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2021 Vol. 38, No. 5

News & Views
The Interannual Variation of Transboundary Contributions from Chinese Emissions of PM2.5 to South Korea
Xiao HAN, Meigen ZHANG
2021, 38(5): 701-706. doi: 10.1007/s00376-021-1003-4
In recent years, several studies pointed out that anthropogenic emission sources in China which significantly contribute to the PM2.5 mass burden was an important cause of particulate pollution in South Korea. However, most studies generally focused upon a single pollution event. It is rare to see comprehensive research that captures those features prone to interannual variations concerning the transboundary pollutant contribution in South Korea using a unified method. In this paper, we establish the emission inventories covering East Asia in 2010, 2015, and 2017, and then conduct the source apportionment by applying a coupled regional air quality model called the Integrated Source Apportionment Module (ISAM). Comparison of simulated and observed PM2.5 mass concentration at 165 CNEMC (China National Environmental Monitoring Center) sites suggests that the PM2.5 concentrations are well represented by the modeling system. The model is used to quantitatively investigate the contribution from emission sources in China to PM2.5 concentrations over South Korea and those features found to be prone to interannual variations are then discussed. The results show that the average annual contribution of PM2.5 has dropped significantly from 28.0% in 2010 to 15.7% in 2017, which strongly suggests that China has achieved remarkable results in the treatment of atmospheric particulates.
Chen-Chao Koo and the Early Numerical Weather Prediction Experiments in China
Jianhua LU
2021, 38(5): 707-716. doi: 10.1007/s00376-021-0268-y
Although the first successful numerical weather prediction (NWP) project led by Charney and von Neumann is widely known, little is known by the international community about the development of NWP during the 1950s in China. Here, a detailed historical perspective on the early NWP experiments in China is provided. The leadership in NWP of the late Professor Chen-Chao Koo, a protégé of C. G. Rossby at the University of Stockholm during the late 1940s and a key leader of modern meteorology (particularly of atmospheric dynamics and physics) in China during the 1950s−70s, is highlighted. The unique contributions to NWP by Koo and his students, such as the ideas of formulating NWP as an “evolution” problem, in which the past data over multiple time steps are utilized, rather than an initial-value problem, and on the cybernetic aspects of atmospheric processes, i.e., regarding the motion of the atmosphere at various time scales as an optimal control system, are also emphasized.
Original Paper
A Case Study of the Initiation of Parallel Convective Lines Back-Building from the South Side of a Mei-yu Front over Complex Terrain
Qiwei WANG, Yi ZHANG, Kefeng ZHU, Zhemin TAN, Ming XUE
2021, 38(5): 717-736. doi: 10.1007/s00376-020-0216-2
Parallel back-building convective lines are often observed extending to the southwest of some mesoscale convective systems (MCSs) embedded in the mei-yu front in China. The convective lines with echo training behavior can quickly develop into a stronger convective group of echoes, resulting in locally heavy rainfall within the mei-yu front rainband. The initiation mechanism of the back-building convective lines is still unclear and is studied based on high-resolution numerical simulation of a case that occurred during 27−28 June 2013. In the present case, the new convection along the convective lines was found to be forced by nonuniform interaction between the cold outflow associated with the mei-yu front MCSs and the warm southerly airflow on the south side of the mei-yu front, which both are modified by local terrain. The mei-yu front MCSs evolved from the western to the eastern side of a basin surrounded by several mesoscale mountains and induced cold outflow centered over the eastern part of the basin. The strong southwest airflow ahead of the mei-yu front passed the Nanling Mountains and impacted the cold outflow within the basin. The nonuniform interaction led to the first stage of parallel convective line formation, in which the low mountains along the boundary of the two airflows enhanced the heterogeneity of their interaction. Subsequently, the convective group quickly developed from the first stage convective lines resulted in apparent precipitation cooling that enhanced the cold outflow and made the cold outflow a sharp southward windshift. The enhanced cold outflow pushed the warm southerly airflow southward and impacted the mountains on the southeast side of the basin, where the roughly parallel mountain valleys or gaps play a controlling role in a second stage formation of parallel convective lines.
