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Lessons Learned from the Tragedy during the 100 km Ultramarathon Race in Baiyin, Gansu Province on 22 May 2021
Qinghong ZHANG, Chan-Pang NG, Kan DAI, Jun XU, Jian TANG, Juanzhen SUN, Mu MU
2021, 38(11): 1803-1810. doi: 10.1007/s00376-021-1246-0
Twenty-one runners died of hypothermia during the 100 km Ultramarathon Mountain race in Baiyin, Gansu Province on 22 May 2021. The hypothermia was caused by a combination of low temperatures, precipitation, and high winds associated with a typical large-scale cold front passing by the race site that morning. Based on historical hourly records of 13 meteorological surface stations over the past six years, temperature (3.0°C) and apparent temperature (−5.1°C) at 1200 LST as well as gust wind speed (11.2 m s−1) at 1100 LST on the day of the tragedy were found to be within the top or bottom 5th percentile for the month of May. The precipitation was only moderate at this time, but when temperature lower than 3.0°C, gust wind speed greater than 11.2 m s−1, and precipitation greater than 0.1 mm for any adjacent three hours were combined together, 1200 LST 22 May fell within the top 0.1% of cases. The European Centre for Medium-range Weather Forecasting model produced reasonably good forecasts of the low temperature and high wind one day and seven days before the event, respectfully. Based on this study, lessons that can be learned from this tragedy are summarized from an academic perspective: Hazard and impact forecasts of high-impact weather events should be developed to increase the value of weather forecasts. Probability forecasts should be issued by government weather agencies and communicated well to the public. And more importantly, knowledge of how to evaluate the impact of weather should be delivered to the public in the future.   We would like to extend our deepest condolences to the families and loved ones of the people who lost their lives in this tragedy, including 21 runners and one officer. May our efforts honor those who lost their lives by highlighting the value of weather forecasting and calling for greater action in the future.
Original Paper
Homogenization of the Daily Land Surface Temperature over the Mainland of China from 1960 through 2017
Dan WANG, Aihui WANG, Xianghui KONG
2021, 38(11): 1811-1822. doi: 10.1007/s00376-021-1038-6
Land surface temperature (LST) is one of the most important factors in the land-atmosphere interaction process. Raw measured LSTs may contain biases due to instrument replacement, changes in recording procedures, and other non-climatic factors. This study attempts to reduce the above biases in raw daily measurements and achieves a homogenized daily LST dataset over China using 2360 stations from 1960 through 2017. The high-quality land surface air temperature (LSAT) dataset is used to correct the LST warming biases especially evident during cold months in regions north of 40ºN due to the replacement of observation instruments around 2004. Subsequently, the Multiple Analysis of Series for Homogenization (MASH) method is adopted to detect and then adjust the daily observed LST records. In total, 3.68 × 103 effective breakpoints in 1.65 × 106 monthly records (about 20%) are detected. A large number of these effective breakpoints are located over large parts of the Sichuan Basin and southern China. After the MASH procedure, LSTs at more than 80% of the breakpoints are adjusted within +/– 0.5ºC, and of the remaining breakpoints, only 10% are adjusted over 1.5ºC. Compared to the raw LST dataset over the whole domain, the homogenization significantly reduces the mean LST magnitude and its interannual variability as well as its linear trend at most stations. Finally, we perform preliminary analysis upon the homogenized LST and find that the annual mean LST averaged across China shows a significant warming trend [0.22ºC (10 yr)–1]. The homogenized LST dataset can be further adapted for a variety of applications (e.g., model evaluation and extreme event characterization).
