ISSN 1000-0526
CN 11-2282/P

Volume 48,Issue 6,2022 Table of Contents

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  • 1  Preliminary Exploration on the Origin of Large-Scale Medium-Range Forecast Errors over East Asia in the CMA Global Numerical Prediction System: A Case Study
    PENG Fei LI Xiaoli ZHAO Bin CHEN Jing
    2022, 48(6):665-676. DOI: 10.7519/j.issn.1000-0526.2021.090601
    [Abstract](64) [HTML](15) [PDF 6.76 M](181)
    Global operational numerical models have the problem of occasional occurrences of extreme large medium-range forecast errors, and tracing these error sources could provide important references for improving the model itself and data assimilation system. In this study, errors of the operational forecasts in medium range (6 days) from the China Meteorological Administration high-resolution global assimilation and forecast system (CMA-GFS) and global ensemble prediction system (CMA-GEPS) with a lower resolution are analyzed during the period spanning from January to February 2020. The error origin for a particular case (initialized on 12 UTC 8 February 2020) with extreme large medium-range forecast errors over East Asia is investigated by employing the method of ensemble sensitivity analysis. From the spatial-temporal evolution characteristics of forecast errors in CMA-GFS and results from ensemble sensitivity analysis based on CMA-GEPS, a preliminary deduction was acquired about the key forecast error source region, i.e., an up-stream region of East Asia located in the Atlantic Ocean and western Europe (20°-90°N, 90°W-60°E). When the initial conditions of the control forecast of CMA-GEPS are replaced with that of the best ensemble member but confined to the above-mentioned key error source region, the medium-range forecast error of 500 hPa potential height was reduced greatly over East Asia, less than 50% of the original control forecast error. This further confirms the effectiveness of the identified key error source region.
    2  The Trigger and Maintenance Mechanism of Local Flash Heavy Rain Under the Control of Subtropical High in Hebei
    YANG Xiaoliang WANG Xiuming YANG Min ZHU Gang
    2022, 48(6):677-690. DOI: 10.7519/j.issn.1000-0526.2022.040101
    [Abstract](75) [HTML](13) [PDF 29.26 M](573)
    A local flash heavy rain process occurred in central Hebei Province in the evening of August 5, 2018, causing a flash flood and death of two people. The trigger and maintenance mechanisms of the flash heavy rain are analyzed using conventional observation data, ground automatic station data, NCEP reanalysis data, VDRAS data, FY-4 meteorological satellite and Doppler weather radar data. The results show that the flash heavy rain occurred under the control of the Western Pacific subtropical high at 500 hPa which was stronger and more northward than usual. Affected by the inverted trough of Typhoon Jongdari landing northward, the water vapor content above Hebei was abnormally abundant. The surface dew point temperature was as high as 28~29℃, and the atmospheric precipitable water deviated from the mean value by more than 4σ. In addition, the atmosphere was unstable and the environmental wind was weak, so the environmental conditions were conducive to the heavy rain. The physical processes leading to the torrential rain in three places in central Hebei were different. In the southeast of Bao-ding, the heavy rain was caused by the gust front moving southward and eastward to form a mesoscale convergence center, where thunderstorm cells merged, enhanced and propagated backward forming a quasi-stationary rain belt. The flash heavy rain in the west of Baoding was triggered by the enhancing easterly airflow in the boundary layer lifted by the windward slope of Taihang Mountain, where the echo maintained as a result of the cold pool formed by precipitation confronting with the easterly wind. The flash heavy rain in the northeast of Baoding experienced two stages: the north-south convective cloud belt and the east-west convective cloud belt, which was the heaviest precipitation at night. At 01:00 BT 6 August, the north-south convergence line formed by the south airflow strengthened at a low altitude (1500 m above the ground) on the east side of Baoding and the south airflow on the west side caused the north-south radar echo to develop. As the southeast airflow turned into a southerly airflow, the quasi-east-west-oriented warm shear line formed with the northeast airflow caused the echo to turn to east-west direction. The enhanced low-level south airflow at night dominated the strengthening and maintenance of the heavy rainfall echo in the northeastern part of Baoding, which was the key factor in determining the location of the flash heavy rain.
