Effects of Low-frequency Oscillation on the Persistent Extreme Precipitation in Sichuan Basin

图  1  降水合成小波功率谱(实线)

(阴影区表示达到0.05显著性水平，虚线表示边界影响区，横坐标上“0”表示降水开始)

Fig. 1  Composited wavelet power spectrum of persistent extreme precipitation events (the solid line)

(the shaded denotes passing the test of 0.05 level, the dotted line denotes the boundary influence area)

图  2  1981年5—9月四川盆地逐日降水量、15~30 d降水分量及30~60 d降水分量

(下参照线为强降水阈值，上参照线为低频曲线0值)

Fig. 2  Time series of daily precipitation, 15-30-day and 30-60-day precipitation from May to Sep in 1981 in Sichuan Basin

(the lower horizontal line marks the threshold of extreme precipitation, while the upper horizontal line represents the value of 0 for low-frequency components)

图  3  四川盆地持续性降水期间850 hPa 15~30 d低频风场(a)和滤波前风场(b)

(灰色表示达到0.05显著性水平区域，箭头矢量为距平风场，“A”代表反气旋，“C”代表气旋，黑色表示青藏高原)

Fig. 3  15-30-day wind fields(a) and wind field before filtering(b) at 850 hPa during the persistent extreme precipitation period

(the gray denotes passing the test of 0.05 level, "A" and "C" denote anticyclonic and cyclonic anomalies, and the black denotes the Tibetan Plateau)

图  4  四川盆地续性降水期间500 hPa 15~30 d低频高度场(a)和滤波前高度场(b)

(填色表示距平；等值线表示滤波前高度，单位：gpm；黑点表示达到0.05显著性水平区域)

Fig. 4  15-30-day geopotential fields(a) and geopotential fields before filtering(b) at 500 hPa during the persistent extreme precipitation period

(the shaded denotes the geopotential anomaly; the contour denotes the original height, unit:gpm; the black dots denote passing the test of 0.05 level)

图  5  四川盆地降水期间15~30 d低频风场垂直纬度剖面和涡度垂直剖面

(a)沿102°~105°E平均风场(等值线为经向风分量，正值表示南风，负值表示北风；阴影表示纬向风分量; 单位：m·s^-1)，(b)28°~34°N，102°~105°E平均涡度场(单位：10^-6 s^-1)

Fig. 5  Cross-sections of 15-30-day composited wind field and 15-30-day composited vorticity during the period of persistent extreme precipitation events

(a)wind field averaged along 102°-105°E (the contour denotes meridional wind, the positive denotes south wind and the negative denotes north wind; the shaded denotes zonal wind, unit:m·s^-1), (b)vorticity field averaged 28°-34°N, 102°-105°E (unit:10^-6s^-1)

图  6  降水前第30天至降水后第15天850 hPa沿100°E~120°E平均的15~30 d低频风场剖面

(单位：m·s^-1，方框为四川盆地降水区, 下同)
(a)纬向风，(b)经向风

Fig. 6  Cross-section of 15-30-day wind field along 100°-120°E at 850 hPa from 30 days before precipitation to 15 days after precipitation

(unit:m·s^-1, the box denotes the target area of Sichuan Basin, hereinafter)
(a)zonal wind, (b)meridional wind

图  7  降水前第7天至降水后第1天850 hPa合成的15~30 d低频风场演变

(单位：m·s^-1, “A”为关键低频反气旋，“C”为关键低频气旋，黑色代表青藏高原)

Fig. 7  Evolution of 15-30-day wind fields at 850 hPa from 7 days before the precipitation to the first day after onset (unit:m·s^-1, "A" and "C" denote anticyclonic and cyclonic, respectively, and the black denotes the Tibetan Plateau)

图  8  降水前第30天至降水后第15天500 hPa沿80°~100°E平均的15~30 d位势高度(填色)和风场(等值线, 单位：m·s^-1)剖面

(a)位势高度和纬向风，(b)位势高度和经向风

Fig. 8  Cross-section of 15-30-day geopotential height (the shaded) and wind field (the contour, unit:m·s^-1) along 80°-100°E at 500 hPa from 30 days before precipitation to 15 days after precipitation

(a)geopotential height and zonal wind, (b)geopotential height and meridional wind

图  9  降水前第14天至降水后第1天500 hPa合成的15~30 d低频高度场(单位：gpm)

Fig. 9  Evolution of 15-30-day geopotential height at 500 hPa from 14 days before the precipitation to the first day after onset (unit:gpm)