function [vec] = rho_center( u, v, ang )
% center and rotate vectors
% [vec] = rho_center( u, v, ang )

% this assumes the Adriatic orientation
u = squeeze(u);
v = squeeze(v);
u = [u(:,1) u u(:,end)];
v = [v(1,:);v;v(end,:)];
vec=( u(:,1:end-1) + u(:,2:end) )/2  + ...
    ( v(1:end-1,:) + v(2:end,:) )/2  * sqrt(-1);
vec=vec .* exp( sqrt(-1).*ang );