Symbiotic Relationship between Meiyu Rainfall and the Morphology of Meiyu Front

Observational evidences from a heavy precipitation event of the 2020 extreme Meiyu season are presented here to reveal a symbiotic relationship between Meiyu rainfall and the morphology of Meiyu front. The two influence each other through dynamical and thermodynamic feedbacks and evolve in a coherent way to generate cyclic behaviors. Specifically, an intense and band-shaped Meiyu front leads to symmetrical instability in the lower atmospheric layer and convective instability in the middle atmospheric layer, forming a rain band along the front. The Meiyu front and its associated instability subsequently weakens as a result of rainfall and the front is bent by the process of tilting frontolysis. Deep convective instability in the middle and lower layers develops in the warm-humid prefrontal area, and triggers isolated heavy rainfall replacing the original rain band south of the bent front. This warm sector precipitation then strengthens the front through tilting and diabatic heating frontogenesis. A stronger front recovers its initial band shape and the associated rainfall also resumes the form of rain band along the front. Analyses of potential energy associated with instability, water vapor convergence, and cross-frontal circulation are carried out to illustrate key processes of this Meiyu front-rainfall cycle. The implications of this symbiotic relationship for simulating and predicting extreme rainfall associated with Meiyu fronts are presented.

Climatological study of a new air stagnation index (ASI) for China and its relationship with air pollution

Air stagnation index (ASI) is a vital meteorological measure of the atmosphere's ability to dilute air pollutants. The original metric adopted by US National Climatic Data Center (NCDC) is found not well suitable to China, because the decouple between upper and lower atmospheric layer results in a weak link between the near surface air pollution and upper-air wind speed. Therefore, a new threshold for ASI is proposed, consisting of daily maximal ventilation in the atmospheric boundary layer, precipitation and real latent instability. In the present study, the climatological features of this newly defined ASI is investigated. It shows that the spatial distribution of the new ASI is similar to the original one; that is, annual mean stagnations occur most often in the northwest and southwest basins, i.e., Xinjiang and Sichuan basins (more than 180 days), and the least over plateaus, i.e., Qinghai–Tibet and Yunnan plateaus (less than 40 days). However, the seasonal cycle of the new ASI is changed. Stagnation days under new metric are observed to be maximal in winter and minimal in summer, which is positively correlated with air pollution index (API) during 2000–2012. The correlation between ASI and concentration of fine particulate matter (PM2.5) during January 2013 of Beijing is also investigated. It shows that the new ASI matches the day-by-day variation of PM2.5 concentration very well and is able to catch the haze episodes in that month.

Results of research on pollen fall-out in the atmospheric ground layer

The paper covers results of observations on the distribution of tree and herbaseous plants' pollen in the lower atmospheric layer at various heights in the three-year period 1966-1968. On the basis of the material obtained a calendar of pollen appearance was prepared.