Decadal Change in the Influence of the Western North Pacific Subtropical High on Summer Rainfall over the Yangtze River Basin in the Late 1970s
Xinyu LI, Riyu LU
2021, 38(11): 1823-1834. doi: 10.1007/s00376-021-1051-9
It is well known that on the interannual timescale, the westward extension of the western North Pacific subtropical high (WNPSH) results in enhanced rainfall over the Yangtze River basin (YRB) in summer, and vice versa. This study identifies that this correspondence experiences a decadal change in the late 1970s. That is, the WNPSH significantly affects YRB precipitation (YRBP) after the late 1970s (P2) but not before the late 1970s (P1). It is found that enhanced interannual variability of the WNPSH favors its effect on YRB rainfall in P2. On the other hand, after removing the strong WNPSH cases in P2 and making the WNPSH variability equivalent to that in P1, the WNPSH can still significantly affect YRB rainfall, suggesting that the WNPSH variability is not the only factor that affects the WNPSH–YRBP relationship. Further results indicate that the change in basic state of thermal conditions in the tropical WNP provides a favorable background for the enhanced WNPSH–YRBP relationship. In P2, the lower-tropospheric atmosphere in the tropical WNP gets warmer and wetter, and thus the meridional gradient of climatological equivalent potential temperature over the YRB is enhanced. As a result, the WNPSH-related circulation anomalies can more effectively induce YRB rainfall anomalies through affecting the meridional gradient of equivalent potential temperature over the YRB.
PV Perspective of Impacts on Downstream Extreme Rainfall Event of a Tibetan Plateau Vortex Collaborating with a Southwest China Vortex
Guanshun ZHANG, Jiangyu MAO, Yimin LIU, Guoxiong WU
2021, 38(11): 1835-1851. doi: 10.1007/s00376-021-1027-9
An extreme rainfall event occurred over the middle and lower reaches of the Yangtze Basin (MLY) during the end of June 2016, which was attributable to a Tibetan Plateau (TP) Vortex (TPV) in conjunction with a Southwest China Vortex (SWCV). The physical mechanism for this event was investigated from Potential Vorticity (PV) and omega perspectives based on MERRA-2 reanalysis data. The cyclogenesis of the TPV over the northwestern TP along with the lower-tropospheric SWCV was found to involve a midtropospheric large-scale flow reconfiguration across western and eastern China with the formation of a high-amplitude Rossby wave. Subsequently, the eastward-moving TPV coalesced vertically with the SWCV over the eastern Sichuan Basin due to the positive vertical gradient of the TPV-related PV advection, leading the lower-tropospheric jet associated with moisture transport to intensify greatly and converge over the downstream MLY. The merged TPV−SWCV specially facilitated the upper-tropospheric isentropic-gliding ascending motion over the MLY. With the TPV-embedded mid-tropospheric trough migrating continuously eastward, the almost stagnant SWCV was re-separated from the overlying TPV, forming a more eastward-tilted high-PV configuration to trigger stronger ascending motion including isentropic-gliding, isentropic-displacement, and diabatic heating-related ascending components over the MLY. This led to more intense rainfall. Quantitative PV diagnoses demonstrate that both the coalescence and subsequent re-separation processes of the TPV with the SWCV were largely dominated by horizontal PV advection and PV generation due to vertically nonuniform diabatic heating, as well as the feedback of condensation latent heating on the isentropic-displacement vertical velocity.
Understanding Interannual Variations of the Local Rainy Season over the Southwest Indian Ocean
Hanying LI, Peng HU, Qiong ZHANG, Ashish SINHA, Hai CHENG
2021, 38(11): 1852-1862. doi: 10.1007/s00376-021-1065-3
Located at the southern boundary of the tropical rainfall belt within the South Africa monsoon regime, Rodrigues Island, ~2500 km east of East Africa, is ideally located to investigate climatic changes over the southwest Indian Ocean (SWIO). In this study, we investigate the climatic controls of its modern interannual rainfall variability in terms of teleconnection and local effects. We find that increased rainfall over the SWIO tends to occur in association with anomalously warm (cold) SSTs over the equatorial central Pacific (Maritime Continent), resembling the central Pacific El Niño, closely linked with the Victoria mode in the North Pacific. Our analyses show that the low-level convergence induced by warm SST over the equatorial central Pacific leads to anomalous low-level divergence over the Maritime Continent and convergence over a large area surrounding the Rodrigues Island, which leads to increased rainfall over the SWIO during the rainy season. Meanwhile, the excited Rossby wave along the tropical Indian Ocean transports more water vapor from the tropical convergence zone into the SWIO via intensified northwest wind. Furthermore, positive feedback induced by the Rossby wave response to the increased rainfall in the region contributes to the large interannual variations over the SWIO.
