Characterizing urban planetary boundary layer dynamics using 3-year Doppler wind lidar measurements in a western Yangtze River Delta city, China

Abstract

The planetary boundary layer (PBL) dynamics play a critical role in shaping urban atmospheric processes by governing the exchange of energy, momentum, and mass within the lower atmosphere. This study investigates PBL dynamics in Hefei, a city in the western Yangtze River Delta (YRD), using 3 years (June 2019 to June 2022) of Doppler wind lidar data. Seasonal and diurnal variations in key parameters, including wind profiles, shear intensity, turbulent mixing, low-level jets (LLJs), and mixing layer height (MLH), are analyzed. Results show that horizontal wind speeds accelerate more rapidly above 3 km with the predominant westerly winds (270°±15°) in all seasons. The vertical depth of the high-wind zone (>8 m s-1) during the day is found to be generally deeper than at night, particularly in winter. In Hefei, LLJs primarily form at sunset and dissipate by noon, typically at altitudes between 0.5 and 0.6 km throughout the year, except in July. LLJ occurrences are the most frequent in spring (31.7 %), followed by summer (24.7 %), autumn (22.3 %), and winter (21.3 %). Summer LLJs are the most intensified, extending up to 1.5 km. The larger wind gradient below the jets significantly enhances turbulence and shear intensity near the ground at night. The seasonal average MLH peaks between 14:00 and 15:00 UTC+8, reaching approximately 1.2 km in spring and summer. Cloud cover raises MLH by about 100 m at night but decreases it by 200 m at the afternoon peak. This study provides insights into lidar-based PBL dynamics and highlights implications for local standards concerning low-altitude economic activities.