Jianning Sun

Long-term observation of air pollution-weather/climate interactions at the SORPES station: a review and outlook

This work presents an overall introduction to the Station for Observing Regional Processes of the Earth System–SORPES in Nanjing, East China, and gives an overview about main scientific findings in studies of air pollution-weather/climate interactions obtained since 2011. The main results summarized in this paper include overall characteristics of trace gases and aerosols, chemical transformation mechanisms for secondary pollutants like O3, HONO and secondary inorganic aerosols, and the air pollution–weather/climate interactions and feedbacks in mixed air pollution plumes from sources like fossil fuel combustion, biomass burning and dust storms. The future outlook of the development plan on instrumentation, networking and data-sharing for the SORPES station is also discussed.

Characteristics of the Drag Coefficient in the Roughness Sublayer over a Complex Urban Surface

The statistics of momentum exchange in the urban roughness sublayer are investigated. The analysis focuses on the characteristics of the dimensionless friction velocity,

Simulation study on light propagation in an isotropic turbulence field of the mixed layer.

{u_{*}}/U

Observation-based estimation of aerosol-induced reduction of planetary boundary layer height

u∗/U, which is defined as the square root of the drag coefficient. The turbulence observations were made at a height of 47 m above the ground on the 325-m meteorological tower, which is located in a very inhomogeneous urban area in Beijing. Under neutral conditions, the dependence of the drag coefficient on wind speed varies with wind direction. When the airflow is from the area of densely built-up buildings, the drag coefficient does not vary with wind speed, while when the airflow is from the area covered by vegetation, the drag coefficient appears to decrease with increasing wind speed. Also, the drag coefficient does not vary monotonically with the atmospheric stability. Both increasing stability and increasing instability lead to the decrease of the drag coefficient, implying that the roughness length and zero-plane displacement may vary in urban areas.

Contribution of Low-Frequency Motions to Sensible Heat Fluxes over Urban and Suburban Areas

Field measurements of the momentum flux and wind velocity gradient were carried out at three urban sites in the cities of Nanjing, Changzhou, and Suzhou, China. The observational data in the urban roughness sublayer are analyzed to derive the momentum flux-gradient relations in terms of framework of the local similarity theory with a least squares fit, and the relations are then compared to the classical similarity relations of Businger et al. (1971). The results show that the momentum flux varies with height; thus, the constant-flux assumption in the Monin-Obukhov Similarity Theory is not met in the urban roughness sublayer. However, the dimensionless wind velocity gradient may be described by the local similarity theory, and the derived flux-gradient relations have the same form as the classical similarity relations do. Under stable conditions, the coefficient βm in the fitted relations increases with height and gradually approaches the value of 4.7 in the classical similarity relations. The trend suggests that the turbulent flow may be described by the classical similarity relations once the height increases and reaches the top of the urban roughness sublayer, and thus, the height of the top of the urban roughness sublayer may be estimated by linear extrapolation of the derived relations in this study. The relation between the critical Richardson number Ric and the coefficient βm is derived as Ricu2009=u2009Prt/βm, where Prt is the turbulent Prandtl number. In the urban roughness sublayer, the value of Ric is larger than 0.2.

Scaling Characteristics of Developing Sea Breezes Simulated in a Water Tank

Water tank experiments and numerical simulations are employed to investigate the characteristics of light propagation in the convective boundary layer (CBL). The CBL, namely the mixed layer (ML), was simulated in the water tank. A laser beam was set to horizontally go through the water tank, and the image of two-dimensional (2D) light intensity fluctuation formed on the receiving plate perpendicular to the light path was recorded by CCD. The spatial spectra of both horizontal and vertical light intensity fluctuations were analyzed, and the vertical distribution profile of the scintillation index (SI) in the ML was obtained. The experimental results indicate that 2D light intensity fluctuation was isotropically distributed in the cross section perpendicular to the light beam in the ML. Based on the measured temperature fluctuations along the light path at different heights, together with the relationship between temperature and refractive index, the refractive index fluctuation spectra and the corresponding turbulence parameters were derived. The obtained parameters were applied in a numerical model to simulate light propagation in the isotropic turbulence field. The calculated results successfully reproduce the characteristics of light intensity fluctuation observed in the experiments.