    3  Mesoscale Mechanisms and Forecast Bias of the Extreme Rainstorm in Sichuan Basin on 21 May 2018
    ZHANG Fang ZHANG Fanghua KONG Qi XU Jun CHEN Tao
    2022, 48(6):691-704. DOI: 10.7519/j.issn.1000-0526.2021.123001
    [Abstract](55) [HTML](10) [PDF 20.10 M](554)
    A rare extreme rainstorm occurred in Sichuan Basin on 21 May 2018. Both subjective and objective forecasts failed to capture this disastrous event. In this study, the largescale circulation, the triggering and developing mechanisms of mesoscale systems and the possible causes of forecast biases are analyzed based on the observational and reanalysis data. During this extreme rainstorm event, the westward extension of the western Pacific subtropical high and the abnormally strong Mongolia cold vortex jointly led to the southward invasion of the cold air. The water vapor and unstable energy in the southern Sichuan Basin were extremely stronger than normal, which favored the occurrence of convective heavy precipitation. The special topography near the Sichuan Basin was closely related to the triggering and maintaining of mesoscale convective systems. The northerly wind converged and was lifted up over the unique “bell mouth” terrain, forming a mesoscale convergence line and mesoscale low around the gorge. The temperature gradient on the west and south edges of the basin was increased due to the basin terrain and upwind heavy rainfall. Then, the ascending motion was induced in the large temperaturegradient area. The persistent convergence of the northerly wind and the topographic obstruction made the convective systems move slowly, resulting in the increase of accumulated precipitation. However, the convections in the southern basin were mainly affected by the synopticscale systems. The underestimate of convective precipitation by the EC model was possibly caused by the deviation of wind in the lower troposphere in the basin and the undetailed description of the basin terrain.
    4  Analysis of a Continuous Low-Cloud Process Causing Spring Temperature Forecast Errors in Beijing
    QIN Qingchang YU Bo XU Luyang JI Bin WANG Yuanyuan ZHANG Linna
    2022, 48(6):705-718. DOI: 10.7519/j.issn.1000-0526.2022.030702
    [Abstract](42) [HTML](5) [PDF 3.46 M](126)
    During 2-4 March 2021, a continuous low-cloud process in Beijing caused large deviation of temperature forecast. Both the global/regional numerical models and the forecasters failed to predict this process. Using the conventional meteorological data, ERA5 reanalysis data and FY-4A high-resolution visible cloud images, combined with the data of ceilometer and cloud radar, this paper discusses the formation and maintenance mechanism of the low cloud. The results are as follows. The favorable background for the formation of the low cloud was that there was no influence of obvious cold air after precipitation and the ground humidity was not well removed in the boundary layer. Meantime, the low cloud developed and maintained with stable atmospheric stratification, weak ascending motion and topographic effect. The warm advection and the growth of wind speed at 925 hPa destroyed the stable stratification, leading to the enhancement of mixing activity in the boundary layer and thus the dissipation of the low clouds. Detailed information of the cloud base height and cloud structure can be obtained via the ceilometer and cloud radar, which can be served as a useful complement to conventional observation.
