Birgitte
Yes, DSD in this context does refer to the distribution or frequency spectrum of raindrop sizes. The issue is that the same radar reflectivity can result from any number of DSD's and one must assume a particular DSD to be able to deduce a rainfall rate. Practically, this amounts to assuming a "type" of rainfall (i.e. convective, tropical, stratiform, etc) and using the appropriate Z-R relationship for that type. Methods have been developed using dual polarization data to evaluate the relative proportions of large vs small drops and thereby estimate the DSD and Z-R relationship.