic conception of a front is adopted, where a cold front is identified as the warm boundary of the frontal zone with a suitable horizontal temperature gradient and cold advection. We combine the traditional thermal front parameter with temperature advection to first identify the cold frontal zone, and then its eastern and southern boundaries are objectively plotted as a cold front in Eurasia. By comparing different cold front identification methods, the results from this two-step cold front identification method and subjective analysis are more consistent, and the positions of the cold front identified with our method are more reasonable. This objective technique is also applied to a nationwide cold wave event over China. Results show that the horizontal extent and movement of the cold front are in good agreement with the related circulation and the associated cold weather. The proposed method and results in this study may shed light on the rapid identification of cold fronts in operational weather analysis and facilitate further research on the long-term activity characteristics of continental cold fronts.
Dominant Modes of Interannual Variability in Atmospheric Water Vapor Content over East Asia during Winter and Their Associated Mechanisms
Wenyue HE, Bo SUN, Huijun WANG
2021, 38(10): 1706-1722. doi: 10.1007/s00376-021-0014-5
Atmospheric water vapor content (WVC) is a critical factor for East Asian winter precipitation. This study investigates the dominant modes of interannual variability in WVC over East Asia during winter and their underlying mechanisms. Based on the empirical orthogonal function (EOF) method, the leading mode (EOF1, R2 = 28.9%) of the interannual variability in the East Asian winter WVC exhibits a meridional dipole pattern characterized by opposite WVC anomalies over northeastern China and eastern China; the second mode (EOF2, R2 = 24.3%) of the interannual variability in the East Asian winter WVC exhibits a monopole pattern characterized by consistent WVC anomalies over eastern China. EOF1 is mainly modulated by two anomalous zonal water vapor transport (WVT) branches over northeastern China and eastern China, which are associated with an anomalous atmospheric wave train over Eurasia affected by sea ice cover in the Kara Sea-Barents Sea (SIC-KSBS) area in the preceding October-November (ON). EOF2 is mainly modulated by an anomalous westerly WVT branch over eastern China, which is associated with a circumglobal atmospheric zonal wave train in the Northern Hemisphere. This circumglobal zonal wave train is modulated by concurrent central and eastern tropical Pacific sea surface temperature anomalies. The SIC-KSBS anomalies in ON and the concurrent SST anomalies over tropical Pacific may partially account for the interannual variability of EOF1 and EOF2 winter WVC, and thus may provide a theoretical basis for improving the prediction of winter climate over East Asia.
Record Arctic Ozone Loss in Spring 2020 is Likely Caused by North Pacific Warm Sea Surface Temperature Anomalies
Yan XIA, Yongyun HU, Jiankai ZHANG, Fei XIE, Wenshou TIAN
2021, 38(10): 1723-1736. doi: 10.1007/s00376-021-0359-9
Record ozone loss was observed in the Arctic stratosphere in spring 2020. This study aims to determine what caused the extreme Arctic ozone loss. Observations and simulation results are examined in order to show that the extreme Arctic ozone loss was likely caused by record-high sea surface temperatures (SSTs) in the North Pacific. It is found that the record Arctic ozone loss was associated with the extremely cold and persistent stratospheric polar vortex over February–April, and the extremely cold vortex was a result of anomalously weak planetary wave activity. Further analysis reveals that the weak wave activity can be traced to anomalously warm SSTs in the North Pacific. Both observations and simulations show that warm SST anomalies in the North Pacific could have caused the weakening of wavenumber-1 wave activity, colder Arctic vortex, and lower Arctic ozone. These results suggest that for the present-day level of ozone-depleting substances, severe Arctic ozone loss could form again, as long as certain dynamic conditions are satisfied.
The Role of Ocean Dynamics in the Cross-equatorial Energy Transport under a Thermal Forcing in the Southern Ocean
Fukai LIU, Yiyong LUO, Jian LU, Xiuquan WAN
2021, 38(10): 1737-1749. doi: 10.1007/s00376-021-1099-6
Under external heating forcing in the Southern Ocean, climate models project anomalous northward atmosphere heat transport (AHT) across the equator, accompanied by a southward shift of the intertropical convergence zone (ITCZ). Comparison between a fully coupled and a slab ocean model shows that the inclusion of active ocean dynamics tends to partition the cross-equatorial energy transport and significantly reduce the ITCZ shift response by a factor of 10, a finding which supports previous studies. To understand how ocean dynamics damps the ITCZ’s response to an imposed thermal heating in the Southern Ocean, we examine the ocean heat transport (OHT) and ocean circulation responses in a set of fully coupled experiments. Results show that both the Indo-Pacific and the Atlantic contribute to transport energy across the equator mainly through its Eulerian-mean component. However, different from previous studies that linked the changes in OHT to the changes in the wind-driven subtropical cells or the Atlantic meridional overturning circulation (AMOC), our results show that the cross-equatorial OHT anomaly is due to a broad clockwise overturning circulation anomaly below the subtropical cells (approximately bounded by the 5°C to 20°C isotherms and 50°S to 10°N). Further elimination of the wind-driven component, conducted by prescribing the climatological wind stress in the Southern Ocean heat perturbation experiments, leads to little change in OHT, suggesting that the OHT response is predominantly thermohaline-driven by air-sea thermal interactions.
