浙北地区多普勒雷达逆风区与风廓线雷达边界层大风提前量研究

浙江地区汛期雷暴大风天气多发频发,常导致经济损失和人员伤亡。为提高短临预报水平,文章利用地面自动气象站、多普勒天气雷达、风廓线雷达资料,选取2022年浙北地区雷暴大风个例,探究了雷达逆风区与边界层大风的关系并计算了地面大风的预警提前量。结果表明:多普勒天气雷达能提前探测到中低空和边界层逆风区的发生发展,较地面大风出现有平均16.9 min和11.4 min的提前量。通过2020~2022年的典型强对流大风个例分析,多普勒雷达逆风区–风廓线雷达边界层大风–自动站地面大风三者之间能形成短临预报提前量时间链,对地面雷暴大风的短临预警有指示意义。此外,2022年7月12日发生的下击暴流事件中,多普勒雷达最低仰角探测水平风场变化不明显,但风廓线雷达探测到高空风速突然增大,早于地面大风出现时间约33 min,表明风廓线雷达有利于提高下击暴流的提前预报预警能力。

Atmospheric boundary layer sources for upper tropospheric air over the Asian summer monsoon region

Previous satellite measurements and model simulations have shown that the Asian summer monsoon（ASM） anticyclone is co-located with higher concentrations of pollutants, which are emitted in the continental atmospheric boundary layer（ABL）. Backward trajectory calculations show that the air at the 150-hPa level has the maximum frequency of ABL sources within 30 days over the most intensive convection regions and their downwind areas, which are not located within the ASM anticyclone,but rather at the southern flank or periphery of the ASM anticyclone. The upper tropospheric airs originated from the ABL sources include two parts： one from the ocean, which has the dominant impact to the south of 20°N, particularly over the South China Sea（SCS） and the west tropical Pacific Ocean; and another from the continent, which is dominant between 10°N and 30°N, particularly over the Bay of Bengal（BoB）, continental India, the Arabian Sea, and the Arabian Peninsula. It is the latter part that forms the higher pollutant concentration within the ASM anticyclone as shown by satellite measurements. Air in the ABL sources（both polluted and unpolluted） converges to the intensive convection region in the lower troposphere, and then traverses the middle troposphere through a wide group of upward pipes, and finally to the upper troposphere. These pipes in the middle troposphere are defined by the ASM intensive convections and cover the south of continental India,the BoB, the Tibetan Plateau, the Indochina Peninsula, the SCS, and the Philippine Sea.

Marine Atmospheric Boundary Layers Associated with Summer Monsoon Onset over the South China Sea in 1998

The variations of the marine atmospheric boundary layer (MABL) associated with the South China Sea Summer Monsoon were examined using the Global Positioning System (GPS) sounding datasets obtained four times daily during May-June 1998 on board Research Vessels Kexue 1 and Shiyan 3. The MABL height is defined as the height at the lowest level where virtual potential temperature increases by 1 K from the surface. The results indicate that the MABL height decreased over the northern South China Sea (SCS) and remained the same over the southern SCS, as sea surface temperature (SST) fell for the northern and rose for the southern SCS after the monsoon onset. Over the northern SCS, a decrease in both the SST and the surface latent-heat flux after the onset resulted in a reduction of the MABL height as well as a decoupling of MABL from clouds. It was found that MABL height reduction corresponded to rainfall occurrence. Over the southern SCS, a probable reason for the constant increase of SST and surface heat flux was the rainfall and internal atmospheric dynamics.

Gustiness and coherent structure under weak wind period in atmospheric boundary layer

Statistical analysis of turbulent and gusty characteristics in the atmospheric boundary layer under weak wind period has been carried out.The data used in the analysis were from the multilevel ultrasonic anemometer-thermometers at 47 m,120 m,and 280 m levels on Beijing 325 m meteorological tower.The time series of 3D atmospheric velocity were analyzed by using conventional Fourier spectral analysis and decompose into three parts:basic mean flow(period > 10 min),gusty disturbances(1 min < period < 10 min)and turbulence fluctuations(period < 1 min).The results show that under weak mean wind condition:1)the gusty disturbances are the most strong fluctuations,contribute about 60% kinetic energy of eddy kinetic energy and 80% downward flux of momentum,although both the eddy kinetic energy and momentum transport are small in comparison with those in strong mean wind condition;2)the gusty wind disturbances are anisotropic;3)the gusty wind disturbances have obviously coherent structure,and their horizontal and vertical component are negatively correlated and make downward transport of momentum more effectively;4)the friction velocities related to turbulence and gusty wind are approximately constant with height in the surface layer.

