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2021 Vol. 38, No. 4

News & Views
Upper Ocean Temperatures Hit Record High in 2020
Lijing CHENG, John ABRAHAM, Kevin E. TRENBERTH, John FASULLO, Tim BOYER, Ricardo LOCARNINI, Bin ZHANG, Fujiang YU, Liying WAN, Xingrong CHEN, Xiangzhou SONG, Yulong LIU, Michael E. MANN, Franco RESEGHETTI, Simona SIMONCELLI, Viktor GOURETSKI, Gengxin CHEN, Alexey MISHONOV, Jim REAGAN, Jiang ZHU
2021, 38(4): 523-530. doi: 10.1007/s00376-021-0447-x
Comprehensive Marine Observing Experiment Based on High-Altitude Large Unmanned Aerial Vehicle (South China Sea Experiment 2020 of the “Petrel Project”)
Xuefen ZHANG, Liangxu LI, Rongkang YANG, Ran GUO, Xia SUN, Jianping LUO, Hongbin CHEN, Daxin LIU, Kebing TANG, Wenwu PENG, Xiaodong HAN, Qiyun GUO, Xiaoxia LI, Xikun FEI
2021, 38(4): 531-537. doi: 10.1007/s00376-020-0314-1
In collaboration with 12 other institutions, the Meteorological Observation Center of the China Meteorological Administration undertook a comprehensive marine observation experiment in the South China Sea using the Yilong-10 high-altitude large unmanned aerial vehicle (UAV). The Yilong-10 UAV carried a self-developed dropsonde system and a millimeter-wave cloud radar system. In addition, a solar-powered unmanned surface vessel and two drifting buoys were used. The experiment was further supported by an intelligent, reciprocating horizontal drifting radiosonde system that was deployed from the Sansha Meteorological Observing Station, with the intent of producing a stereoscopic observation over the South China Sea. Comprehensive three-dimensional observations were collected using the system from 31 July to 2 August, 2020. This information was used to investigate the formation and development processes of Typhoon Sinlaku (2020). The data contain measurements of 21 oceanic and meteorological parameters acquired by the five devices, along with video footage from the UAV. The data proved very helpful in determining the actual location and intensity of Typhoon Sinlaku (2020). The experiment demonstrates the feasibility of using a high-altitude, large UAV to fill in the gaps between operational meteorological observations of marine areas and typhoons near China, and marks a milestone for the use of such data for analyzing the structure and impact of a typhoon in the South China Sea. It also demonstrates the potential for establishing operational UAV meteorological observing systems in the future, and the assimilation of such data into numerical weather prediction models.
IAP’s Solar-Powered Unmanned Surface Vehicle Actively Passes through the Center of typhoon Sinlaku (2020)
Hongbin CHEN, Jun LI, Wenying HE, Shuqing MA, Yingzhi WEI, Jidong PAN, Yu ZHAO, Xuefen ZHANG, Shuzhen HU
2021, 38(4): 538-545. doi: 10.1007/s00376-021-1006-1
The solar-powered marine unmanned surface vehicle (USV) developed by the USV team of the Institute of Atmospheric Physics is a rugged, long-duration, and autonomous navigation vessel designed for the collection of long-range, continuous, real-time, meteorological and oceanographic measurements, especially under extreme sea conditions (sea state 6–7). These solar-powered USVs completed a long-term continuous navigation observation test over 26 days. During this time, they coordinated double-USV observations and actively navigated into the path of Typhoon Sinlaku (2020) before collecting data very close to its center during the 2020 USV South China Sea Typhoon Observation Experiment. Detailed high temporal resolution (1 min) real-time observations collected by the USV on the typhoon were used for operational typhoon forecasting and warning for the first time. As a mobile meteorological and oceanographic observation station capable of reliable, automated deployment, data collection, and transmission, such solar-powered USVs can replace traditional observation platforms to provide valuable real-time data for research, forecasting, and early warnings for potential marine meteorological disasters.
