Observation of Boundary-Layer Jets in the Northern South China Sea by a Research Vessel

Boundary-layer jets (BLJs) in the South China Sea play an important role in heavy rainfall in South China, yet observations in maritime locations are still limited. This study examines the vertical structures and temporal evolutions of BLJs in the northern South China Sea using intensive radiosonde observations from a research vessel from 15 to 18 June 2022 and evaluates the performance of various reanalysis datasets in capturing these features. Observations identified BLJs with jet cores at altitudes of approximately 500-700 m. Wind speeds slightly decreased from 15 to 16 June and then significantly increased after 17 June, showing double peaks on 17 June below 1 km at altitudes of 250 and 700 m. Among the reanalysis datasets, ERA5 exhibited more accurate results on average, followed by MERRA2, both of which outperformed JRA55 and FNL. ERA5 and MERRA2 had mixed performances in depicting BLJ characteristics. ERA5 accurately captured the initial decrease in wind speeds and their subsequent enhancement, while MERRA2 initially faltered but improved later. On the diurnal scale, neither MERRA2 nor ERA5 accurately represented the wind speed peaks observed at 2300 and 1100 LST, whereas ERA5 roughly reflected the nocturnal acceleration of the BLJs. During the observation period, the intensification of BLJs in the northern SCS, influenced by an eastward-moving high-pressure system and a southward-moving low-pressure vortex, led to enhanced precipitation in South China that gradually moved northward from the coastline to inland regions. This study provides new insights into the detailed characteristics of marine BLJs based on direct observations.