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Investigation of Derechos in China Part Ⅰ: Spatiotemporal distribution,environmental characteristics,and morphology of Derechos producing convective systems
CHEN Xiaoxin, YU Xiaoding, WANG Xiuming
 doi: 10.11676/qxxb2021.067
[Abstract](37) [FullText HTML](4) [PDF 9256KB](25)
Based on the 36 Derechos that occurred in China during 18 years from 2002 to 2019, a study on the spatiotemporal distribution and environmental characteristics of Derechos as well as the morphology of Derechos producing convective systems have been conducted using proximity soundings, surface observations, satellite images, single Doppler weather radar data, and weather radar mosaics data. The results are as follows. (1) Derechos mainly occur in the eastern half of China, including North China, East China, South China, and regions to the south of the Yangtze River. The regions of high occurrence frequency show remarkable seasonal variation, which is manifested as the northward movement during the first phase and the southward movement during the second phase from spring to summer. Derechos activity has significant seasonal changes. They mainly occur from March to August with the highest frequency in June and the lowest in August. The convective systems producing the Derechos tend to initiate around noon, while the Derechos themselves tend to occur between mid-afternoon and early midnight. (2) The main characteristics of environmental parameters of Derechos in China are as follows: the 50th percentile of CAPE is 1420 J/kg, the 50th percentile of the 0—6 km vertical shear is 18.0 m/s. The 50th percentile of DCAPE is 1090 J/kg. (3) The weather pattern of Derechos can be divided into the following four types: the subtropical high periphery type, the weak trough type, the high level dry-cold advection forced type and the strong trough type, among which the strong trough type has the highest frequency and the high level dry-cold advection forced type has the lowest frequency. (4) During the period when the maximum wind gust occurs, the most frequent convective storm type is squall line, the second frequent storm type is multi-cell cluster storm and the third frequent storm type is supercell storm.
Thunderstorm climatology over mainland China based on hourly observations during 1971—2010
ZHANG Hengjin, ZHENG Yongguang
 doi: 10.11676/qxxb2022.004
[Abstract](49) [FullText HTML](15) [PDF 3188KB](21)
Up to now there is no study on thunderstorm climatology based on long-term hourly observations over mainland China. Using hourly thunderstorm observations collected at 796 national-level stations during 1971—2010, the temporal and spatial evolution and duration characteristics of hourly thunderstorms over China are revealed, and some new findings are obtained. Over China, the spatial distribution pattern of annual average thunderstorm hours is close to that of annual average thunderstorm days. However, the situation is different in the Qinghai-Tibet Plateau, where the number of annual average thunderstorm days is high but the number of annual average thunderstorm hours is low, indicating that the duration of convective activity is shorter than that in the plain area. It is also found that the annual relative variability in areas of high annual average thunderstorm days and hours is significantly lower than that in areas of low annual average thunderstorm days and hours. Among the four seasons, the number of annual average thunderstorm hours in winter is the smallest, and many thunderstorms occur in the first half of the night. They account for the largest proportion of thunderstorms in the whole day. Nocturnal thunderstorms occur most frequently in summer, but their proportion of the thunderstorms in the whole day is the lowest. Nocturnal thunderstorms in Sichuan Basin are significant all year round. The diurnal peak of thunderstorm hours at each station mostly occurs in the afternoon and the bottom largely occurs in the morning, and there are more thunderstorm hours in the first half of the night than in the second half. Affected by the East Asian summer monsoon, over most parts of China to the south and east of the Hu Huanyong line, the number of thunderstorm hours per thunderstorm day is not less than 3 hours, whereas the number is significantly less than 3 hours to the north and west of the Hu Huanyong line. For thunderstorms of different durations, thunderstorms that can last for 2 hours occur most frequently over China and over different regions. The decadal variations of thunderstorm hours over China and over different regions all show a decreasing trend, but nocturnal thunderstorm hours have increased significantly since 2000. The significant decrease in thunderstorm hours in China is mainly due to the significant decrease of thunderstorm hours in the afternoon, and the increase in thunderstorm hours since 2000 is attributed to the increase in thunderstorm hours at night.
