[{"account_id":"1234","cities":{"cityname1":{"param1":1,"param2":2}}},{"account_id":"2345","cities":{"cityname2":{"param1":3,"param2":3}}},{"account_id":"3456","cities":{"cityname3":{"param1":8,"param2":6}}}]

db.collection.aggregate([{"$set":{cities:{"$objectToArray":"$cities"}}},{"$unwind":"$cities"},{"$group":{"_id":{account_id:"$account_id","cityname":"$cities.k"},"sumOfParam1":{"$sum":"$cities.v.param1"},"sumOfParam2":{"$sum":"$cities.v.param2"}}}])