I lived with a cistern and septic in the Manitoba prairie winter for 12 years, so I can relate to your issue. I won't bore you with the details of my set up, but the concrete septic tank was about 20% above grade, which gave rise to some similar issues. Based on my experience, what I would suggest to you is:
1. Your cistern won't freeze below the frost line depth, but the top portion will freeze. If you are using a float switch in the tank, one inch of ice can not only immobilize your float switch, it can cause you to bust the switch trying to chip it out....
2. You can insulate the above grade part in several ways (I used a combination of square bales and earthen planters) and that definitely helps. But you will still need a heat source.
3. Because my situation was a septic tank and not fresh water, I used a suspended heat lamp instead of a livestock submersible heater. If I was in your situation, I would look into the latter.
4. The amount of insulation and the strength of the heat source will be influenced by how quickly you are using and replenishing the water. When I had a 5000 gallon cistern, we could live comfortably on 90 gallons per day. So I could go 2 months before I had to truck more water in. If your cistern is being fed by a well on a float switch, you will get more regular water movement which will help.
Good luck.