Abstract
The planetary boundary layer (PBL) is an atmospheric region near the Earth’s surface. It is significant for weather forecasting and for the study of air quality and climate. In this study, the top of nocturnal residual layers—which are what remain of the daytime mixing layer—are estimated by an elastic backscatter Lidar in Wuhan (30.5°N, 114.4°E), a city in Central China. The ideal profile fitting method is widely applied to determine the nocturnal residual layer height (RLH) from Lidar data. However, the method is seriously affected by an optical thick layer. Thus, we propose an improved iterative fitting method to eliminate the optical thick layer effect on RLH detection using Lidar. Two typical case studies observed by elastic Lidar are presented to demonstrate the theory and advantage of the proposed method. Results of case analysis indicate that the improved method is more practical and precise than profile-fitting, gradient, and wavelet covariance transform method in terms of nocturnal RLH evaluation under low cloud conditions. Long-term observations of RLH performed with ideal profile fitting and improved methods were carried out in Wuhan from 28 May 2011 to 17 June 2016. Comparisons of Lidar-derived RLHs with the two types of methods verify that the improved solution is practical. Statistical analysis of a six-year Lidar signal was conducted to reveal the monthly average values of nocturnal RLH in Wuhan. A clear RLH monthly cycle with a maximum mean height of about 1.8 km above ground level was observed in August, and a minimum height of about 0.7 km was observed in January. The variation in monthly mean RLH displays an obvious quarterly dependence, which coincides with the annual variation in local surface temperature.
Highlights
The planetary boundary layer (PBL) is an atmospheric region near the Earth’s surface affected by surface frictional resistance [1,2,3]
The convectively-driven PBL is known as a convective boundary layer (CBL) that develops about half an hour after sunrise, and nocturnal residual layers (RL) are what remain of the daytime CBL [1]
residual layer height (RLH) are higher than the corresponding improved RLHs, which may contribute to the influence of the low cloud layer
Summary
The planetary boundary layer (PBL) is an atmospheric region near the Earth’s surface affected by surface frictional resistance [1,2,3]. The IPF method is utilized to obtain RLH by fitting the Lidar signal or particle backscatter coefficient to an idealized profile [12]. Lidar is affected by strong backscatter layers, such as clouds and optical thick aerosols (e.g., mineral dust aerosol) [14,17,18]. These layers cause strong peaks in the range-corrected Lidar signal, and these peaks may cause large errors in RLH detection. We propose an improved iterative fitting method based on the IPF method to eliminate the optical thick layer effect on RLH detection using.
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