Parameterized Forward Operators for Simulation and Assimilation of Polarimetric Radar Data with Numerical Weather Predictions
Guifu ZHANG, Jidong GAO, Muyun DU
2021, 38(5): 737-754. doi: 10.1007/s00376-021-0289-6
Many weather radar networks in the world have now provided polarimetric radar data (PRD) that have the potential to improve our understanding of cloud and precipitation microphysics, and numerical weather prediction (NWP). To realize this potential, an accurate and efficient set of polarimetric observation operators are needed to simulate and assimilate the PRD with an NWP model for an accurate analysis of the model state variables. For this purpose, a set of parameterized observation operators are developed to simulate and assimilate polarimetric radar data from NWP model-predicted hydrometeor mixing ratios and number concentrations of rain, snow, hail, and graupel. The polarimetric radar variables are calculated based on the T-matrix calculation of wave scattering and integrations of the scattering weighted by the particle size distribution. The calculated polarimetric variables are then fitted to simple functions of water content and volume-weighted mean diameter of the hydrometeor particle size distribution. The parameterized PRD operators are applied to an ideal case and a real case predicted by the Weather Research and Forecasting (WRF) model to have simulated PRD, which are compared with existing operators and real observations to show their validity and applicability. The new PRD operators use less than one percent of the computing time of the old operators to complete the same simulations, making it efficient in PRD simulation and assimilation usage.
Iterative Methods for Solving the Nonlinear Balance Equation with Optimal Truncation
Qin XU, Jie CAO
2021, 38(5): 755-770. doi: 10.1007/s00376-020-0291-4
Two types of existing iterative methods for solving the nonlinear balance equation (NBE) are revisited. In the first type, the NBE is rearranged into a linearized equation for a presumably small correction to the initial guess or the subsequent updated solution. In the second type, the NBE is rearranged into a quadratic form of the absolute vorticity with the positive root of this quadratic form used in the form of a Poisson equation to solve NBE iteratively. The two methods are rederived by expanding the solution asymptotically upon a small Rossby number, and a criterion for optimally truncating the asymptotic expansion is proposed to obtain the super-asymptotic approximation of the solution. For each rederived method, two iterative procedures are designed using the integral-form Poisson solver versus the over-relaxation scheme to solve the boundary value problem in each iteration. Upon testing with analytically formulated wavering jet flows on the synoptic, sub-synoptic and meso-α scales, the iterative procedure designed for the first method with the Poisson solver, named M1a, is found to be the most accurate and efficient. For the synoptic wavering jet flow in which the NBE is entirely elliptic, M1a is extremely accurate. For the sub-synoptic wavering jet flow in which the NBE is mostly elliptic, M1a is sufficiently accurate. For the meso-α wavering jet flow in which the NBE is partially hyperbolic so its boundary value problem becomes seriously ill-posed, M1a can effectively reduce the solution error for the cyclonically curved part of the wavering jet flow, but not for the anti-cyclonically curved part.
Impact of the Monsoonal Surge on Extreme Rainfall of Landfalling Tropical Cyclones
Dajun ZHAO, Yubin YU, Lianshou CHEN
2021, 38(5): 771-784. doi: 10.1007/s00376-021-0281-1
A comparative analysis and quantitative diagnosis has been conducted of extreme rainfall associated with landfalling tropical cyclones (ERLTC) and non-extreme rainfall (NERLTC) using the dynamic composite analysis method. Reanalysis data and the tropical cyclone precipitation dataset derived from the objective synoptic analysis technique were used. Results show that the vertically integrated water vapor transport ( Q vt) during the ERLTC is significantly higher than that during the NERLTC. The Q vt reaches a peak 1−2 days before the occurrence of the ERLTC and then decreases rapidly. There is a stronger convergence for both the Q vt and the horizontal wind field during the ERLTC. The Q vt convergence and the wind field convergence are mainly confined to the lower troposphere. The water vapor budget on the four boundaries of the tropical cyclone indicates that water vapor is input through all four boundaries before the occurrence of the ERLTC, whereas water vapor is output continuously from the northern boundary before the occurrence of the NERLTC. The water vapor inflow on both the western and southern boundaries of the ERLTC exceeds that during the NERLTC, mainly as a result of the different intensities of the southwest monsoonal surge in the surrounding environmental field. Within the background of the East Asian summer monsoon, the low-level jet accompanying the southwest monsoonal surge can increase the inflow of water vapor at both the western and southern boundaries during the ERLTC and therefore could enhance the convergence of the horizontal wind field and the water vapor flux, thereby resulting in the ERLTC. On the other hand, the southwest monsoonal surge decreases the zonal mean steering flow, which leads to a slower translation speed for the tropical cyclone associated with the ERLTC. Furthermore, a dynamic monsoon surge index (DMSI) defined here can be simply linked with the ERLTC and could be used as a new predictor for future operational forecasting of ERLTC.
