Novel Methods for the Quantification of Atmospheric Turbulence Strength in the Atmospheric Surface Layer

Download or read book Novel Methods for the Quantification of Atmospheric Turbulence Strength in the Atmospheric Surface Layer written by Raymond James Oermann and published by . This book was released on 2014 with total page 336 pages. Available in PDF, EPUB and Kindle. Book excerpt: Atmospheric turbulence is a random and chaotic process brought about by energy transport mechanisms in the atmosphere. Of particular focus in this research program is the energy transport occurring in the atmospheric surface layer between ground heated air parcels and the air directly above. This dynamic process is impacted by so many apparently random initial conditions and boundary states that a precise analytical description of the process is beyond current computation. The process of turbulence can, however, be statistically analysed and characterised empirically. Observable phenomena such as scintillation have been found to have a range dependence and even the micro-meteorological structure of the turbulent eddies (chiefly responsible for the convective energy transport) have had their spatial distribution, scale size and power spectra examined in detail. As complex as the field is, one is left in no doubt, when viewing a scene through strong turbulence, that this dynamic and random process disrupts and perturbs the passage of visible radiation and is seen to severely degrade the quality of imagery. So the question raised is: "How can the strength of atmospheric turbulence be determined from either physical measurements of meteorological data or from sequences of perturbed imagery?" In order for imagery to be corrected for turbulence-induced degradation, or even the reliability of the imagery for uses such as object recognition and identification, an accurate measure of the current turbulence strength is required. After exploring the turbulence phenomenon and its impact upon laser spots and imagery, three parallel approaches to the determination of the current turbulence strength were pursued in this research program. The first was a controlled laboratory experiment where an artificial turbulent air- stream generated above a hot plate was investigated. Using sensitive temperature probes the temperature structure function (a quantity specifically developed to describe the spatial nature of a turbulent field) in a region of relatively high turbulence strength was determined. The homogeneity and isotropy of this turbulent space was then assessed. The instrument and processes developed were also applied in the field to studies of an ambient turbulent field. Measurements of turbulence strength made in this way were compared to results from a commercial scintillometer. The second line of research was field based. It involved the assessment of a range of observable features of the effects of turbulent air masses upon visible imagery. These features included analysis of the apparent motion of an incoherent light source and features of the scene. These observable effects were assessed along a horizontal path 1.5 m above the ground for their ability to infer the strength of the turbulent air mass through which the radiation had passed. The third line of research involved the development of an understanding of the impact of specific local meteorological parameters on the strength of turbulence. A major outcome from over one hundred days of data was the development of an empirical model of turbulence strength that improves upon previous models from the literature. The merits and shortcomings of each of these approaches to the characterisation of atmospheric turbulence conditions are compared. Ultimately, the ability to use imagery sequences acquired through turbulence, the temperature structure function of a sample volume or an empirical model using meteorological parameters to determine the strength of that turbulence, were assessed. This information is a necessary first step in the development of techniques for the mitigation of turbulence-induced, image distortion.