256 Ice particle fallspeed derived from in-situ observations of cirrus uncinus

Wednesday, 9 July 2014
Richard J. Cotton, UK Met Office, Exeter, United Kingdom; and K. Furtado

Ice particle terminal fallspeeds are often represented as a power law relating the fallspeed to the particle diameter. In weather prediction models, the fallspeed parametrisation is used to calculate sedimentation rates and other processes such as riming and depositional growth. There is a range of fallspeed-diameter relationships in the literature, derived from ground and aircraft-based observations.

In this study, we attempt to derive the fallspeed relation directly from in-situ observations of mid-latitude cirrus uncinus, or cirrus fallstreaks. These fallstreaks were sampled with a 'Lagrangian descent' flight path where the aircraft drifted with the mean horizontal wind and also descended at a rate close to the mass-weighted fallspeed of ice aggregates. The aim was to repeatedly sample the same population of ice particles while they sediment and aggregate. The fallspeed relation is then derived from the changing particle size distribution during the descent.

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