Evaluation of three different gust diagnostic schemes in the CMA-BJ for gale forecasting over Beijing
QUAN Jiping, LI Qingchun, ZHONG Jiqin, ZHANG Shuting, CHEN Min, ZHANG Xinyu, QI Miao
 doi: 10.11676/qxxb2022.001
[Abstract](45) [FullText HTML](7) [PDF 19749KB](15)
Numerical prediction is one of the important ways to forecast gust. This study analyzed and evaluated the performance of the three gust diagnostic schemes (AFWA, UPP, and IUM) in the CMA-BJ for gale forecasting over Beijing. Subjective analysis on two extreme gale processes as well as objective validation on batch experiments of each season indicated that there are significant differences in the gust forecasting using the three schemes, and the forecasting with the IUM scheme shows significant advantages. The IUM scheme performs relatively well on forecasting gales associated with both cold air and thunderstorms. For the gale on 18 March 2020, which was induced by cold air invasion, the scheme shows good performance on the forecast of starting and ending time, the location and evolution, and the maximum value during the entire period of the gale. For the gale on 2 August 2020, which was generated by a thunderstorm, the scheme overestimates the gale area and shows certain biases in the location of the gale. However, the scheme issues warning for gale, which is the most significant. Moreover, the gust speed predicted by the IUM scheme has positive biases at all seasons, but the forecasts of gales equal to or greater than level 5 are more consistent with observations. In general, the IUM scheme performs better than the other two schemes for the gale forecasting over Beijing, and can provide strong support for the gale forecasting.
Multi-element deep learning fusion correction method for numerical weather prediction
ZHANG Yanbiao, CHEN Mingxuan, HAN Lei, SONG Linye, YANG Lu
 doi: 10.11676/qxxb2021.066
[Abstract](135) [FullText HTML](10) [PDF 6448KB](35)
As the mainstream technology of modern weather forecast, numerical weather prediction (NWP) has been developing in the direction of refinement in recent years, yet the prediction error is still unavoidable. Therefore, it is of great significance to improve the accuracy of numerical weather forecast by revising the results. A traditional method of prediction correction, i.e., the Anomaly Numeral-correction with Observations (ANO), is used to correct the forecast based on statistics of historical data. Results indicate that this method has a good effect. As an emerging method, deep learning has been gradually applied to the field of meteorology in recent years, and has achieved significant results in precipitation prediction and cloud image recognition. Domestic scholars in China used CU-Net, a deep learning model to correct the deviations of the model grid point forecast data of 2 m temperature, 2 m relative humidity and 10 m wind respectively from the European Centre for Medium-Range Weather Forecast (ECMWF), which significantly improved the forecast compared with the ANO method. Based on the above tests, this paper uses dense convolutional structure network model to improve the CU-Net model and forms a new deviation correction model for NWP, which is named as Dense-CUnet, and further develops a deviation correction model named Fuse-CUnet to integrates multiple meteorological elements from NWP and topographic features. Deviation correction tests and comparative analysis of these different models have been carried out. Root mean square error (RMSE) and mean absolute error (MAE) are used as the scoring metrics. By comparing with the original prediction results of ECMWF and the results revised by the ANO and CU-Net methods, it is found that the dense-convolution structure network model Dense-CUnet can be used to effectively modify the positive effect. Moreover, the Fuse-CUnet model that integrates multiple elements can greatly improve the revision effect.
