Han Zou

The limited understanding of the structure of the urban surface atmospheric boundary layer can be attributed to its inherent complexity, as well as a deficiency in comprehensive measurements. We analyzed one year of meteorological data and Doppler wind lidar measurements in Beijing to explore how atmospheric stability is influenced by wind speed, radiation, turbulence, and pollution levels. Results indicate that the predominant state of the urban boundary layers in Beijing is an active condition (characterized by strong unstable and unstable stability regimes) throughout the day, attributed to the significant heat storage capacity of the urban canopy. Strong stable regimes are more frequently observed during winter and autumn, peaking during transitions from night to day. Furthermore, both strong unstable and strong stable regimes occur under very weak wind conditions (indicating weak dynamic instability), with strong instability associated with high net radiation levels while strong stability correlates with low net radiation conditions (indicative of robust thermal stability). The unstable regime manifests under strong winds (reflecting strong dynamic instability) alongside moderate net radiation environments, characterized by elevated values of turbulence kinetic energy and urban boundary height, highlighting the critical role of mechanical turbulence generation during periods of high wind activity. Additionally, six instances of pronounced stable conditions observed during daytime can be partially attributed to low radiation coupled with high pollutant concentrations near the surface, resulting from prolonged temperature inversions due to intense radiative cooling effects and weak dynamic forcing. Our findings presented herein are expected to have urban boundary layer climate and environment implications for other cities with high pollution and dense urban infrastructure all over the world.

Atmospheric ducts could disturb radar detection and radio communication in the Arctic and risk increasing human activities under Arctic warming. It is an urgent need to understand atmospheric ducts in the Arctic, with distinct atmospheric systems and environment. This study attempts to analyze the surface atmospheric ducts over Svalbard, as a critical region in the Arctic Ocean, and discuss the relation to atmospheric and oceanic conditions, based on observation and reanalysis data. The results show an occurrence of 12.6 percent, mean strength of 1.30 MU, and mean depth of 13 m for the surface duct in Svalbard winter from December 2018 to February 2020. The surface duct is characterized as two types: T-type ducts mainly constructed by temperature inversion and H-type ducts constructed by a humidity gradient. The T-type duct dominates the surface duct in Svalbard winter, with an appearance percentage of 80 percent, indicating an importance of temperature inversion in surface duct construction. The surface duct is closely related to atmospheric and oceanic conditions in Svalbard winter, because the T-type duct occurs in colder, drier, and northeasterly weather and the H-type duct occurs in warmer, wetter, and southeasterly weather with higher sea surface temperature.

The diurnal evolution of the atmospheric boundary layer—the lowermost part of the atmosphere where the majority of human activity and meteorological phenomena take place—is described by its depth. Additionally, the boundary layer height (BLH) and the turbulence intensity strongly impact the pollutant diffusion, especially during transition periods. Based on integrated observations from a 325-m meteorological tower and a Doppler Wind lidar in the center of Beijing, the entire diurnal cycle of urban BLH in December 2016 was characterized. Results highlight that the Doppler lidar exhibited it is well suited for monitoring convective BLH while it trudges in monitoring stable BLH, while a 325-m meteorological tower provided an important supplement for Doppler lidar under nocturnal boundary layer and heavily polluted conditions. For the diurnal cycle, under light wind condition, the pattern of urban BLH was largely modulated by thermal forcing of solar radiation and may partly be affected by wind speed. While under strong wind condition, the pattern of urban BLH was largely modulated both by thermal forcing and dynamical forcing. The present work also presented evidence for several new features in the morning and afternoon transitions of the urban boundary layer, showing the duration of the morning transition varied between 1 and 5 h, with the largest value occurring under weak wind with high PM_2.5 concentration; while the afternoon transition ranged from 3 to 6 h, which was positively (negatively) correlated to wind speed (PM_2.5 concentration). Our work highlights that weak wind speed (weak dynamic motion) and heavy aerosol pollution (weak thermal forcing due to the effect of cooling) can dramatically affect the evolution of the boundary layer.

Surface heating is important in terms of the thermal effect of the Tibetan Plateau on atmospheric systems in Asia. The complex mountain terrain across the Tibetan Plateau introduces small-scale heterogeneity in the local atmosphere and land–air heat exchange that biases our understanding of the Tibetan thermal effect and misleads boundary-layer parameterization for numerical studies and predictions. It is therefore essentially important to evaluate heterogeneity in the Tibetan mountains. An observational experiment was conducted in a small region with a complex landscape in the eastern Himalayas from June 9 to July 9, 2013. This study analyzes the observations of three major land covers and discusses the observed heterogeneity of the local atmosphere and land–air heat exchange. A heterogeneity rate is defined to approximately quantify the heterogeneity observed in the atmospheric properties and land–air heat exchange. Obvious heterogeneity is found in the local atmosphere across the complex terrain of the Eastern Himalayas, which could introduce uncertainties up to 33.3 percent in our knowledge related to the local weather and climate. A large heterogeneity is revealed in the land–air heat exchange, which would bias our understanding of total surface heating in this region up to 16 percent and of heating properties up to 62 percent. Therefore, attention should be given to the uncertainties introduced by small-scale heterogeneity in any weather and climate studies across a complex terrain, such as the eastern Himalayas. In addition, the heterogeneity rate provided could be a useful measure to generally identify the uncertainties in our understanding of the local atmosphere and land–air heat exchange across a complex terrain.

The Southeast Tibet is an important region of the Tibetan Plateau bearing the interaction between the Tibetan and the neighbor atmospheric systems. The South Asian summer monsoon (SASM) as a basic climate system in Asia could impact the local atmosphere and the near‐surface heat exchange process in the Southeast Tibet. An observational campaign, Observation on the Surface‐to‐air Exchange Processes in Southeast Tibet (OSEP2013), was carried out in this region during SASM in 2013. The atmospheric parameters and turbulent heat fluxes were observed and averaged over three different land surfaces of the inhomogeneous landscape during the observation campaign. Results show clear SASM impacts on the local atmosphere and near‐surface heat exchange in the Southeast Tibet. The South Asian summer monsoon was onset on 1 June 2013 and experienced a south phase and north phase during OSEP2013. The convection and humidity were increased in the Southeast Tibet by SASM, especially during the north phase. The observation domain received low radiation energy due to the convective clouds brought by SASM, and the soil and air temperatures were lowered as consequence. In addition, the air humidity was increased over this region by the wet air transportation of SASM circulation. The sensible and latent heat transfers were decreased by the low land‐air temperature difference and high air humidity during SASM. The latent heat transfer dominated the total heat transfer in the Southeast Tibet due to the low sensible heat transfer in the SASM situation, and the domination was increased as the sensible heat transfer was further decreased during the SASM north phase.