Wind profiles and low-level jet structures over the coastal waters of Japan

Abstract

Accurate characterization of coastal wind conditions is essential for offshore wind energy development; however, atmospheric structures in Japan's nearshore regions remain poorly understood. This study analyzed year-long vertical light detection and ranging (lidar) observations at closely located onshore and offshore sites along the Aomori coast to clarify the differences in wind profiles and their seasonal and directional dependence. Offshore wind speeds showed strong correlations (r > 0.8) with onshore data, indicating that, although direct substitution is inappropriate, onshore observations can effectively serve as reference data for offshore extrapolation when using the measure–correlate–predict (MCP) method. Low-level jets (LLJs) were more frequently observed under wind directions parallel to the coastline, with occurrence rates of 12 %–29 % for southerly winds (SW–SE) and 8 %–10 % for northerly winds (N–NE), compared to other directions. The fraction of days with LLJ occurrences was also higher in spring and summer, reaching about 40 % in MAM and 37 % in JJA. Case analyses revealed that diurnal transitions associated with land–sea breeze circulation modulate vertical mixing and surface friction, promoting the development of LLJs. These results advance our understanding of nearshore boundary-layer dynamics and provide a basis for improving assessments of offshore wind resources, turbine designs, and LLJ forecasting strategies.