Vol.35, NO.1, 2024
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Chen Xunlai, Xu Ting, Wang Rui, et al. Fine observation characteristics and causes of '9·7' extreme heavy rainstorm over Pearl River Delta, China. J Appl Meteor Sci, 2024, 35(1): 1-16. DOI:  10.11898/1001-7313.20240101.
Citation: Chen Xunlai, Xu Ting, Wang Rui, et al. Fine observation characteristics and causes of "9·7" extreme heavy rainstorm over Pearl River Delta, China. J Appl Meteor Sci, 2024, 35(1): 1-16. DOI:  10.11898/1001-7313.20240101.
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Fine Observation Characteristics and Causes of "9·7" Extreme Heavy Rainstorm over Pearl River Delta, China

DOI: 10.11898/1001-7313.20240101
  • 1.

    Shenzhen Meteorological Service, Shenzhen 518040

  • 2.

    Shenzhen Key Laboratory of Severe Weather in South China, Shenzhen 518040

  • 3.

    Shenzhen National Climate Observatory, Shenzhen 518040

  • Received Date: 2023-11-13
  • Rev Recd Date: 2023-12-19
  • Publish Date: 2024-01-31
    Abstract
  • On 7-8 September 2023, the Pearl River Delta experiences an extremely heavy rainstorm, known as "9·7" extreme rainstorm. Multi-source data are comprehensively utilized, including high-density automatic weather station data, sounding data, wind profiler data, Doppler radar data, high-resolution measurements from FY-4B satellite, and the fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) atmospheric reanalysis (ERA5), to analyze the fine precipitation characteristics and causes of this case. Results indicate that the extremely heavy rainstorm is characterized by area of coverage, wide coverage area, long duration, and substantial rainfall. The extremely heavy rainstorm is caused by the combined interaction of 200 hPa upper-level divergence, the middle-level weak guiding flow, the lower-level southwest monsoon, and the residual vortex of Typhoon Haikui (2311). It is generated by the long-term horizontal scale of about 100 km banded mesoscale convective complex, with significant train effect and warm cloud precipitation characteristics. The centroid of intense echoes with an intensity greater than 45 dBZ is located below 4 km during the most intense precipitation stage, while intense echoes with an intensity greater than 30 dBZ can last for up to 21 hours in Shenzhen. In terms of raindrop distribution characteristics extreme rainfall is mainly caused by a high density of small and medium-sized raindrops. When the rainfall intensity exceeds 20 mm·h-1, the size of raindrop particles increases, but the numerical concentration significantly decreases. Results in an increase in raindrop size but a decrease in the number of concentrations. The duration, intensity, and area of extreme rainstorms have a strong correlation with the fluctuation of the low-level jet in the boundary layer and the location of the core area of the jet. Heavy rainfall occurs within 1-2 hours after a rapid strengthening of the low-level jet index. After the low-level jet index decreases, the intensity of heavy precipitation diminishes. Variations in the low-level jet and low-level jet index have significant implications for heavy rainfall. The prolonged presence of Typhoon Haikui residual vortex in the Pearl River Delta is the synoptic-scale cause of this extremely heavy rainstorm. The residence time of the lingering vortex exceeds 16 hours. During that time, the deep boundary layer low-level jet continuously transfers warm water vapor to the lingering vortex. Simultaneously, the water vapor from the western Pacific, carried by the northeast airflow of Typhoon Yunyeung, and the southwest monsoon water vapor transfers through the Bay of Bengal, Indochina Peninsula, and the South China Sea, ultimately results in the formation of a stable mesoscale convergence line near the Pearl River Delta, causing an extremely heavy rainstorm.
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  • Fig. 1  Rainfall(unit:mm)(a) and maximum rainfall intensity(unit:mm·h-1)(b) from 1600 BT 7 Sep to 1600 BT 8 Sep in 2023

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    Fig. 2  Hourly rainfall of typical stations from 1700 BT 7 Sep to 1600 BT 8 Sep in 2023

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    Fig. 3  Wind(the barb), wind speed(the shaded) and geopotential height(the red contour, unit:dagpm)from 5 Sep to 8 Sep in 2023

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    Fig. 4  FY-4B TBB from 7 Sep to 8 Sep in 2023

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    Fig. 5  Radar combination reflectivity of Guangdong from 7 Sep to 8 Sep in 2023

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    Fig. 6  Time-height section of radar combination reflectivity(the shaded) and rainfall intensity(the black solid line)at Xiaowutong Station of Shenzhen from 1800 BT 7 Sep to 2000 BT 8 Sep in 2023

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    Fig. 7  Raindrop number concentration(the shaded), raindrop diameter(height of the shaded)and rainfall intensity(the black solid line) at Shiyan Station of Shenzhen from 1600 BT 7 Sep to 1600 BT 8 Sep in 2023

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    Fig. 8  Scatter plots of rainfall intensity with raindrop mass-weighted average diameter(a)and standardized number concentration(b) at Shiyan Station of Shenzhen from 1600 BT 7 Sep to 1600 BT 8 Sep in 2023

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    Fig. 9  Wind at 850 hPa(the red solid line denotes converging line) from 7 Sep to 8 Sep in 2023

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    Fig. 10  Wind profile of radar(the shaded denotes wind speed no less than 12 m·s-1) at Longgang Station of Shengzhen(a) and low jet index and hourly rainfall(b) from 7 Sep to 8 Sep in 2023

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    Fig. 11  Wind(the vector) and divergence(the shaded denotes less than -2×10-5 s-1)at 975 hPa from 7 Sep to 8 Sep in 2023

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    Fig. 12  Water vapor transport at 925 hPa(the vector, unit:g·cm-1·hPa·s-1)from 7 Sep to 8 Sep in 2023(the shaded denotes water vapor flux)

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    • Received : 2023-11-13
    • Accepted : 2023-12-19
    • Published : 2024-01-31
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