Theory and observations of ice particle evolution using Doppler radar

Aggregation is the dominant growth mechanism in thick stratiform ice clouds. The literature is awash with different mass-size power law relationships for such particles, but there has been little study of why such power laws occur and what they tell us about the physical process of aggregation. Recent theoretical modelling of the aggregation process has led to the prediction that the exponent of this power law is controlled by the hydrodynamic flow regime of the particles, as characterised by the exponent in the power law relationship between Reynolds and Best numbers. This proposed feedback has a particular form which leads to the prediction of exponential growth of the average particle size as a function of fall time (ie the time which the 'average' particle has been falling for).

We use radar observations of ice clouds, and for a given altitude 'z' we construct a characteristic fall time using the doppler velocity profile through the cloud between z and the cloud top. We then use measurements of the reflectivity as a proxy for particle size (raised to some power), given the assumption that in aggregation dominated conditions mass flux density is approximately conserved. We find that plots of reflectivity against fall time show a clear exponential trend in many of our case studies. We believe that this exponential behaviour is evidence for the proposed link between aggregate geometry and fall regime.