Round shiny object emitting three rows of blue pulsing arc rays.

I was with my friend at San Francisco's Lake Merced while doing some night time fishing. The night was cool and there was some overcast clouds as we were talking and fishing, then I spotted some lights overhead and thought they were coming from hi-beams such as an automobile. I thought they might be police patrol car spotlights (we were not supposed to be fishing at night because the lake was closed after dark) So now we were on the alert and thinking  we would get busted by cops so we remained silent for a minute or two. Then, across from the lake we saw a bright round metal ball about 20 to 25 feet in diameter peek out from behind a tree top, while hovering some 30 to 40 feet above the ground. At first I thought it was a spotlight that someone was projecting onto the trees, but almost simultaneously I realized that there was no attached beam that could be making the spotlight, plus , it was partially obscured by the tree tops. My friend and I looked at eachother and   we could not believe our own eyes! Then, the light slowly went back behind the trees and suddenly was gone. A few moments afterwards, the light appeared directly above the lake (approx. 100 feet or less above the water) but this time the shiny round object was obscured by the overcast clouds/fog but we could still see it shining. The next thing that happened will stay in my memory forever, this thing started emitting three sets of light blue arced pulse-typed  light rays. They were coming down in three rows and when they hit the surface of the water they simultaneously bounced back up through the arc stream for about 10 to 15 seconds and this thing made absolutely no noise. After this , it took off and was gone forever.This event happened 12 years ago but I remember most of the details clearly as if it just happened. After watching Peter Jennings' report on UFO's two nights ago, I learned of the National UFO Reporting Center and this was my experience.
Witness indicates that date of sighting is approximate.  PD