Applying a value to a circle shape in a 2D contour plot

I have a plot of length (x) against length (y) with contours (T) over the top. I would like to impose a value of T in the shape of a semi-circle, but I am really struggling. I know how to plot circles on their own but I cant put them into this plot.

All help appreciated,

Alex

Answer 1

[Alexander]

Anyone???