That's pretty similar to where I live and we bury lines 2-3' deep with no trouble. I put either a 2' or 3' insulated earth tube under the Ritchie in our front field, and added a 5 gallon bucket notched on the side to get a little extra length and protect the line at the elbow. That was probably 20 years ago and the lines/valve assy have never frozen. When it gets down in the single digits, I've had the balls freeze up on top if one of them isn't sealing properly and allowing cold air in. Biggest complaint I have is the plastic material has started to degrade over the last few years and the whole top piece (all of the red parts) needs to be replaced.
Long story short, I'd be willing to bet that adding an earth tube to yours and sealing out the air front entering between the base and pad will solve your freezing problems. You're going to have a lot of heat loss with the wind in your area otherwise.