Scale Properties of Anisotropic and Isotropic Turbulence in the Urban Surface Layer

The convective atmospheric boundary layer was modeled in the water tank. In the entrainment zone (EZ), which is at the top of the convective boundary layer (CBL), the turbulence is anisotropic. An anisotropy coefficient was introduced in the presented anisotropic turbulence model. A laser beam was set to horizontally go through the EZ modeled in the water tank. The image of two-dimensional (2D) light intensity fluctuation was formed on the receiving plate perpendicular to the light path and was recorded by the CCD. The spatial spectra of both horizontal and vertical light intensity fluctuations were analyzed. Results indicate that the light intensity fluctuation in the EZ exhibits strong anisotropic characteristics. Numerical simulation shows there is a linear relationship between the anisotropy coefficients and the ratio of horizontal to vertical fluctuation spectra peak wavelength. By using the measured temperature fluctuations along the light path at different heights, together with the relationship between temperature and refractive index, the one-dimensional (1D) refractive index fluctuation spectra were derived. The anisotropy coefficients were estimated from the 2D light intensity fluctuation spectra modeled by the water tank. Then the turbulence parameters can be obtained using the 1D refractive index fluctuation spectra and the corresponding anisotropy coefficients. These parameters were used in numerical simulation of light propagation. The results of numerical simulations show this approach can reproduce the anisotropic features of light intensity fluctuations in the EZ modeled by the water tank experiment.

Impact of Eddy Characteristics on Turbulent Heat and Momentum Fluxes in the Urban Roughness Sublayer

Radiative aerosols are known to influence the surface energy budget and hence the evolution of the planetary boundary layer. In this study, we develop a method to estimate the aerosol-induced reduction in the planetary boundary layer height (PBLH) based on two years of ground-based measurements at a site, the Station for Observing Regional Processes of the Earth System (SORPES), at Nanjing University, China, and radiosonde data from the meteorological station of Nanjing. The observations show that increased aerosol loads lead to a mean decrease of 67.1 W m−2 for downward shortwave radiation (DSR) and a mean increase of 19.2 W m−2 for downward longwave radiation (DLR), as well as a mean decrease of 9.6 Wm−2 for the surface sensible heat flux (SHF) in the daytime. The relative variations of DSR, DLR and SHF are shown as a function of the increment of column mass concentration of particulate matter (PM2.5). High aerosol loading can significantly increase the atmospheric stability in the planetary boundary layer during both daytime and nighttime. Based on the statistical relationship between SHF and PM2.5 column mass concentrations, the SHF under clean atmospheric conditions (same as the background days) is derived. In this case, the derived SHF, together with observed SHF, are then used to estimate changes in the PBLH related to aerosols. Our results suggest that the PBLH decreases more rapidly with increasing aerosol loading at high aerosol loading. When the daytime mean column mass concentration of PM2.5 reaches 200 mg m−2, the decrease in the PBLH at 1600 LST (local standard time) is about 450 m.摘要大气气溶胶通过吸收和散射太阳辐射影响地表能量平衡, 从而影响行星边界层的发展. 本文利用南京大学 SORPES (the Station for Observing Regional Processes of the Earth System) 超级站的两年地面观测数据以及南京探空站的遥感观测数据, 估算大气气溶胶含量对行星边界层高度的影响. 地面观测结果表明: 白天大气气溶胶的存在导致向下短波辐射的年平均值下降 67.1 W m−2; 向下长波辐射的年平均值上升 19.2 W m−2; 地面感热通量的年平均值下降 9.6 W m−2. 并且大气辐射量的变化与 PM2.5 柱浓度的增加有很强的相关性. 利用地面感热通量与 PM2.5 柱浓度之间的统计学关系, 本文估算出气溶胶含量极低 (背景天) 时地面感热通量的大小. 并利用估算出的地面感热通量以及其观测结果, 计算出因气溶胶含量增加而导致的边界层高度的降低量. 本文的结果认为随着大气中气溶胶含量的增加, 边界层高度加速降低. 当白天气溶胶平均柱浓度达到 200 mg m−2时, 16 时 (当地时间) 时边界层高度下降约 450 m.