a =Plot3D[{-x^2-y^2 + 400},{x,-20,25},{y,-150,50},RegionFunction->Function[{x,y,z},x^2+y^2≤400],MeshFunctions->{#3&},Boxed->False,Mesh->{{{0},{100},{200},{300},{400}}},MeshStyle->{Pink,Thick}, MeshFunctions->(((#1)^2)+((#2)^2)&),PlotStyle->{{Opacity[0.2],Pink}},PlotPoints->50]

b = ContourPlot[{400-x^2-y^2},{x,-20,25},{y,-50,50},ContourShading->False,Contours-> {0, 100, 200, 300, 400},ContourStyle-> {Pink}]

Plot3D[{-300,-x^2-y^2 + 400},{x,-20,25},{y,-150,50},RegionFunction->Function[{x,y,z},x^2+y^2≤400],MeshFunctions->(((#1)^2)+((#2)^2)&),Boxed->False,Mesh->{{{0},{100},{200},{300},{400}}},MeshStyle->{Pink,Thick}, PlotStyle->{{Opacity[0.2],Pink}},PlotPoints->50,AspectRatio->Automatic]