Intra-seasonal summer precipitation anomaly over eastern China and evolution characteristics of its associated tropical and mid-to-high latitudes atmospheric circulation
LU Xiaojuan, FANG Jiabei, YANG Xiuqun, HU Haibo
 doi: 10.11676/qxxb2022.003
[Abstract](52) [FullText HTML](14) [PDF 17932KB](15)
Based on CPC(Climate Prediction Center) daily precipitation data, NCEP/NCAR daily reanalysis data and NOAA outgoing longwave radiation data from May to August for the period 1981—2020, the characteristics of main modes of summer intra-seasonal precipitation anomalies in eastern China (named Southern type and Jiang-huai type precipitation anomalies) and the evolution features of associated atmospheric intra-seasonal oscillation (ISO) signals in tropical and mid-to-high latitudes are analyzed using EOF decomposition and lead-lag composite analysis. The causes of intra-seasonal precipitation anomalies are preliminarily discussed as well. The results show that: (1) The Southern type precipitation anomaly events are almost evenly distributed in early, middle and late summer, while the Jiang-huai type precipitation anomaly events mainly occur in middle summer. (2) In early summer, the Southern type precipitation anomalies are mainly presented as precipitation anomalies in the south of the Yangtze River, and in middle and late summer, while the precipitation anomalies in the north of the Yangtze River are also significant. (3) The Southern type precipitation anomaly events are affected by atmospheric ISO signals in the tropical and mid-to-high latitudes. The warm and moist air transport carried by the tropical atmospheric convection and the cold air activity accompanied by the propagation of the Rossby wave train in the mid-to-high latitudes generate water vapor convergence in the southern region, which is conducive to the development and maintenance of precipitation anomalies. In addition, atmospheric ISO signals in the tropical and mid-to-high latitudes are modified by intra-seasonal variations of sea surface temperature, subtropical high and jet stream. From early summer, mid-summer to late summer, the ISO source and propagation path of the tropical atmosphere have changed, and the propagation path and intensity of the Rossby wave train in the upper troposphere at mid-to-high latitudes are also different. (4) Reversed change of the middle and lower reaches of the Yangtze River and South China coast appears during Jiang-huai type precipitation anomaly events, accompanied by the east-west movement of the Western Pacific Subtropical High. The convective anomaly propagating northward and northwestward from the equatorial western Pacific Ocean and the intra-seasonal combined variation of the blocking highs over the Ural Mountains and the Sea of Okhotsk at mid-to-high latitudes are the main reasons for the formation of Jiang-huai type precipitation anomalies.
WANG Chenpeng, HUANG Mengtian, ZHAI Panmao
 doi: 10.11676/qxxb2022.017
[Abstract](75) [FullText HTML](3) [PDF 927KB](7)
This paper reviews the definition and type of droughts, as well as their driving factors, historical changes, attributions and projections based on the Working Group I (WGI) contribution to the Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report (AR6) on the physical science basis of climate change. The IPCC AR6 new results show that: (1) The changes in meteorological and agricultural droughts are not significant on a global scale, but display an increasing trend in some regions. The increasing trend shows that anthropogenic climate change plays an important role in exacerbating droughts. For meteorological droughts, the understanding of the influence of human activities is of low confidence. (2) Agricultural and ecological droughts in most regions can be attributed to human activities (medium to high confidence); for hydrological droughts, in addition to human-induced climate change, regional water resources management and land use are also important factors (medium confidence). (3) In the future projections, more regions around the world will experience more severe and frequent drought events in the future. Agricultural and ecological droughts will also become more frequent and intense as the temperature rises. (4) In the analysis of assessment of changes in different types of droughts, the key variable of atmospheric evaporation demand (AED) is emphasized. Changes in AED are not only a direct response to climate warming, but also a driving factor for drought changes, affecting the physiological processes of vegetation. Meanwhile, changes in AED also feedback to evapotranspiration. The interactions between different types of droughts will become more complicated under the background of intensifying climate change. In the future, the research and operational service of droughts in China should deepen our understanding of connections between changes of different types of droughts, strengthen multi-disciplinary cooperation and explore feedback loops between drought changes, local human activities and ecological processes based on multiple datasets and various evidences.