The Asymmetric Atmospheric Response to the Decadal Variability of Kuroshio Extension during Winter
Jianqi ZHANG, Chongyin LI, Xin LI, Chao ZHANG, Jingjing CHEN
2021, 38(5): 785-799. doi: 10.1007/s00376-020-0264-7
The Kuroshio extension (KE) exhibits interdecadal variability, oscillating from a stable state to an unstable state. In this paper, ERA-Interim reanalysis data are used to discuss the possible reasons for the asymmetric response of the atmosphere to symmetric sea surface temperature anomaly (SSTA) during periods of differential KE states. The analysis has the following results: the SSTA presents a nearly symmetrical distribution with opposite signs during the KE stable and unstable periods. During the KE stable period, the storm track is located north of 40°N and is significantly enhanced in the northeast Pacific Ocean. The atmospheric response is similar to the West Pacific/North Pacific Oscillation teleconnection (WP/NPO like pattern) and presents a barotropic structure. The inversion results of the potential vorticity equation show that the feedback of transient eddy vorticity manifests a WP/NPO like pattern and presents a barotropic structure, which is the main reason for bringing about the response of the WP/NPO like pattern. The magnitude of the feedbacks of both diabatic heating and transient eddy heating is small, which can offset one another. During the KE unstable period, the main body of the storm track is located to the south of 40°N, and there is no significant response signal in the atmosphere, except near the west coast of North America. Compared with the KE stable period, the asymmetry of response of the transient eddy vorticity is the main reason for the asymmetric response of the atmosphere.
On the Mechanism of a Terrain-Influenced Snow Burst Event during Midwinter in Northeast China
Na LI, Baofeng JIAO, Lingkun RAN, Xinyong SHEN, Yanbin QI
2021, 38(5): 800-816. doi: 10.1007/s00376-020-0104-9
Short-duration snow bursts with heavy snow represent one type of hazardous weather in winter which can be easily missed by the winter weather warnings but often results in great hazards. In this paper, the mechanism for the occurrence of such events was investigated with the aid of a localized terrain-influenced snow burst event in Northeast China. The snow burst was produced by an eastward-moving cold-frontal snowband which encountered the downstream complex terrain of the Changbai Mountains and intensified. To ascertain the role of orography on the snow burst, numerical experiments, together with a parallel sensitivity experiment removing Changbai Mountains, were performed to attempt to distinguish the contributions of cold-frontal system and orographic effects to produce the heavy snow. Diagnosis showed that without the influence of Changbai Mountains, the release of conditional instability (CI) and inertial instability (II) within a weak frontogenetical environment was responsible for the snowband maintenance. Orographic effects played important roles in enhancing the snowband and increasing the snowfall intensities. The enhancement mechanism was related to the interactions of the cold-frontal snowband and the topography. On the one hand, orographic frontogenesis and persistent ascent, created by orographic gravity waves over the terrain, greatly enhanced the orographic lifting. The intensification of the lifting promoted the release of CI and thus enhanced the snowfall. On the other hand, pre-existing orographic instabilities were released due to the passing of the cold-frontal snowband, which could also serve to intensify the snowband over terrain and thus increase the snowfall.
