Dynamics of downdrafts and their role in tropical rainfall

Lead Research Organisation: University of Cambridge
Department Name: Applied Maths and Theoretical Physics


Under the impacts of climate change, variations in rainfall intensities and patterns can cause huge human, agricultural, ecological and economic costs. This rainfall is associated with strong convective storms that create descending columns of cold air and rain. It is unclear how the various dynamical processes interact to produce the observed rainfall distributions, what determines the initial properties of the cold pool that results from the impact of the downdraft with the ground, and how a collision of two or more pools triggers secondary convection. The student will carry out laboratory experiments to determine the main physical processes that control the generation of secondary convection and rainfall patterns and intensities. These experiments will be interpreted in terms of mathematical models and then extrapolated to the meteorological context. The student will receive training in laboratory experiments including PIV, digital image processing and data analysis, and in mathematical modeling. The student will also be trained to use the Met Office numerical models and interpret the results


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Description When dense fluid is released from a tube in an ambient environment the fall exhibits 3 different regimes where the duration of the first two regimes depends on the length on the tube, and the height the tube is from the ground. We have also shown that depending on which regime a falling thermal impacts the ground in will affect the speed and height of the spread of the dense fluid. To obtain these results a novel extension to the dye attenuation technique has been developed in order to measure flows with very high dilution rates.
Exploitation Route Our experiment is a crude representation of a downdraft and we believe that the existence of these three regimes and their effect on resulting gravity currents could help to explain the high speeds associated with downdrafts which may be useful to the MET Office for weather prediction purposes.
Sectors Environment

Title Dye Attenuation 
Description An extension of the classic dye attenuation technique used in experimental fluid mechanics whereby instead of using a single colour lighting and dye e.g. red and methylene blue. 3 coloured LED's are used, in this case red/green/blue and methylene blue. This allows accurate dye attenuation measurements to be taken for a larger range of concentrations of dye using a single camera. 
Type Of Material Data analysis technique 
Year Produced 2018 
Provided To Others? No  
Impact I will be able to take accurate measurements of the density field of a thermal as it descends and impacts the ground and spreads as a gravity current, whereas with the original technique I was only partially able to get accurate measurements for part of the descent.