ISSN 1000-0526
CN 11-2282/P

Volume 46,Issue 11,2020 Table of Contents

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  • 1  Characteristics and Possible Causes for the Extreme Meiyu in 2020
    LIU Yunyun DING Yihui
    2020, 46(11):1393-1404. DOI: 10.7519/j.issn.1000-0526.2020.11.001
    [Abstract](319) [HTML](779) [PDF 4.34 M](1328)
    The extreme Meiyu in 2020 was featured with earlier onset, later retreat, longer duration of rainy season, wider rainfall range, abundant accumulated rainfall, and more severe rainfall processes than normal. Through the analysis of simultaneous atmospheric circulation systems, we have found that the average locations of the key East Asian monsoon circulation systems were relatively stable, with significant characteristic of quasi-biweekly oscillation (QBWO) in the Meiyu season of 2020. The onset and retreat of Meiyu, the northward shift and stagnation of the rainfall belt, and the occurrence and persistence of severe rainstorm processes had a good relationship with the QBWO. During the Meiyu season, the western Pacific subtropical high experienced six periodic oscillations of northward migration and southward retreat. Meanwhile, the upper- and low-level monsoon circulation systems strengthened for five times corresponding with the QBWO in the Meiyu season. Especially, the strengthening of the southwesterly low-level jet, add the repeated establishment of the large-value centers of southerly with relatively consistent latitude made the water vapor transport from the tropics strengthened again and again, and the water vapor convergence and upward movement developed repeatedly, leading to the long-term persistence of Meiyu in the Yangtze-Huaihe River Valley (YHRV) and frequent occurrences of rainstorms. On the other hand, two blocking highs maintained in the mid- and high latitudes, which was characterized by the spatial pattern of “two ridges and one trough” over Eurasia, and the low trough along the East Asian coast was also active. The cold air flew continuously southward through the northwestern and/or northeastern paths, frequently merging with the repeatedly strengthened warm moisture from the low latitude over the YHRV. So, this is another important factor causing the extreme Meiyu in 2020. Moreover, by comparing the spatio-temporal distribution of Meiyu and the corresponding flood disaster in 2020 with other super Meiyu years since 1951, we have also found that the Meiyu in 2020 was weaker than that of 1954, but stronger than that of 1998 and 1991. Thanks to the improvement of both the accuracy of climate prediction and the ability of disaster prevention and mitigation, the flood and disaster losses caused by the 2020 extreme Meiyu in the YHRV were significantly less than before.
    2  Extreme Features of Severe Precipitation in Meiyu Period over the Middle and Lower Reaches of Yangtze River Basin in June-July 2020
    ZHANG Fanghua CHEN Tao ZHANG Fang SHEN Xiaolin LAN Yu
    2020, 46(11):1405-1414. DOI: 10.7519/j.issn.1000-0526.2020.11.002
    [Abstract](267) [HTML](259) [PDF 7.43 M](1199)
    Severe flood occurred in the middle and lower reaches of the Yangtze River Basin (MLYRB) in June-July 2020 (JJ2020). In this paper, the extreme features of the severe flood are analyzed based on the daily precipitation data from national stations since 1951 and the hourly observations from national and regional stations in JJ2020. The range of anomalous precipitation (anomaly percentage more than 50% or 100%) in MYLRB and the total amount are significantly larger than those in 1998 and 2016, ranked the second in recent 70 years following 1954. There are three rainfall large-value centers with maximal accumulated precipitation exceeding 1 500 mm, which refer to Dabie Mountain Area, southern Anhui and southwestern Hubei. The anomalous-precipitation-caused flood is closely related to the extreme long-term persistence of Meiyu in MLYRB. The Meiyu period in 2020 lasts for 52 days, 23 days more than the climatology. Moreover, in JJ2020, positive precipitation anomaly exists in each dekad, which has never been experienced before since 1961, reflecting the extreme features of the flood. The analysis of rain days of different grades shows that total stations having received moderate rain (10-24.9 mm·d-1), heavy rain (25-49.9 mm·d-1), torrential rain (50-99.9 mm·d-1) and severe torrential rain (≥ 100 mm·d-1) in JJ2020 are the most since 1961, especially the severe precipitations in the last three grades are more significant. The extremity of the severe precipitation can also be found in the short-duration heavy rain processes. In JJ2020, the accumulated station number and days with hourly precipitation over 20 mm and 50 mm are respectively 1612 and 100, both of which are the highest since 1981. At the four stations with total precipitation exceeding 1 500 mm, frequencies of hourly precipitation more than 20 mm are 2-4 times that of the same period in normal years. Accumulation of the first 43, 21, 19 and 27 heaviest hourly precipitation at the four stations exceeds their seasonal averages, respectively. The above results suggest that the significant extremity of the JJ2020 severe precipitation in Meiyu period in MLYRB exists in the total rainfall, the duration and the short-term intensity of severe precipitation.