A Logistic-growth-equation-based Intensity Prediction Scheme for Western North Pacific Tropical Cyclones
Yanchen ZHOU, Jiuwei ZHAO, Ruifen ZHAN, Peiyan CHEN, Zhiwei WU, Lan WANG
2021, 38(10): 1750-1762. doi: 10.1007/s00376-021-0435-1
Accurate prediction of tropical cyclone (TC) intensity remains a challenge due to the complex physical processes involved in TC intensity changes. A seven-day TC intensity prediction scheme based on the logistic growth equation (LGE) for the western North Pacific (WNP) has been developed using the observed and reanalysis data. In the LGE, TC intensity change is determined by a growth term and a decay term. These two terms are comprised of four free parameters which include a time-dependent growth rate, a maximum potential intensity (MPI), and two constants. Using 33 years of training samples, optimal predictors are selected first, and then the two constants are determined based on the least square method, forcing the regressed growth rate from the optimal predictors to be as close to the observed as possible. The estimation of the growth rate is further refined based on a step-wise regression (SWR) method and a machine learning (ML) method for the period 1982−2014. Using the LGE-based scheme, a total of 80 TCs during 2015−17 are used to make independent forecasts. Results show that the root mean square errors of the LGE-based scheme are much smaller than those of the official intensity forecasts from the China Meteorological Administration (CMA), especially for TCs in the coastal regions of East Asia. Moreover, the scheme based on ML demonstrates better forecast skill than that based on SWR. The new prediction scheme offers strong potential for both improving the forecasts for rapid intensification and weakening of TCs as well as for extending the 5-day forecasts currently issued by the CMA to 7-day forecasts.
The Simulation of Five Tropical Cyclones by Sample Optimization of Ensemble Forecasting Based on the Observed Track and Intensity
Jihang LI, Zhiyan ZHANG, Lu LIU, Xubin ZHANG, Jingxuan QU, Qilin WAN
2021, 38(10): 1763-1777. doi: 10.1007/s00376-021-0353-2
The quality of ensemble forecasting is seriously affected by sample quality. In this study, the distributions of ensemble members based on the observed track and intensity of tropical cyclones (TCs) were optimized and their influence on the simulation results was analyzed. Simulated and observed tracks and intensities of TCs were compared and these two indicators were combined and weighted to score the sample. Samples with higher scores were retained and samples with lower scores were eliminated to improve the overall quality of the ensemble forecast. For each sample, the track score and intensity score were added as the final score of the sample with weight proportions of 10 to 0, 9 to 1, 8 to 2, 7 to 3, 6 to 4, 5 to 5. These were named as “tr”, “91”, “82”, “73”, “64”, and “55”, respectively. The WRF model was used to simulate five tropical cyclones in the northwestern Pacific to test the ability of this scheme to improve the forecast track and intensity of these cyclones. The results show that the sample optimization effectively reduced the track and intensity error, “55” usually had better performance on the short-term intensity prediction, and “tr” had better performance in short-term track prediction. From the overall performance of the track and intensity simulation, “91” was the best and most stable among all sample optimization schemes. These results may provide some guidance for optimizing operational ensemble forecasting of TCs.
Detecting Regional Deep Ocean Warming below 2000 Meter Based on Altimetry, GRACE, Argo, and CTD Data
Yuanyuan YANG, Min ZHONG, Wei FENG, Dapeng MU
2021, 38(10): 1778-1790. doi: 10.1007/s00376-021-1049-3
The deep ocean below 2000 m is a large water body with the sparsest data coverage, challenging the closure of the sea-level budget and the estimation of the Earth’s energy imbalance. Whether the deep ocean below 2000 m is warming globally has been debated in the recent decade. However, as the regional signals are generally larger than the global average, it is intriguing to investigate the regional temperature changes. Here, we adopt an indirect method that combines altimetry, GRACE, and Argo data to examine the global and regional deep ocean temperature changes below 2000 m. The consistency between high-quality conductivity-temperature-depth (CTD) data from repeated hydrographic sections and our results confirms the validity of the indirect method. We find that the deep oceans are warming in the Middle East Indian Ocean, the subtropical North and Southwest Pacific, and the Northeast Atlantic, but cooling in the Northwest Atlantic and Southern oceans from 2005 to 2015.
Data Description Article
A Potential Risk Index Dataset for Landfalling Tropical Cyclones over the Chinese Mainland (PRITC dataset V1.0)
Peiyan CHEN, Hui YU, Kevin K. W. CHEUNG, Jiajie XIN, Yi LU
2021, 38(10): 1791-1802. doi: 10.1007/s00376-021-0365-y
A dataset entitled “A potential risk index dataset for landfalling tropical cyclones over the Chinese mainland” (PRITC dataset V1.0) is described in this paper, as are some basic statistical analyses. Estimating the severity of the impacts of tropical cyclones (TCs) that make landfall on the Chinese mainland based on observations from 1401 meteorological stations was proposed in a previous study, including an index combining TC-induced precipitation and wind (IPWT) and further information, such as the corresponding category level (CAT_IPWT), an index of TC-induced wind (IWT), and an index of TC-induced precipitation (IPT). The current version of the dataset includes TCs that made landfall from 1949–2018; the dataset will be extended each year. Long-term trend analyses demonstrate that the severity of the TC impacts on the Chinese mainland have increased, as embodied by the annual mean IPWT values, and increases in TC-induced precipitation are the main contributor to this increase. TC Winnie (1997) and TC Bilis (2006) were the two TCs with the highest IPWT and IPT values, respectively. The PRITC V1.0 dataset was developed based on the China Meteorological Administration’s tropical cyclone database and can serve as a bridge between TC hazards and their social and economic impacts.