You should try to state more clearly what is not working. Give an example set of initial values, say what you expect the result to be, and say what result you actually get. Without this, it's really hard to help you.
All the same, here's a guess. Maybe you need:
uw(ra >= 1.12246*sigma) = 0.0;
This applies the test to each element of ra and sets the corresponding element of uw to zero if the element of ra passes the test.