function P = ana_pair(t,g,param,opt)
% recalculate analytical air pressure used in rissage application
%
% pair = ana_pair(ocean_time,grd,parameters,option)
%
% for option 1 (tanh shape step pressure change)
%    dp = param(1);
%    dir = param(2);
%    spd = param(3);
%    Lfront = param(4)

switch opt
  
  case 1
    % single step function in pressure
    dp = param(1);
    dir = param(2);
    spd = param(3);
    R = param(4);
    pha = (g.x_rho-176.2e3)*cos(dir)+(g.y_rho-4632e3)*sin(dir)-spd*t;
    P = 1020 - dp*0.5*(1+tanh(pha/R));
    
  case 2
    % other things we might try
    
end