The formation of typical drylines in Hetao and its role in triggering convection
ZHANG Yiping, YU Xiaoding, WANG Di, HAO Xiaozhen, WANG Jinlan
 doi: 10.11676/qxxb2022.009
[Abstract](48) [FullText HTML](8) [PDF 280KB](16)
Not many researches have been conducted on typical cases of convection triggered by drylines in the Hetao region of China. The formation of three typical drylines and their roles in triggering convection in the Hetao region of Yellow River are analyzed in detail based on upper air soundings, intensive surface observations, EC-ERA5 (0.25º×0.25º) reanalysis data and FY2 meteorological satellite images (the resolution of subsatellite point in the visible images is 1.25 km while that in the infrared images is 5 km). The results are as follows. (1) The three cases occurred under the background of the development of the Mongolia cyclone, which was induced by the eastward movement of the upper trough in the middle and upper troposphere. The structure of the forward-tilting trough indicates that the upper-level northwesterly flows and cold advection were superimposed on the lower-level warm zone, providing a favorable environmental condition for the occurrence of severe convection over large areas. (2) All the three cases occurred along the gentle slope of the Loess Plateau, which is high in the west and low in the east, and under the condition of the Mongolia cyclone development, presenting significant regional characteristics. Besides, the drylines with a length of about 600—800 km and a width of about 80—100 km were oriented along north-northeast to south-southwest direction and coincided with the orientation of the 1300 m contour line in the Hetao region. Furthermore, the main cause for drylines generation is the effect of the dry-warm air produced by the near adiabatic descending warming and the rapid diabatic warming and dehumidification in the western Hetao area. (3) The drylines show obvious diurnal characteristics. The western part of the dryline warmed up rapidly in the daytime and the dryline moved eastward, while it cooled off faster than the eastern part from the nighttime to early morning and the dryline retreated westward. The drylines present their most marked features around 14: 00 BT with the dew point temperature difference between wet and dry sides reaching 10 ℃/(100 km) or more with an obvious convergent flow field of the westerly and the southerly wind. (4) Due to impact of the dryline and its associated convergent flow field, the initial convection was generated near the dryline from 13: 00 BT to 14: 00BJT in the afternoon, which were then reinforced on both sides of the dryline and the linear convective band formed. The convective band moved eastward and gradually stayed away from the drylines under the steering of upper-level westerly airflows and continued to move eastward, expanding to a range of about 500 km to the east of the ground drylines under favorable environmental conditions in the eastern Hetao region. Severe convections such as large-scale thunderstorms, strong winds, local hails and even tornadoes subsequently developed over central and northern Shannxi province and some areas in North China. Finally, the synoptic conceptual model of the generation of typical drylines in Hetao and the areas prone to severe convection are summarized according to the common characteristics of the three typical drylines in Hetao area. This study provides a reference for analysis and forecast of drylines that could trigger convections in the similar situation.
Local circulation characteristics and mechanism analysis of Haituo Mountain in Beijing during winter 2019
WANG Qianqian, QUAN Jiannong, CHENG Zhigang, ZHANG Man, XUE Haile, WU Yongxue
 doi: 10.11676/qxxb2022.005
[Abstract](42) [FullText HTML](7) [PDF 2214KB](20)
The Haituo Mountain, one of the main outdoor venues of the Winter Olympic (Paralympic) Games, presents a severe challenge for refined prediction of wind field due to its complex terrain. Therefore, it is imperative to carry out intensive wind field observations to improve our understanding of local circulation characteristics and influence mechanism of complex terrain, and provide data support for the improvement of refined prediction and service in the competition area. Based on data obtained in the observational experiment at Haituo Mountain in 2019, the horizontal distribution, vertical structure, mountain-valley wind conversion time and local circulation characteristics under the influences of different background wind fields are statistically analyzed using multi-source data such as data collected by automatic weather stations, Doppler wind lidars, eddy covariance systems, ceilometer and cloud radars, etc. The influence mechanism for local circulation in the Haituo Mountain is discussed from three perspectives, i.e., topographic dynamic and thermodynamic impacts and background wind field influences. The results show that the local circulation characteristics in the Haituo Mountain are significant under calm weather conditions when the background winds are weak. The local circulation is dominated by thermally-driven valley wind circulation superimposed by the influence of background airflow passing over the mountain. At low-altitude (<1650 m) stations, mountain and valley winds show opposite directions; at mid- (1650—1800 m) and high- (>1800 m) altitude stations located near the mountain ridges and peaks, mountain and valley winds both are dominated by westerly winds. Valley wind is significantly stronger than mountain wind at the low- and mid-altitude stations, while the opposite is true at high-altitude stations. In contrast, under the influence of strong background winds, local circulation characteristics disappear and the wind direction in the whole valley tends to be consistent with the background wind, while the difference in wind direction between the daytime and nighttime is small and wind speed decreases with increasing height.
