%  function EPSA = calc_EPSA(Ts,Ta,RHa,CO2)
%
% Computes the emissivity of atmospheric column at 
% average temperature Ta (K) and surface temperature Ts (K),
% CO2 concentration CO2 (in ppm), assuming fixed relative humidity.
%
% EPSA = 1 - exp(-TAUinf) = emissivity of a gray layer
% TAUinf = ALPHA*CO2 + GAMMA*Qa = optical depth
% ref values: ALPHA = .001  (CO2 in ppm)
%             GAMMA = .7   (Qa in g/kg)
%
% Ref: Emanuel, 2002.

   function EPSA = calc_EPSA(Ts,Ta,RHa,CO2)
   
   % Read Parameter
   Param;
   
   % Computes (low level) specific humidity 
   Pa = 750; %low level pressure (mb)
   [Esat,Qsat] = Clausius_Clapeyron((Ts+Ta)/2,Pa),
   Qa = 1000*RHa * Qsat; %in g/kg
   
   % Test
   %ALPHA*CO2, GAMMA*Qa, ' **'
   
   % Computes optical depth
   TAUinf = ALPHA*CO2 + GAMMA*Qa;
   
   % Computes emissivity
   EPSA = 1-exp(-TAUinf),