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Phase Two of the Integrative Monsoon Frontal Rainfall Experiment (IMFRE-II) was conducted over the middle and lower reaches of the Yangtze River during the period 16 June to 19 July 2020. During IMFRE-II, ten heavy rainfall events occurred and were observed by seven ground-based observational systems, the Shanxi KingAir (KA350) aircraft, and Fengyun and global precipitation measurement (GPM) satellites. The primary goal of IMFRE-II is to investigate the spatiotemporal evolution of mei-yu frontal systems, such as their horizontal and vertical distributions, from west to east over the middle and lower reaches of the Yangtze River through comprehensive observations from ground-based and spaceborne passive and active remote sensors, and aircraft in-situ measurements, as well as model simulations.
IMFRE-II was motivated by IMFRE-I, which was conducted from 10 June to 10 July 2018 over the middle reaches of the Yangtze River and organized by the Wuhan Institute of Heavy Rain (IHR). IMFRE-I maximized the use of our observational capacity enabled by a suite of ground-based remote sensing instruments, most notably the IHR Mesoscale Heavy Rainfall Observing System (MHROS), including different wavelengths of radars, microwave radiometers, and disdrometers. The KA350 aircraft participating in the campaign is equipped with Ka-band cloud radar and different probes. The comprehensive datasets from both the MHROS and aircraft instruments are combined with available satellite observations and model simulations to answer the three scientific questions proposed in IMFRE-I.
The 2020 mei-yu season was an extreme one—its rainfall intensity and coverage broke records. The heavy precipitation events during the 2020 mei-yu season can be briefly summarized by the following three characteristics.
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The 2020 mei-yu season over the Yangtze River basin (Hubei) started on 8 June and ended on 21 July, which is eight days earlier and 11 days later than its climatological mean start date (16 June) and end date (10 July), respectively. The 2020 mei-yu season lasted for 44 days, which is 19 days longer than the climatological mean, and ranks as the eighth longest mei-yu season since 1951. During the 2020 mei-yu season, the rainfall centers were primarily located over the middle and lower reaches of the Yangtze River. Some regions, such as southern Chongqing, eastern Hubei, northern Hunan, northwestern Jiangxi, central and southern Anhui, central and southern Jiangsu, and northern Zhejiang, received more than 600 mm rainfall, which is 50% higher than normal years. The accumulated rainfall amounts along the Yangtze River were twice as much as or even more than their climatological means. Total rainfall amounts in the southern and central-eastern parts of Hubei Province and the central and southern parts of Anhui Province exceeded 1000 mm (Fig. 1). The major rainbelt was located in the Yangtze River−Huaihe River valley in early June, shifted southward from middle and late June to early July, and maintained steadily along the Yangtze River and over its northern part. In mid-July, the rainbelt was primarily located over the Yangtze River with slight meridional fluctuation.
Figure 1. Spatial distributions of (a) the 2020 mei-yu cumulative precipitation (units: mm) and (b) the precipitation anomaly (units: %) against the climatological mean during the period 8 June to 20 July 2020 in the Yangtze River basin.
Statistical results of precipitation observed at 735 national weather stations in the Yangtze River basin show that the accumulated precipitation amount during the 2020 mei-yu season exceeded 200 mm over the entire basin (except for a few areas in the middle and upper reaches of the Jinsha River). The regional average precipitation amount during the 2020 mei-yu season over the middle and lower reaches of the Yangtze River was 435.1 mm, which is the largest one since 1961. The second largest precipitation amount is 382.7 mm in 2016. Note that the 60-yr average precipitation amount during the same period is 263.1 mm. The average number of heavy rainfall days in 2020 was 2.3 days/station, the highest one during the same period since 1961.
