ISSN 1000-0526
CN 11-2282/P
CN 11-2282/P
Comparison of Thermal and Dynamic Characteristics of Two Bohai sea-effect snowfall Weather Events in Shandong Peninsula
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Abstract:
Abstract:On 17th (hereinafter referred to as “1217”)and from 24th to 26th December 2021 (hereinafter referred to as “1225”), Shandong Peninsula experienced two Bohai sea-effect snowfall events.Compared with “1217”,the cold air intensity was stronger and snowfall duration was longer for “1225”, but the total snowfall amount was less and intensity of snowfall was weaker than “1217”. In order to clarify the reasons, thermodynamic and dynamical characters were analyzed and compared by using data of moored buoy station,S-band Doppler weather radar,conventional sounding, hourly snowfall observation data from ground automatic meteorological stations and ERA5( ECMWF Reanalysis v5). The main results are as follows:During the first snowfall phase of "1225", sea-air temperature difference(temperature difference between sea surface and 850hPa air) was 28°C, which exceeded "1217" (24°C). During the second snowfall phase, both events exhibited comparable sea-air temperature between 24°C and 25°C. However, throughout the snowfall period,"1225" displayed an?intermittent weakening of low-level cold advection, with?advection intensities consistently weaker than "1217". This weaker thermal advection?impaired the development of?lower-level thermal and moisture conditions?for the "1225". During "1217", the intense core of cold advection extended vertically to 700～500 hPa, whereas during "1225" it was confined to 875～750 hPa and was weaker than"1217". Consequently, the inversion layer developed above the cold advection center in "1225" displayed both lower height and reduced thickness relative to "1217". This structural difference further suppressed depth of the conditionally unstable (or neutral) layer beneath the inversion,thereby limiting the vertical extent of shallow convective clouds and ultimately diminishing snowfall intensity. During the second snowfall phase of "1225", steering wind direction (mean wind direction from 1000 hPa to 700 hPa) over the Bohai Sea and its upstream regions was 327°, deviating by 11° from "1217" with a larger westerly component. Consequently, the low-level tongue, mesoscale shear line and heavy snow echo band all displayed larger zonal component and positioned predominantly offshore resulting in inconspicuous snowfall over the peninsula ultimately. Cold advection constitutes a primary physical driver modulating the thermal structure of sea-effect snowfall. Simultaneously, it indirectly governs the initiation and evolution of mesoscale shear lines by inducing a low-level warm tongue, thereby providing dynamical lifting and organizational mechanism over snowfall echo structures,ultimately shaping the dynamical structure of snowfall. For forecasting applications, it is essential to make an integrated dynamic analysis of cold advection developmental stages, three-dimensional spatial structure and intensity evolution on low-level. Focus on sea-air temperature difference,absolute moisture content within the boundary layer, inversion layer height, thickness of the conditionally(or neutral) unstablelayer and steering wind direction. Diagnostics of mesoscale characteristics such as the position, orientation and intensity of low-level warm tongue and shear line are imperative.
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