[Abstract](9) [PDF 2838KB](4)
Assessment of FY-3D MERSI/NDVI global product
WANG Yuanyuan, LI Guicai
 doi: 10.11676/qxxb2022.007
[Abstract](43) [FullText HTML](9) [PDF 2578KB](8)
FY-3D MERSI/NDVI is a critical operational product used in many studies (ecosystem monitoring, climate change, agriculture drought, etc.), and it is essential to obtain a comprehensive assessment of this product’s quality. In this paper, the first assessment results of global MERSI/NDVI for the period from May 2019 to December 2020 are reported using Terra MODIS/NDVI as the reference. Quantitative measures of APU (Accuracy, Precision, and Uncertainty) are calculated and the variations associated with different factors (seasons, land cover types and NDVI values) are analyzed. Results indicate that generally the two products share a high similarity concerning the spatial pattern and temporal profiles features. The dynamic range of MERSI/NDVI is slightly narrower because it overestimates NDVI for barren land and underestimates NDVI for dense vegetation. Sensitivity analysis indicates that the overestimation is mainly attributed to overestimation of NIR reflectance, whereas the underestimation is mainly attributed to overestimation of red reflectance. Phonological features conveyed by the two NDVI products are consistent, but there are slightly noisier fluctuations in MERSI/NDVI time series probably caused by cloud contamination during growing season. Over the 20 months period checked in this study, the global mean of the Accuracy value ranges within −0.02―0, and the global mean of the Precision and Uncertainty values generally range within 0.06―0.08. With respect to the spatial pattern, APU values are the highest in forests, moderate in grassland/shrubland, and lowest in desert. Linear regression models with MODIS/NDVI as independent variables and MERSI/NDVI as dependent variables achieve high accuracies (R2: 91%—95%, RMSE: 0.048—0.068), confirming that it is feasible to build a compatible long-term NDVI dataset using both products. This study is the first cross-sensor comparison study using almost all of the global MERSI/NDVI data available since the operational application of the FY-3D satellite. Overall, the MERSI/NDVI data are of very high quality and can be effectively used for deriving vegetation phonological and greenness information. Its performance on the global scale will be monitored continually.
Research on radar echoextrapolation nowcasting method combined with physical constraints of the mesoscale model
[Abstract](8) [PDF 2097KB](0)
New series of daily maximum and minimum temperature observations for Beijing, China since 1841
SI Peng, GUO Jun, ZHAO Yufei, WANG Ji, CAO Lijuan, WANG Min, WANG Qi, FENG Jing
 doi: 10.11676/qxxb2022.008
[Abstract](47) [FullText HTML](3) [PDF 3357KB](12)
Continuous daily observations over as long a period as possible are an important basis for the study of extreme climate events and their characteristics over the past hundred years. Due to the scarcity of reliable and continuous daily observations over century-long scale, the pattern of extreme climate change before the 1950s still can not be well interpreted both in China and abroad. New series of daily maximum and minimum temperature observations for Beijing from 1 January 1841 (for minimum) and 1 January 1880 (for maximum) to 31 December 2019 have been constructed. Two daily observational datasets are obtained from China Meteorological Administration (CMA) as the basic sources: (1) The digital daily observations covering the period of 1 January 1841—31 December 1950 from 5 in-situ sites and (2) the daily data observed by new weather and climate systems over the period of 1 January 1951—31 December 2019. Errors caused by manual observations, instrument malfunctions and digital inputs are removed by quality control. Those missing daily maximum and minimum temperature data before 1951 at Beijing meteorological observation station are filled by interpolations using standardized series method. Several significant breakpoints caused by changes in station locations and instrument manufacturers are detected by penalized maximal t test (PMT) and penalized maximal F test (PMF). The homogenized daily maximum and minimum temperature series for Beijing are obtained by Quantile Matching (QM) adjustment with reference series from Berkeley Earth-daily data. It is found that the decadal variabilities of the new daily temperature series over century-long scale are consistent with those of Berkeley Earth-monthly, CRUTS4.03 and GHCNV3 data in Beijing. The warming trend estimated based on the annual mean temperature series averaged from the new daily maximum and minimum temperatures is 0.154±0.018°C/(10 a) (at the 95% confidence level for statistical uncertainties) during the period of 1911—2019 in Beijing, which is similar to that over China. Therefore, the time series of the homogenized daily maximum and minimum temperatures over century-long scale in Beijing city can to a certain extent provide a new data source for the study of climate extremes.
