Turbulence Intermittency for Cloud Physics (TITCHY)

Lead Research Organisation: Imperial College London
Department Name: Aeronautics


Since the dawn of humankind people have looked up at the sky, perhaps projected every day images into the dazzling variety of shapes that cumulus clouds produce, and asked "why do clouds form and then disappear?" and "why does it rain?" To this day these questions remain unanswered, although of course our understanding of the physics of clouds has advanced enormously. It has been provocatively asked "can we understand clouds without turbulence?" to which my response is a resounding "no!" Clouds grow by entraining environmental air across the sharply defined visible boundary of the cloud. Similarly they decay through precipitation, and more importantly the detrainment of air back to the environment. Neither of these processes are well understood. In recent years I have jump started the field of entrainment between two adjacent regions of turbulence, or turbulent/turbulent entrainment (TTE) which is precisely the scenario that occurs for a warm cumulus cloud in the turbulent atmospheric boundary layer. Entrainment dilutes a cloud and fundamentally alters its microphysics, yet TTE for a cloud is not understood in part because of its inherent intermittency. Without understanding the TTE of water mass, energy, momentum, buoyancy, and heat into a cloud it is not possible to parameterise it and thereby improve weather/climate forecasts. TITCHY will do this, through a carefully co-articulated campaign of state-of-the-art experiments and simulations specifically devised to assess the importance of my TTE paradigm to cloud microphysics. The second thrust of TITCHY is to examine the physics of water droplets within a cloud; in particular the forces that act on them and how they affect the collision/coalescence process that ultimately yields raindrops. These forces are subject to intermittent turbulent physics hitherto neglected but potentially of critical importance. Based on my transformative new ideas, TITCHY seeks to tackle a centuries-old problem with a modern outlook.


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