Meiyu in the Yangtze River Delta and its extended-range forecast associated with intraseasonal evolution of atmospheric circulation
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摘要: 基于1981—2020年长江三角洲(简称长三角)地区62个国家基本气象站的逐日降水量资料及NCEP/NCAR全球大气逐日再分析资料,分析了长三角地区梅汛期降水与前期大气环流季节内协同演变的关系,在此基础上利用改进的时空投影方法(STPM)构建了针对该地区梅汛期降水的延伸期预报模型。结果表明:(1)长三角地区梅汛期降水存在显著的10—80 d季节内振荡,且振荡强度有明显的空间差异和年际变化,降水量越大对应的季节内振荡越强。(2)梅汛期降水发生前15—10天大气环流发生季节内调整,热带低频对流活跃并出现经向传播,在西北太平洋、长江流域至黄淮—日本海的对流层低层(高层)激发反气旋(气旋)—气旋(反气旋)—反气旋(气旋)的低频波列,建立起低层辐合、高层辐散的环流配置,长三角地区对流增强;大气低频响应导致南亚高压季节内南北振荡和东西进退明显,西太平洋副热带高压在长三角东南侧稳定维持,上述低、中和高纬度环流的季节内动态协同演变共同促进了该地区梅汛期低频降水的发生。(3)将影响梅汛期降水的前期大气环流季节内动态演变过程作为预报因子,基于STPM方法训练得到长三角梅汛期降水的延伸期逐候预报模型,近10年的独立回报评估显示该模型对梅汛期未来10—25 d降水有较高的预报技巧。Abstract: Based on daily precipitation data obtained from 62 national basic meteorological stations in the Yangtze River Delta (YRD) and the NCEP/NCAR reanalysis dataset from 1981 to 2020, the relationship of intraseasonal precipitation in Meiyu season with the evolution of atmospheric circulation over the YRD is studied. The extended-range precipitation forecast model based on atmospheric circulation evolution is then constructed by using improved Spatial-temporal Projection Method (STPM). 10—80 d intraseasonal oscillation is found in the daily variation of Meiyu precipitation, and the amplitude differs in spatial and temporal scales. The stronger intraseasonal oscillation denotes heavier precipitation. From 15 to 10 d lead, the tropical intraseasonal convections become active with meridional propagations. The anomalously strong convections over the YRD enhance. The intraseasonal anticyclone (cyclone) — cyclone (anticyclone) — anticyclone (cyclone) wave train is triggered in the lower troposphere (upper troposphere), allowing divergence (convergence) to gradually establish. The intraseasonal atmosphere response leads to a stable Western Pacific subtropical high and obvious north-south and east-west oscillations of the South Asian High. All these cooperative intraseasonal changes of atmosphere from different levels and latitudes accelerate the development of precipitation. Based on the dynamic intraseasonal evolution of atmospheric circulation, the extended-range pentadly precipitation forecast model for the Meiyu season is constructed by using STPM. The independent evaluation of historic forecasts shows that the model has useful forecast skills at lead time of 10—25 d in the YRD.
