An enclosed array of lights mounted on top of emergency vehicles for the purpose of either moving traffic out of the way or alerting the public to the presence of an emergency.
Lightbars can come in different types. The most common consists of rotating spherical mirrors around a light to cause quite a light display. Sometimes these rotators are spread out such as in a V-shape, a vector lightbar, most notably used by the NYPD. Recently LED lightbars emerged into emergency lighting due to their more efficient energy usage.
The cop turned on the lightbar and blasted the streets with the siren.