A COMPARISON OF INTRASEASONAL RAINFALL FEATURES OVER THE MIDDLE AND LOWER REACHES OF YANGTZE RIVER DURING SUMMERS OF 1991, 1998 AND 2016
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摘要: 针对长江中下游三个大水年1991、1998和2016年，利用NCEP/NCAR大气环流再分析资料和CMAP降水资料，对比了夏季降水的季节内特征，分析了引起降水季节内变化的大气环流季节内振荡ISO演变及源地。小波分析表明，三年季节内降水周期差异明显，分别为20~30 d、20~40 d和10~20 d。随之，以东亚季风区季节内振荡指数及热带外Rossby波活动通量，诊断了引起三年季节内活动异常的热带和中纬度ISO变率特点。结果显示影响三年季节内降水的ISO差异较大。1991年受到来自印度洋10~30 d和中纬度高层Rossby波10~30 d的ISO共同影响，造成周期为20~30 d的低频降水；1998年ISO来源路径单一，受中北太平洋30~60 d和10~30 d的ISO西传叠加作用，降水表现为20~40 d的振荡；引起2016年季节内降水异常的ISO源地较多，既有来自印度洋向东北传播30~60 d的ISO，又有来自太平洋向西北传播10~30 d的ISO，还有来自热带外10~30 d的ISO，三者在长江中下游汇合，引起降水10~20 d的振荡。研究结果对认识长江中下游夏季集中降水的形成有重要意义。Abstract: By using the NCEP/ NCAR reanalysis data and CMAP daily precipitation data, we compared intraseasonal oscillation (ISO) characteristics of rainfall during the summers of 1991, 1998 and 2016, three summers of the severest floods, over the Middle and Lower reaches of the Yangtze River (MLYR). Then we analyzed the evolution and source of atmospheric ISO. A wavelet analysis shows the difference in the periodicity of intraseasonal rainfall in these three years, which are 20~30 d, 20~40 d and 10~20 d, respectively. One diagnosis based on the boreal summer intraseasonal oscillation (BSISO) indices and the Rossby wave activity indices suggest that: (1) In 1991, two ISO, one from the tropical Indian Ocean and the other from the mid-latitudinal Eurasia, lasted for a period of 10~30 d. The ISO propagated toward northeast and southeast, and merged in the MLYR, causing a 20~30 d fluctuation in rainfall. (2) In 1998, the ISO originated from the central North Pacific. It propagated westward with two characteristic periods of 30~60 d and 10~30 d, leading to a 20~40 d oscillation in the rainfall over the MLYR. (3) In 2016, the origination of ISO was relatively more complicated. The ISO was a combination of those from the tropical Indian Ocean with a period of 30~60d, those from the tropical Pacific with a period of 10~30 d, and those from the mid latitudinal Eurasia with a period of 10~30 d. They propagated simultaneously toward the MLYR, resulting in a 10~20 d oscillation in rainfall.
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图 1 1991(a)、1998(b)和2016(c)年夏季长江中下游十站平均降水的逐日（直方图）及逐候演变（红线）
单位：mm/day。站点为宜昌、常德、长沙、武汉、南昌、安庆、南京、常州、杭州、上海。
图 2 1991、1998和2016年夏季总降水（左列，单位：mm）、降水距平（中列，单位：mm）以及日降水标准差（右列）的分布（单位：mm/d）
红框为所研究的长江中下游地区。
图 3 1991(a、d)、1998(b、e)和2016(c、f)年长江中下游夏季逐日降水距平序列（上排，直方图, 单位：mm）及其小波功率谱（下排，阴影表示通过0.05显著性检验），同时给出滤波后的季节内降水序列（上排红线, 单位：mm，右上角数字代表滤波周期）
图 4 1991 (a)、1998(b)和2016年(c)通过小波分析显著置信度水平为0.05的季节内降水时段（红线）
数字为根据正负一个（疏虚线）和两个（密虚线）标准差划分的八个位相，其中位相3（7）代表降水中断期（活跃期）。
图 5 1991(a、d、g)、1998(b、e、h)和2016(c、f、i)年28~32 °N平均(a、b、c)、110~120 °E平均(d、e、f)、5 °S~5 °N平均(g、h、i)、滤波后OLR异常的时间-经度剖面图（单位：W/m2）
Y轴数字为显著降水时段的循环位相，括号内（外）数字代表第几次循环（降水位相），箭头指代负OLR异常的传播。
图 7 1991（第一列）、1998（第二列）及2016（第三列）年显著降水时段分别经20~30 d、20~40 d及10~20 d滤波后的OLR在8个位相的合成分布（阴影，单位：W/m2）
红线为500 hPa等压面上5 880等位势高度线（单位：gpm）。
图 8 1991（第一列），1998（第二列）和2016（第三列）年不同降水位相200 hPa位势高度异常（等值线，单位：gpm）、水平波活动通量（箭头，单位：m2/s2）和波活动通量散度（阴影，单位：10-5 m/s2）的分布
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