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Yunnan–Guizhou quasi-stationary front (YGQSF) is a unique weather phenomenon that frequently occurs during the winter half year over the Yunnan–Guizhou Plateau in southwestern China. Most of the previous studies analyzed it only with synoptic cases. This study investigates the structure, variation, and impact of this front from a climate perspective, using long-term high-resolution atmospheric reanalyses and high-density station records for 1981–2016. An objective method quantifying YGQSF is proposed and three indexes are defined to measure its intensity, frequency, and location, respectively, with horizontal gradient of air potential temperature at a terrain-following level of sigma 0.995. With these indexes, climatological structure, subseasonal variability as well as climatic impact of the front are comprehensively examined. In climatology, YGQSF as a north–south-oriented low-level front is found to occur the most frequently during January–February–March, determined predominately by the coldness from the east of the front. The structure of the front identified essentially reflects an obstruction of high-terrain Yunnan (the western part of the plateau) to the low-level cold air mass, which makes near-surface cold northeasterly winds cease westward intruding and veer upward over relatively low-terrain Guizhou, transporting moisture upward and forming low clouds. A sharp climate contrast is thus formed between two sides of the front: cold, sunless, and continuously rainy Guizhou versus warm and sunny Yunnan. Furthermore, the front features significant subseasonal variations with periods at around 30 days and 60 days largely in its intensity. Anomalously strong YGQSF events which are caused 75% by the cold anomaly from the east but no more than 17% by the warm anomaly from the west yield different anomalous structures, but consistently amplify the sharp climate contrast between Yunnan and Guizhou. [ABSTRACT FROM AUTHOR]

Interaction between synoptic eddy and mean flow plays a crucial role in maintaining midlatitude westerly jet. In this study, climatologies of synoptic eddy activities and their feedback onto midlatitude jet for 1980–2016 are evaluated and compared through analyzing daily data from five atmospheric reanalyses with different resolutions including one coarse-resolution reanalysis (NCEP2) and four fine-resolution reanalyses (ERA-Interim, JRA-55, MERRA-2, and CFSR). Horizontal resolutions of the atmospheric models generating those reanalyses are approximately equivalent to 210, 79, 60, 50, and 38 km, respectively. Results show that the eddy activities and their feedback onto the midlatitude jet in those fine-resolution reanalyses are consistently and significantly stronger than those in the coarse-resolution reanalysis (NCEP2). The maximal relative increases that are found to occur primarily in the midlatitudes of the Southern Hemisphere are estimated to be up to 55% for the baroclinicity, 53% for the eddy energetics, 59% for the eddy forcing, and even 85% for the eddy feedback onto the mean flow. Those increases are reasonably conjectured to be related to increased model resolutions, since the synoptic eddy genesis is proportional to the low-level atmospheric meridional temperature gradient which is sensitive to the meridional resolution of atmospheric models. Although the coarse-resolution reanalysis resolves synoptic eddies insufficiently and thus underestimates their feedback onto the mean flow, the magnitudes of eddy-driven jets are almost the same among five reanalyses, implying a mismatch between the eddy feedback and the eddy-driven jet in the coarse-resolution reanalysis. Therefore, the results of this study imply the importance of using fine-resolution reanalyses in accurately understanding the midlatitude synoptic eddy–mean flow interaction. [ABSTRACT FROM AUTHOR]