The multipath phenomena mostly occur for indoor or in-town propagation when signal power is relatively high and reflecting objects are close to signal propagation path. Rain causes a dispersion resulting in an additional loss and depolarization. Multipath dispersion by rain drops does exist but does not contribute by a discernible amount at microwaves or mm-waves. You can devote a study to this problem as the technology moves towards terahertz spectrum where rain multipath may become interesting.