Temporal and spatial characteristics of mesoscale convective systems associated with abrupt heavy rainfall events over Southwest China during May-August
WANG Jingyu, CUI Chunguang, CHEN Yangruixue, WANG Xiaofang
 doi: 10.11676/qxxb2022.006
[Abstract](51) [FullText HTML](12) [PDF 12510KB](23)
Based on TBB data from FY2E (2010—2014) and FY2G (2015—2018) and 468 rain gauges, temporal and spatial characteristics of the mesoscale convective systems associated with abrupt heavy rainfall events (AHR-MCS) over southwest China (SWC) in May-August of 2010—2018 are investigated according to whether the hourly cloud tops of mesoscale convective systems (MCS) cover the gauged-based abrupt heavy rainfall. AHR-MCSs appear most frequently in July with high frequencies over 4 discrete subregions located in Sichuan basin (SR-A), western Hunan (SR-B), northern Guangxi (SR-C) and southwestern Guangxi (SR-D). Abnormal meridional circulations, large divergence anomalies of vertically integrated water vapor flux, high moist static energy anomalies and strong wind shear(June-August) between surface and 700 hPa (especially in June-August) are favorable for AHR-MCSs. AHR-MCSs are the MCSs that have longer lifespan over SWC. They mainly move eastward, but those located in the western (eastern) part of SWC over SR-A and SR-D (SR-B and SR-C) more frequently move southeastward (northeastward). When they are mature, the MCSs in SR-A usually have smaller cloud areas and lower cloud tops, while those in SR-B and SR-C have larger areas and those in SR-D have higher cloud tops. AHR-MCSs appear most frequently during 2000—2300BT (at Beijing Time) with diurnal unimodal structure, and the peak occurs later as the lifespan is longer. There is only one peak over both SR-A and SR-D, but the peak occurrence time of the former is 6-8 hours later than that of the latter. Besides, multi-peak structure of AHR-MCS frequency appears over SR-B and SR-C. The maximum hourly precipitation mostly appears during the development stage of the AHR-MCS, and often corresponds to the time when the minimum temperature of brightness blackbody (TBB) shows up, earlier than the mature time. Gauges within maximum hourly precipitation are often located in the Ⅰ and Ⅲ quadrants of clouds when the minimum TBB occurs. The maximum hourly precipitation not only frequently appears in the cloud areas of lower TBB (<−51℃), but also appears in the cloud areas of higher TBB gradient (> 0.4℃·km−1), and these two make similar contributions to heavy rainfall.