    3  Synoptic Analysis of Extreme Meiyu Precipitation over Yangtze River Basin During June-July 2020
    CHEN Tao ZHANG Fanghua YU Chao MA Jie ZHANG Xidi SHEN Xiaolin ZHANG Fang LUO Qi
    2020, 46(11):1415-1426. DOI: 10.7519/j.issn.1000-0526.2020.11.003
    [Abstract](330) [HTML](1299) [PDF 16.25 M](2356)
    Extreme Meiyu struck Yangtze River Basin (YRB) from June to July 2020. The accumulated precipitation in the middle and lower reaches of Yangtze River exceeded that of the same peroid in 1998 and was the most accumulated precipitation since 1961. The Meiyu rainfall events this year were characterized by long-time duration, short intervals and spatial overlapping with nocturnal rainfall peak. Accumulated precipitation centers were obviously correlated with topographical features of Dabie Mountain, Wannan Mountain and mountainous territory in the west of Hubei Province. Analysis reveals that main rainbelt swung significantly over YRB in June with quasi-stationary rainbelt along Yangtze River in July. Staggering of Meiyu front over YRB was correlated with persistence of strong blocking high pressure systems in mid-high latitude zones of Asia and abnormal position of the Western Pacific subtropical high staging at about 20°N during July with significant positive vapor flux anomaly beyond 3σ over YRB. Due to the interaction of low-level southwest jet, upper-level westerly jet and upper-level easterly jet, displacement of low-level convergence with upper-level divergence was conducive to intense precipitation along Meiyu front. Heavy rainfall events on Meiyu front could be identified into two synoptic types. One type was quasi-stationary Meiyu front, and the other type featured mesoscale cyclogenesis on Meiyu frontal zone. The former type was dominated by steady quasi-zonal upper-level flow, and staggered low-level wind shear zones with quasi-stationary rainbelts. The other type of rainstorm events developed with distinct cyclonegenesis along Meiyu front, and was accompanied by long-life mesoscale convective systems organized with complicated meso-β scale convective rainbelt adjoining to low-level vortexes. Forecast verifications indicate that there is high predictivity for the first type of heavy rain events, while the second type of Meiyu events has low predictivity and more uncertainty. Obvious forecast deviations tend to occur due to unrealistic positive feedback between over-intensified low-level vortex, precipitation latent heat releasing and low-level jet enhancing in ECMWF-HR forecast.
