Linkage between surface energy balance non‐closure and horizontal asymmetric turbulent transport

A number of studies have reported that the traditional eddy covariance (EC) method generally underestimated vertical turbulent fluxes, leading to an outstanding non-closure problem of the surface energy balance (SEB). Although it is recognized that the enlarged surface energy imbalance frequently coincides with the increasing wind shear, the role of large eddies in affecting the SEB remains unclear. On analyzing data collected by an EC array, considerable horizontal inhomogeneity of kinematic heat flux is observed. The results show that the combined EC method that incorporates the spatial flux contribution increases the kinematic heat flux by 21% relative to the traditional EC method, improving the SEB closure. Additionally, spectral analysis indicates that large eddies with scales ranging from 0.0005 to 0.01 (in the normalized frequency) mainly account for the horizontal inhomogeneity of kinematic heat flux. Under unstable conditions, this process is operating upon large eddies characterized by enlarged asymmetric turbulent flux transport. With enhanced wind shear, the increment of flux contribution associated with sweeps and ejections becomes disproportionate, contributing to the horizontal inhomogeneity of kinematic heat flux, and thus may explain the increased SEB non-closure

TURBULENCE STRUCTURES ACROSS LAYERS IN STABLE BOUNDARY LAYER

The stable atmospheric boundary layer (SBL), most often occurring at night and with the presence of a vertical temperature inversion, is characterized by very low levels of turbulence. Consequently, pollutants released from the surface are mainly trapped within the SBL, causing adverse effect on human health. Compared to the daytime convective boundary layer, the dynamics of the SBL remain poorly understood. This lack of understanding directly impacts our ability to model the SBL and the effects on pollutant transport and diffusion. Advancement in our understanding of the interaction of SBL state and turbulence can be fueled from a more in-depth study of SBL regime classification, variation of turbulence structures, and mechanisms regulating flux exchanges, which are the main subjects of this dissertation. This work can be divided into three parts. In the first part, eddy covariance data collected at four different heights are used to characterize the state of the SBL in terms of how upper and lower portions of the SBL may be coupled or not. The results indicate that the transport of momentum and heat across SBL layers is determined by large eddies. Therefore, distinct SBL coupling states are delineated by unique turbulence structures. Large gradients in momentum and heat fluxes are frequently observed, violating the ‘constant-flux’ assumption. In the second part, it is shown that increased vertical gradients of wind speed and potential temperature can cause flux gradients. This process is operated upon by downward penetrating large eddies with altered phase difference, contributing unevenly to fluxes and thus causing flux gradients. In the third part, the role of large eddies induced by wind profile distortion (WPD) in regulating flux exchanges and the transition of SBL states is examined. Results showed that the WPD-induced large eddies can penetrate downward to the surface, enhancing vertical mixing. As the WPD is intensified, both turbulent fluxes and flux transport efficiencies associated with large eddies are enhanced, causing a transition from strongly to weakly stable regimes. These results have important implications for how the SBL can be parameterized in numerical weather and pollutant transport models

Improved Quadrant Analysis for Large-Scale Events Detection in Turbulent Transport

Quadrant analysis has been widely used to investigate the turbulent characteristics in the atmospheric boundary layer (ABL). Although quadrant analysis can identify turbulent structures that contribute significantly to turbulent fluxes, the approach to the hyperbolic hole and its parameter, referred to as hole size, remains uncertain and varies among different studies. This study discusses an improved quadrant analysis with an objective definition of the hole size for the isolation of large coherent structures from small-scale background fluctuations. Eddy covariance data collected 50 m above the grass canopy were used to analyze and evaluate the proposed method. This improved quadrant analysis ensures that the detected large coherent eddies play a dominant role in transporting fluxes, occupying 10% of the total time, with mean flux contributions ranging from 62% to 95% for momentum and 35–104% for scalars. The separated background small-scale eddies are isotropically characterized by a comparable time duration and flux contributions in each quadrant. It is observed that under an unstable atmosphere, large-scale ejections are more active than sweeps, while under stable conditions, they are equally important. Furthermore, mechanical-driven transport under near-neutral conditions only enhances ejection and sweep motions of momentum. In contrast, the buoyancy-driven scenarios under unstable conditions enhance the large-scale activities of sensible heat alone

Impact of Aerosol Mixing State and Hygroscopicity on the Lidar Ratio

The lidar ratio (LR) is a key parameter for the retrieval of atmospheric optical parameters from lidar equations. In this study, we simulated the optical parameters to investigate the impact factors of the LR using a three-component optical aerosol assumption based on the Mie model. The simulated LR was generally related to the overall particle size of the aerosols, the proportion of elemental carbon (EC), as well as aerosol mixing states and hygroscopicity. The LR was positively correlated with the particle size and volume fraction of elemental carbon (fEC). The LR increased more than three-fold with the increase in fEC from 0% to 40%. The LR of the core-shell (CS) mixing state and homogeneously internal (INT) mixing state was greater than that of the external (EXT) mixing state. The LR of all mixing states increased monotonically with hygroscopicity when the fEC was below 10%, while the LR of the core-shell mixing state (homogeneously internal mixing state) initially decreased (increased) and then increased (decreased) with increasing hygroscopicity when the fEC was more than 20%. These results will help in selecting a reasonable LR for practical applications

Impact of Aerosol Mixing State and Hygroscopicity on the Lidar Ratio

The lidar ratio (LR) is a key parameter for the retrieval of atmospheric optical parameters from lidar equations. In this study, we simulated the optical parameters to investigate the impact factors of the LR using a three-component optical aerosol assumption based on the Mie model. The simulated LR was generally related to the overall particle size of the aerosols, the proportion of elemental carbon (EC), as well as aerosol mixing states and hygroscopicity. The LR was positively correlated with the particle size and volume fraction of elemental carbon (fEC). The LR increased more than three-fold with the increase in fEC from 0% to 40%. The LR of the core-shell (CS) mixing state and homogeneously internal (INT) mixing state was greater than that of the external (EXT) mixing state. The LR of all mixing states increased monotonically with hygroscopicity when the fEC was below 10%, while the LR of the core-shell mixing state (homogeneously internal mixing state) initially decreased (increased) and then increased (decreased) with increasing hygroscopicity when the fEC was more than 20%. These results will help in selecting a reasonable LR for practical applications