GAO Qianqian LIU Yu GUO Zengyuan PENG Yanyu
[Abstract](40) [PDF 4249KB](13)
Climatic characteristics and environmental conditions of tornadoes in Liaoning under the background of cold vortex
CAI Kuizhi, YAO Xiuping, SUN Xiaowei, CHENG Pan
 doi: 10.11676/qxxb2021.063
[Abstract](38) [FullText HTML](5) [PDF 2780KB](15)
In order to study the relationship between cold vortex and tornado in Liaoning, the characteristics of tornado occurrence in Liaoning under the background of cold vortex are investigated. Using tornado observations and weather disasters data in Liaoning province and the ERA5 atmospheric reanalysis data of the European Center for Mediumrange Weather Forecasts (ECMWF) from 1951 to 2020, tornado cases in Liaoning under the cold vortex background are collected and sorted out. Differences between the physical parameters of EF2—4 (EF2+) and EF0—1 (EF1−) tornadoes under the cold vortex background are compared. The results are as follows. (1) Tornadoes under the cold vortex background mainly occur in the coastal area and the central and northern plain of Liaoning province. The average distance between the tornado and the center of the cold vortex is nearly 900 km. The stronger the cold vortex, the farther its center is from the tornado; the weaker the cold vortex, the closer the distance between the center of the cold vortex and the tornado. Under the background of cold vortex, tornadoes in Liaoning province mainly occur between the southeast and southwest quadrants of the cold vortex, and frequently occur in the enhancement stage of the cold vortex. (2) The number of tornados in Liaoning province in the cold vortex background accounts for nearly 50% of the total samples from 1951 to 2020, and the number of tornados is positively correlated with the number of cold vortex days. The distance between the mean location of the tornadoes in Liaoning under the background of the cold vortex and the average center of the cold vortexes has obvious monthly changes with the closest distance in August. (3) Comparing EF2—4 (EF2+) with EF0—1 (EF1−) tornadoes under the background of cold vortex, the thermal and dynamic environmental parameters of EF2+ tornadoes are 40% and 65% higher than that of EF1- tornadoes. Comparing the tornadoes in Liaoning under the background of cold vortex and tropical cyclone tornadoes, it is found that the convective available potential energy of the cold vortex tornadoes is about 3 times that of the tropical cyclone tornadoes, but the storm relative helicity of the cold vortex tornadoes is only half that of the tropical cyclone tornado. (4) Energy helicity index (EHI) and Significant tornado parameter (STP) can be used to determine the tornado level in Liaoning under the cold vortex background, which is far below the comprehensive parameter threshold of the American tornado. Therefore, the intensity and position of the cold vortex are closely related to the occurrence of tornado in Liaoning. The physical quantity parameters of the EF2—4 (EF2+) tornado under the cold vortex background are significantly greater than those of the EF0—1 (EF1−) tornado. The thermal and dynamic environmental parameters of tornado in Liaoning under the background of cold vortex and tropical cyclone tornado are quite different. Adjusting the Energy helicity index (EHI) and Significant tornado parameter thresholds can improve the POD of strong tornado in Liaoning under the background of cold vortex.
Error Analysis and Modeling of C-Band Dual Polarization Radar Quantitative Precipitation Estimation
 doi: 2022.016
[Abstract](14) [PDF 5983KB](7)
Effects of Environmental Factors on Bowen Ratio of Typical Farmland Ecosystem in Loess Plateau
[Abstract](24) [PDF 1746KB](4)
Typhoon initialization in the GRAPES--_GFS global forecast system
[Abstract](14) [PDF 2077KB](2)
Review of Research on Boundary Layer Convergence Lines Triggering Convection
[Abstract](23) [PDF 2154KB](17)
Study on the Vertical Structure and Macro- and Micro Characteristics and Differences of Precipitation in the Sichuan Basin and Surrounding Areas.
[Abstract](49) [PDF 3487KB](19)
Assimilation of radar data based on cloud-dependent background error covariance and its impact on rainfall forecasting
[Abstract](50) [PDF 3408KB](16)
An Intercomparison on the Algorithms of Cloud Liquid Water Using Advanced Technology Microwave Sounder(ATMS)Observations
[Abstract](25) [PDF 2629KB](3)
Meiyu in Yangtze River Delta and its Extended-range Forecast Associated with Intraseasonal Evolution of Atmospheric Circulation
[Abstract](33) [PDF 4871KB](9)
2021year No.6
Display Method:
A method to approximate the surface frontal line on the Yunnan-Guizhou Plateau by linear fitting is proposed. Through comprehensive analysis of the frontal line position, the spatial distribution and temporal variation of meteorological elements around the frontal line, as well as changes in the position and direction of the frontal line for long-lasting frontal line events, the movement characteristics of the frontal line on the Yunnan-Guizhou Plateau are systematically revealed. The cold (warm) frontal lines are concentrated in 102.5°—105°E (104.5°—105.75°E) and the maximum cooling (heating) zone is located on the eastern (western) side of the frontal line. The changes in meteorological elements around the frontal line are closely related to the movement of the frontal line. The westward moving frontal line usually brings cooling condition, increases surface air pressure, and reduces sunshine hours around the frontal line, while the eastward-moving frontal line often leads to opposite changes. According to the movement of long-lasting frontal line events, frontal line events can be divided into three types: Stationary, westward and eastward. The stationary frontal lines are the most in all the three types. The westward moving frontal line can advance continuously and quickly and is accompanied by a clockwise swing of the frontal line. The eastward moving frontal line has a lower frequency and a relatively low speed. Overall, with the objective and quantitative description of the frontal line, especially its dynamic characteristics, the above results can provide an important reference for the fine forecast of various meteorological elements on the Yunnan-Guizhou Plateau.