    4  Application of SC Dual-Polarization Radar to a Convective Weather Case in South to Yangtze River
    LAI Chen ZHI Shulin LI Jie YANG Gang
    2020, 46(11):1427-1439. DOI: 10.7519/j.issn.1000-0526.2020.11.004
    [Abstract](190) [HTML](273) [PDF 2.90 M](690)
    On 21 March 2019, a large-scale convective weather disaster occurred in the southern part of the south to Yangtze River, and it was also the first regional severe convection case encountered after the dual-polarization transformation of the Ji’an SC Doppler Radar in southern Jiangxi. Based on the radar detection data, the distribution and evolution characteristics of the dual-polarized radar parameters were analyzed, and compared with the observation data of cloud-to-ground flashes (CGs). Besides, the differences in performance characteristics between short-time severe rain and hailstorm were explored. The results show that in this case the increase of the horizontal reflectivity ZH and the differential reflectivity ZDR at low elevation angles are significantly ahead of the change of ground rainfall, while the changes of CGs frequency are more advanced, over 6 min earlier than ZH and about 5-10 min earlier than ZDR. The peak time of KDP lags behind lightning for about 15 min, which could help to predict the evolution trend of ground rain intensity in this process. During the hailstrom process, the moment when ZH begins to increase is about 15 min before the ground sag, and the other polarizations advance even earlier. At the same time, CGs increase rapidly when hail occurs. These characteristics have a good indication for hail predictions in this process. In addition, this paper also explores the relationship between the precipitation value M of ice particles and hail. It was found that it surges 15 min ahead of falling to the ground, and the higher the altitude, the earlier the M peak appears. This could provide reference for predicting the possibility moment of hail.
    5  Analysis of Water Vapor Variation and Transformation During the Two Airport Thunderstorms in Urumqi and Chengdu
    XU Haolin ZHENG Jiafeng JIANG Tao LI Qian ZENG Zhengmao ZHANG Jie ZHU Keyun
    2020, 46(11):1440-1449. DOI: 10.7519/j.issn.1000-0526.2020.11.005
    [Abstract](158) [HTML](116) [PDF 1.37 M](702)
    The high-temporal-resolution water vapor density, integral water vapor content (V) and liquid water path (L) measured by ground-based microwave radiometer have important application potential and value in the prediction and research of severe convective precipitation. The paper uses these data to study the water vapor distribution, evolution and vapor-liquid conversion in different stages before and after the two thunderstorms that happened in Urumqi and Chengdu airports, respectively. During the July 4 thunderstorm in Urumqi, under the action of water vapor transport and vertical motion, the low-level water vapor density significantly increased before precipitation and recovered rapidly after precipitation. Before the July 15 thunderstorm precipitation in Chengdu, the whole-layer water vapor experienced the evolution process of increasing first and then decreasing. During the process of water vapor accumulation, the maximum increment was 4.99 g·m-3. During the process of water vapor conversion, the whole-layer water vapor decreased rapidly, of which the water vapor density decreased more significantly at the height of clouds. The cloud water vapor content (IWVc) inversion in the text is better than V and L in indicating the onset and end of precipitation. Before the Urumqi July 4 precipitation, IWVc increased by 1.8 times and 2.2 times, respectively. After the end of precipitation, IWVc decreased rapidly. Before the precipita-tion in Chengdu on July 15, IWVc increased by 1.3 times and 1.5 times, respectively. During the severe precipitation, the growth rate of water vapor in the cloud was lower than that of water vapor conversion. In addition, the increase or decrease of IWVc can also be good indicators for the precipitation intensity of the two thunderstorm processes. For the stable precipitation of the Urumqi July 4 thunderstorm, as the IWVc increased, the surface precipitation intensity increased. Moreover, the greater the increment of water vapor in clouds, the higher the surface precipitation intensity. In the period when the IWVc decreased, the precipitation amount was less than 0.01 mm. For the showery precipitation during the Chengdu July 15 thunderstorm, the accumulation of IWVc was ahead of the occurrence of surface precipitation. The more the IWVc accumulated, the severer the surface precipitation happened. After turning into stable precipitation, the relationship between IWVc and surface precipitation returned to the corresponding increase or decrease, and the decline of IWVc increment or decrement also indicated the 〖JP2〗weakening and end of the precipi-〖JP〗tation.
