Atmospheric Boundary Layer Processes, Characteristics and Parameterization

The atmospheric boundary layer is distinguished from the rest of the atmosphere due to its unique characteristics, i.e., its direct interaction with the Earth’s surface and active turbulence. Understanding the dynamic and chemical processes in the boundary layer is of great importance in weather and air quality forecasting. Recently, with the improvements in observation and simulation techniques, our understanding of atmospheric boundary layer processes and characteristics has significantly improved. For example, ultrasonic anemometers and large-aperture scintillometers can provide information about turbulent exchanges, while the large eddy simulation technique simulates the detailed structure of turbulent eddies. This Special Issue is dedicated to reporting new findings with regard to atmospheric boundary layer processes, characteristics, and parametrization methods, including, but not limited to, turbulent exchange, transportation, and parametrization; boundary layer jets; local atmospheric circulation; surface energy partitioning; atmospheric stability conditions; pollutant distribution and transportation; etc.

This Special Issue has published 12 papers reporting new findings on various aspects of the atmospheric boundary layer.

Four papers were dedicated to understanding the characteristics and parametrization methods of the tropical cyclone atmospheric boundary layer. One of the four papers looked into the vertical eddy diffusivity in the atmospheric boundary layer during landfall of a tropical cyclone, which was observed by three-dimensional ultrasonic anemometers. An exceptional finding in this paper was the variation in the turbulent parameters with regard to the distance to the tropical cyclone center: outside three times of the radius of maximum wind (RMW) from the tropical cyclone center, the turbulent kinetic energy and eddy diffusivity values increased with increasing wind speed; however, in the area that was within one to three times the RMW from the tropical cyclone center, these values decreased slowly with increasing wind speed [1]. The other three papers investigated the turbulent fluxes exchanged over the ocean under tropical cyclone conditions through aircraft eddy-covariance measurements [2], dropsondes observations [3], and numerical simulations [4]. Gao et al. proposed new equations to parameterize the surface drag coefficient over the ocean surface through aircraft eddy-covariance measurements [2]; Ye et al. showed from the dropsondes observations that the relationship between the surface drag coefficient and wind speed varied with the distance from the tropical cyclone center [3]; and Ye et al. also showed from simulations that the surface flux scheme option, which overestimated the enthalpy exchange coefficient, leads to excessive inflow within the boundary layer and larger eyewall updrafts [4].

Three papers reported improvements in models or parameterizations related to the atmospheric boundary layer. The paper by Zhang et al. proposed using the random forest model to correct the simulation results of the Simple Biosphere Model 2, which improved the coefficient of determination between the calculations and measurements by 13–68% [5]. Nofal et al. introduced a more efficient parameterization to obtain the appropriate concentration boundary layer height and internal integral calculation intervals, and the new parameterizations showed an ability to save about 78% of the computation time compared to the original algorithm [6]. Reilly et al. put forward their understanding of the performance of different turbulence length-scale parameterization methods in the numerical weather prediction models using turbulence kinetic energy schemes, and they recommended using the turbulence length-scale formulations which considered the boundary layer height, turbulent kinetic energy and stratification, as these formulations had a satisfactory performance in different flow regimes [7].

Five papers investigated the air–land–sea interface phenomenon and the associated mechanisms through numerical simulations and/or observations. The paper by Wang et al. analyzed the impact erosion of aeolian sand saltation in the Gobi Desert and found that the maximum value of the saltation erosion rate increased due to a power law relationship with the friction velocity [8]. Zhang et al. presented how the atmospheric convective boundary layer varied with different wind shear settings in large eddy simulations, and they found that the increasing wind shear not only enlarged the variances of horizontal winds and temperature, but also enhanced large-scale coherent structures [9]. Moreira et al. demonstrated the spatial characteristics of the winter atmospheric boundary height and the aerosol layer aloft in São Paulo with observations from two simultaneous Lidar methods, and they found that the boundary layer height differences were affected by the cloud and sea breeze mostly, together with other influential factors including the development stages, topographic effects, and the presence of aerosol layers associated with biomass burning events [10]. Han et al. clarified how the periodic tidal elevations modified the sea surface fluxes by making observations from a near-coast platform in the East China Sea, and they found about a 1.5–3.5% mean difference of surface fluxes caused by the tide-dominated sea surface elevation [11]. Zhu et al. revealed that the Madden–Julian oscillation, partially through modifying the local-scale land–sea circulations, affected the diurnal variation and offshore propagation of the Sumatra western coast rainfall [12].

To summarize, the present SI provides not only new findings, but also new methods and new insights regarding the processes, characteristics and parameterization of the atmospheric boundary layer.