Original Paper
Diversity of the Coupling Wheels in the East Asian Summer Monsoon on the Interannual Time Scale: Challenge of Summer Rainfall Forecasting in China
Congwen ZHU, Boqi LIU, Kang XU, Ning JIANG, Kai LIU
2021, 38(4): 546-554. doi: 10.1007/s00376-020-0199-z
Two types of three-dimensional circulation of the East Asian summer monsoon (EASM) act as the coupling wheels determining the seasonal rainfall anomalies in China during 1979–2015. The first coupling mode features the interaction between the Mongolian cyclone over North Asia and the South Asian high (SAH) anomalies over the Tibetan Plateau at 200 hPa. The second mode presents the coupling between the anomalous low-level western Pacific anticyclone and upper-level SAH via the meridional flow over Southeast Asia. These two modes are responsible for the summer rainfall anomalies over China in 24 and 7 out of 37 years, respectively. However, the dominant SST anomalies in the tropical Pacific, the Indian Ocean, and the North Atlantic Ocean fail to account for the first coupling wheel’s interannual variability, illustrating the challenges in forecasting summer rainfall over China.
Distinctive MJO Activity during the Boreal Winter of the 2015/16 Super El Niño in Comparison with Other Super El Niño Events
Xuben LEI, Wenjun ZHANG, Pang-Chi HSU, Chao LIU
2021, 38(4): 555-568. doi: 10.1007/s00376-020-0261-x
Many previous studies have demonstrated that the boreal winters of super El Niño events are usually accompanied by severely suppressed Madden-Julian oscillation (MJO) activity over the western Pacific due to strong descending motion associated with a weakened Walker Circulation. However, the boreal winter of the 2015/16 super El Niño event is concurrent with enhanced MJO activity over the western Pacific despite its sea surface temperature anomaly (SSTA) magnitude over the Niño 3.4 region being comparable to the SSTA magnitudes of the two former super El Niño events (i.e., 1982/83 and 1997/98). This study suggests that the MJO enhanced over western Pacific during the 2015/16 super El Niño event is mainly related to its distinctive SSTA structure and associated background thermodynamic conditions. In comparison with the previous super El Niño events, the warming SSTA center of the 2015/16 super El Niño is located further westward, and a strong cold SSTA is not detected in the western Pacific. Accordingly, the low-level moisture and air temperature (as well as the moist static energy, MSE) tend to increase in the central-western Pacific. In contrast, the low-level moisture and MSE show negative anomalies over the western Pacific during the previous super El Niño events. As the MJO-related horizontal wind anomalies contribute to the further westward warm SST-induced positive moisture and MSE anomalies over the western tropical Pacific in the boreal winter of 2015/16, stronger moisture convergence and MSE advection are generated over the western Pacific and lead to the enhancement of MJO convection.
Monthly Variations of Atmospheric Circulations Associated with Haze Pollution in the Yangtze River Delta and North China
Xinyu ZHANG, Zhicong YIN, Huijun WANG, Mingkeng DUAN
2021, 38(4): 569-580. doi: 10.1007/s00376-020-0227-z
Haze pollution in early winter (December and January) in the Yangtze River Delta (YRD) and in North China (NC) are both severe; however, their monthly variations are significantly different. In this study, the dominant large-scale atmospheric circulations and local meteorological conditions were investigated and compared over the YRD and NC in each month. Results showed that the YRD (NC) is dominated by the so-called Scandinavia (East Atlantic/West Russia) pattern in December, and these circulations weaken in January. The East Asian December and January monsoons over the YRD and NC have negative correlations with the number of haze days. The local descending motion facilitates less removal of haze pollution over the YRD, while the local ascending motion facilitates less removal of haze pollution over NC in January, despite a weaker relationship in December. Additionally, the monthly variations of atmospheric circulations showed that adverse meteorological conditions restrict the vertical (horizontal) dispersion of haze pollution in December (January) over the YRD, while the associated local weather conditions are similar in these two months over NC.
Which Features of the SST Forcing Error Most Likely Disturb the Simulated Intensity of Tropical Cyclones?
