中国东部西南低空急流日变化特征及其机制

本文利用Final Global Data Assimilation System (FNL) 6小时再分析数据集分析了西南低空急流的日变化特征及其影响因子,结果表明:西南低空急流具有明显的日变化特征,在夜间和早晨(02LST,08LST)中国东南大部分地区急流发生频率较高,而在白天和傍晚(14LST,20LST)低空急流发生频率较低.经向地转风分量在一天内基本保持稳定,经向非地转分量在02LST最强,占实际风场强度50%以上,而在14LST和20LST,经向风场近似满足地转平衡.对风场非定常性、风速在流动方向上的非均匀性、流线弯曲和大气斜压性产生的地转偏差的分析结果表明,经向非地转风的日变化主要是由局地变压、水平风场涡度、垂直运动和温度梯度的日变化产生,副热带高压强度和位置的变化、青藏高原大地形加热效应和昼夜间海陆热力性质差异是造成经向非地转风夜间加强的重要原因.在中国东部地区,风速在流动方向的非均匀性虽然有利于非地转风的产生,但其没有明显的日变化,不是经向非地转风在夜间加强的主要原因.

Diurnal variation of the south-westerly low-level jet over eastern China and its mechanism

Based on the atmospheric data from the Final Global Data Assimilation System (FNL) reanalysis datasets spanning from 2000 to 2009, the diurnal variation of the south-westerly low-level jet (LLJ) over eastern China and its development and evolution mechanisms have been analyzed. The major conclusions are as follows. The occurrence frequency of LLJ over southeast China has significant diurnal cycle, most LLJ occurs in the nighttime (02LST and 08LST), while in the daytime (14LST and 20LST), the occurrence frequency of LLJ is quite low. The analysis of the diurnal variation of geostrophic meridional wind shows that geostrophic wind flow has little diurnal cycle, the high nocturnal occurrence frequency of LLJ results mainly from increased nocturnal ageostrophic wind. The intensity of ageostrophic meridional wind is near 4 m/s at 02LST, which composes 50%~60% of the full wind, but only 0.5 m/s at 14LST and 20LST, which is small compared with the full wind. According to the ageostrophic equation, ageostrophic wind is represented in four items, which are the unconstant wind field, inhomogeneous flow in the wind direction, curved streamline and vertical wind shear. Quantitative analyses of these four items show that the diurnal variation of ageostrophic meridional wind is mainly caused by the diurnal variations of local pressure change, horizontal vorticity, vertical motion and temperature gradient, while the substantial reasons for the enhancement of geostrophic wind at night are heating effect of Tibetan Plateau and the reverse land-sea thermal contrast from daytime to nighttime and the influence of Subtropical High. In East China, inhomogeneous flow in the wind direction, which is in favor of ageostrophic wind but with no significant diurnal variation, tends not to be the major reason for the enhancement of geostrophic wind at night.