A numerical investigation of the influence of land-sea breeze on the atmospheric effluents released at a coastal site

ABSTRACT. Mesoscale features of a coastal atmospheric boundary layer such as the land-sea circulation and the thermal internal boundary layer (TIBL) structure have been simulated using a two-dimensional numerical boundary layer model. Using Boussinesq approximation for horizontal momentum equations and hydrostatic approximation for vertical momentum equation the model solves the 'shallow water' equations year over a grid domain 80 km length on either side of the coastline and 2 km height. The influence of the land-sea breezes on the dispersion of pollutants released from a continuous point source located at the roast has been studied. The fumigation of pollutants from an offshore source into TIBL over the land has also been illustrated. The limitations associated with the model are also discussed.

Boundary-layer evolution over long wind farms

The structure of the internal boundary layer above long wind farms is investigated experimentally. The transfer of kinetic energy from the region above the farm is dominated by the turbulent flux of momentum together with the displacement of kinetic energy operated by the mean vertical velocity: these two have comparable magnitude along the farm opposite to the infinite-farm case. The integration of the energy equation in the vertical highlighted the key role of the energy flux, and how that is balanced by the growth of the internal boundary layer in terms of energy thickness with a small role of the dissipation. The mean velocity profiles seem to follow a universal structure in terms of velocity deficit, while the Reynolds stress does not follow the same scaling structure. Finally, a spectral analysis along the farm identified the leading dynamics determining the turbulent activity: while behind the first row the signature of the tip vortices is dominant, already after the second row their coherency is lost and a single broadband peak, associated with wake meandering, is present until the end of the farm. The streamwise velocity peak is associated with a nearly constant Strouhal number weakly dependent on the farm layout and free stream turbulence condition. A reasonable agreement of the velocity spectra is observed when the latter are normalised by the velocity variance and integral time scale: nevertheless the spectra show clear anisotropy at the large scales and even the small scales remain anisotropic in the inertial subrange.

Internal boundary layer development over lake surface in case of very young waves

<p>An internal boundary layer (IBL) may develop above lakes due to surface roughness change. The water surface has significantly less resistance to wind flow compared to the aerodynamically rough land surface. As a result, the wind speed increases along the fetch in the IBL over the lake surface. Consequently, the wind shear stress, which is the main driving force of waves and currents in lakes, also varies along the fetch. Measurements were carried out for six weeks in 2018 within a Croatian-Hungarian observational campaign in Lake Balaton in order to explore the IBL characteristics and establish a simple but reliable IBL model that can reproduce wind shear stress variability over the lake. One wind measurement station was installed on land and three over the lake along the fetch of the prevailing wind direction. On the landside, the wind profile was observed by a sodar from which characteristic land surface roughness lengths were derived by logarithmic profile fitting. On the waterside, momentum fluxes were measured with eddy-covariance (EC) technique at fetches of ~0.1, ~3.5, and ~6 km. To describe the water surface roughness dynamics, waves were simultaneously recorded with an underwater acoustic surface tracking at the middle station. An analytic IBL model is fitted to the measured wind speed and stress data employing wind speed classes. In the model, the wind stress development is dynamically coupled with the wave state by a wave age dependent roughness length function which is valid for highly fetch limited conditions and very young wave ages of ~2-15. The model is able to quantitatively reproduce wind speed, wind stress, and wave state development over the lake surface based on land observation of wind speed if the land roughness length is also known. Based on our model and measurements, we found a considerable spatial variability of momentum flux due to the change of wave state and wind speed along the fetch. The variation of momentum flux also influences the evolving sensible heat flux, which was also compared to the EC measurements.</p>

Internal boundary layer in a singularly perturbed problem of fractional derivative

This paper is devoted to the study of internal boundary layer. Such motions are often associated with effect of boundary layer, i.e. low flow viscosity affects only in a narrow parietal layer of a streamlined body, and outside this zone the flow is as if there is no viscosity - the so-called ideal flow. Number of exponentials in the boundary layer is determined by the number of non-zero points of the limit operator spectrum. In the paper we consider the case when spectrum of the limit operator vanishes at the point To study the problem the Lomov regularization method is used. The original problem is regularized and the main term of asymptotics of the problem solution is constructed as the low viscosity tends to zero. Numerical results of solutions are obtained for different values of low viscosity.

