Sempere-Torres, Daniel (Universitat Politecnica de Catalunya. Dpt. d'Enginyeria Hidraulica Maritima i Ambiental. Barcelona, Spain)
Porra, J.M. (Universitat de Barcelona. Dpt. de Fisica Fonamental. Barcelona, Spain)
Creutin, J.D. (LTHE, Universite J.Fourier, INPG, CNRS, Grenoble, France)
Rain is basically a population of falling drops interacting with each other and with their environment. For many applications, such as microwave detection or numerical modelling, the explicit description of drop interactions is not necessary and the size distribution of drops is enough to describe the population behaviour. Drop Size Distribution makes possible to compute rain bulk properties like water content, rain intensity or radar reflectivity. It is also the basic descriptor in the numerical parameterization and modelling of rain microphysics.
A body of previous theoretical and experimental studies suggests it exists a general formulation for raindrop size distributions able to describe a wide diversity of rain conditions. The formulation basically relates the evolution of the drop populations to a rain bulk variable like the rain rate. This paper proposes a simple to carry out testing methodology in order to seek experimental evidence of the validity of such a description. Nineteen Autumn storms from the Barcelona region (Spain) are analysed. The results show a good general agreement between data and theory to the sense of the defined test criteria. The general shape of the raindrop size distributions is shown to be acceptably represented by an exponential function. This limits the number of modelling parameters to two and has important consequences in the derivation of rain bulk variable relationships, as the one that relates reflectivity and rainfall rate, allowing to use weather radar to estimate rainfall intensity.