Question: plot vs plot3d vs contourplot (disconnect in 3d plot?)

Hi everyone,

restart:with(plots):

lambda1:=(S+sqrt(S^2+4*alpha))/(2);

lambda2:=(S-sqrt(S^2+4*alpha))/(2);

plot3d({lambda1,lambda2}, S=-10..10, alpha=-5..5, contours=50,grid=[100,100]);

plot([subs(alpha=-5,lambda1),subs(alpha=-4,lambda1),subs(alpha=-3,lambda1),subs(alpha=-2,lambda1)

,subs(alpha=-1,lambda1),subs(alpha=-0.5,lambda1),subs(alpha=0.0,lambda1),subs(alpha=0.5,lambda1)

,subs(alpha=1,lambda1),subs(alpha=2,lambda1),subs(alpha=3,lambda1),subs(alpha=4,lambda1)

,subs(alpha=5,lambda1),subs(alpha=-5,lambda2),subs(alpha=-4,lambda2),subs(alpha=-3,lambda2)

,subs(alpha=-2,lambda2),subs(alpha=-1,lambda2),subs(alpha=-0.5,lambda2),subs(alpha=0.0,lambda2)

,subs(alpha=0.5,lambda2),subs(alpha=1,lambda2),subs(alpha=2,lambda2),subs(alpha=3,lambda2)

,subs(alpha=4,lambda2),subs(alpha=5,lambda2)],S=-10...10,axes=box,color=[red,green]);

contourplot({lambda1,lambda2}, S=-10..10, alpha=-5..5, contours=50,grid=[100,100]);

How to minimize the disconnect in the 3d plot and countourplot to get a smooth plot?