Marine Boundary Layers above Heterogeneous SST: Across-Front Winds

Turbulent flow in a weakly convective marine atmospheric boundary layer (MABL) driven by geostrophic winds Ug = 10 m s−1 and heterogeneous sea surface temperature (SST) is examined using fine-mesh large-eddy simulation (LES). The imposed SST heterogeneity is a single-sided warm or cold front with temperature jumps Δθ = (2, −1.5) K varying over a horizontal distance between [0.1, −6] km characteristic of an upper-ocean mesoscale or submesoscale regime. A Fourier-fringe technique is implemented in the LES to overcome the assumptions of horizontally homogeneous periodic flow. Grid meshes of 2.2 × 109 points with fine-resolution (horizontal, vertical) spacing (δx = δy, δz) = (4.4, 2) m are used. Geostrophic winds blowing across SST isotherms generate secondary circulations that vary with the sign of the front. Warm fronts feature overshoots in the temperature field, nonlinear temperature and momentum fluxes, a local maximum in the vertical velocity variance, and an extended spatial evolution of the boundary layer with increasing distance from the SST front. Cold fronts collapse the incoming turbulence but leave behind residual motions above the boundary layer. In the case of a warm front, the internal boundary layer grows with downstream distance conveying the surface changes aloft and downwind. SST fronts modify entrainment fluxes and generate persistent horizontal advection at large distances from the front.

Study of the Vertical Structure of the Coastal Boundary Layer Integrating Surface Measurements and Ground-Based Remote Sensing

The understanding of the atmospheric processes in coastal areas requires the availability of quality datasets describing the vertical and horizontal spatial structure of the Atmospheric Boundary Layer (ABL) on either side of the coastline. High-resolution Numerical Weather Prediction (NWP) models can provide this information and the main ingredients for good simulations are: an accurate description of the coastline and a correct subgrid process parametrization permitting coastline discontinuities to be caught. To provide an as comprehensive as possible dataset on Mediterranean coastal area, an intensive experimental campaign was realized at a near-shore Italian site, using optical and acoustic ground-based remote sensing and surface instruments, under different weather characteristic and stability conditions; the campaign is also fully simulated by a NWP model. Integrating information from instruments responding to different atmospheric properties allowed for an explanation of the development of various patterns in the vertical structure of the atmosphere. Wind LiDAR measurements provided information of the internal boundary layer from the value of maximum height reached by the wind profile; a height between 80 and 130 m is often detected as an interface between two different layers. The NWP model was able to simulate the vertical wind profiles and the eight of the ABL.

Estimativa da altura da camada limite planetária no Centro de Lançamento de Alcântara

Sea and land breeze circulation and the internal boundary layer are some aspects that make it difficult to determine the coastal Planetary Boundary Layer (PBL) height (h). This paper evaluates the h estimation for the Alcantara Launch Center (CLA) in Maranhão state using 14 months of remote measurements obtained by a ceilometer and by the ERA5 reanalysis. This response depends on the concentration of aerosols and atmospheric humidity present. Mean results indicated that dry months (September to November, with = 637 488 m) tend to have less hourly and seasonal variability of h compared to wet months (March to May, with = 770 912 m). A higher mean error in the wet season was obtained with ERA5 PBL h (h = 708 53 m over the land; e h = 648 46 m over the ocean) than in the dry season (h = 18 89 m; e h = 46 77 m). The greater amount of atmospheric humidity during the rainy season increases the estimation uncertainty. This condition was more frequent at night due to typical rainfall in the place.