Abstract:On 17 and during 24-26 December 2021, Shandong Peninsula experienced two BohaiSea sea-effect snowfall events. The cold air intensity was stronger and snowfall duration was longer during the 24-26 process than that on 17, but the total snowfall amount was less and intensity of snowfall was weaker. In this paper, thermodynamic and dynamical characteristics are analyzed and compared based on the data of moored buoy station, S-band Doppler weather radar, conventional sounding, hourly snowfall observation from ground automatic meteorological stations and ERA5 reanalysis data. The results are as follows. In 24-26 process, air-sea temperature difference between sea surface and 850 hPa was 28℃, exceeding that on 17 (24℃). However, the snowfall process in 24-26 displayed an intermittent weakening of low-level cold advection, with advection intensities consistently weaker than on 17. This weaker thermal advection impacted the development of low-level thermodynamic conditions. On 17, the intense core of cold advection extended vertically to 700-500 hPa, whereas during the snowfall from 24 to 26, it was confined to 875-750 hPa. Consequently, the inversion layer developed upward above the cold advection center and influenced the depth of conditionally unstable layer beneath the inversion layer, which limited the vertical extent of shallow convective clouds and ultimately diminished snowfall intensity. During the second snowfall phase from 24 to 26, steering wind direction over the Bohai Sea and its upstream regions had a larger westerly component. Thus, the low-level warm tongue, mesoscale shear line and heavy snowfall echo band all displayed larger zonal component and positioned predominantly offshore, resulting in inconspicuous snowfall over the Shandong Peninsula. Cold advection constituted a primary physical driver modulating the thermal structure of sea-effect snowfall. It indirectly controlled the initiation and evolution of mesoscale shear lines by inducing a low-level warm tongue, thereby providing dynamical lifting and organi-zational mechanism over snowfall echo structures and ultimately shaping the dynamical structure of snowfall. When forecasting such snowfall, it is essential to make an integrated dynamic analysis of cold advection developmental stages and its three-dimensional spatial structure and intensity evolution on low-level. To be specific, the air-sea temperature difference, absolute moisture content within the boundary layer, inversion layer height, thickness of the conditionally unstable layer and steering wind direction should be considered more. Diagnosing mesoscale characteristics such as the position, orientation and intensity of low-level warm tongue and shear line is also imperative.