    5  Formation Mechanism of Extreme Winds in Hangzhou Bay Caused by Convection at the Bottom of Cold Vortex
    PENG Xiayun ZHANG Lina LIU Hanhua LI Wenjuan HUANG Xinqing HUANG Xuanxuan
    2022, 48(6):719-728. DOI: 10.7519/j.issn.1000-0526.2022.051601
    [Abstract](49) [HTML](7) [PDF 4.10 M](185)
    On April 12, 2020, affected by the cold vortex, a large-scale thunderstorm and gale occurred in East China, including an extreme gale exceeding the Beaufort scale 12 in Hangzhou Bay. Before the occurrence of this severe convection process, the high and low levels had consistent northwest air flow, with low water vapor content and weak energy conditions, so it was difficult to predict, thus larger deviations appeared in coastal sea surface wind forecast. Based on conventional observation data and Doppler weather radar and wind profile radar data, combined with ERA5 reanalysis data, this paper analyzes the characteristics of thunderstorm gale in this process and the evolution characteristics of convective system before and after its moving into Hangzhou Bay, and focuses on the possible causes of extreme gale in the northeast of Hangzhou Bay. The research shows that the formation of extremely strong wind in Hangzhou Bay was the result of the joint action of many factors. Hangzhou Bay had certain energy conditions, and the temperature reduction rate of the lower atmosphere was close to the dry adiabatic decline rate, which was conducive to the formation of strong downdraft in the convective system and the surface strong wind caused by subsidence divergence. There was a northwest wind jet in the middle layer. Under the action of convective subsidence, the high momentum in the middle layer was brought to the ground, enhancing the ground wind speed. During the process of the convective system moving towards Hangzhou Bay, the cold pool enhanced, and the friction on the water surface of Hangzhou Bay was smaller than that on land. Therefore, the intense flow in the cold pool spread faster, which was also one of the important reasons for the increasing of wind speed in Hangzhou Bay. The occurrence of gales above Beaufort scale 13 in the northeast of Hangzhou Bay was also related to the uneven distribution of marine thermal and dynamic conditions and the variation of echo shape after the convection moved into the sea.
    6  Analyses on Correlation and Causes for the Consecutive Thunderstorm Gust Fronts over the Bohai Sea Bay
    XU Changyi BU Qingjun HUANG Anning
    2022, 48(6):729-745. DOI: 10.7519/j.issn.1000-0526.2022.040202
    [Abstract](31) [HTML](14) [PDF 29.02 M](288)
    To understand more about the diversities of thunderstorm gust fronts, based on Doppler radar data from Tianjin and Cangzhou, wind profiler radar data from Xiqing and Huanghua, Tianjin Meteorological Tower, surface observations data, as well as VDRAS data, this paper comparatively analyzes the causes and correlation of two consecutive gust fronts that occurred in the Bohai Sea Bay on 10 June 2016. The results show that the structures are quite different between these two gust fronts. For the first gust front (GFⅠ), the near-surface meso-γ scale vortices tended to appear along the leading of GFⅠ, and the strong southwesterly warm-humid air flows from boundary layer and lower-level were transported to the thunderstorm along GFⅠ. For the second gust front (GF Ⅱ), dynamic structure was characterized by two strong inflows. One was boundary layer rear northeast inflow at 150-750 m height, another strong southwest inflow was at 990-2 190 m height. The two inflows induced two separated vertical clockwise circulations at the front and rear of GF Ⅱ. The configuration of cold pool and low-level vertical wind shear played an important role in formation and maintenance of these two gust fronts. The strength of cold pool at the rear of GFⅠ is relatively weak, and the 0-3 km vertical wind shear was stronger than the propagation speed of cold pool. The thunderstorm inclined to a stratiform cloud structure, which was not conducive to further development. Comparatively, for GF Ⅱ, the cold pool developed stronger, the propagation of cold pool was more than the 0-3 km vertical wind shear, thus the updraft in the thunderstorm was more vertical, resulting in intensified storm. Meanwhile, the interaction and inherent correlation between meso-γ scale vortex and cold pool were also obvious. Due to the collision of meso-γ scale vortices, strong low-level convergence and updraft occurred between the two front gusts. Moreover, the consolidation of the two cold pools aggravated low-layer instability, which increased the rotation of the upper and lower layers, and formed strong horizontal vorticity, finally leading to the rapid strengthening of convective storm and evolution into bow echo.
