Let me pass on an actual observation of rain on a wing - from a perspective not available from the front office....
Right in the middle of my primary flight training, I flew as pax on (coincidentally) a 737 from MEM to IND. On approach, IND had numerous summer thunderboomers in the area, with ceilings under the CBs at about 4000, and intermittent strong downpours (we passed through about 5 on the inbound leg to 23R).
Each time we entered the heavy rain, I could see, from my window seat just above the right wing, a "bead" of water pulsating along the leading edge of the deployed slat. Or at least the "edge" that I could see (the actual leading edge was likely so low as to be out of sight from my location about 6 feet back).
That build-up of water pulsated between 1 and 2 inches in thickness (2.5-5cm) in the heaviest parts of the rain; reducing, of course, to nothing when we exited each rain column.
For a beginning pilot, it was an impressive demonstration of just how much water can build up from the forward movement of the wing through a torrential downpour.