Near-surface Atmospheric Boundary Layer Research Articles

Experimental investigation of the interaction between near-surface atmospheric boundary layer winds and downburst outflows

This article focuses on experimental investigation of the interaction between downburst (DB) and near-surface atmospheric boundary layer (ABL) winds. The flow field of DB immersed into ABL winds (DBABL) is compared against the outflow produced by an isolated DB without ABL winds. The diameter of investigated downdrafts was 3.2 ​m. The study demonstrates that there is a profound difference between the radial velocity components in the DBABL and DB outflows in terms of peak and mean velocities, vertical profile of radial velocity, as well as the overall temporal signature of the velocity records. The asymmetry of the DBABL outflow in the along-ABL wind direction is similar to that observed in real traveling downbursts. The turbulence intensity in the near-surface DBABL winds can exceed 40% in some parts of the outflow where the downburst propagates against ABL winds. The DBABL wind profile is characterized by smaller nose-shape curvature than the DB winds in the part of the outflow where downburst and ABL winds have the same direction. The steady-state ABL wind segments and transient downburst segments in the DBABL velocity records are similar to anemometer records of real downbursts. The paper also shows that the velocity addition DBABL→≠DB→+ABL→ throughout the outflow.

Seasonal Variations of the Near-Surface Atmospheric Boundary Layer Structure in China’s Gurbantünggüt Desert

As the largest fixed and semifixed desert in China, the Gurbantünggüt Desert has a longperiod of snow in winter and the rapid growth of ephemeral plants in spring, presentingthe obvious seasonal changes in the underlying desert surface type, which could lead to the significantvariety in the near-surface boundary layer over this desert. To clarify the influence of the underlying surface change on the near-surface atmospheric boundary layer, gradient tower data and Eddy covariance data in 2017 were analyzed. The results were as follows: the wind profile can be divided into the nocturnal stable boundary layer and the daytime unstable boundary in spring, summer, and autumn, while the wind profile dominating nighttime stability in winter. During the study period, the four-season temperature profiles can be divided into four types: night radiation type, morning transition type, daylight solar radiation type, and evening transition type, and the temperature difference between spring and summer is more than that of autumn and winter. The vertical temperature lapse rate can reach 4.5°C/100 m in spring and summer, while the vertical temperature lapse rate is 0.5°C/100 m in winter. The special humidity value in summer and spring is greater than autumn and winter. The profile is almost in the inverse humidity state at almost all periods in winter. The inverse humidity phenomenon occurred on the autumn night. Besides, the specific humidity is closely related to the temperature and the near-surface wind speed. The “rapid change” of the underlying surface of the spring desert region affects the surface energy budget, which affects the turbulent energy and the stability of the near-surface layer, thus affecting the changes in temperature, humidity, and wind profile.

Open-jet boundary-layer processes for aerodynamic testing of low-rise buildings

Investigations on simulated near-surface atmospheric boundary layer (ABL) in an open-jet facility are carried out by conducting experimental tests on small-scale models of low-rise buildings. The objectives of the current study are: (1) to determine the optimal location of test buildings from the exit of the open-jet facility, and (2) to investigate the scale effect on the aerodynamic pressure characteristics. Based on the results, the newly built open-jet facility is well capable of producing mean wind speed and turbulence profiles representing open-terrain conditions. The results show that the proximity of the test model to the open-jet governs the length of the separation bubble as well as the peak roof pressures. However, test models placed at a horizontal distance of 2.5H (H is height of the wind field) from the exit of the open-jet, with a width that is half the width of the wind field and a length of 1H, have consistent mean and peak pressure coefficients when compared with available results from wind tunnel testing. In addition, testing models with as large as 16% blockage ratio is feasible within the open-jet facility. This reveals the importance of open-jet facilities as a robust tool to alleviate the scale restrictions involved in physical investigations of flow pattern around civil engineering structures. The results and findings of this study are useful for putting forward recommendations and guidelines for testing protocols at open-jet facilities, eventually helping the progress of enhanced standard provisions on the design of low-rise buildings for wind.

Observations of relative humidity in the near-wake of a wind turbine using an instrumented unmanned aerial system

ABSTRACTSimulation, modeling, and limited observations have shown that wind farms have an impact on the near-surface atmospheric boundary layer as turbulent wakes enhance vertical mixing of momentum, heat, and moisture. The few observational datasets that do exist lack high spatial resolution due to their use of a limited number of meteorological sensors or remote sensing techniques. This study utilizes an instrumented small unmanned aerial system to gather high-resolution in situ field measurements in order to differentially map near-wake changes to relative humidity. Observations show that downstream relative humidity is differentially altered in the vertical, spanwise and downstream directions.

