Characteristics and Formation Mechanisms of Low-Level Jets in Northeastern China.

This study examines low-level jets (LLJs) across Northeastern China during both warm (June–September) and cold seasons (December–March) from 1957 to 2021, using fifth generation of the European Centre for Medium-Range Weather Forecasts reanalysis data with 25-km resolution. LLJs manifest in two prominent regions, one along the leeward flank of the Da Hinggan Ling Mountains in the cold season and another at the center of Northeastern China in the warm season. The intricate interplay between ambient circulation and terrain shapes LLJ distribution, altitudes, wind directions, diurnal cycles, and seasonal diversities. During the warm season, prevailing southwesterly LLJs are found at 925 hPa, while the cold season features stronger and more frequent northwesterly LLJs at 875 hPa. Analysis of the diurnal patterns reveals distinctive behaviors of LLJs in the cold and warm seasons. During the warm season, the single peak in LLJ occurrence emerges around midnight; conversely, in the cold season, LLJs are most frequent shortly before midnight, with an additional sub-peak in the morning. A momentum budget analysis establishes mechanisms underlying these two distinct diurnal variations. In both seasons, the diurnal variation of LLJs is predominately driven by an inertial oscillation and mountain-valley circulations. However, the sub-peak observed in the cold-season morning arises from the thermodynamic and dynamic interaction between the low-level atmosphere and complex terrain. [ABSTRACT FROM AUTHOR]