Jiawei YAO, Wansuo DUAN, Xiaohao QIN
2021, 38(4): 581-602. doi: 10.1007/s00376-020-0073-z
Among all of the sources of tropical cyclone (TC) intensity forecast errors, the uncertainty of sea surface temperature (SST) has been shown to play a significant role. In the present study, we determine the SST forcing error that causes the largest simulation error of TC intensity during the entire simulation period by using the WRF model with time-dependent SST forcing. The SST forcing error is represented through the application of a nonlinear forcing singular vector (NFSV) structure. For the selected 12 TC cases, the NFSV-type SST forcing errors have a nearly coherent structure with positive (or negative) SST anomalies located along the track of TCs but are especially concentrated in a particular region. This particular region tends to occur during the specific period of the TCs life cycle when the TCs present relatively strong intensity, but are still intensifying just prior to the mature phase, especially within a TC state exhibiting a strong secondary circulation and very high inertial stability. The SST forcing errors located along the TC track during this time period are verified to have the strongest disturbing effect on TC intensity simulation. Physically, the strong inertial stability of TCs during this time period induces a strong response of the secondary circulation from diabatic heating errors induced by the SST forcing error. Consequently, this significantly influences the subsidence within the warm core in the eye region, which, in turn, leads to significant errors in TC intensity. This physical mechanism explains the formation of NSFV-type SST forcing errors. According to the sensitivity of the NFSV-type SST forcing errors, if one increases the density of SST observations along the TC track and assimilates them to the SST forcing field, the skill of TC intensity simulation generated by the WRF model could be greatly improved. However, this adjustment is most advantageous in improving simulation skill during the time period when TCs become strong but are still intensifying just prior to reaching full maturity. In light of this, the region along the TC track but in the time period of TC movement when the NFSV-type SST forcing errors occur may represent the sensitive area for targeting observation for SST forcing field associated with TC intensity simulation.
Characteristics of Lake Breezes and Their Impacts on Energy and Carbon Fluxes in Mountainous Areas
Lujun XU, Huizhi LIU, Qun DU, Yang LIU, Jihua SUN, Anlun XU, Xiaoni MENG
2021, 38(4): 603-614. doi: 10.1007/s00376-020-0298-x
In mountainous lake areas, lake–land and mountain–valley breezes interact with each other, leading to an “extended lake breeze”. These extended lake breezes can regulate and control energy and carbon cycles at different scales. Based on meteorological and turbulent fluxes data from an eddy covariance observation site at Erhai Lake in the Dali Basin, southwest China, characteristics of daytime and nighttime extended lake breezes and their impacts on energy and carbon dioxide exchange in 2015 are investigated. Lake breezes dominate during the daytime while, due to different prevailing circulations at night, there are two types of nighttime breezes. The mountain breeze from the Cangshan Mountain range leads to N1 type nighttime breeze events. When a cyclonic circulation forms and maintains in the southern part of Erhai Lake at night, its northern branch contributes to the formation of N2 type nighttime breeze events. The prevailing wind directions for daytime, N1, and N2 breeze events are southeast, west, and southeast, respectively. Daytime breeze events are more intense than N1 events and weaker than N2 events. During daytime breeze events, the lake breeze decreases the sensible heat flux (Hs) and carbon dioxide flux (\begin{document}${{F}}_{{\rm{CO}}_2} $\end{document}) and increases the latent heat flux (LE). During N1 breeze events, the mountain breeze decreases Hs and LE and increases \begin{document}${{F}}_{{\rm{CO}}_2} $\end{document}. For N2 breeze events, the southeast wind from the lake surface increases Hs and LE and decreases \begin{document}${{F}}_{{\rm{CO}}_2} $\end{document}. Results indicate that lakes in mountainous areas promote latent heat mixing but suppress carbon dioxide exchange.
