The Initial Development of a Thermal and the Application to Downdraughts

Downward moving cold air within thunderstorms, known as downdraughts, can be used to determine the severity of a storm. Therefore an understanding of them is useful for weather forecasting. Typically in weather forecasting these downdraughts are modelled using the theory of a plume from from Morton, Taylor and Turner (1956), which inherently assumes that the plume is long and thin. Downdraughts are generally wider than they are high and hence deviate from the Morton, Taylor and Turner theory.

We perform laboratory experiments using finite releases of dense fluid from cylinders of varying lengths. We use an extension of the dye attenuation technique to take measurements of the edges and center of mass of the release. This enables us to look at how the vertical velocity and shape develops during the descent. In doing so we see the development of the shape factor and spreading rate during the draining and thermal phases of the descent. Using previously developed theory and assumptions of the shape this allows us to analyse the entrainment during both phases and asses the adequacy of the Morton Taylor Turner (1956) entrainment assumption in the context of downdraughts.