maximum and minimum

Hi

I want to set a minimum in NCL if some values of an array are out of normal range. I was trying to use the conditional command as below, but failed. The message shows that "fatal:Conditional statements (if and do while) require SCALAR logical values, see all and any functions
fatal:Execute: Error occurred at or near line 31 in file". how to set minimum in NCL? thanks

a= new( (/46,72/), float)
..
if (a.lt.0) then
a = 0.
end if