Properties of High-Order Finite Difference Schemes and Idealized Numerical Testing
Daosheng XU, Dehui CHEN, Kaixin WU
2021, 38(4): 615-626. doi: 10.1007/s00376-020-0130-7
Construction of high-order difference schemes based on Taylor series expansion has long been a hot topic in computational mathematics, while its application in comprehensive weather models is still very rare. Here, the properties of high-order finite difference schemes are studied based on idealized numerical testing, for the purpose of their application in the Global/Regional Assimilation and Prediction System (GRAPES) model. It is found that the pros and cons due to grid staggering choices diminish with higher-order schemes based on linearized analysis of the one-dimensional gravity wave equation. The improvement of higher-order difference schemes is still obvious for the mesh with smooth varied grid distance. The results of discontinuous square wave testing also exhibits the superiority of high-order schemes. For a model grid with severe non-uniformity and non-orthogonality, the advantage of high-order difference schemes is inapparent, as shown by the results of two-dimensional idealized advection tests under a terrain-following coordinate. In addition, the increase in computational expense caused by high-order schemes can be avoided by the precondition technique used in the GRAPES model. In general, a high-order finite difference scheme is a preferable choice for the tropical regional GRAPES model with a quasi-uniform and quasi-orthogonal grid mesh.
Satellite Observations of Reflectivity Maxima above the Freezing Level Induced by Terrain
Aoqi ZHANG, Weibiao LI, Shumin CHEN, Yilun CHEN, Yunfei FU
2021, 38(4): 627-640. doi: 10.1007/s00376-020-0221-5
Previous studies have recognized reflectivity maxima above the freezing level (RMAF) within stratiform precipitation over mountain slopes, however, quantitative studies are limited due to the lack of adequate identification criteria. Here, we establish an identification method for RMAF precipitation and apply it to the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar (PR) observations. Using the TRMM 2A25 product from 1998 to 2013, we show that the RMAF structure in reflectivity profiles can be effectively identified. RMAF exists not only in stratiform precipitation but also in convective precipitation. RMAF frequency is positively correlated with elevation, which is thought to be caused by enhanced updrafts in the middle layers of stratiform precipitation, or in the low to middle layers of convective precipitation over mountains. The average RMAF heights in stratiform and convective precipitation were 1.35 and 2.01 km above the freezing level, respectively, which is lower than previous results. In addition, our results indicate that the RMAF structure increased the echo top height and enhanced precipitation processes above the RMAF height, but it suppressed the downward propagation of ice particles and the near-surface rain rate. Future studies of orographic precipitation should take into account the impact of the RMAF structure and its relevant dynamic triggers.
Diurnal Variations of Precipitation over the Steep Slopes of the Himalayas Observed by TRMM PR and VIRS
Xiao PAN, Yunfei FU, Sen YANG, Ying GONG, Deqin LI
2021, 38(4): 641-660. doi: 10.1007/s00376-020-0246-9
This study investigates diurnal variations of precipitation during May–August, 1998–2012, over the steep slopes of the Himalayas and adjacent regions (flat Gangetic Plains–FGP, foothills of the Himalayas–FHH, the steep slope of the southern Himalayas–SSSH, and the Himalayas-Tibetan Plateau tableland–HTPT). Diurnal variations are analyzed at the pixel level utilizing collocated TRMM precipitation radar and visible infrared data. The results indicate that rain parameters (including rain frequency, rain rate, and storm top altitude) are predominantly characterized by afternoon maxima and morning minima at HTPT and FGP, whereas, maximum rain parameters at FHH typically occur in the early morning. Rain parameters at SSSH are characterized by double peaks; one in the afternoon and one at midnight. Over HTPT and FGP, convective activity is strongest in the afternoon with the thickest crystallization layer. Over FHH, the vertical structure of precipitation develops most vigorously in the early morning when the most intense collision and growth of precipitation particles occurs. Over SSSH, moist convection is stronger in the afternoon and at midnight with strong mixing of ice and water particles. The results of harmonic analysis show that rain bands move southward from lower elevation of SSSH to FHH with apparent southward propagation of the harmonic phase from midnight to early morning. Moreover, the strongest diurnal harmonic is located at HTPT, having a diurnal harmonic percentage variance of up to 90%. Large-scale atmospheric circulation patterns exhibit obvious diurnal variability and correspond well to the distribution of precipitation.
