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# On the Diurnal Cycle of Heavy Rainfall over the Sichuan Basin during 10–18 August 2020

• A sustained heavy rainfall event occurred over the Sichuan basin in southwest China during 10–18 August 2020, showing pronounced diurnal rainfall variations with nighttime peak and afternoon minimum values, except on the first day. Results show that the westward extension of the anomalously strong western Pacific subtropical high was conducive to the maintenance of a southerly low-level jet (LLJ) in and to the southeast of the basin, which favored continuous water vapor transport and abnormally high precipitable water in the basin. The diurnal cycle of rainfall over the basin was closely related to the periodic oscillation of the LLJ in both wind speed and direction that was caused by the combination of inertial oscillation and terrain thermal forcing. The nocturnally enhanced rainfall was produced by moist convection mostly initiated during the evening hours over the southwest part of the basin where high convective available potential energy with moister near-surface moist air was present. The convective initiation took place as cold air from either previous precipitating clouds from the western Sichuan Plateau or a larger-scale northerly flow met a warm and humid current from the south. It was the slantwise lifting of the warm, moist airflow above the cold air, often facilitated by southwest vortices and quasi-geostrophic ascent, that released the convective instability and produced heavy rainfall.
摘要: 2020 年 8 月 10 日至 18 日，中国西南四川盆地发生持续性强降水事件，除第一天外，降水量呈现显著的夜间峰值和下午谷值的特征。累积降水最大中心位于盆地西北部，但夜雨增强首先主要出现在盆地西南部，随后是盆地西北部。研究结果表明，异常强的西太平洋副热带高压西伸，有利于盆地东南侧偏南低空急流的维持，从而有利于持续的水汽输送和盆地异常高的可降水量分布。盆地降水日变化与低空急流风速和风向的周期性变化密切相关，这种周期性变化由惯性振荡和地形热力强迫共同引起。夜间增强的降水大多由盆地西南部新生的湿对流带来。夜间盆地西南部大气具有高的对流有效位能和更湿的近地层空气，由高原西移到盆地的降水云有关的冷空气/偏北气流带来的冷空气与从南边而来的暖空气相遇有利于对流的触发。暖湿气流在冷空气之上斜升，同时有西南涡和准地转抬升的助力，促进了对流不稳定能量的释放和强降水的产生
• Figure 1.  The 8-day accumulated rainfall amount (shaded, mm) from 1800 LST 10 August to 1800 LST 18 August 2020, with black solid diamonds denoting stations recording rainfall greater than 1000 mm. The red rectangle denotes the target region of this study. Colored curves denote terrain heights (km). The names and locations of three sounding stations are shown by magenta crosses. The names of major mountains and other relevant locations appear in black.

Figure 2.  Time series of the regionally averaged hourly rainfall rates (black lines) and the numbers of stations with hourly rainfall rates > 20 mm h−1 (green bars) and 60 mm h−1 (red bars), respectively, over the target region during 10–18 August 2020. There are a total of about 5400 rain gauge stations in the target region. Red and blue arrows indicate the trends of hourly rainfall rates after 2000 LST and around 0800 LST, respectively. The magenta vertical line roughly separates the rainfall episodes of E1 and E2; similarly for the remaining figures.

Figure 3.  Distribution of the averaged (a) 500-hPa and (b) 800-hPa geopotential heights (contoured at 10-gpm intervals) and isotherms (contoured at 1°C intervals), (c) precipitable water (contoured at 5-cm intervals), and (d) 800-hPa specific humidity (contoured at 1 g kg−1 intervals) during the E2 episode. Standardized anomalies with respect to the 30-year mean quantity of the months of August from 1981 to 2010 are shaded. Purple arrows in (b) show the mean horizontal wind > 10 m s−1 at 800 hPa. Green arrows in (d) show the moisture flux anomalies at 800 hPa (g s−1 cm−1 hPa−1). The contours of 5850, 5880 and 1980 gpm are highlighted in purple in (a) and (b). Blue contours in (d) denote specific humidity of 14.5 g kg−1. Gray shading represents terrain higher than (a) 5 km and (b, c, d) 2 km. The red rectangles indicate the target region of this study. The blue rectangle in (b) is used in Fig. 10.

Figure 4.  Daily surface maps at 2000 LST from 10–17 August obtained from the National Meteorological Center of China Meteorological Administration. Time in form of ddhh (LST) is shown in the left-bottom corners. The red rectangles denote the target region of this study.

Figure 5.  The total 3-hourly accumulated rainfall (mm) during the E2 episode. Time period in the form of hh–hh (LST) is shown in the upper-left corners in each panel. Curves denote terrain heights (km). Red ellipses denote the regions with 3-hourly rainfall amounts exceeding that in the previous 3 hours.

Figure 6.  Three-hourly averaged 800-hPa wind (black vectors) and its deviation (red vectors) and divergence (blue shading, 10−4 s−1), and 900-hPa ${\theta }_{\mathrm{e}\mathrm{p}}$ (purple-contoured at 2-K intervals) during the E2 episode. Time period in the form of hh–hh (LST) is shown in the upper-left corner in each panel. Isotachs of 10 and 14 m s−1 are shown by solid and dashed black contours, respectively. Gray shading and curved lines represent terrain higher than 2 km and terrain of 1 km elevation, respectively. Note the ${\theta }_{\mathrm{e}\mathrm{p}}$ over the region with terrain height of greater than 1 km are fake.