Diurnal and seasonal variations of wind farm impacts on land surface temperature over western Texas

This paper analyzes seasonal and diurnal variations of MODerate resolution Imaging Spectroradiometer (MODIS) land surface temperature (LST) data at ~1.1 km for the period of 2003–2011 over a region in West-Central Texas, where four of the world’s largest wind farms are located. Seasonal anomalies are created from MODIS Terra (~10:30 a.m. and 10:30 p.m. local solar time) and Aqua (~1:30 a.m. and 1:30 p.m. local solar time) LSTs, and their spatiotemporal variability is analyzed by comparing the LST changes between wind farm pixels (WFPs) and nearby non wind farm pixels (NNWFPs) using different methods under different quality controls. Our analyses show consistently that there is a warming effect of 0.31–0.70 °C at nighttime for the nine-year period during which data was collected over WFPs relative to NNWFPs, in all seasons for both Terra and Aqua measurements, while the changes at daytime are much noisier. The nighttime warming effect is much larger in summer than winter and at ~10:30 p.m. than ~1:30 a.m. and hence the largest warming effect is observed at ~10:30 p.m. in summer. The spatial pattern and magnitude of this warming effect couple very well with the geographic distribution of wind turbines and such coupling is stronger at nighttime than daytime and in summer than winter. Together, these results suggest that the warming effect observed in MODIS over wind farms are very likely attributable to the development of wind farms. This inference is consistent with the increasing number of operational wind turbines with time during the study period, the diurnal and seasonal variations in the frequency of wind speed and direction distribution, and the changes in near-surface atmospheric boundary layer (ABL) conditions due to wind farm operations. The nocturnal ABL is typically stable and much thinner than the daytime ABL and hence the turbine enhanced vertical mixing produces a stronger nighttime effect. The stronger wind speed and the higher frequency of the wind speed within the optimal power generation range in summer than winter and at nighttime than daytime likely drives wind turbines to generate more electricity and turbulence and consequently results in the strongest warming effect at nighttime in summer. Similarly, the stronger wind speed and the higher frequency of optimal wind speed at ~10:30 p.m. than that at ~1:30 a.m. might help explain, to some extent, why the nighttime LST warming effect is slightly larger at ~10:30 p.m. than ~1:30 a.m. The nighttime warming effect seen in spring and fall are smaller than that in summer and can be explained similarly.

Acoustic Tomography as a Remote Sensing Method to Investigate the Near-Surface Atmospheric Boundary Layer in Comparison with In Situ Measurements

Abstract The acoustic tomography method is applied in the atmospheric surface layer to observe near-surface temperature fields. Important advantages of this technique are the remote sensing capacity and the possibility of directly deriving area-average meteorological quantities. Combined observations of the air temperature using an acoustic tomography system and point measurements were carried out to validate the tomographic method. Results were used to compare representativeness for a designated area of direct measurements with the tomographic solution. The results demonstrate agreement between the two different measurement methods, except for some deviations of absolute values mainly caused by an imperfectly sheltered and ventilated thermocouple device.

Stationary model for resuspension of radionuclides and assessments of 137CS concentration in the near-surface layer for the contaminated areas in the Bryansk Region of Russia and Belarus

After the surface contamination from the Chernobyl accident on 26 April 1986 had been formed, the radioactive contamination in the near-surface atmospheric boundary layer was dependent upon several processes including resuspension. A model for resuspension, dispersion, transport, and redeposition of radioactive materials is described. The model input parameters are discussed, with emphasis on the resuspension rate. Two applications of the model to the southwestern part of the Bryansk Region in Russia and the adjacent area in Belarus are described. The first consists of a series of numerical experiments that served as preparation for the second application: estimates of the resuspension rate in the area surrounding the town of Novozybkov in Russia

On transformation of the near-surface atmospheric boundary layer in the ocean temperature front area

A two-dimensional, quasi-analytical model for the formation of the internal atmospheric boundary layer in the ocean temperature front area is described, developed on the basis of integral relations for motion and thermal conductivity equations. The computed data are matched with wind speed observations, as well as with direct and indirect dynamic velocity measurements in the air, obtained across the Gulf Stream frontal zone.

The mixing layer of Loch Ness

Measurements of the thermal and velocity structure of the near-surface mixing layer of a freshwater lake in moderate wind conditions from fixed or mobile arrays of sensors reveal large-scale coherent structures consisting of narrow fronts across which both the temperature and the horizontal component of the current increase. These fronts are generally transverse to the wind direction and are inclined to the vertical, and appear to be similar to fronts, reported as temperature ‘ramps’, in the near-surface atmospheric boundary layer. The time derivatives of the temperature are skewed in a sense consistent with observations in laboratory and atmospheric boundary layers, and of a magnitude consistent with measurements in the latter. Evidence is presented to show that bubbles generated by breaking waves are carried down in the large-scale pattern of flow associated with the fronts in the mixing layer. The presence of a Langmuir circulation associated with wind rows has not been established in these experiments. The relevance of the observations to the ocean mixing layer is discussed.