Abstract
An abnormal heavy rainfall that occurred on 27 October 2014 in the Sichuan Basin (SB), China, is analyzed. An inverted trough at 850 hPa evolved into a Southwest China Vortex (SWCV), and strong upward motion caused by interaction between the low-level jet (LLJ) at 850 hPa and the upper-level jet (ULJ) at 200 hPa triggered the rainstorm process. Under a large-scale circulation system featuring a westerly trough and subtropical high, there were two cloud bands over the northeast side and south side of the Tibetan Plateau, respectively. Influenced by the eastward-moving trough, the inverted trough, LLJ, and the SWCV, a Mesoscale Convective System (MCS) was generated near the intersection of the two cloud bands, and it was the direct rainstorm system. The MCS strengthened under the situation of the 850 hPa inverted trough, but weakened when the inverted trough evolved to into the SWCV. Eventually, it formed the phenomenon known as “existing vortex without cloud.” Through analysis of the possible reasons why precipitation strengthened (weakened) under the situation of the inverted trough (SWCV), it was found that the strengthening of precipitation was due to a strong tilting updraft in the area of the ULJ and LLJ intersection. On one hand, the upward motion was related to the vorticity advection variation with height and the low-level warm advection forcing; while on the other hand, the dew-point front near the LLJ also played a lifting role in the upward flow of the lower-layer vertical circulation. Meanwhile, the LLJ “head” was a high-value area of water vapor convergence, which provided sufficient water vapor for the rainstorm. During the SWCV, the weakening of precipitation was due to the SWCV weakening gradually; plus, the ULJ was interrupted over the SB, the upper airflow presented downdrafts, and its superposition with the ascending branch of low-level vertical circulation. This airflow structure inhibited the development of strong upward motion, whilst at the same time, the LLJ retreated toward the south and the dew-point front ultimately weakened and disappeared. Subsequently, water vapor convergence weakened and no longer supported the occurrence of heavy rainfall. Therefore, the strong upward motion caused by the ULJ-LLJ intersection and the lower-level dew-point front were the key reasons for the occurrence of this late-autumn rainstorm.
Highlights
China is a country that experiences many rainstorms [1,2,3,4]
The rainstorms that occur in the upstream, steep-terrain region of the Yangtze River are notoriously difficult to understand and forecast and have long represented a difficult problem for meteorological research and operations relating to rainstorms in China [5, 6]. e Sichuan Basin (SB), in Southwest China (Figure 1(a)), is on the leeward slope of the eastern side of the Tibetan Plateau and is characterized by distinct geomorphology such as steep terrain, mountain basins, lakes, and rivers
Observational evidences have considerably demonstrated that the Tibetan Plateau Vortex, shear lines [17], and the Southwest China Vortex (SWCV) are the major weather systems responsible for rainstorms in Sichuan; plus, they have an important in uence on the strong precipitation that occurs in the vast areas downstream of the Tibetan Plateau. e considerable research e ort in recent decades in this regard has led to some meaningful achievements, such as on the generation sources of the Tibetan Plateau Vortex and SWCV in summer [18,19,20], the causes of their formation [8, 21,22,23], and their thermodynamic structure and e ects on rainstorms [24, 25]
Summary
China is a country that experiences many rainstorms [1,2,3,4]. Among them, the rainstorms that occur in the upstream, steep-terrain region of the Yangtze River are notoriously difficult to understand and forecast and have long represented a difficult problem for meteorological research and operations relating to rainstorms in China [5, 6]. e Sichuan Basin (SB), in Southwest China (Figure 1(a)), is on the leeward slope of the eastern side of the Tibetan Plateau and is characterized by distinct geomorphology such as steep terrain, mountain basins, lakes, and rivers. Observational evidences have considerably demonstrated that the Tibetan Plateau Vortex, shear lines [17], and the Southwest China Vortex (SWCV) are the major weather systems responsible for rainstorms in Sichuan; plus, they have an important in uence on the strong precipitation that occurs in the vast areas downstream of the Tibetan Plateau. The occurrence of high frequency of lightning activity and strong precipitation rates or high accumulated precipitation can be of convective results and of baroclinic instability, as well, considering that such events occur during the warm period of the year; plus, it is very rare for such an event to be observed in the cold season of autumn, and it is the latest-occurring regional heavy rainfall event in recorded history (since 1955) in the Sichuan region.
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