6月MJO对广东降水调制与直接影响系统的联系

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

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。

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 precipitati- on 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-lati- tude 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 substan- tiality 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 signifi- cantly 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 in- creases 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 pre- diction indicators for large domain heavy precipitation over Guangdong.