Don't do anything.
Rain isn't clean. It has dissolved dust inside each drop. When the rain dries, that dust will be left on the car, in the form of water spots. You can scrub your car all you want, but there will still be water spots if it continues to rain. Unless there is dirt caked on your car, a soapy rag is what actually cleans the car. Water running over the surface of your car will not clean the car. Your best bet is to wait until the rain stops, and wash it with a microfiber rag (a couple dollars at most) and some dish soap. Hose it off, and dry it with a clean microfiber rag.
If you insist on washing your car in the rain, you'll need a soap to attract the dirt. Otherwise, the dirt will remain stuck to the surface of the paint (and will remain in the microscopic scratches in the paint). Get a clean microfiber rag, and put a little soap on it. Wash the car with the rag, and then hose it off (rain doesn't provide enough volume of water to rinse the soap off). Dry it off once the rain stops, before the car has enough time to air dry. If you let the car air dry, there WILL be water spots.
If you are washing your car in the rain because you don't have access to a hose, try some waterless soaps made specifically for washing vehicles. They aren't very expensive, and they will certainly do better than trying to wash your car in the rain.