Role of the Nocturnal Low-level Jet in the Formation of the Morning Precipitation Peak over the Dabie Mountains
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The diurnal variation of precipitation over the Dabie Mountains (DBM) in eastern China during the 2013 mei-yu season is investigated with forecasts of a regional convection-permitting model. Simulated precipitation is verified against surface rain-gauge observations. The observed morning precipitation peak on the windward (relative to the prevailing synoptic-scale wind) side of the DBM is reproduced with good spatial and temporal accuracy. The interaction between the DBM and a nocturnal boundary layer low-level jet (BLJ) due to the inertial oscillation mechanism is shown to be responsible for this precipitation peak. The BLJ is aligned with the lower-level southwesterly synoptic-scale flow that carries abundant moisture. The BLJ core is established at around 0200 LST upwind of the mountains. It moves towards the DBM and reaches maximum intensity at about 70 km ahead of the mountains. When the BLJ impinges upon the windward side of the DBM in the early morning, mechanical lifting of moist air leads to condensation and subsequent precipitation.
Key wordsnocturnal boundary layer low-level jet diurnal variation precipitation Dabie Mountains mei-yu season
大别山地区是江淮流域一个暴雨中心, 降水日变化呈现典型的双峰结构, 其中清晨降水峰值的形成机制原因较为复杂, 过去存在较大的争议. 本文利用南京大学夏季实时运行的4km WRF, 研究了梅雨期间大别山地区清晨峰值降水的形成机制, 模式预报很好地再现了大别山地区的降水时空分布和日变化特征. 研究表明, 大别山迎风坡区域夜间边界层急流存在明显的惯性振荡, 凌晨5点左右惯性振荡方向与盛行风方向重合, 夜间边界层急流强度达到最强; 当低层暖湿气流输送到大别山地区时, 地形抬升作用进一步增强了降水. 夜间边界层急流增强和大别山地形的抬升作用是该地区清晨降水峰值形成的主要机制.
关键词夜间边界层 低空急流 日变化 降水 大别山 梅雨
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This work was primarily supported by the Special Foundation of the China Meteorological Administration (Grant No. GYHY201506006). The work was also supported by the National Science Foundation of China (Grant Nos. 41405100, 41322032 and 41275031).
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