Boundary conditions with a step (interpolation or import)

[monacellif]

Hi everybody,
sometime I came up with a new question; thank you in advanced for replying.
Assume I have a BC with this kind of trend:
constant for the first part and decreasing as exp in the second part. See the picture for better understanding; where x is the z axis in ELMER [tx(2)] and y is the temperature.
I assume, in ELMER GUI, I should use the if-else construct. I have entered this:

Temperature = if (tx(2)<0.03) 100; else variable coordinate; real MATC "2.7^(-0.3*tx(2))"

but, obviously, it does not work. Which is the right syntax?

Is there also a way to import T values according to tx(2) position? How fine my T, dependant from tx(2), should be?
Thank you