Dynamics of downdraughts and cold pools : an experimental and numerical study

Downward moving cold air within a thunderstorm, known as a downdraught, can be an additional storm hazard, and also prolong the convective lifecycle. An understanding of downdraughts is therefore useful for weather forecasting. Typically, in weather forecasting downdraughts are modelled using the theory of a plume from Morton et al. (1956), which inherently assumes that the plume is long and thin. However, downdraughts are often wider than their height and hence deviate from the Morton, Taylor and Turner theory. More recently they have been compared to thermals rather than plumes, and their flow along the ground after impact as axisymmetric gravity currents, for example in the study performed by Rooney (2015). In this thesis both numerical and laboratory experiments have been performed by releasing finite volumes of dense fluid from cylinders of varying lengths. In the laboratory the cylinders were perspex tubes submerged in a tank filled with sodium chloride at a fixed height above the base of the tank. The tubes were sealed at the base using a latex sheet and filled with sodium nitrate solution which was dyed using methylene blue. The fluid was released by bursting the latex sheet and was analysed using an extension to the dye attenuation technique developed by Cenedese & Dalziel (1998). Red, green and blue LEDs were used to address the need for a larger range of dye concentration than provided by red LEDs alone. The numerical experiments were performed in MONC, the Met Office’s large eddy model. Some basic testing of the model, and comparison with Rooney (2015) was done to determine the set up needed to best reproduce both Rooney (2015) and the laboratory experiments. Edge detection and tracking was used for both sets of experiments to see how the vertical and radial velocities, and shape of the releases change when changing both the length of the cylinder, and height of the cylinder from the ground. Velocities and shape information from edge detection, and information on the volume from dye attenuation was also used to give information on the entrainment of the thermals. Changing the height and length of the tube determined whether or not the dense release had developed into a self-similar thermal, was still draining from the tube, or transitioning between the two, upon impact with the ground. It was found that the slumping phases and heights of the resulting gravity currents behave differently depending on both the height and length of the tube. Theory for the evolution of the flow during the draining of the tube was developed and compared to both the numerical and laboratory experiments. This model combined with existing thermal theory has been used to define more realistic initial conditions for a gravity current model.