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Modulation Effect of MJO on the Precipitation over Guangdong and Its Link with the Direct Impact System in June
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摘要: 利用1979—2008年广东省86个测站逐日降水资料及NCEP-DOE第2套分析资料等，提出影响广东500 hPa环流系统的判别方法，分析6月赤道MJO (季节内振荡) 活动对广东降水的调制作用随中低纬度环流型的变化。结果表明：强MJO第3位相广东出现强降水的概率最高，是8个位相中唯一强降水等级出现日数超过弱降水日数的位相。在直接影响广东的5种500 hPa环流系统 (包括西风槽、西风浅槽、平直西风或高压边缘、副热带高压、热带低压槽) 中，西风槽类型影响时，赤道MJO对广东降水的调制作用最强，其他环流类型影响时，MJO的调制作用很弱。广东在西风槽影响下，当处于MJO第3位相 (第6位相) 时，降水距平百分率达到最高 (低)。MJO对广东降水的调制作用随中低纬度环流系统的变化，主要是环流系统之间的不同配合导致降水所需的动力上升条件和水汽输送条件的相互配合发生变化造成的。Abstract: A distinguishing method of 500 hPa circulation systems influencing Guangdong is established, and the changes in modulation effect of MJO on the precipitation over Guangdong with the low latitude 500 hPa circulation system in June is analyzed using 30-year (1979—2008) 86-station observational daily precipitation of Guangdong and daily atmospheric data from NCEP-DOE Reanalysis 2. It is found that each phase of strong MJO may correspond to various levels of precipitation, of which the third phase has the highest probability of heavy precipitation (49.3%). The third phase is the only phase among 8 phases of MJO that strong precipitation days outnumber weak precipitation days. The 500 hPa circulation systems in low-latitude impacting Guangdong directly mainly include westerly trough, shallow westerly trough, flat westerly or border of subtropical high, subtropical high and tropical low or trough. The most strong modulation effect of MJO on the precipitation over Guangdong occurs in the case impacted by westerly trough, while the modulation effect is quite weak in the other cases. The precipitation anomaly percentage averaged over Guangdong is peak (valley) in the third phase (the sixth phase) under the case impacted by westerly trough. The changing in modulation effect of MJO on the precipitation over Guangdong with the low latitude circulation system is substantiality due to the changing of dynamic ascending motion and the water vapor transport, both of which are necessary for rainfall. In the westerly trough case, the subtropical high is strong and westward, water vapor transport to Guangdong increases significantly, and the dynamic ascending motion and high level divergence conditions are also enhanced, which results in Guangdong precipitation strengthening in the third phase of MJO. But in the sixth phase of MJO, the subtropical high is weak and eastward, water vapor transport to Guangdong decreases significantly although the dynamic ascending motion is enhanced, leading to weaker precipitation than that in the third phase of MJO. However, in the case without westerly trough, although water vapor transport to Guangdong increases in the third phase of MJO, the geopotential height anomaly is positive over East Asia and Western Pacific, therefore the dynamic ascending motion is reduced, so the precipitation is not much stronger than the other MJO phases. Therefore, the modulation of MJO on the precipitation over Guangdong needs the cooperation of westerly trough. The direct impact of westerly trough and remote correlation effect of the third phase of MJO are good prediction indicators for large domain heavy precipitation over Guangdong.
