Analyses of Structural Characteristics of a Sea Fog Event over the Western Yellow Sea in April, 2024 by Using the Millimeter-Wave Radar and Lidar Data

Using the millimeter-wave radar and lidar data, conventional meteorological observation data, and ERA5 reanalysis data, the authers conducted observational analyses of a sea fog event that occurred over the western Yellow Sea in April, 2024. A prolonged sea fog event lasting four days on April 12—15, 2024 occurred in Qingdao and its adjacent coastal waters. It influenced by warm and moist airflows behind the maritime high-pressure system. During the fog development stage, favorable air-sea temperature difference, stable air-sea interface, suitable conditions of inversion layer, weak wind shear within the atmospheric boundary layer, and high static stability collectively facilitated the development and maintenance of the sea fog. The lidar deployed at Huangdao National Basic Meteorological Station successfully captured the variation trend of atmospheric visibility throughout the fog′s formation and dissipation processes. It also captured the onshore propagation of sea fog during its formation stage. However, significant detection attenuation was observed during the mature stage of sea fog event. The millimeter-wave radar revealed the vertical structural characteristics of this sea fog event at different developmental stages. During the formation stage, see fog echo signals were detected below 0.5 km. Longwave radiative cooling at the fog top resulted in a significantly lower reflectivity factor at the fog top edge compared to the middle and lower layers, inhibiting the upward expansion of the fog top. During the mature stage, fine and irregular bright-dark fluctuations emerged within the non-top and non-bottom regions of the fog layer. This phenomenon might have been closely associated with small-scale turbulent motions inside the fog. In the late stage, the coalescence and growth of fog droplets followed by sinking led to echo signals exhibiting a filamentous structure similar to the convective features of weak precipitation, with the echo height rising above 1.2 km. At this time, the conditions were no longer conducive to the maintenance and development of the sea fog. The combined application of conventional and new observational equipment, leveraging their complementary advantages, may contribute to an in-depth understanding of the structural characteristics of sea fog over the Yellow Sea.