Assimilating All-sky Infrared Radiances from Himawari-8 Using the 3DVar Method for the Prediction of a Severe Storm over North China
Dongmei XU, Zhiquan LIU, Shuiyong FAN, Min CHEN, Feifei SHEN
2021, 38(4): 661-676. doi: 10.1007/s00376-020-0219-z
Although radar observations capture storm structures with high spatiotemporal resolutions, they are limited within the storm region after the precipitation formed. Geostationary satellites data cover the gaps in the radar network prior to the formation of the precipitation for the storms and their environment. The study explores the effects of assimilating the water vapor channel radiances from Himawari-8 data with Weather Research and Forecasting model data assimilation system (WRFDA) for a severe storm case over north China. A fast cloud detection scheme for Advanced Himawari imager (AHI) radiance is enhanced in the framework of the WRFDA system initially in this study. The bias corrections, the cloud detection for the clear-sky AHI radiance, and the observation error modeling for cloudy radiance are conducted before the data assimilation. All AHI radiance observations are fully applied without any quality control for all-sky AHI radiance data assimilation. Results show that the simulated all-sky AHI radiance fits the observations better by using the cloud dependent observation error model, further improving the cloud heights. The all-sky AHI radiance assimilation adjusts all types of hydrometeor variables, especially cloud water and precipitation snow. It is proven that assimilating all-sky AHI data improves hydrometeor specifications when verified against the radar reflectivity. Consequently, the assimilation of AHI observations under the all-sky condition has an overall improved impact on both the precipitation locations and intensity compared to the experiment with only conventional and AHI clear-sky radiance data.
Morphological Characteristics of Precipitation Areas over the Tibetan Plateau Measured by TRMM PR
Yilun CHEN, Aoqi ZHANG, Yunfei FU, Shumin CHEN, Weibiao LI
2021, 38(4): 677-689. doi: 10.1007/s00376-020-0233-1
The multidimensional morphological characteristics (including scale, horizontal shape and 3D morphology) of precipitation areas over the Tibetan Plateau in summer were studied using 15 years (1998–2012) of observational data from the precipitation radar onboard the Tropical Rainfall Measuring Mission satellite. As the scale of the precipitation area increased from 20 to 150 km, the near-surface rain rate (RRav) of the precipitation area increased by up to 78% (from ~1.12 to ~2 mm h−1). Linear precipitation areas had the lowest median RRav (~1 mm h−1 over the eastern Tibetan Plateau), whereas square-shaped precipitation areas had the highest median RRav (~1.58 mm h−1 over the eastern Tibetan Plateau). The 3D morphology was defined as the ratio of the average vertical scale to the average horizontal scale, where a large value corresponds to thin and tall, and a small value corresponds to plump and short. Thin-and-tall precipitation areas and plump-and-short precipitation areas had a greater median RRav, whereas the precipitation areas with a moderate 3D morphology had the lowest median RRav. The vertical structure of the precipitation-area reflectivity was sensitive to both size and 3D morphology, but was not sensitive to the horizontal shape. The relationship between RRav and the morphological characteristics was most significant over the southern slopes of the Tanggula Mountains and the Tibetan Plateau east of 100°E. The morphological characteristics of precipitation areas are therefore closely related to the intensity of precipitation and could potentially be used to forecast precipitation and verify numerical models.
Data Description Article
Western North Pacific Tropical Cyclone Database Created by the China Meteorological Administration
Xiaoqin LU, Hui YU, Ming YING, Bingke ZHAO, Shuai ZHANG, Limin LIN, Lina BAI, Rijin WAN
2021, 38(4): 690-699. doi: 10.1007/s00376-020-0211-7
This paper describes the access to, and the content, characteristics, and potential applications of the tropical cyclone (TC) database that is maintained and actively developed by the China Meteorological Administration, with the aim of facilitating its use in scientific research and operational services. This database records data relating to all TCs that have passed through the western North Pacific (WNP) and South China Sea (SCS) since 1949. TC data collection has expanded over recent decades via continuous TC monitoring using remote sensing and specialized field detection techniques, allowing collation of a multi-source TC database for the WNP and SCS that covers a long period, with wide coverage and many observational elements. This database now comprises a wide variety of information related to TCs, such as historical or real-time locations (i.e., best track and landfall), intensity, dynamic and thermal structures, wind strengths, precipitation amounts, and frequency. This database will support ongoing research into the processes and patterns associated with TC climatic activity and TC forecasting.