I think below is fine reference:

Das, S., A. Maitra and A.K. Shukla, 2010. Rain

attenuation modeling in the 10-100 GHz frequency

using drop size distributions for different climatic

zones in tropical India. Progress Electromag. Res. B,

25: 211-224. DOI: 10.2528/PIERB10072707

Hi,

I would suggest you start with the current ITU-R P 618 model for combined effects (including rain attenuation). Although there are several alternative models available, none have been exhaustively tested in the region you are interested in. 

http://www.itu.int/rec/R-REC-P.618/en

regards

Luis