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During the 2020 mei-yu season, the Yangtze River basin experienced a total of ten precipitation events, mostly along or near the Yangtze River. Table 1 lists the time periods and regions of these ten precipitation events, as well as their highest rainfall for each event, where quite a few regions experienced repeatable heavy precipitation events. Short-term torrential rainstorms frequently occurred during the 2020 mei-yu season. For example, on 6 July 2020, 3-h cumulative rainfall of 178.5 mm during 0000−0300 LST (LST=UTC+8) and 24-h cumulative rainfall of 502 mm were observed at Jingzhou, Hubei Province; 6-h cumulative rainfall of 309.4 mm during 0000−0600 LST and 24-h cumulative rainfall of 436 mm were observed at Wuhan, Hubei Province; and 24-h cumulative rainfall of 993.9 mm occurred at Huanggang, Hubei Province. The accumulated rainfall amounts at these three stations all broke their historical records since the establishment of these stations. In addition, several extreme weather events occurred during the 2020 mei-yu season. For example, EF2 tornadoes struck Yangzhou, Jiangsu Province at 1400 LST 12 June and Jingmen in Hubei Province at 1900 LST 27 June.
Order Period Heavy rainfall area Maximum precipitation and occurrence site 1 8−11 June Wujiang River, Dongting Lake, Poyang Lake, Hanjiang River Sile, Hunan, 252.1 mm 2 11−14 June Both banks of the top and lower reaches of the Yangtze River Gao Jiana, Hunan, 353 mm 3 15−18 June Jianglingjiang River, Wujiang River, and the lower reaches of the Yangtze River Changping, Sichuan, 333 mm 4 20−25 June Hanjiang River, Wujiang River, Poyang Lake and the lower reaches of the Yangtze River Shendu, Hunan, 403.5 mm 5 26−30 June The top and lower reaches of the Yangtze River Huaban, Sichuan, 906.1 mm 6 1−4 July The top and lower reaches of Jinsha River, Wujiang River, Poyang Lake and the top reaches of Yangtze River Yutang, Jiangxi, 311.2 mm 7 4−8 July The top and lower reaches of the Yangtze River Bahe, Hubei, 993.9 mm 8 9−12 July Jialing River, Wujiang River Basin, Dongting Lake, Poyang Lake Yonghe, Jiangxi, 480.9 mm 9 14−17 July The upper and lower reaches of the Yangtze River Hong Chiba, Chongqing, 441.9 mm 10 17−19 July The top and lower reaches of the Yangtze River Sungang, Anhui, 414.8 mm Table 1. Brief summary of the 10 precipitation events during the 2020 mei-yu period in the Yangtze River basin.
These extreme heavy rainfall events have caused severe damage to property, threatened human lives, and disrupted daily life with urban and rural flooding, mountain torrents, landslides, and other geological disasters in many places. For instance, the national college entrance exam in Shexian of Anhui Province was delayed due to severe flooding. At 0000 LST 12 July, the water level of Poyang Lake reached 22.53 m, exceeding the 1998 water level and breaking the historical record since the availability of hydrological observations.
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The anomalous large-scale synoptic and dynamic patterns are one of the main factors that led to the extremely heavy rainfall events during the 2020 mei-yu season. The western Pacific subtropical high (WPSH) ridge line was located 10° west of its normal position and was stronger than normal years, as illustrated in Figs. 2a and b. The southwesterlies in the middle and lower troposphere (i.e., the low-level jet, LLJ) were much stronger than in normal years over the southern part of the Yangtze River (Figs. 2a and b). Significant convergences of both wind direction and wind speed developed over the middle and lower reaches of the Yangtze River. Huge amounts of water vapor were transported by the LLJ from the southwest to the northeast on the west side of the WPSH and converged with the dry and cold air masses transported from the north over the middle and lower reaches of the Yangtze River. The strong convergence of water vapor flux in this region provided anomalously high moisture content (Figs. 2c and d) and eventually resulted in extremely severe rainstorms during the 2020 mei-yu period.
