If you have followed the GT series since the ps3 era when Dynamic weather and time was introduced you should know that despite cockpit view had visible rain drops on the windshield.
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The car model/body itself did not include these rain water drops (that being said even in Replay mode or photomode itself). GT5/6 did not show any drops of water on the windshield itself only when going to cockpit view.
This video (GT6 Adaptive tesselation) shows accurately that rain drops were never modeled on the body.
Notice the tiny rain particles when moving the camera around.
In comparison to Driveclub which did modeled rain drops on the car:
Not trying to turn this into a comparison thread just making my point crisp and clear.
If you have prove that GT Sport does indeed model these rain drops (on the Northern Festival mission) and on the car body/model itself I would like to see some screenshots or videos ilustrating this.