Multi-platform Observations Characterizing the Afternoon-to-Evening Transition of the Planetary Boundary Layer in Northern Alabama, USA

摘要: Observations from the University of Alabama in Huntsville campus and ground-based scanning radar for over 140 total spring, summer, autumn cases are studied to contribute relative scarcity long-term datasets documenting afternoon-to-evening transition planetary boundary layer. A sunset frame reference is employed, focusing on period 3 h before 2 after astronomical sunset, several findings consistent with previous investigations. Fluctuating components wind temperature computed nearly collocated surface, Doppler profiler, vertically pointing lidar measurements show a decline as turbulence intensity diminishes through transition. When normalized by their initial values, pattern emerges: variances slowly at first then quite abruptly about 90 min sunset. After begin wane, vertical velocity fluctuations decrease, rate decay increases vigorous thermal structures diminish. The fastest horizontal variance occurs an accelerated display slowest decrease among these quantities. Near-surface humidity mean water vapour mixing ratio steady rise generally beginning 80 prior Composites motion final convective-type towers upward occur hour coherent 800 m (all heights a.g.l.). Lidar 195 decreases more than order magnitude approaching remains below 0.01 m\(^{2}\) s\(^{-2}\) rest time frame. Subtle, but steady, both speed radar-derived convergence above surface layer (at 300 m) span entire 5-h While results broad range, increase clear found be statistically significant. Implications possible transition-effect enhancements pre-existing low-level areas briefly noted.