Figure 2. (a) 500-hPa mean geopotential height (GH, contours), 850-hPa mean winds (vectors) and (c) integrated moisture flux (vectors and shading) derived from NCEP reanalysis during the 2020 mei-yu season (8 June to 20 July 2020) and (b, d) their anomalies (relative to the climatological mean for the period 1981−2010; shaded). The contours in (c) are the averaged GH for the period 1981−2010. The blue box in (a) is the IMFRE-II field region.
1.1. Stronger rainfall intensity and larger areal coverage over a longer period
1.2. More precipitation events with extremely heavy rainfall
1.3. Anomalous large-scale synoptic and dynamic patterns
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IMFRE-II was organized by the Wuhan Institute of Heavy Rainfall, China Meteorological Administration, with joint efforts from the University of Arizona, Georgia Institute of Technology, University of Science and Technology of China, Nanjing University, Jiangsu Meteorological Bureau, Anhui Institute of Meteorological Sciences, Shanxi Weather Modification Office, and Hubei Meteorological Bureau. There are seven surface sites in the middle and lower reaches of the Yangtze River and their locations are shown in Figs. 3 and 4. Four surface sites are located in the middle reaches of the Yangtze River (Zone I in Fig. 4) and the other three sites are located in the lower reaches (Zone II in Fig. 4). The ground-based remote sensors and instruments at each surface site, and the aircraft probes, are introduced as follows.
Figure 3. Schematic diagram showing seven surface sites, satellite and aircraft observations during the IMFRE-Ⅱ field campaign during the period June−July 2020 over the middle and lower reaches of the Yangtze River. The abbreviations represent the names of the seven surface as follows: ZG, Zigui; XY, Xiangyang; JZ, Jingzhou; XN, Xianning; SX, Shouxian; HF, Hefei; NJ, Nanjing. In addition, YC denotes Yichang airport, where the aircraft takes off and lands.
Figure 4. Layout of the surface sites and the airport during IMFRE-Ⅱ over the middle and lower reaches of the Yangtze River. The squares indicate the locations of the seven surface sites, with the numbers 1−7 representing Zigui, Xiangyang, Jingzhou, Xianning, Shouxian, Hefei, and Jiangning, respectively. The circular marks indicate the locations of precipitation weather radars, the yellow triangle represents Yichang airport, and the yellow rectangle indicates the flight area (over the Jingzhou surface site). The numbers I and II represent observing target areas in the middle and lower reaches of the Yangtze River, respectively.
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The ground-based observations include those obtained from MHROS, with additional observations from traditional weather radars, regular soundings, and surface meteorological variables located at the four surface sites in Hubei Province (Zone I in Fig. 4). IHR MHROS consists of mobile X-POL precipitation radars, millimeter wavelength cloud radars, fixed S-band dual-polarimetric precipitation radars, micro-rain radars, GNSS/MET network, microwave radiometers, radiosonde soundings, wind profiler radars, and Parsivel and 2DVD disdrometers. The other three surface sites in Zone II have similar instruments as IHR MHROS, but not all. The surface remote sensors and instruments at each site used in this study are listed in Table 2.
