Abstract
Turbulent mixing is critical in affecting urban climate and air pollution. Nevertheless, our understanding of it, especially in a cloud-topped boundary layer (CTBL), remains limited. High-temporal resolution observations provide sufficient information of vertical velocity profiles, which is essential for turbulence studies in the atmospheric boundary layer (ABL). We conducted Doppler Light Detection and Ranging (LiDAR) measurements in 2019 using the 3-Dimensional Real-time Atmospheric Monitoring System (3DREAMS) to reveal the characteristics of typical daytime turbulent mixing processes in CTBL over Hong Kong. We assessed the contribution of cloud radiative cooling on turbulent mixing and determined the altitudinal dependence of the contribution of surface heating and vertical wind shear to turbulent mixing. Our results show that more downdrafts and updrafts in spring and autumn were observed and positively associated with seasonal cloud fraction. These results reveal that cloud radiative cooling was the main source of downdraft, which was also confirmed by our detailed case study of vertical velocity. Compared to winter and autumn, cloud base heights were lower in spring and summer. Cloud radiative cooling contributed ~32% to turbulent mixing even near the surface, although the contribution was relatively weaker compared to surface heating and vertical wind shear. Surface heating and vertical wind shear together contributed to ~45% of turbulent mixing near the surface, but wind shear can affect up to ~1100 m while surface heating can only reach ~450 m. Despite the fact that more research is still needed to further understand the processes, our findings provide useful references for local weather forecast and air quality studies.
Highlights
Turbulent mixing is a crucial part of the atmospheric boundary layer (ABL), which modulates the variation in temperature, flow velocity, moisture, and atmospheric composition and acts as a bridge between the top of the ABL and the surface [1,2]
Downdrafts were more common in spring while more updrafAtss wreevreeaolebdsebryveFdigiunreau3tau, mthne.highest cloud fraction was observed in spring while the lowest was obtained in autumn
Downdrafts were more common in spring while more updrafts were observed in autumn, revealing that cloud radiative cooling is the main source of downdraft
Summary
Turbulent mixing is a crucial part of the atmospheric boundary layer (ABL), which modulates the variation in temperature, flow velocity, moisture, and atmospheric composition and acts as a bridge between the top of the ABL and the surface [1,2]. The Doppler LiDAR employed in this study operated round the clock and has been set up for an optimized vertical resolution of 30 m in the boundary layer up to around 3 km altitude. It was operated in velocity azimuth display (VAD) scanning mode for obtaining one horizontal wind profiles every 10 min using 6 beams around a circle at an elevation angle of 75◦. Results show that the percentage difference between the averaged horizontal wind speeds provided by LiDAR and upper air sounding data for heights less than 1.00 km was less than 10%, which indicates a sufficient agreement between both data sources [33]
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