CM has done a good job for the 2013 game rain scenes. But if you prefer less visible rain streaks, you can use this mod to customise. Just replace weather_fx.xml in the respective circuits (excluding Estoril and Imola). You can choose either negligible or some-rain-streaks version. As this is a visual mod, it will not affect saved games. Make backup as usual.
Video showing some-rain-streaks version