Impacts of Irrigation and Vegetation Growth on Summer Rainfall in the Taklimakan Desert
Dongze XU, Yanluan LIN
2021, 38(11): 1863-1872. doi: 10.1007/s00376-021-1042-x
In recent decades, a greening tendency due to increased vegetation has been noted around the Taklimakan Desert (TD), but the impact of such a change on the local hydrological cycle remains uncertain. Here, we investigate the response of the local hydrological cycle and atmospheric circulation to a green TD in summer using a pair of global climate model (Community Earth System Model version 1.2.1) simulations. With enough irrigation to support vegetation growth in the TD, the modeling suggests first, that significant increases in local precipitation are attributed to enhanced local recycling of water, and second, that there is a corresponding decrease of local surface temperatures. On the other hand, irrigation and vegetation growth in this low-lying desert have negligible impacts on the large-scale circulation and thus the moisture convergence for enhanced precipitation. It is also found that the green TD can only be sustained by a large amount of irrigation water supply since only about one-third of the deployed water can be “recycled” locally. Considering this, devising a way to encapsulate the irrigated water within the desert to ensure more efficient water recycling is key for maintaining a sustainable, greening TD.
Implications from Subseasonal Prediction Skills of the Prolonged Heavy Snow Event over Southern China in Early 2008
Keyue ZHANG, Juan LI, Zhiwei ZHU, Tim LI
2021, 38(11): 1873-1888. doi: 10.1007/s00376-021-0402-x
An exceptionally prolonged heavy snow event (PHSE) occurred in southern China from 10 January to 3 February 2008, which caused considerable economic losses and many casualties. To what extent any dynamical model can predict such an extreme event is crucial for disaster prevention and mitigation. Here, we found the three S2S models (ECMWF, CMA1.0 and CMA2.0) can predict the distribution and intensity of precipitation and surface air temperature (SAT) associated with the PHSE at 10-day lead and 10−15-day lead, respectively. The success is attributed to the models’ capability in forecasting the evolution of two important low-frequency systems in the tropics and mid-latitudes [the persistent Siberian High and the suppressed phase of the Madden−Julian Oscillation (MJO)], especially in the ECMWF model. However, beyond the 15-day lead, the three models show almost no skill in forecasting this PHSE.  The bias in capturing the two critical circulation systems is responsible for the low skill in forecasting the 2008 PHSE beyond the 15-day lead. On one hand, the models cannot reproduce the persistence of the Siberian High, which results in the underestimation of negative SAT anomalies over southern China. On the other hand, the models cannot accurately capture the suppressed convection of the MJO, leading to weak anomalous southerly and moisture transport, and therefore the underestimation of precipitation over southern China.  The Singular Value Decomposition (SVD) analyses between the critical circulation systems and SAT/precipitation over southern China shows a robust historical relation, indicating the fidelity of the predictability sources for both regular events and extreme events (e.g., the 2008 PHSE).
Effect of the Vertical Diffusion of Moisture in the Planetary Boundary Layer on an Idealized Tropical Cyclone
Hongxiong XU, Dajun ZHAO
2021, 38(11): 1889-1904. doi: 10.1007/s00376-021-1016-z
Previous numerical studies have focused on the combined effect of momentum and scalar eddy diffusivity on the intensity and structure of tropical cyclones. The separate impact of eddy diffusivity estimated by planetary boundary layer (PBL) parameterization on the tropical cyclones has not yet been systematically examined. We have examined the impacts of eddy diffusion of moisture on idealized tropical cyclones using the Advanced Research Weather Research and Forecasting model with the Yonsei University PBL scheme. Our results show nonlinear effects of moisture eddy diffusivity on the simulation of idealized tropical cyclones. Increasing the eddy diffusion of moisture increases the moisture content of the PBL, with three different effects on tropical cyclones: (1) an decrease in the depth of the PBL; (2) an increase in convection in the inner rain band and eyewall; and (3) drying of the lowest region of the PBL and then increasing the surface latent heat flux. These three processes have different effects on the intensity and structure of the tropical cyclone through various physical mechanisms. The increased surface latent heat flux is mainly responsible for the decrease in pressure. Results show that moisture eddy diffusivity has clear effects on the pressure in tropical cyclones, but contributes little to the intensity of wind. This largely influences the wind–pressure relationship, which is crucial in tropical cyclones simulation. These results improve our understanding of moisture eddy diffusivity in the PBL and its influence on tropical cyclones, and provides guidance for interpreting the variation of moisture in the PBL for tropical cyclone simulations.
