3 - Is rain fractal?

Summary

ABSTRACT Some scaling properties of the distribution of rain rate in space are investigated. The scales of concern range from the radar coverage range (of the order of 400 km) down to individual raindrops. Preliminary results show that over this wide range of scales rain fields exhibit: preferential scales in the range of a few tens of kilometers; behaviour compatible with multifractal structure between scales of 0.5 to 12 km and some clustering properties of the distribution of the small raindrops in space probably related to raindrop collisions and breakup.

INTRODUCTION

Rain rate fields exhibit variability at all scales down to individual raindrops. In this sense the answer to the title question is positive. However, our interest focuses more on restrictive properties, such as scale invariance, that could serve in modelling the process. One should not expect the restrictive properties (if they exist) to extend uniformly over all scales of the rainfall fields. On scales smaller then the size of individual cumulus turbulence probably prevails in determining the distribution of water substance. On larger scales other phenomena will affect markedly the distribution of precipitation in space. For example, orography (including the distribution of humidity sources) plays undoubtedly a role in organizing convective elements.

The ability to study rain at all scales is limited. A single radar does not cover a typical precipitation system. Quantitative data from networks of radars covering extended areas are now only becoming operational.