Atmospheric Flow Fields: Theory, Numerical Methods And Software Tools

This first chapter is an introduction that provides all the basic information that the reader will need for a profitable use of the book. It reviews the vertical structure of the atmosphere, the surface-energy budget and concepts of atmospheric stability. The atmosphere is conventionally divided into layers based on the vertical structure of density, pressure and temperature fields. This is an important issue in understanding the dynamics of the middle atmospheric and variations of the main meteorological parameters.

Atmospheric dynamics is governed by complex mathematical relationships that have been extensively modeled and discussed in recent decades. The aim of this chapter is to quickly recall the basic governing equations along with their main applications in order to better understand Modeling references. In the bibliography, a set of reference handbooks is provided for an exhaustive presentation of these topics.

The role of orography in a complex area is of great importance, even more so when the study area comprises a coastal region where there is a critical need for topographical information due to the presence of the sea (or, in general, large water masses). The surface boundary condition is generally defined by means of a Digital Elevation Model (DEM), which provides information about terrain heights in the area of interest. Both DEM and computational domains are discrete representations of reality, and need algorithms capable of translating the continuous reality into a discrete model. This study illustrates the care that must be taken when defining the surface boundary condition in order to avoid significant misinterpretation.

The atmospheric boundary layer (ABL) height a fundamental parameter characterising the structure of the lower troposphere. It is one of the important parameters requested by different dispersion models as input data for forecasting air quality. The aim of this chapter is to review various methods for the mixed layer height estimate in a complex coastal valley area and compare them to achieve critical awareness of their application. In this chapter, selected case studies of complex terrain meteorology are presented.

A dispersion model is essentially a computational procedure for predicting pollutant concentrations downwind of a source. Current models are based on knowledge of the emission characteristics (stack exit velocity, plume temperature, stack diameter, etc.), terrain (surface roughness, local topography, nearby buildings), and state of the atmosphere (wind speed, stability, mixing height, etc.). The main purpose of this chapter is to provide an overview of different dispersion models. The objective of dispersion Modeling is to predict the rate of spread of the pollutant cloud, and the consequent decrease in mean concentration. The model must be able to predict diffusion rates based on measurable meteorological variables such as wind speed, atmospheric turbulence, and thermodynamic effects. Therefore, algorithms at the heart of air pollution models are based on mathematical equations describing these various phenomena, which, can be used to predict concentration distributions downwind of a source when combined with empirical (field) data.

Analytical solutions of equations are of fundamental importance in understanding and describing physical phenomena. We provide a short review of the analytical solutions of the advection-diffusion equation. Two new solutions are presented, adopting novel analytical approaches named Generalized Integral Laplace Transform Technique (GILTT) and Advection Diffusion Multilayer Model (ADMM). The GILTT method is an analytical series solution of the advection-diffusion equation including the solution of an associate Sturn-Liouville problem, expansion of the pollutant concentration in a series in terms of the attained eigefunction, replacement of this expansion in the advection-diffusion equation and, finally, taking moments. This procedure leads to a set of differential ordinary equations that is solved analytically by Laplace transform technique. The ADMM method is an analytical integral solution of the advection-diffusion equation based on a discretization of the PBL in N sub-layers; in each sub-layers the advection-diffusion equation is solved by the Laplace transform technique, considering an average value for eddy diffusivity and the wind speed.

Purpose of this chapter was to analyze the planetary boundary layer (PBL) using remote sensing tools. Sodar techniques allowed us to obtain speed and wind direction profiles from a height ranging from 30/40 m. to 1000 m. The SODAR (Sound Detection And Ranging) is, in fact, an alternative to the use of cup anemometers and offers the possibility of measuring both the wind speed distribution with height and the wind direction. In particular, these instruments play a key role both to assess the wind resource in a specific area and to estimate the alteration of the wind field caused by the complex orography of the region.