Natural Ventilation by Air Captors and Extractors Sheds in Hospitals in Brazil： Wind Tunnel Measurements

Natural ventilation is an efficient design strategy for the passive cooling of buildings, especially in tropical countries such as Brazil. Among the ventilation strategies, sheds can be highlighted. These structures consist of roof openings that work as air captors or extractors depending on their location in relation to the prevailing wind directions. The hospitals of the Sarah Network, designed by the Brazilian architect Joao Filgueiras Lima, Lele, are worldwide known for using these elements to improve natural ventilation. This paper analyses the natural ventilation performance of sheds for air collecting and extracting in two Sarah hospitals located in the cities of Salvador and Rio de Janeiro. In each building, the sheds were analyzed for air extracting and collecting. The analyses were carried out by reduced physical models in an atmospheric boundary layer wind tunnel. The wind velocity was measured at external and internal points of the buildings, using hot-wire anemometers. The results show that the wards in Rio de Janeiro hospital are 17% more ventilated than the ones in the Salvador hospital. However, this difference occurs not only because of the collecting sheds but also because of set of openings and the configuration of the covering in hospitals in Rio de Janeiro.

Roll Vortices in the Boundary Layer Caused by a Concave Wind Profile: A Theoretical Study

The present study solves a two-layer atmospheric wave equation model with a lower atmosphere concave wind profile and cold-air outbreak over sea, while simultaneously proving that such a wind shear may cause neutral boundary layer roll vortices in the presence of disturbing sources upstream. Without thermal effects, the wind shear-induced waves have band structures at the top of the boundary layer that are similar to cloud street patterns observed over sea. This study proves that dynamic and thermal effects can act independently to initiate the roll vortices in the lower atmosphere. At the same time, a quantitative comparison shows that dynamic effects play a large role in the formation of roll vortices in the initial stage of cold-air outbreak and will be surpassed by thermal effects soon after surface heating commences.

Interaction between the atmospheric boundary layer and a standalone wind turbine in Gansu—Part I： Field measurement

Experiments and numerical simulations of the wake field behind a horizontal-axis wind turbine are carried out to investigate the interaction between the atmospheric boundary layer and a stand-alone wind turbine. The tested wind turbine（33 k W） has a rotor diameter of 14.8 m and hub height of 15.4 m. An anti-icing digital Sonic wind meter, an atmospheric pressure sensor, and a temperature and humidity sensor are installed in the upstream wind measurement mast. Wake velocity is measured by three US CSAT3 ultrasonic anemometers. To reflect the characteristics of the whole flow field, numerical simulations are performed through large eddy simulation（LES） and with the actuator line model. The experimental results show that the axial velocity deficit rate ranges from 32.18% to 63.22% at the three measuring points. Meanwhile, the time-frequency characteristics of the axial velocities at the left and right measuring points are different. Moreover, the average axial and lateral velocity deficit of the right measuring point is greater than that of the left measuring point. The turbulent kinetic energy（TKE） at the middle and right measuring points exhibit a periodic variation, and the vortex sheet-pass frequency is mostly similar to the rotational frequency of the rotor. However, this feature is not obvious for the left measuring point. Meanwhile, the power spectra of the vertical velocity fluctuation show the slope of-1, and those of lateral and axial velocity fluctuations show slopes of-1 and-5/3, respectively.However, the inertial subranges of axial velocity fluctuation at the left, middle, and right measuring points occur at 4, 7, and7 Hz, respectively. The above conclusion fully illustrates the asymmetry of the left and right measuring points. The experimental data and numerical simulation results collectively indicate that the wake is deflected to the right under the influence of lateral force. Therefore, wake asymmetry can be mainly attributed to the lateral force exerted by the wind turbine on the fluid.