|MadSci Network: Earth Sciences|
I am not sure that I understand your question. In general rainfall in rainforests is not regular but very seasonal. Most rainforests have a rainy season and a drier or less rainy season. The rain occurs largely because of the large amounts of energy available to evaporate water into the atmosphere. This in turn increase cloudiness in the tropics which influences the global albedo (reflectivity). Clouds reflect solar energy but they alo hold in thermal energy so the effect on the temperature is largely a function of the height of the clouds. All tropical forests are not rain forests and in fact the area of seasonal and dry forests (or what would be dry forests if they had not been converted to rangeland) probably exceed the area of rainforest. If we could alter the amount or timing of rainfall in tropical forests it would most likely happen by a cooling of the earth. Or to put it another way, if we cooled the earth, then the amount and timing of rainfall would probably decrease on average. This is simply because more heat causes more evapotranspiration and vice versa.
Try the links in the MadSci Library for more information on Earth Sciences.