    6  Mesoscale Analysis of Severe Downslope Windstorm Caused by Gap Jet in Tianshan Mountain Canyon
    TANG Hao LU Hancheng CHU Changjiang SUN Mingjing JU Chenxiang
    2020, 46(11):1450-1460. DOI: 10.7519/j.issn.1000-0526.2020.11.006
    [Abstract](152) [HTML](136) [PDF 3.19 M](679)
    Severe downslope windstorm triggered by the interaction between the gap jet and the asymmetrical topography in the Tianshan Mountain canyon caused severe disasters of trains rollover in the Turpan Depression in Xinjiang on February 28, 2007. To understand the mechanism of downslope windstorm between the interaction of large-scale circulation background, mesoscale system and complex topography in this extreme windstorm, we use a WRF model and conduct numerical simulation and mesoscale diagnostic analysis. The results show that under the pressure gradient between north-south sides of the Tianshan Mountain, the air parcel climbed windward slope and flew into the Tianshan Mountain canyon and then formed gap jet due to effect of funnelling, and at the same time, the jet generated gravity waves forced by the asymmetric terrain of the Tianshan Mountain canyon, and produced lee waves in the leeward, which transmitted the energy of the gap jet to the ground, forming the severe downslope windstorm finally. In this process, the turbulence formed by the wave breaking and the critical layer absorbed the upper layer energy downward, strengthening the energy of the gap jet. Moreover, the atmospheric stability stratification exacerbated the sinking movement which sank energy to the surface.
    7  Application of GFDL Vortex Tracker Technique to the Zhejiang Regional NWP Models
    YU Zhenshou JI Chunxiao ZHANG Honglei DUAN Jingjing
    2020, 46(11):1461-1473. DOI: 10.7519/j.issn.1000-0526.2020.11.007
    [Abstract](173) [HTML](172) [PDF 3.96 M](629)
    The most common tropical cyclone (TC) center identification method for searching for minimum sea level pressure (MSLP) still has some shortcomings. In this paper, TC center identification method and intensity estimation method based on GFDL vortex tracker technique (GVT) are studied in the regional numerical weather prediction model application. Detailed comparisons are conducted between GVT and three traditional TC center location and intensity estimation techniques which include MSLP, the method of using maximum vorticity value at 500 hPa (VT500), and the method of using minimum geopotential height at 500 hPa (H500). The results show that using H500, VT500 and MSLP methods to determine TC center location, can generate some degrees of zigzagshaped irregular swing paths, especially the VT500 method. The VT500 method has the largest irregular swing amplitude and the positioning result is the worst. GVT method can better correct the zigzagshaped swing path and make the model prediction path closer to the observed one, so its positioning result is the best. However, the positioning results of H500, MSLP and GVT are relatively close when the typhoon location is far away from the coastline. The errors of the minimum pressure of TC center by these four methods show that the VT500 method has the largest error, the GVT method has the smallest error, and the errors by MSLP and H500 methods are between those of VT500 and GVT. The errors of maximum wind speed near the TC center by four methods show the errors of the four methods are not much different. The operational application assessment in 2016-2018 show that the Zhejiang Typhoon Vortex Tracker System based on GVT technology can improve the prediction quality of Zhejiang WRFADAS Regional Model System and Zhejiang WRFADAS Rapid Refresh System.
    8  Updates in TRAMS 3.0 Model Version and Its Verification on Typhoon Forecast
    XU Daosheng CHEN Zitong ZHANG Yanxia DAI Guangfeng ZHONG Shuixin ZHANG Banglin CHEN Dehui WU Naigeng WU Kaixin LI Haorui
    2020, 46(11):1474-1484. DOI: 10.7519/j.issn.1000-0526.2020.11.008
    [Abstract](170) [HTML](244) [PDF 7.87 M](602)
    Based on the operational TRAMS (Tropical Region Assimilation Model for the South China Sea) 2.0 version model, a series of technical updates were conducted, including model resolution, dynamical frame, and physical process, thus TRAMS 3.0 version model has been formed. Verifications of the typhoons in 2017 showed that track errors and intensity errors were reduced significantly, the 72 h mean track error was decreased by 7% (about 13.6 km), and the intensity was decreased by 10.5% (about 1.2 hPa). The meteorological elements were also improved by the newly TRAMS 3.0 model. Increasement of model resolution and revised cumulus scheme provided major contribution to forecast improvement in TRAMS 3.0 version.