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图 1 6月气候平均和影响广东5种环流系统的500 hPa位势高度场合成 (单位：gpm)
(a) 气候平均, (b) 平直西风或副热带高压边缘, (c) 西风浅槽, (d) 西风槽, (e) 热带低压槽, (f) 副热带高压
Fig. 1 Composite 500 hPa geopotential height for climatology and five-circulation types influencing Guangdong in June (unit: gpm)
(a) climatic state, (b) flat westerly or border of subtropical high, (c) shallow westerly trough, (d) westerly trough, (e) tropical low or trough, (f) subtropical high
图 2 强MJO事件第3位相出现强降水 (a) 和弱降水 (b) 的500 hPa位势高度场合成 (单位: dagpm), 出现强降水 (c) 和弱降水 (d) 的500 hPa位势高度场距平合成 (单位:dagpm;阴影区表示达到0.05显著性水平)，出现强降水 (e) 和弱降水 (f) 的850 hPa水汽通量 (矢量，单位：10 kg·hPa-1·m-1·s-1) 及其标量 (填色区，单位：10 kg·hPa-1·m-1·s-1), 水汽通量散度 (等值线，只显示散度不大于0的等值线, 单位: 10-7 kg·hPa-1·m-2·s-1)距平合成
Fig. 2 Composite 500 hPa geopotential height (unit: dagpm) for strong rainfall (a) and weak rainfall (b), 500 hPa geopotential height anomaly (unit: dagpm； the shaded indicates passing test of 0.05 level) for strong rainfall (c) and weak rainfall (d) in the third phase of strong MJO, composite anomalous fields of 850 hPa moisture flux (vector, unit: 10 kg·hPa-1·m-1·s-1), its scalar quantity (the shaded, unit: 10 kg·hPa-1·m-1·s-1) and moisture flux divergence (contour: equal to and less than zero, unit: 10-7 kg·hPa-1·m-2·s-1) for strong rainfall (e) and weak rainfall (f) in the third phase of strong MJO
图 3 500 hPa不同环流型条件下广东平均日降水距平百分率随强MJO位相的变化
Fig. 3 Variation of daily precipitation anomaly percentage averaged over Guangdong with strong MJO phases for different 500 hPa circulation types
图 4 西风槽情形下强MJO各位相广东降水距平百分率分布 (括号内为合成分析日数)
Fig. 4 Composite distributions of daily precipitation anomaly percentage over Guangdong in strong MJO phases in the westerly trough case (the number in brackets indicates the number of days of composite analysis)
图 5 西风槽情形下强MJO第3位相及第6位相的500 hPa位势高度场 (单位：dagpm)、位势高度场距平 (单位：dagpm) 和垂直速度距平 (单位：Pa·s-1;阴影区表示达到0.05显著性水平)
(a) 第3位相高度场，(b) 第6位相高度场，(c) 第3位相高度场距平，(d) 第6位相高度场距平，(e) 第3位相垂直速度距平，(f) 第6位相垂直速度距平
Fig. 5 Composite fields of 500 hPa geopotential height (unit: dagpm), anomaly of 500 hPa geopotential height (unit: dagpm) and anomaly of 500 hPa vertical speed (unit: Pa·s-1) in the third phase and the sixth phase of strong MJO during the westerly trough case (the shaded indicates passing the test of 0.05 level)
(a) geopotential height in the third phase, (b) geopotential height in the sixth phase, (c) anomaly of geopotential height in the third phase, (d) anomaly of geopotential height in the sixth phase, (e) anomaly of pressure vertical speed in the third phase, (f) anomaly of pressure vertical speed in the sixth phase
图 6 西风槽与非西风槽情形下强MJO第3位相与第6位相的850 hPa位势高度场距平 (等值线, 单位: dagpm), 水汽通量距平 (矢量，单位：10 kg·hPa-1·m-1·s-1) 及其标量场距平 (填色区，单位：10 kg·hPa-1·m-1·s-1)
(a) 西风槽, 第3位相，(b) 西风槽, 第6位相，(c) 非西风槽, 第3位相，(d) 非西风槽, 第6位相
Fig. 6 Composite anomalous fields of 850 hPa geopotential height (contour, unit: dagpm), water vapor flux (vector, unit: 10 kg·hPa-1·m-1·s-1) and its scalar anomaly (the shaded, unit: 10 kg·hPa-1·m-1·s-1) in the third phase and the sixth phase of strong MJO with the westerly trough and without westerly trough
(a) westerly trough, the third phase, (b) westerly trough, the sixth phase, (c) without westerly trough, the third phase, (d) without westerly trough, the sixth phase
表 1 强MJO事件8个位相各等级降水出现日数
Table 1 The number of days of different precipitation grade in 8 phases of strong MJO
强MJO位相 弱降水日数/d 平均值附近降水日数/d 强降水日数/d 强降水所占百分比/% 第1位相 41 12 30 36.1 第2位相 43 8 23 31.1 第3位相 29 9 37 49.3 第4位相 41 8 19 27.9 第5位相 41 4 18 28.6 第6位相 42 6 20 29.4 第7位相 8 2 7 41.2 第8位相 27 6 19 36.5 
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