Vertical profiles of meteorological parameters obtained from balloon-borne GPS Radiosonde for a period of more than two years are analyzed for characterization of the coastal atmospheric boundary layer (CABL) over Thumba ( , , India). The study reports seasonal variability in the thickness of three different sublayers of the CABL, namely, mixed layer, turbulent flow, and sea breeze flow. Among the three, the vertical thickness of sea breeze flow showed considerable dominance on the other two throughout the year. Mixed layer heights derived through gradients in virtual potential temperature ( ) showed large seasonal variability with a peak in the Summer and Post-Monsoon. On the other hand, the vertical thickness of turbulent flow remained steady all through the year. Results from the present study indicate that the magnitudes of mixed layer heights are often larger than the turbulent flow thickness. 1. Introduction In Physics and Fluid Dynamics, a boundary layer is generally defined as a layer of the fluid in the immediate vicinity of a bounding surface, where viscosity variations are dealt in detail. The atmospheric boundary layer (ABL) is defined as the lowest part of the Earth's atmosphere, where physical quantities such as flow velocity, temperature, and moisture display rapid fluctuations and vertical mixing is strong [1–3]. The ABL processes control exchanges of momentum, water, and trace substances between the Earth's surface and the free troposphere. Substances, once emitted into the ABL, are gradually dispersed horizontally and vertically through turbulent processes and become completely mixed over this layer. All the ABL processes largely dependent on atmospheric turbulence are represented in parameterized form in atmospheric models. The thickness of ABL is commonly used to characterize the vertical extent of mixing within the boundary layer and the level at which exchange with the free troposphere occurs [4–6]. In principle, the ABL thickness can be retrieved from atmospheric global circulation models since they contain algorithms which determine the intensity of the turbulence as a function of height [7, 8]. However, these data are not routinely available or on a vertical resolution which is too crude in view of the application. In the field of boundary layer meteorology and air pollution dispersion, the vertical thickness of ABL remain one of the important parameters governing the dispersion of pollutants and measured concentrations of trace gases and atmospheric aerosols [4–6]. Therefore, accurate knowledge and broad overview of the factors
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