CMIP6 Evaluation and Projection of Temperature and Precipitation over China
Xiaoling YANG, Botao ZHOU, Ying XU, Zhenyu HAN
2021, 38(5): 817-830. doi: 10.1007/s00376-021-0351-4
This article evaluates the performance of 20 Coupled Model Intercomparison Project phase 6 (CMIP6) models in simulating temperature and precipitation over China through comparisons with gridded observation data for the period of 1995–2014, with a focus on spatial patterns and interannual variability. The evaluations show that the CMIP6 models perform well in reproducing the climatological spatial distribution of temperature and precipitation, with better performance for temperature than for precipitation. Their interannual variability can also be reasonably captured by most models, however, poor performance is noted regarding the interannual variability of winter precipitation. Based on the comprehensive performance for the above two factors, the “highest-ranked” models are selected as an ensemble (BMME). The BMME outperforms the ensemble of all models (AMME) in simulating annual and winter temperature and precipitation, particularly for those subregions with complex terrain but it shows little improvement for summer temperature and precipitation. The AMME and BMME projections indicate annual increases for both temperature and precipitation across China by the end of the 21st century, with larger increases under the scenario of the Shared Socioeconomic Pathway 5/Representative Concentration Pathway 8.5 (SSP585) than under scenario of the Shared Socioeconomic Pathway 2/Representative Concentration Pathway 4.5 (SSP245). The greatest increases of annual temperature are projected for higher latitudes and higher elevations and the largest percentage-based increases in annual precipitation are projected to occur in northern and western China, especially under SSP585. However, the BMME, which generally performs better in these regions, projects lower changes in annual temperature and larger variations in annual precipitation when compared to the AMME projections.
Background Error Covariance Statistics of Hydrometeor Control Variables Based on Gaussian Transform
Tao SUN, Yaodeng CHEN, Deming MENG, Haiqin CHEN
2021, 38(5): 831-844. doi: 10.1007/s00376-021-0271-3
Use of data assimilation to initialize hydrometeors plays a vital role in numerical weather prediction (NWP). To directly analyze hydrometeors in data assimilation systems from cloud-sensitive observations, hydrometeor control variables are necessary. Common data assimilation systems theoretically require that the probability density functions (PDFs) of analysis, background, and observation errors should satisfy the Gaussian unbiased assumptions. In this study, a Gaussian transform method is proposed to transform hydrometeors to more Gaussian variables, which is modified from the Softmax function and renamed as Quasi-Softmax transform. The Quasi-Softmax transform method then is compared to the original hydrometeor mixing ratios and their logarithmic transform and Softmax transform. The spatial distribution, the non-Gaussian nature of the background errors, and the characteristics of the background errors of hydrometeors in each method are studied. Compared to the logarithmic and Softmax transform, the Quasi-Softmax method keeps the vertical distribution of the original hydrometeor mixing ratios to the greatest extent. The results of the D′Agostino test show that the hydrometeors transformed by the Quasi-Softmax method are more Gaussian when compared to the other methods. The Gaussian transform has been added to the control variable transform to estimate the background error covariances. Results show that the characteristics of the hydrometeor background errors are reasonable for the Quasi-Softmax method. The transformed hydrometeors using the Quasi-Softmax transform meet the Gaussian unbiased assumptions of the data assimilation system, and are promising control variables for data assimilation systems.
