Having measured all the air flow parameters involved, the lowest part of the stack has 3x the airflow of that at the top, at speed. The air's source is not important, it can sometimes come from far away, only the static pressure at the specific location, that alone defines heat exchanger efficiency, when all other variables are fixed. this is also why the basis for these changes is not intuitive, and without measurement, represents a lawn dart game.
For example, at 60 mph, the static pressure at the grill top slot location is .45 inches water, and at the lowest grill point, it is 1.6 iw. This means that most of the cooling system is doing business on the lowest half, 70% of the heat rejection occuring down low.
The lowest portion is the "usual" location for a cooler for this reason. That pic, is a valid way of introducing more flow to the cooler alone, without depriving any other component (much) in the process. Adding fins, and thickness, to an already air starved stack, is a poor approach, and irresponsible unless airflow is a addressed simultaneously.