近四十年渤海及周边典型城市夏季开始时间变化机制研究OA

Based on the 1982–2022 high-resolution operational sea surface temperature(SST)and sea ice analysis data-set(OSTIA)and the ERA5 global reanalysis dataset released by the European Centre for Medium-Range Weather Fore-casts(ECMWF),this research investigates the variation characteristics of summer onset dates in the Bohai Sea and surround-ing typical cities.It further explores the dominant cloud layer influencing this variation by comparing cloud cover and cloud radiative temperature effects at different altitudes.The results show that over the past four decades,summer onset dates in the nearshore shallow waters and deep-water areas of the Bohai Sea have advanced by 17 and 11 days,respectively.In the Bohai surrounding typical cities,summer onset based on air temperature has advanced by 12 days,while that based on soil tem-perature by 11 days.Compared with middle-and low-level clouds,high-level cloud cover exhibited the strongest correlation with summer onset dates and showed the greatest reduction,which was closest to the decrease in total cloud cover over the same period.The variation in cloud temperature effect influenced by high clouds was also the greatest over the past four dec-ades.The reduction in high-level cloud cover has increased the concurrent surface downward shortwave radiation,ultimately influencing the long-term trend of summer onset dates over both sea and land.