    7  Discrimination Criteria of Winter Precipitation Types in and Around Beijing
    JING Hao YU Bo ZHANG Linna XING Nan FU Jiaolan DAI Yi DU Jia
    2022, 48(6):746-759. DOI: 10.7519/j.issn.1000-0526.2021.122401
    [Abstract](47) [HTML](5) [PDF 3.11 M](131)
    The 2012-2020 surface observation and sounding data from Beijing Meteorological Observatory, Zhangjiakou Station and Laoting Station in winter as well as other data are used to establish a set of discrimination criteria for different precipitation types by statistically analyzing vertical temperature and humidity profiles. From forecast perspective, we focus on two key factors of snowflakes forming and melting, and adopt the cloud-top temperature and the 0℃-layer height as new indicators of discrimination criteria for precipitation types. The results show that cloud-top temperature below or equal to -14℃ is a key threshold to generate adequate snowflakes or other ice particles in clouds. This is a necessary condition for snowfall. At the same time, the 0℃-layer height higher than or equal to 0.5 km and lower than or equal to 0.1 km is a threshold for the snowflakes to hardly melt and completely melt, respectively. Sleet or rain is likely to appear with cloud top-temperature between -14℃ and -4℃. The 0℃-layer height at 0.1 km is a threshold of distinguishing sleet from rain. The TS scores of discrimination criteria based on cloud-top temperature and 0℃-layer height in this paper are significantly improved compared with those by using the combination of temperatures at specific layers. The TS scores for snow, sleet and rain reach (improved) 0.93 (0.11), 0.57 (0.39) and 0.86 (0.43), respectively. The results of this study could provide a new reference for forecast and amendment of winter precipitation type.
    8  Quality Control and Effect Evaluation of Range Sidelobe Echo of Millimeter Wave Cloud Radar
    ZENG Zhengmao ZHENG Jiafeng LYU Qiaoyi YANG Hui CAI Yiyou
    2022, 48(6):760-772. DOI: 10.7519/j.issn.1000-0526.2022.032103
    [Abstract](28) [HTML](7) [PDF 3.33 M](128)
    According to the actual performance characteristics of the millimeter wave cloud measuring radar in Pinghe Area of Fujian Province, and in response to the problem of range sidelobe artifacts in operation, an improved data quality control method is proposed. Using the observation data in Pinghe of Fujian Province from September 2018 to August 2020, the quality of the radar is quantitatively evaluated to study the actual impact of data quality control on cloud-precipitation detection. The results show that the proposed method can well improve the radar detection measurements, and the range sidelobe artifacts can be effectively filtered. The range sidelobe also significantly affect the radar’s detection of airborne cloud and rain echoes. The impact is most concentrated near the starting height above the blind zone of the two wide pulse modes, namely 1.50-2.28 km and 3.63-7.74 km, and the frequency of range sidelobe artifact decreases as altitude increases. The range sidelobe artifact mainly interferes with radar detection of low and medium level weak clouds, thus leading to underestimation of cloud base and overestimation of cloud top and cloud thickness. The average errors of three cloud parameters can reach -0.53、0.74 and 0.73 km respectively.
    9  Visibility Forecast and Influence Factor Analysis Based on Regional Modeling
    ZHAO Cuiguang ZHAO Shengrong LIN Jian LYU Zhongliang YAO Li WEI Qing
    2022, 48(6):773-782. DOI: 10.7519/j.issn.1000-0526.2022.033001
    [Abstract](43) [HTML](6) [PDF 2.08 M](125)
    Based on rotated empirical orthogonal function analysis of the daily observational data of visibility from 2008 to 2018, the objective division of visibility in different seasons is obtained. Taking the global numerical prediction model of ECMWF from 2017 to 2019 as the prediction factor, the visibility prediction model for different regions and seasons is built and the regional model is applied to the station for prediction. Then the ECMWF model forecast data in 2020 are used as an independent sample, and the seasonal forecast of visibility in China is carried out. The results show that using the comprehensive algorithm of multiple linear regression, regression estimate of event possibility and discriminant analysis, the visibility forecast of model output statistics based on regional model output statistics is much better in different seasons and different forecast projections than the model direct output (DMO). The underestimation of DMO is corrected, and the improvement of winter forecast score is the most obvious. The model shows high prediction skills in the prediction of low visibility below 1 km, especially at 05:00 BT. Factor analysis shows the high-frequency factors affecting visibility mainly include temperature, pressure, humidity and wind that are closely related to boundary layer conditions, as well as surface thermal conditions, precipitation related factors and stability. The high-frequency factors selected for visibility prediction of different orders in different seasons are different. Spring is sensitive to temperature-related factors, the factors related to precipitation are selected more frequently in summer, and the unstable factors in autumn and winter are more important.