    9  Verification of ECMWF 10 m Wind Forecast for Coastal Zhejiang Province
    SHEN Huayu FANG Yanying TU Xiaoping LYU Jinwen LU Yun JIANG Lulu ZHENG Zheng
    2020, 46(11):1485-1494. DOI: 10.7519/j.issn.1000-0526.2020.11.009
    [Abstract](158) [HTML](207) [PDF 1.49 M](735)
    Using the observation data from Zhejiang coastal stations, the ECMWF 10 m wind forecast from 2015 to 2018 was verified and evaluated. The results are as follows. The forecast error is closely related to the offshore distance. The farther the station is from the coastline, the higher the consistency is. For farshore stations, the predicted wind speeds are lower than the observed values and the wind directions have counterclockwise deviations. However, for nearshore stations, the results are opposite. With forecast lead time extending, the correlation coefficient of wind speed gradually decreases, while the rootmeansquare error of wind speed and wind direction gradually increases, and this change is more pronounced for far offshore stations. Further analysis on buoy stations indicates that the forecasts of NW, N and NE winds prevailing in winter have significant systematic deviations. The predicted wind speeds are weaker for strong winds, and wind directions have clockwise deviations for light winds. The forecast dispersions of wind speed and wind direction are NW>N>NE. The prediction errors of S and SW winds prevailing in summer are relatively smaller than that of the winter NW and N wind directions. Regarding the three typical gale systems, the cold air gales obtain the best forecast outcome, with the accuracy reaching 97% in 12 h lead time and over 70% in 96 h lead time. The accuracy of tropical cyclone wind forecast declines most significantly with lead time. The accuracy is 85% in 36 h lead time, but decreases sharply to less than 50% beyond 96 h. For low pressures or reverse trough systems, the forecast accuracy remains less than 60% within 144 h lead time. The maximum wind predictions and observations at buoy stations during cold air and tropical cyclone systems are basically in line with the linear distribution. The correlation coefficient for the cold air system can pass the 0.05 significance test within 144 h, while, for tropical cyclones, it can pass the 0.05 significance test only within 48 h. The linear regression method is used to correct the maximum wind speed forecast at buoy stations for cold air systems, and the independent sample tests prove that this method is effective in reducing prediction errors.
    10  Topography-Based Dynamic Critical Arearainfall Threshold for Small to Middle-Sized River Flood Warning
    BAO Hongjun LIN Jian CAO Shuang WANG Meng
    2020, 46(11):1495-1507. DOI: 10.7519/j.issn.1000-0526.2020.11.010
    [Abstract](147) [HTML](196) [PDF 1.93 M](681)
    A topography-based dynamic critical arearainfall threshold model was developed for solving flood warning of the ungauged and small to middle-sized basin in this paper. The exponential equation model between dynamic critical arearainfall threshold and four main characteristic factors was established. The four main characteristic factors include basin area, channel slope, land use and soil type, which affect the flood generation processes primarily in small to middle-sized basins. Dynamic critical arearainfall threshold in the ungauged basin is calculated with the developed exponential equation model of the gauged basins and four main characteristic factors of the ungauged basin. Five small to middle sized basins in the subtropical monsoon climate region of eastern China were selected as the test basins, 〖JP2〗including Huangchuan Basin of Huaihe River, Tunxi and Yuliang basins of Qiantang River, Xitiaoxi and Nantiaoxi basins of Taihu Lake. Taking the warning flood as an example, dynamic critical arearainfall thresholds of Huangchuan, Yuliang, Xitiaoxi and Nantiaoxi basins were inversed with the long-term hydrological and meteorological data by the GMKHM hydrological distributed model. According to the developed critical threshold model, dynamic critical arearainfall threshold of Tunxi Basin was calculated 〖JP2〗based on the developed exponential equation model.〖JP〗 The topography-based dynamic critical arearainfall threshold was applied to flood warning verification of 35 representative flood events in Tunxi Basin. The results show that flood warning hit rate based on the topography-based dynamic critical arearainfall threshold is 91.4%, which is close to that on the basis of dynamic critical arearainfall threshold calculated with long-term hydrological and meteorological data in Tunxi Basin. The developed topography-based dynamic critical arearainfall threshold model has certain reference significance for similar flood warning of ungauged basins and flash flood warning.
