Experimental Study of an Artificial Thermal Plume in the Boundary Layer. Part II: Some Aspects of the Plume Thermodynamical Structure

Abstract

Some aspects of the mean and turbulent structures of artificial thermal plumes in the boundary layer (BL) are presented. This analysis is based mainly on measurements with an instrumented aircraft. As initial conditions for plume rise, the characteristics of the BL during the 10 experiments are summarized. Under neutral conditions, plume rise in the BL follows approximately the classical prediction. Plume structure aspects are analyzed inside the upwind active zone corresponding to the region of strongest gradients within the convective column. In an attempt to compare the various experiments, scaling parameters are derived from energetic considerations for both dissipative and first-order parameters. Individual and normalized vertical profiles are given. The statistics presented discriminate between dry and condensed plumes. For both populations, vertical profile tendencies are rather similar in the BL. However, condensed plumes show higher values of the perturbed variables inside the whole column. Mean and turbulent structure within the plume is sensitive to the stratification of the environment. In the BL, the main structural aspects consist of quasi-constant profiles of vertical velocity and specific humidity excess, and of rapid decreases of temperature excess, dissipation rate and temperature structure parameter, such decreases are due to mixing with the surrounding atmosphere. Penetration into the drier stable layers results in a reversal of the sign of ?p, an increase in the specific humidity excess and a small decrease in the dissipative parameters. The intensity of the plume turbulence is found to be more dependent on wind speed and condensation processes than on atmospheric turbulence. The oscillations inside the horizontal part of the plume in the upper stable layer are modulated by mean advection and by possible interaction between the buoyant cells and natural waves.