The antenna appears to be a 1/2 wave UHF antenna, and they don't appear to have their own license in the FCC database, so I'm guessing they are on any one of several UHF systems operated by the local comm shops. A key to help try to figure it out is which manufacturer and model number of a radio in one of their vehicles. For example, if it's a Motorola XPRxxxx, it could be a DMR or analog system. A Hytera mobile could point to an XPT system (similar to DMR). A Kenwood or Icom radio could point towards a NEXEDGE or IDAS system. They could also be operating on an older legacy LTR system, although a lot of the LTR systems here in the Capital Region are going silent.
Another thing you can try, if your scanner is capable of it, is using the Close Call feature if you happen to be close to one of their vehicles when the radio is transmitting, but I realize that could be very hit or miss.
I know the info I provided doesn't give you the exact answer to your question, but those clues might be able to help you figure out what channel, system or talk group they use.