    11  Evaluation on the Performance of BCC SecondGeneration Climate System Model for East Asian Summer Climate Prediction
    CHENG Zhi GAO Hui ZHU Yuejia SHI Yueling LIU Junjie WANG Xujia
    2020, 46(11):1508-1519. DOI: 10.7519/j.issn.1000-0526.2020.11.011
    [Abstract](181) [HTML](202) [PDF 5.10 M](644)
    Based on circulation and precipitation hindcast and realtime forecast data of BCC SecondGeneration Climate System Model developed by National Climate Centre, the evaluation of the ability of the model for East Asian summer climate prediction is carried by using the synchronized and lag correlation, partial correlation and combined EOF decomposition analysis. The results show that the main spatial distribution of high and lowlevel circulation and precipitation climatical fields can be well predicted, particularly the location of the heavy rainfall center and members of the monsoon system, but with some systematic biases. The model has poor performance in predicting the precipitation interannual variability in East Asia, especially in China, but has decent prediction ability for the interannual variability of 500 hPa geopotential height, and its skill improves with the approaching to the starting month. In addition, the model can predict the temporal and spatial distribution of the main modes of interannual variations of the East Asian climate with high skills. It can also catch the main characteristics of the response of East Asian climate variability to the developing El Ni〖AKn~D〗o and decaying El Ni〖AKn~D〗o period. This skill mainly comes from the accurate grasp of the east patern El Ni〖AKn~D〗o events. Overall, the BCC SecondGeneration Climate System Model can predict the East Asian summer climate with decent skill, which can benefit the shortterm climate prediction.
    12  Analysis of the August 2020 Atmospheric Circulation and Weather
    LIU Da ZHANG Ling
    2020, 46(11):1520-1528. DOI: 10.7519/j.issn.1000-0526.2020.11.012
    [Abstract](188) [HTML](141) [PDF 8.17 M](825)
    The main characteristics of the general atmospheric circulation in August 2020 are as follows. There were two polar vortex centers in the Northern Hemisphere and they were a little weaker than normal. The circulation at middlehigh latitudes of the Eurasian showed straight, and the general circulation was of latitudinal type with small longitude. The Western Pacific subtropical high was stronger than that of the climatological normal, and the location was west compared to normal years. The monthly mean temperature was 21.6℃, 〖JP2〗higher than normal (20.8℃) by 0.8℃. The monthly mean precipitation was 134.5 mm, 〖JP〗28% higher than normal (105.2 mm). During this month, 9 torrential rain processes with some extreme records occurred in China, causing flood disasters across the regions of southwest, northwest, Huanghuai and some other places. There were 7 tropical cyclones active over South China Sea and Western North Pacific, among which typhoon Hagupit (2004), Mekkhala (2006) and Higos (2007) made landfall in China. The total and landing numbers of typhoon were more than normal. Longlasting high temperature events happened in Jiangnan, Jianghan and Jianghuai regions with the duration reaching or exceeding historical extreme records in some regions.

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