Climatic Features Of The South-westerly Low-Level Jet Over Southeast China And Its Association With Precipitation Over East China

Based on the atmospheric data from the Final Global Data Assimilation System (FNL) reanalysis datasets spanning from2000to2009, the atmospheric data from the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCAR/NCAR) reanalysis datasets spanning from1961to2000and the precipitation data of China from the National Meteorological Information Center, the major features of the south-westerly low-level jet (LLJ) in the lower troposphere over Southeast China and its annual, seasonal, diurnal evolutions, the climatic impacts from large-scale conditions, the mechanisms of the development and evolution of LLJ and the possible impact of LLJ on precipitation have been analyzed. The major conclusions are as follow:1. Climatology of LLJ over Southeast ChinaThe LLJ appears more frequently in April and July than in autumn and winter. Two largest values of the LLJ occurrence frequency peaks occur in April and June/July, the June/July peak is the main peak. The correlation between the LLJ occurrence frequency in April and July is insignificant, which reflects that the occurrence of LLJ in April and July may be influenced by different climatic element.Examination on the geostrophic wind and ageostrophic wind over the LLJ region shows that, the geostrophic meridional zonal wind peaks in April, the geostrophic zonal wind peaks in July. The seasonal variation of ageostrophic meridional wind is consistent with the variation of LLJ occurrence frequency. But, the ageostrophic zonal wind is easterly before July and makes the full jet a little weaker. The investigation on the zonal temperature gradient shows that, at800hPa, the positive zonal gradient replaced by a weak negative zonal temperature gradient, corresponds to the maximum southerly wind at800hPa, which is consistent with the level of the jet core.2. The diurnal cycle of LLJ and mechanism of LLJ formation The occurrence frequency of LLJ over southeast China has significant diurnal cycle, most LLJ occurs in the nighttime (02LST and08LST), and in the daytime (14LST and20LST), the occurrence frequency of LLJ is quite low. The analysis of the diurnal of geostrophic meridional wind shows that geostrophic wind flow has little diurnal cycle, the high nocturnal occurrence frequency of LLJ is mainly resulting from increased nocturnal ageostrophic wind. The intensity of ageostrophic meridional wind is near4m/s at02LST, which composes50%-60%of the full wind, only0.5m/s at14LST and20LST, 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. Quantitatively analysises 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.The developing and decaying of LLJ over Southeast China can be regarded as the results of inertial oscillation of ageostrophic wind around geostrophic flow. As the variation of geostrophic flow is much weaker than that of ageostrophic wind, multiple LLJs occur continuously because of the inertial oscillation of ageostrophic wind while the jet intensity in the mature period become weak progressively. Howerver, if ageostrophic wind undergoes a significant geostrophic adjustment process, geostrophic flow obviously strengthens with the development of LLJ whose life cycle becomes short and ageostrophic wind with the inertial oscillation disappears quickly towards geostrophic balance.3. Possible impact of LLJ on precipitationAnalysis of the climatic effects of LLJ on precipitation distribution in3rainy seasons over Southeast China indicates that, the seasonal variation of the latitude of maximal LLJ center is consistent with the variation of rainfall belt movement over Eastern China. Further analysis indicates a significant positive correlation between the wind intensity over the LLJ region and the precipitation over Southeast China and signifies that the rainfall events with strong intensity correspond to strong low-level winds, most heavy rainfall events are associated with LLJs. Mechanism analysis shows that the LLJ affects the precipitation over Southeast China by transporting water vapor and triggering upward motion. Rainfall regions well corresponds to the regions of the moisture convergence and strong upward motion triggered by LLJ. Negative wind divergence anomalies at850hPa and positive wind divergence anomalies at200hPa over the Yangtze-Huaihe River Valley strengthen the upward motion over this region, which are conductive to produce more precipitation over the Yangtze-Huaihe River Valley.Analysis of the correlation between the precipitation of Meiyu and the annual variation of LLJ for the same period shows that quite different correlation coefficients are found around1982. The correlation coefficient before1982is much higher than that after1982. Further investigation shows that before1982the distinct contrasting area of height and wind field at850hPa between wet years and dry years locates at Northwest Pacific instead of landmass. However, after1982the anomaly center of height field significantly enhances and shifts westward with anomalous southwest flow existing over South China; the anomaly center of wind field overlaps the climatologic position of southwest LLJ. Meanwhile, analysis of the mean meridional wind shows that the contrast of meridional wind at the middle and lower level of troposphere locates on the north side of Meiyu zone while positive contrasting center locates at the south side. The enhanced LLJ from the Meiyu zone and south side increases the water vapor transportation, the weakened southerly (or strengthened northerly) on the north side of Meiyu zone restrains the northward vapor transportation, the enhanced vapor convergence are conductive to produce more precipitation in Meiyu zone.