In order to develop the four-dimensional variational data assimilation (4DVar) system that can be used in regional numerical weather prediction, the framework of the incremental 4DVar is developed in this study on the basis of the recently developed perturbation forecast model GRAPES_PF. At the current stage, this 4DVar framework does not include physical schemes such as short-wave and long-wave radiation, planetary boundary layer, cumulus convection, cloud microphysics, etc. Compared to the operational GRAPES 3DVar system, air temperature is chosen as an extra analysis control variable in the new framework. The linear balance equation, which relates the balanced Exner pressure with stream function, is deduced and solved numerically on the terrain-following vertical coordinate. The adjoint of perturbation Helmholtz equation is solved using the iterative generalized conjugate residual (GCR) approach. To evaluate the validity of this framework, a suite of idealized numerical experiments using pseudo radiosonde data have been carried out to simulate typhoon Mujigae, which occurred over South China Sea in October 2015. The experiments reveal that the 4DVar framework offers results in line with theoretical expectations, i.e., by ingesting more observations in time and through the constraint of perturbation forecast model, the 4DVar leads to more obvious improvements than the 3DVar in both analysis and forecast. This study provides a reasonable framework of four-dimensional variational data assimilation, which can be further implemented with full linear physical package soon.
The high-resolution numerical simulations and verifications of 10 convective cases that occurred in Beijing have been conducted by assimilating surface observations in a four-dimensional Variational Doppler Radar Analysis System (VDRAS) based on the rapid-refresh 4D variational assimilation (RR4DVar) technique of multi-radar observations and three-dimensional cloud-scale numerical model. Compared with the surface observations fusion scheme, the verification results show that the surface observations assimilation obviously can improve analysis results below 1 km boundary layer height, and the root mean square errors (RMSE) of simulated wind speed and wind direction are respectively reduced by 0.1 m/s and 7.2° on average, the RMSE of temperature is reduced by 0.2℃. The RMSE of wind speed is decreased by 0.5 m/s at the lowest model level of 100 m, and the error of wind speed increases with height below 3 km. The RMSEs of wind direction and temperature are respectively reduced by 15.5° and 0.4℃ at the lowest model level, and the errors of temperature are decreased at all levels below 1.5 km height. The RMSEs of 10 m surface wind speed and wind direction are respectively reduced by 0.2 m/s and 10.8°, and the error of 2 m surface temperature is also reduced. In addition, the surface observations assimilation can to a certain extent improve the 1 hour forecast of surface fields, whereas the RMSEs of regional surface temperature and wind field increase with forecast time. Combined with the detailed analysis of localized and rapidly intensified convection case that occurred in Beijing on 17 May, 2019, it is found that the surface observations assimilation can better represent the dynamical and thermodynamical characteristics in the lower atmosphere with more details, and thus improves the forecast of low-level meteorological variables. Further investigation of the local trigger mechanism of convection indicates that the interaction between the convergence line of sea breeze and the urban condition to some extent has affected the trigger and development of local convection in Beijing. This method can further improve the nowcasting of localized and rapidly intensified convection in Beijing.

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Supervisor: China Meterological Administration

Sponsor: China Meteorological Society



Editor In Chief: Yihui DING