Location Geographic
coordinatesInstrument Observed variables Operation mode Range Resolution Zigui 110°42′39″E,
30°51′48″N,
altitude: 1807 mX-band dual-polarization radar Z, V, W, ZDR, ΦDP, ρHV Volume scan 150 km Radial: 150 m
Temporal: 6 min2-D video disdrometer Raindrop spectrum, falling speed,
shape, rain intensity, etc.High-speed camera observation 100 × 100 mm Horizontal: 0.19 mm
Vertical: 0.2 mm (< 10 m s−1)
Temporal: 1 minMicro-rain radar Profiles of raindrop spectrum, terminal
velocity, intensity, etc.Zenith pointing 0−24 km Vertical: 100 m
Temporal:1 minXiangyang 112°4′48′′E, 32°N,
altitude: 163.4 mLaser ceilometer Cloud base height Zenith pointing 0−12 km Vertical: 15 m Ka-band cloud radar Z, V, W, LDR Zenith pointing 0.03−15 km Vertical: 30 m
Temporal: 5 sMicro-rain radar Profiles of raindrop spectrum, terminal
velocity, intensity, etc.Zenith pointing 0−3 km Vertical: 100 m
Temporal: 1 minJingzhou 112°26′23″E,
30°06′39″N,
altitude: 25 mMicrowave radiometer Cloud LWP and PWV. Zenith pointing 0−10 km Temporal: 2 min Micro-rain radar Profiles of raindrop spectrum, terminal
velocity, intensity, etc.Zenith pointing 0−6 km Vertical: 50 m
Temporal: 1 minLidar Wind profile, turbulence intensity, aerosol
backscattering intensity, extinction
coefficient, cloud base height, PBL heightZenith pointing 0−6 km Vertical: 30 m
Temporal:10 sTropospheric wind profiler radar Profiles of horizontal wind speed and direction,
vertical wind velocity, $C_n^2$Zenith pointing 0−14 km Vertical: 120−480 m
Temporal: 6 min2-D video disdrometer Raindrop spectrum, falling speed, shape, rain intensity, etc. High-speed camera observation 100 × 100 mm Horizontal: 0.19 mm
Vertical: 0.2 mm (< 10 m s−1)
Temporal: 1 minKa-band cloud radar Z, V, W, LDR Zenith pointing 0.01−20 km Vertical: 10 m
Temporal: 5 sJingzhou S-band dual-polarization radar Z, V, W, ZDR, ΦDP, ρHV Volume scan 230 km Radial: 150 m
Temporal: 6 minJianli X-band dual-polarization radar Z, V, W, ZDR, ΦDP, ρHV Volume scan 150 km Radial: 75 m
Temporal: 6 minXianning 114.3°E, 29.8°N,
altitude: 86 mMicrowave radiometer Cloud LWP and PWV Zenith pointing 0−10 km Temporal: 2 min Ka-band cloud radar Z, V, W, LDR Zenith pointing 0.01−20 km Vertical: 10 m
Temporal: 5 sMicro-rain radar Profiles of raindrop spectrum,
terminal velocity, intensityZenith pointing 0−3 km Vertical: 100 m
Temporal: 1 minDisdrometer Raindrop spectrum, falling speed,
intensity, etc.High resolution observation Ground observation Temporal: 1 min Tropospheric wind profiler radar Profiles of horizontal wind speed and direction,
vertical wind velocity, $C_n^2$Zenith pointing 0−16 km Vertical: 120−480 m
Temporal: 6 minLaser ceilometer Cloud base height Zenith pointing 0−15 km Shouxian 116°47′15′′E,
32°26′9′′N,
altitude: 25.7 mMicrowave radiometer Cloud LWP and PWV Zenith pointing 0−10 km Temporal: 2 min Tropospheric wind profiler radar Profiles of horizontal wind speed
and direction, vertical wind
velocity, $C_n^2$Zenith pointing 0−6 km Vertical: 120−480 m
Temporal: 6 minC-band radar Z, V, W, LDR Zenith pointing 0−15 km Vertical: 30 m
Temporal: 3 sX-band dual-polarization radar Z, V, W, ZDR, ΦDP, ρHV VOL 0−24 km Temporal: 1 min Micro-rain radar Profiles of raindrop spectrum, terminal
velocity, intensity, etc.Zenith pointing 0−3.1 km Vertical: 100 m
Temporal: 1 minHefei 117°03′26′′E
31°57′20′′N,
