Modulation Effect of MJO on the Precipitation over Guangdong and Its Link with the Direct Impact System in June

Lin Ailan; Li Chunhui; Zheng Bin; Gu Dejun

Vol.24, NO.4, 2013

Article Contents

Article Navigation > Journal of Applied Meteorological Science > 2013 > 24(4): 397-406

Lin Ailan, Li Chunhui, Zheng Bin, et al. Modulation effect of MJO on the precipitation over Guangdong and its link with the direct impact system in June. J Appl Meteor Sci, 2013, 24(4): 397-406.

Citation:

Lin Ailan, Li Chunhui, Zheng Bin, et al. Modulation effect of MJO on the precipitation over Guangdong and its link with the direct impact system in June. J Appl Meteor Sci, 2013, 24(4): 397-406.

Lin Ailan, Li Chunhui, Zheng Bin, et al. Modulation effect of MJO on the precipitation over Guangdong and its link with the direct impact system in June. J Appl Meteor Sci, 2013, 24(4): 397-406.

Citation:

Lin Ailan, Li Chunhui, Zheng Bin, et al. Modulation effect of MJO on the precipitation over Guangdong and its link with the direct impact system in June. J Appl Meteor Sci, 2013, 24(4): 397-406.

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Modulation Effect of MJO on the Precipitation over Guangdong and Its Link with the Direct Impact System in June

Key Laboratory of Regional Numerical Weater Prediction, Institute of Tropical and Marine Meteorology, CMA, Guangzhou 510080

Received Date: 2012-11-22
Rev Recd Date: 2013-05-08
Publish Date: 2013-08-31

Abstract

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.
- MJO;
- precipitation over Guangdong in June;
- low-latitude circulation system;
- westerly trough

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References(44)

Figures and Tables

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

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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

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Fig. 3 Variation of daily precipitation anomaly percentage averaged over Guangdong with strong MJO phases for different 500 hPa circulation types

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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)

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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

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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

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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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