An Observational Study on the Local Climate Effect of the Shangyi Wind Farm in Hebei Province
Yonghong LIU, Bing DANG, Yongming XU, Fuzhong WENG
2021, 38(11): 1905-1919. doi: 10.1007/s00376-021-0290-0
Zhangjiakou is an important wind power base in Hebei Province, China. The impact of its wind farms on the local climate is controversial. Based on long-term meteorological data from 1981 to 2018, we investigated the effects of the Shangyi Wind Farm (SWF) in Zhangjiakou on air temperature, wind speed, relative humidity, and precipitation using the anomaly or ratio method between the impacted weather station and the non-impacted background weather station. The influence of the SWF on land surface temperature (LST) and evapotranspiration (ET) using MODIS satellite data from 2003 to 2018 was also explored. The results showed that the SWF had an atmospheric warming effect at night especially in summer and autumn (up to 0.95°C). The daytime air temperature changes were marginal, and their signs were varying depending on the season. The annual mean wind speed decreased by 6%, mainly noted in spring and winter (up to 14%). The precipitation and relative humidity were not affected by the SWF. There was no increase in LST in the SWF perhaps due to the increased vegetation coverage unrelated to the wind farms, which canceled out the wind farm-induced land surface warming and also resulted in an increase in ET. The results showed that the impact of wind farms on the local climate was significant, while their impact on the regional climate was slight.
Robust Solution for Boundary Layer Height Detections with Coherent Doppler Wind Lidar
Lu WANG, Wei QIANG, Haiyun XIA, Tianwen WEI, Jinlong YUAN, Pu JIANG
2021, 38(11): 1920-1928. doi: 10.1007/s00376-021-1068-0
Although coherent Doppler wind lidar (CDWL) is promising in detecting boundary layer height (BLH), differences between BLH results are observed when different CDWL measurements are used as tracers. Here, a robust solution for BLH detections with CDWL is proposed and demonstrated: mixed layer height (MLH) is retrieved best from turbulent kinetic energy dissipation rate (TKEDR), while stable boundary layer height (SBLH) and residual layer height (RLH) can be retrieved from carrier-to-noise ratio (CNR). To study the cause of the BLH differences, an intercomparison experiment is designed with two identical CDWLs, where only one is equipped with a stability control subsystem. During the experiment, it is found that the CNR could be distorted by instrument instability because the coupling efficiency from free-space to the polarization-maintaining fiber of the telescope is sensitive to the surrounding environment. In the ML, a bias up to 2.13 km of the MLH from CNR is found, which is caused by the CNR deviation. In contrast, the MLH from TKEDR is robust as long as the accuracy of wind is guaranteed. In the SBL (RL), the CNR is found capable to retrieve SBLH and RLH simultaneously and robustly. This solution is tested during an observation period over one month. Statistical analysis shows that the root-mean-square errors (RMSE) in the MLH, SBLH, and RLH are 0.28 km, 0.23 km, and 0.24 km, respectively.