altitude: 49.8 mMicrowave radiometer Cloud LWP and PWV Zenith pointing 0−10 km Temporal: 3 min Boundary layer wind profiler radar Profiles of horizontal wind speed and direction, vertical wind velocity, $C_n^2$ Zenith pointing Above 60 m−6 km Vertical: 50−200 m
Temporal: 6 minDisdrometer Raindrop spectrum, rain intensity Zenith pointing 0.001− 1200 mm h−1 1 min Ka-band cloud radar Z, V, W, LDR Zenith pointing 0−20 km 6 s, 30 m Aerosol/water vapor lidar Water vapor mixing ratio Zenith pointing > 2 km (in daytime)
> 10 km (at night)7.5−300 m
1−30 minJiangning 118°53′58″E,
31°55′54″N,
altitude: 39 m2-D video disdrometer Raindrop spectrum, shape, rain intensity High-speed camera observation 100 × 100 mm Horizontal: 0.19 mm
Vertical: 0.2 mm (< 10 m s−1)
Temporal: 1 minMicro-rain radar Profiles of raindrop spectrum, terminal velocity, intensity, etc. High-speed camera observation 0−3 km Vertical: 100 m
Temporal: 1 minNote: that Z = reflectivity factor; V = Doppler radial velocity; W = Doppler spectrum width; ZDR = differential reflectivity; ΦDP = differential phase shift; ρHV = correlation coefficient; LDR = linear depolarization ratio; $C_n^2$ = structure constant of atmospheric refractive index; LWP = liquid water path; PWV = precipitable water vapor; PBL = planetary boundary layer. Table 2. List of surface sites and their corresponding sensors and instruments used in IMFRE-II
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The KA350 aircraft participating in the IMFRE-II field campaign is equipped with the following sensors: cloud particle probe (CDP), cloud imaging probe (CIP), and precipitation imaging probe (PIP) for measuring cloud and precipitation droplet size distributions; PCASP and CCN-200 for measuring aerosol properties and cloud condensation nuclei; Ka-band cloud radar for measuring cloud and light precipitation with a total of 17 flight hours during IMFRE-II (Table 3). The aircraft was based out of the Yichang Three Gorges International Airport (denoted by the yellow triangle in Fig. 4), which is 104 km away from the Jingzhou surface site. The flight area is a grid box of 10 km × 10 km centered at the Jingzhou surface site (site 3 and yellow rectangular area in Fig. 4). Figure 5 illustrates some probes and cloud radar aboard the KA350 aircraft.
Name Detecting range (μm) Number of channels Cloud parameters CDP (cloud particle probe) 2−50 30 Cloud droplets, ice crystals CIP (cloud imaging probe) 12.5−1562.5 62 Large cloud droplets, snow and ice crystals PIP (precipitation imaging probe) 50−6250 62 Precipitation particles, sleet, snow and ice crystals PCASP 0.095−3.1 30 Aerosols CCN-100 0.25−10.25 20 Cloud condensation nuclei AIMMS Temperature, pressure, humidity, winds, etc. KPR (Ka-band cloud radar) −35 to 35 dBZ Cloud, light precipitation WCM (total water content) 10−4 to 10 g m−3 Clouds and precipitation Table 3. KingAir probes and Ka-band cloud radar.
The KA350 aircraft flew primarily in horizontal flight patterns at different altitudes with one to two vertical flights. Specifically, after taking off, the aircraft ascended to the cloud top or a maximum height of 7300 m and then circled around over the targeted area. It maintained a steady, horizontal flight pattern at the highest altitude and then descended 300 m for another horizontal circular flight. The aircraft repeatedly passed through the clouds at different altitudes until it reached a minimum altitude of 1500 m and then climbed up to 7300 m.
