
Let PHPHB =  PH(:,:,:,:)+PHB(:,:,:,:), with 27 vertical levels. We want to interpolate to 26 vertical levels. Like this

PHPB(:,3,:,:)=((PHPB(:,3,:,:)+PHPB(:,4,:,:))/2

PHPB(:,4,:,:)=((PHPB(:,4,:,:)+PHPB(:,5,:,:))/2

PHPB(:,5,:,:)=((PHPB(:,5,:,:)+PHPB(:,6,:,:))/2