Figure 7.  Time series of the vertical distribution of relative vorticity (10−5 s−1) averaged over the target region with terrain elevations of less than 700 m.

Figure 8.  Vertical cross sections of the averaged moisture flux (g s−1 cm−1 hPa−1) during the E2 episode across the (a) western, (b) southern, (c) eastern, and (d) northern boundaries of the target region shown in Fig. 1. Gray shadings represent terrain. Time series are shown of (e) the averaged moisture flux across each boundary (107 kg s−1), and (f) the net moisture flux through the four boundaries (107 kg s−1) from 10 to 18 August 2020.

Figure 9.  Time series of the horizontal wind speed at 800 hPa (shaded, m s−1) averaged over 105°–108°E during 10 to 18 August 2020. Deviations from the temporal means during E2 are shown as vectors (m s−1). The white line denotes the southern border of the target region.

Figure 10.  Time series of area-averaged (25°–28°N, 107°–110°E; see the location in Fig. 3b) geostrophic wind speed (${V}_{\mathrm{g}}$, ${\mathrm{m}\;\mathrm{s}}^{-1}$) and horizontal wind speed ($V$, ${\mathrm{m}\mathrm{ }\mathrm{s}}^{-1}$) at 1 km above the ground.

Figure 11.  Zonal cross sections of the deviation temperature (shaded, K), deviation wind [composite of zonal wind (${\mathrm{m}\;\mathrm{s}}^{-1}$) and vertical wind (${\mathrm{c}\mathrm{m}\;\mathrm{s}}^{-1}$)], and deviation vertical velocity (contour, ${\mathrm{c}\mathrm{m}\;\mathrm{s}}^{-1}$) averaged between 26.5°‒27.5°N at (a) 1400, (b) 2000, (c) 0200, and (d) 0800 LST during the E2 episode. Gray shadings denote terrain along 26.5°N.

Figure 12.  Time series of the meridionally averaged TBB (°C) between 28°N and 33°N from 10 to 18 August 2020. The two vertical red lines indicate the western and eastern boundaries of the target region.

Figure 13.  Distribution of TBB (°C) at selected times. Time in the form of ddhh (LST) is shown above each panel. Three times were selected for each daily cycle, representing the time immediately before the convection in the target region was triggered or strengthened, when it was triggered or strengthened, and when the cloud was the deepest during its lifetime. The solid ellipses denote the positions of newly initiated convection. Light beige shadings represent terrain above 1 km. The blue rectangles in the second and fifth columns are used to plot the cross sections shown in Fig. 15. The red rectangles denote the target region of this study

Figure 14.  Time series of the (a) convective available potential energy (CAPE, J kg−1) and (b) surface dew-point depression over the Wenjiang, Shapingba and Yibin sounding stations (see their locations in Fig. 1).

Figure 15.  Zonal (a, b, d, e, f) or meridional (c, g, h) vertical cross sections of the in-plane flow vectors [a composite of meridional/zonal wind ($\mathrm{m}\;{\mathrm{s}}^{-1}$) and veritical wind ($\mathrm{c}\mathrm{m}\;{\mathrm{s}}^{-1}$)], ${\theta }_{\mathrm{e}\mathrm{p}}$ (red-contoured at 2-K intervals), relative vorticity (purple contours of 1, 2 and 3 × 10−4 s−1), and divergence of horizontal wind (blue shading, 10−4 s−1) taken along the blue rectangles shown in Fig. 13 at the selected times. Gray shadings represent terrain. Purple double arrowed lines roughly indicate the location of convection initiation or strengthening during the evening hours shown in Fig. 13.

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## Manuscript History

Manuscript revised: 12 October 2021
Manuscript accepted: 14 October 2021
###### 通讯作者: 陈斌, bchen63@163.com
• 1.

沈阳化工大学材料科学与工程学院 沈阳 110142

## On the Diurnal Cycle of Heavy Rainfall over the Sichuan Basin during 10–18 August 2020

###### Corresponding author: Yali LUO, ylluo@cma.gov.cn; yali.luo@qq.com;
• 1. State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Beijing 100081, China
• 2. Department of Atmospheric and Oceanic Science, University of Maryland, College Park, College Park, Maryland 20742, USA

Abstract: A sustained heavy rainfall event occurred over the Sichuan basin in southwest China during 10–18 August 2020, showing pronounced diurnal rainfall variations with nighttime peak and afternoon minimum values, except on the first day. Results show that the westward extension of the anomalously strong western Pacific subtropical high was conducive to the maintenance of a southerly low-level jet (LLJ) in and to the southeast of the basin, which favored continuous water vapor transport and abnormally high precipitable water in the basin. The diurnal cycle of rainfall over the basin was closely related to the periodic oscillation of the LLJ in both wind speed and direction that was caused by the combination of inertial oscillation and terrain thermal forcing. The nocturnally enhanced rainfall was produced by moist convection mostly initiated during the evening hours over the southwest part of the basin where high convective available potential energy with moister near-surface moist air was present. The convective initiation took place as cold air from either previous precipitating clouds from the western Sichuan Plateau or a larger-scale northerly flow met a warm and humid current from the south. It was the slantwise lifting of the warm, moist airflow above the cold air, often facilitated by southwest vortices and quasi-geostrophic ascent, that released the convective instability and produced heavy rainfall.

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