A Case Study on MJO Energy Transport Path in a Local Multi-scale Interaction Framework
Yuanwen ZHANG, Guiwan CHEN, Jian LING, Shenming FU, Chongyin LI
2021, 38(11): 1929-1944. doi: 10.1007/s00376-021-1098-7
A new local kinetic energy (KE) budget for the Madden−Julian Oscillation (MJO) is constructed in a multi-scale framework. This energy budget framework allows us to analyze the local energy conversion processes of the MJO with the high-frequency disturbances and the low-frequency background state. The KE budget analysis is applied to a pronounced MJO event during the DYNAMO field campaign to investigate the KE transport path of the MJO. The work done by the pressure gradient force and the conversion of available potential energy at the MJO scale are the two dominant processes that affect the MJO KE tendency. The MJO winds transport MJO KE into the MJO convection region in the lower troposphere while it is transported away from the MJO convection region in the upper troposphere. The energy cascade process is relatively weak, but the interaction between high-frequency disturbances and the MJO plays an important role in maintaining the high-frequency disturbances within the MJO convection. The MJO KE mainly converts to interaction KE between MJO and high-frequency disturbances over the area where the MJO zonal wind is strong. This interaction KE over the MJO convection region is enhanced through its flux convergence and further transport KE to the high-frequency disturbances. This process is conducive to maintaining the MJO convection. This study highlights the importance of KE interaction between the MJO and the high-frequency disturbances in maintaining the MJO convection.
Optimal Gridding Process for GMI Brightness Temperature Using the Backus−Gilbert Method
Guangcan CHEN, Yunfei FU
2021, 38(11): 1945-1957. doi: 10.1007/s00376-021-0358-x
Satellite microwave instruments have different field of views (FOVs) in different channels. A direct average technique (“direct method”) is frequently used to generate gridded datasets in the earth science community. A large FOV will measure radiance from outside the area of a designated grid cell. Thus, the direct method will lead to errors in a measurement over a grid cell because some pixels covering areas outside of the cell are involved in the averaging process. The Backus−Gilbert method (BG method) is proposed and demonstrated to minimize those uncertainties. Three sampling resolutions (6.5 km × 6.0 km, 11.5 km × 6.0 km, 13.0 km × 6.0 km) are analyzed based on the scanning characteristics of the Global Precipitation Measurement (GPM) Microwave Imager (GMI) 18.9-GHz channel. Brightness temperatures (TBs) at 0.5 km × 0.5 km resolution over eastern China are used to obtain synthetic 18.9-GHz TBs at the three sampling resolutions. The direct and BG methods are both applied to create a 25 km × 25 km gridded dataset and their related uncertainties are analyzed. Results indicate the error variances with the direct method are 3.00, 3.68 and 4.99 K2 at the three sampling resolutions, respectively. By contrast, the BG method leads to a much smaller error variance than the direct method, especially over areas with a large TB gradient. Two GMI orbital measurements are applied to verify the BG method for gridding process is reliable. The BG method could be utilized for general purpose of creating a gridded dataset.
Evaluation of Surface Relative Humidity in China from the CRA-40 and Current Reanalyses
Jingpeng ZHANG, Tianbao ZHAO, Zhi LI, Chunxiang LI, Zhen LI, Kairan YING, Chunxiang SHI, Lipeng JIANG, Wenyu ZHANG
2021, 38(11): 1958-1976. doi: 10.1007/s00376-021-0333-6
Recently, the China Meteorological Administration (CMA) released a new Global Atmospheric Reanalysis (CRA-40) dataset for the period 1979−2018. In this study, surface relative humidity (RH) from CRA-40 and other current reanalyses (e.g., CFSR, ERA5, ERA-Interim, JRA-55, and MERRA-2) is comprehensively evaluated against homogenized observations over China. The results suggest that most reanalyses overestimate the observations by 15%−30% (absolute difference) over the Tibetan Plateau but underestimate the observations by 5%−10% over most of northern China. The CRA-40 performs relatively well in describing the long-term change and variance seen in the observed surface RH over China. Most of the reanalyses reproduce the observed surface RH climatology and interannual variations well, while few reanalyses can capture the observed long-term RH trends over China. Among these reanalyses, the CFSR does poorly in describing the interannual changes in the observed RH, especially in Southwest China. An empirical orthogonal function (EOF) analysis also suggests that the CRA-40 performs better than other reanalyses to capture the first two leading EOF modes revealed by the observations. The results of this study are expected to improve understanding of the strengths and weaknesses of the current reanalysis products and thus facilitate their application.