I've never had any trouble with snow building on air coolers, and I live in a climate where from time to time we measure the snow in feet. I guess the heat from the process and the upflow from the fans is enough to keep them clear.
You should probably provide some heating coils to temper very cold air prior to contacting the bundles and provide freeze protection for cold weather shutdowns.
Using variable speed drivers for fan motors is the best solution I found so far. And don't forget to equip your system with accurate temperature measurements - especially tube rows being the closest to the motor(s). In some very extreme conditions, winterization coils have to be employed.
I'd suggest you to take a look at Hudson website and you will find quite good references there.
Sometimes, for sloped (roof-type) air coolers, employing variable speed drivers is not enough to keep you on the safe side of operation regarding freezing protection. Going for heating coils (especially if you are in very very cold area) could be the most cost-effective solution.
Look at Hudson's solutions for setting hot air recirculation assembly.
I work in an oil refinery in Central Minnesota. We have heavy snow and extreme cold. All of our fin-fan coolers are forced draft. I would not recommend induced draft because of problems with fan bearings and motor drivers on the hot side of the coil. Induced draft also results in great maintenance problems for repairs made in hot weather. We have never had any problems with snow buildup on our forced draft fans. The Hudson reference is a good one. Most of our new fans have been Rainey. I'm sure that they have comparable support available.
Let the fan manufacturer know your weather conditions and they can make appropriate design changes to accomodate the cold and snow.