Drag experiments with cylinders of varying roughness related to flow around thunderstorm cells

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The drag on a storm by environmental winds, which vary with height, is considered by analogy with a hydrodynamic experiment of vortex shedding due to flow past smooth and rough cylinders. The vortex shedding rate for rough cylinders is shown experimentally to be smaller than that for smooth cylinders, and this difference is equivalent to an increase in fluid viscosity. This result is at variance with earlier predictions that roughness-induced turbulence would destabilize the flow and cause transition to occur at a lower Reynolds number. The eddy viscosity influences the drag on a storm during its growth. If the hydrodynamic analogy between the rough cylinder experiment and the storm cloud is valid, then increased roughness at the storm cloud edge increases the eddy viscosity locally, which would be indicative of increased mixing. The increased mixing at the periphery of the storm between ambient air and storm air causes the storm motion to respond more to the momentum of the flow. The vertical variation in ‘roughness’ of the storm may cause a number of different responses in the direction of motion.