3.1. Experimental area
3.2. Ground-based observational system
3.3. Aircraft probes and flight plan
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IMFRE-II was conducted over the middle and lower reaches of the Yangtze River during the period 16 June to 19 July 2020. During IMFRE-II, 10 mei-yu frontal systems were observed by different platforms, including both ground-based and spaceborne active and passive remote sensors, KA350 probes, and onboard cloud radar. A large amount of remote sensing observations and in-situ measurements related to heavy rainfall processes and properties during the mei-yu period were obtained. Some highlights from IMFRE-II are presented as follows:
(1) To the best of our knowledge, IMFRE-II is the first field campaign in China to use ground-based, airborne, and spaceborne platforms to obtain comprehensive observations over the middle and lower reaches of the Yangtze River. Seven ground-based observational systems, such as MHROS, with the addition of traditional weather radars, regular soundings, and surface meteorological variables, were used to collect data related to heavy rainfall processes during the mei-yu period. These ground-based observations combined with Fengyun and GPM satellite observations can provide the horizontal and vertical distributions of rainfall processes and properties, which are crucial in studying the spatiotemporal evolution of mei-yu frontal systems from west to east over the middle and lower reaches of the Yangtze River. The microphysical properties of cloud and precipitation were observed and retrieved from the active and passive ground-based remote sensors and instruments, including different wavelengths of radars, microwave radiometers, and Parsivel and 2DVD disdrometers.
(2) Seven aircraft flights were successfully carried out with a total of 17 hours and the spectra of ice particles, cloud droplets, and raindrops at different altitudes were obtained. Of the seven flights, two were in moderate rain, two were in light rain, and three flew through cloudy and clear-sky conditions. As illustrated in Fig. 6 and listed in Table 4, the KA350 aircraft flew from ~1500 m to 7300 m and collected the spectra of ice particles, cloud droplets, and raindrops in clouds at different altitudes. Figure 7 shows an example of Ka-band cloud radar reflectivity onboard the KA350 aircraft and their associated cloud and rain drop size distributions (DSDs) at different altitudes on 21 June 2020 during IMFRE-II. The vertical distributions of radar reflectivity and DSDs have demonstrated a typical warm rain process in a mei-yu precipitation event. Cloud droplets normally grow with height through the condensational process in updrafts and become rain-sized drops through the collision−coalescence and autoconversion processes near the cloud top. These raindrops fall from near the cloud top where the gravitational force exceeds the buoyancy force and grow further by collecting cloud droplets and small raindrops through the collision−coalescence process. As raindrops fall, the spectra of the DSD become wider and the accretion process becomes increasingly important, and eventually some of the raindrops become larger raindrops down to the surface. These in-situ measurements, as well as the onboard cloud radar measurements, will provide a “cloud truth” to validate the ground-based and satellite retrieved cloud and precipitation properties, and quantitatively estimate their retrieval uncertainties. These integrative datasets provide a solid basis for the development of a warm (and/or cold) rain conceptual model in order to better understand the cloud-to-rain conversion and accretion processes in mei-yu precipitation events.
Order Date Measurement
period (LST)Minimum flight
altitude (m)Maximum flight
altitude (m)Freezing level
altitude (m)Weather 1 18 June 1040−1240 1499 7335 5420 Rainy 2 21 June 1845−1955 3075 7318 5782 Rainy 3 28 June 1510−1705 1463 6326 5668 Rainy 4 1 July 0940−1140 1511 7326 5426 Cloudy 5 2 July 1825−2025 1480 7308 5355 Clear sky 6 4 July 1050−1205 4723 7339 5382 Rainy 7 4 July 1640−1758 1492 7331 5638 Clear sky Table 4. Summary of the aircraft flights during IMPFRE-II.
Figure 7. Left-hand column: Ka-band (35.64 GHz) cloud radar reflectivity onboard the KingAir aircraft during four time periods at different altitudes with a descending mode on 21 June 2020 over the Jingzhou surface site during IMFRE-II. Right-hand column: The corresponding cloud and rain drop size distributions (DSDs) from the merged product of CDP (2−50 µm), CIP (25−1550 µm) and PIP (100−6200 µm) during the four times and altitudes.
Acknowledgements. The IMFRE-II field campaign was primarily supported by the National Natural Science Foundation of China (Grant Nos. 41620104009 and 91637211), the Key Program for International S&T Cooperation Projects of China (Grant No. 2016YFE0109400), and the National Key R&D Program of China (Grant No. 2018YFC1507200).