This is the driver of a maroon Hummer. Some of the snowball fight participants, mostly among those on the east side of 14th Street near U, targeted this vehicle when it stopped at the intersection.
The driver got out and, after waving a walky-talky (hinting that maybe he's an official of some sort), he begins to wave a gun. No one near me seemed to know, as we'd later find out, that he was with the police. He did not identify himself until later, when more police had arrived, his weapon was re-holstered, and arguments ensued.
[Please see set description and whole set for context.]