Replace negative values in an numpy array

Is there a simple way of replacing all negative values in an array with 0?

I'm having a complete block on how to do it using a NumPy array.

E.g.

a = array([1, 2, 3, -4, 5])

I need to return

[1, 2, 3, 0, 5]

a < 0 gives:

[False, False, False, True, False]

This is where I'm stuck - how to use this array to modify the original array.