There are two different boundary conditions: 1) a Temperature boundary condition which enforces a specific temperature on a surface and 2) a Convection boundry condition, which has an Ambient Temperature input and a Film Coefficient. When you say Ambient temperature above, are you referring to boundary condition of type 1 or type 2 because it's not clear the way you wrote your reply.
Also, are you doing a 2D problem or a 3D problem. That is also not clear because a surface in 2D is the edge of the rectangle, while in 3D it is the face of the rectangle.
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