    10  Cause Analysis of Drought in Autumn, Winter and Spring in Guangdong from 2020 to 2021
    WU Hongyu WU Yao GUO Yao
    2022, 48(6):783-793. DOI: 10.7519/j.issn.1000-0526.2022.040801
    [Abstract](24) [HTML](7) [PDF 5.64 M](177)
    The drought occurred in autumn, winter and spring in South China from October 2020 to May 2021, especially in Guangdong, where the duration of drought and the degree of precipitation reduction were rare in recent 58 years. Using the daily precipitation and temperature data of 86 national meteorological stations in Guangdong, NCEP/NCAR and NOAA reanalysis data, this paper studies the abnormal characteristics of atmospheric circulation and SST in autumn, winter and spring in Guangdong from 2020 to 2021 by using correlation and synthesis analysis methods, and discusses its relationship with the drought in autumn, winter and spring in Guangdong. The results show that the drought in autumn, winter and spring in Guangdong from 2020 to 2021 occurred under the climate background of less precipitation, and was closely related to the anomalies of atmospheric circulation and SST. In the autumn, winter and spring of 2020-2021, the high-level subtropical westerly jet in the Northern Hemisphere was weakened, the southern section of the Middle East Asia trough was significantly weakened, the ridge line of the Western Pacific subtropical high was significantly northward, the low level south branch trough was weak, the cold air affecting Guangdong was weak, Guangdong is in the water vapor divergence area, and the North Pacific high pressure was significantly enhanced. A La Ni〖AKn~D〗a event with moderate intensity occurred in the equatorial Middle East Pacific from August 2020 to March 2021, the atmosphere responded significantly to the La Ni〖AKn~D〗a event, the Walker circulation strengthened, the upward movement near the Philippines was obvious, and there was a downward movement in Guangdong, so that a cyclonic abnormal circulation formed near the tropical Western Pacific, the South China Sea and the Philippines in the lower troposphere. The cyclonic abnormal circlution blocked the transport of southwest water vapor from the Bay of Bengal and the South China Sea to Guangdong. At the same time, the temperature was significantly higher. At last, the high temperature and less rain led to the occurrence of drought in Guangdong.
    11  Analysis of the March 2022 Atmospheric Circulation and Weather
    MAI Zi ZHANG Tao
    2022, 48(6):794-800. DOI: 10.7519/j.issn.1000-0526.2022.051001
    [Abstract](51) [HTML](6) [PDF 7.17 M](171)
    The main characteristics of the general atmospheric circulation in March 2022 are as follows. There were two polar vortex centers in the Northern Hemisphere. The 500 hPa geopotential height presented the distribution of an anomalous four-wave pattern in the mid-high latitude of Northern Hemisphere and the air flow was straight in Asia. The strength of Western Pacific subtropical high was a little higher than that in normal years, and the south branch trough was a little weaker. There was only one strong cold front process in March. The national average temperature in March was 7.2℃, 2.4℃ higher than the normal and recorded the first in the same period since 1961. The monthly mean precipitation amount was 36.7 mm, 25% more than in normal period (29.4 mm). Three rainfall processes occurred in this month, causing severe weather such as heavy rain and thunderstorm. In addition, there were two large-scale sand-dust weather processes in northern China. Droughts appeared and developed in Huanghuai Region and Jianghuai Region.

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