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图 3 1981—2020年长三角地区梅汛期 (a)降水10—80 d季节内分量方差 (单位:mm)、(b)候平均降水量 (单位:mm) 空间分布及 (c) 候降水量与季节内变化分量散点、(d) 季节内振荡强度与总降水量的逐年变化
Figure 3. Spatial distribution in the Meiyu season over the Yangtze River Delta from 1981 to 2020 (a. the variance of 10—80 d intraseasonal precipitation components,unit:mm;b. pentad mean precipitation,unit:mm) and (c) the scatter distribution of pentad mean precipitation and 10—80 d intraseasonal component of precipitation,(d) the annual variation of regional average 10—80 d intraseasonal component of precipitation oscillation intensity and total precipitation
图 4 1981—2010年长三角区域平均的梅汛期逐日降水与前35—0 天的向外长波辐射对流活动 (a) 和700 hPa相对湿度 (b) 季节内分量的时间相关系数空间分布场 (×表示通过95%置信水平的显著性t检验;下标数字1—5分别表示前35、25、15、10、0天)
Figure 4. Temporal correlation coefficient maps of outgoing longwave radiation (a) and 700 hPa relative humidity (b) with the intraseasonal component of daily precipitation averaged over the Yangtze River Delta at lead times of 35 to 0 days (cross areas denote values at the 95% confidence level by t-test;the numbers 1—5 denote lead 35,25,15,10,0 d,respectively)
图 5 1981—2010年长三角区域平均的梅汛期逐日降水与前35—0天的850 hPa (a) 和200 hPa (b) 矢量风场季节内分量的时间相关系数场空间分布 (加粗箭头表示矢量相关系数通过了95%置信水平的显著性t检验;色阶为根据风矢量相关计算的散度相关系数;“A”“C”分别代表反气旋、气旋;下标数字1—5分别表示前35、25、15、10、0天)
Figure 5. Temporal correlation coefficient maps of 850 hPa (a) and 200 hPa (b) winds with the intraseasonal component of daily precipitation over the Yangtze River Delta at lead times of 35 to 0 days (the bold vector arrows indicate the vector correlation coefficients at the 95% confidence level by t-test,and the shaded areas are divergence values calculated based on the zonal and meridional winds correlation coefficients;the "A" "C" stands for anticyclone/cyclone; the numbers 1—5 denote lead 35,25,15,10,0 d,respectively)
图 6 1981—2010年长三角区域平均的梅汛期逐日降水与前35—0天的500 hPa (a) 和200 hPa (b) 位势高度季节内分量的时间相关系数空间分布场 (×表示通过了95%置信水平的显著性t检验;下标数字1—5分别表示前35、25、15、10、0天)
Figure 6. Temporal correlation coefficient maps of 500 hPa (a) and 200 hPa (b) geopotential height with the intraseasonal component of daily precipitation averaged over the Yangtze River Delta at lead times of 35 to 0 days (cross areas denote values at the 95% confidence level by t-test;the numbers 1—5 denote lead 35,25,15,10,0 d,respectively)
图 8 2011—2020年长三角地区梅汛期季节内降水独立回报值和实际观测值的时间相关系数空间分布 (a—f分别为提前10、15、20、25、30、35 d起报,打点区域表示通过了95%置信水平的显著性t检验)
Figure 8. Spatial distributions of temporal correlation coefficient between independent forecasts and observations of intraseasonal precipitation in Meiyu season over the Yangtze River Delta from 2011 to 2020 (a—f are for forecasts at lead times of 10,15,20,25,30,35 d,respectively;dotted areas denote values at the 95% confidence level by t-test)
图 9 季节内振荡偏强年 (a1—f1) 和偏弱年 (a2—f2) 长三角地区梅汛期季节内降水独立回报值和实际观测值的时间相关系数空间分布 (a—f分别为提前10、15、20、25、30、35 d起报,打点区域表示通过了95%置信水平的显著性t检验)
Figure 9. Spatial distributions of temporal correlation coefficient between independent forecasts and observations of intraseasonal precipitation in Meiyu season over the Yangtze River Delta in strong (a) and weak (b) intraseasonal oscillation years (a—f are for forecasts at lead times of 10,15,20,25,30,35 d,respectively;dotted areas denote values at the 95% confidence level by t-test)
表 1 长三角梅汛期降水延伸期预报的大气环流关键区经纬度
Table 1. Key areas of atmospheric circulation for extended-range precipitation forecast in Meiyu season over the Yangtze River Delta
大气环流要素 关键区 大气向外长波辐射(olr) 7.5°—57.5°N,95°—140°E 700 hPa相对湿度(rhum700) 7.5°—47.5°N,95°—140°E 850 hPa纬向风(u850) 5°—42.5°N,107.5°—127.5°E 850 hPa经向风(v850) EQ—45°N,95°—135°E 200 hPa纬向风(u200) 5°—45°N,102.5°—147.5°E 200 hPa经向风(v200) EQ—45°N,100°—145°E 500 hPa高度(h500) 2.5°—42.5°N,100°—135°E 200 hPa高度(h200) 2.5°—45°N,102.5°—127.5°E -
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