function tau=stresstc(sp,z,Ta,rhoa)

% STRESSTC: computes the neutral wind stress following Smith (1988).

% tau = STRESSTC(sp,z,Ta,rhoa) computes the neutral wind stress given the 

% wind speed and air temperature at height z following Smith (1988),

% J. Geophys. Res., 93, 311-326. Air temperature and density are optional

% inputs. 

%

%   INPUT:  sp    - wind speed   [m/s]

%           z     - measurement height  [m]

%           Ta    - air temperature (optional) [C]

%           rhoa  - air density (optional)  [kg/m^3]

%

%   OUTPUT: tau   - wind stress  [N/m^2]



%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% 3/8/97: version 1.0

% 8/26/98: version 1.1 (revised by RP)

% 4/2/99: versin 1.2 (air density option added by AA)

% 8/5/99: version 2.0

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%



% load constants

as_consts;



if nargin == 2,

  Ta   = Ta_default;

  rhoa = rho_air;

elseif nargin == 3

  rhoa = rho_air; 

end



[cd,u10] = cdntc(sp,z,Ta);

tau = rhoa*(cd.*u10.^2);