Case Study on Generation Mechanism of Extreme Ocean-Effect Snowstorm in the East of Shandong Peninsula in January
Analysis is conducted on the generation mechanism of ocean-effect snowstorm in Bohai which occurred from 9 to 11 January 2018, based on multiple observation data of buoy station, automatic weather station, Doppler weather radar, L-band radar, NCEP/NCAR 6 h reanalysis and precipitation, combined with horizontal wind data retrieved from EVAP technology. The results are as follows: This ocean-effect snowstorm process was an extreme snowfall event, characterized by long duration of heavy snowfall, large snowfall amount and near meso-γ scale distribution of snowstorm. The temperature of Bohai Sea and Shandong Peninsula decreased continuously caused by two strong cold air effect before and after the snowstorm. The 850 hPa temperature dropped to -18 to -16℃， which was helpful to the strong ocean-effect snowstorm generation. The cold air strength of this process was obviously stronger than December ocean-effect snowstorm. It was found that the median cold air strength of January ocean-effect snowstorm was about 5℃ lower than in December. Obvious difference of air and SST led to prominent sense heat flux with maximum 226.8 W·m-2 when strong cold air was active, and convective instability occurred over Bohai Sea and the coastal areas of northern Shandong Peninsula. It was a thin convection limited below 850 hPa. There was an obvious thermal ridge of pseudo-equivalent potential temperature during the snowstorm. The radar reflectivity factor graphs show up the obvious “train effect”. NE and WSW winds appeared over a small coastal area of northern Shandong Peninsula, causing wind direction shear lines of NW and NE, WSW and NE. The lower shear line led to the narrow-band echo which triggered the snowstorm. And the NE wind reached the height of less than 1.2 km, mostly under 0.6 km. Characteristics of the January ocean-effect snowstorm that less appeared are revealed though the study of this extreme snowstorm process. Its characteristics such as the circulation pattern, thermal instability and dynamical condition are similar to those of the December ocean-effect snowstorm. The major difference is that the cold air strength is stronger than in December, which could be the primary forecasting focus of January ocean-effect snowstorm. Buoy data and EVAP radar retrieved wind can quantitatively reflect the mesoscale feature of ocean-effect snowstorm.