function [u_star,flag] = charnock(u10,alpha_c,b)

% flag = 1	did NOT converge
% flag = 0	converged!

% model parameters
% alpha_c 	Charnock constant (0.011 - Liu & Tang; 0.018 ECMWF; 0.013 Zeng etc.
% b		smooth flow constant (0.11 all but NCEP)
%alpha_c = 0.011;
%b = 0.11;
% permanent constants
% nu is air kinematic viscosity
g = 9.81;
nu = 1.5e-5;

% initialize
u_star = 0.04 * u10;


flag =1 ;
for n = 1:10
   z0 = alpha_c* u_star^2/g + b*nu/u_star;
   u_star_new = u10 *.4 / log(10/z0);
   small = abs((u_star_new - u_star)/(u_star+eps));
   if small < 1e-6
      flag = 0;
      break
   else
      u_star = u_star_new;
   end
end