Symbolism, Imagery, Allegory
In the novel, rain works as an introduction to the sections narrating Pedro's flashbacks. Look at the first one:
Water dripping from the roof tiles was forming a hole in the sand of the patio. Plink! plink! and then another plink! as drops struck a bobbing, dancing laurel leaf caught in a crack between the adobe bricks. (6.1)
This is a pretty handy way of keeping track of where in time you are as you read through the chapters—if it's raining we're probably in Pedro's youth—and also tells us a story about the town's past versus its present.
The symbol of rain works to show the reader how Comala wasn't always a bad place, but its fertility and lushness shriveled and wilted after Pedro came to power. Or, in a more nuanced reading of the symbol, we see how Pedro's memories of Comala, much like Dolores', are rose-tinted and bear little resemblance to the inhospitable hellscale of Comala in reality