An Experiment on the Prediction of the Surface Wind Speed in Chongli Based on the WRF Model: Evaluation and Calibration
Na LI, Lingkun RAN, Dongdong SHEN, Baofeng JIAO
2021, 38(5): 845-861. doi: 10.1007/s00376-021-0201-4
In this study, the ability of the Weather Research and Forecasting (WRF) model to generate accurate near-surface wind speed forecasts at kilometer- to subkilometer-scale resolution along race tracks (RTs) in Chongli during the wintertime is evaluated. The performance of two postprocessing methods, including the decaying-averaging (DA) and analogy-based (AN) methods, is tested to calibrate the near-surface wind speed forecasts. It is found that great uncertainties exist in the model’s raw forecasts of the near-surface wind speed in Chongli. Improvement of the forecast accuracy due to refinement of the horizontal resolution from kilometer to subkilometer scale is limited and not systematic. The RT sites tend to have large bias and centered root mean square error (CRMSE) values and also exhibit notable underestimation of high-wind speeds, notable overestimation or underestimation of the near-surface wind speed at high altitudes, and notable underestimation during daytime. These problems are not resolved by increasing the horizontal resolution and are even exacerbated, which leads to great challenges in the accurate forecasting of the near-surface wind speed in the competition areas in Chongli. The application of postprocessing methods can greatly improve the forecast accuracy of near-surface wind speed. Both methods used in this study have comparable abilities in reducing the (positive or negative) bias, while the AN method is also capable of decreasing the random error reflected by CRMSE. In particular, the large biases for high-wind speeds, wind speeds at high-altitude stations, and wind speeds during the daytime at RT stations can be evidently reduced.
Data Description Article
CAS-LSM Datasets for the CMIP6 Land Surface Snow and Soil Moisture Model Intercomparison Project
Binghao JIA, Longhuan WANG, Yan WANG, Ruichao LI, Xin LUO, Jinbo XIE, Zhenghui XIE, Si CHEN, Peihua QIN, Lijuan LI, Kangjun CHEN
2021, 38(5): 862-874. doi: 10.1007/s00376-021-0293-x
The datasets of the five Land-offline Model Intercomparison Project (LMIP) experiments using the Chinese Academy of Sciences Land Surface Model (CAS-LSM) of CAS Flexible Global-Ocean-Atmosphere-Land System Model Grid-point version 3 (CAS FGOALS-g3) are presented in this study. These experiments were forced by five global meteorological forcing datasets, which contributed to the framework of the Land Surface Snow and Soil Moisture Model Intercomparison Project (LS3MIP) of CMIP6. These datasets have been released on the Earth System Grid Federation node. In this paper, the basic descriptions of the CAS-LSM and the five LMIP experiments are shown. The performance of the soil moisture, snow, and land-atmosphere energy fluxes was preliminarily validated using satellite-based observations. Results show that their mean states, spatial patterns, and seasonal variations can be reproduced well by the five LMIP simulations. It suggests that these datasets can be used to investigate the evolutionary mechanisms of the global water and energy cycles during the past century.
The Assessment of Global Surface Temperature Change from 1850s: The C-LSAT2.0 Ensemble and the CMST-Interim Datasets
Wenbin SUN, Qingxiang LI, Boyin HUANG, Jiayi CHENG, Zhaoyang SONG, Haiyan LI, Wenjie DONG, Panmao ZHAI, Phil JONES
2021, 38(5): 875-888. doi: 10.1007/s00376-021-1012-3
Based on C-LSAT2.0, using high- and low-frequency components reconstruction methods, combined with observation constraint masking, a reconstructed C-LSAT2.0 with 756 ensemble members from the 1850s to 2018 has been developed. These ensemble versions have been merged with the ERSSTv5 ensemble dataset, and an upgraded version of the CMST-Interim dataset with 5° × 5° resolution has been developed. The CMST-Interim dataset has significantly improved the coverage rate of global surface temperature data. After reconstruction, the data coverage before 1950 increased from 78%−81% of the original CMST to 81%−89%. The total coverage after 1955 reached about 93%, including more than 98% in the Northern Hemisphere and 81%−89% in the Southern Hemisphere. Through the reconstruction ensemble experiments with different parameters, a good basis is provided for more systematic uncertainty assessment of C-LSAT2.0 and CMST-Interim. In comparison with the original CMST, the global mean surface temperatures are estimated to be cooler in the second half of 19th century and warmer during the 21st century, which shows that the global warming trend is further amplified. The global warming trends are updated from 0.085 ± 0.004°C (10 yr)–1 and 0.128 ± 0.006°C (10 yr)–1 to 0.089 ± 0.004°C (10 yr)–1 and 0.137 ± 0.007°C (10 yr)–1, respectively, since the start and the second half of 20th century.