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call randominit(dt,1.39e-4)
subroutine randominit (dt,dcoef)
c
c Given the timestep size "dt" and rolloff frequency "dcoef",
c this function provides a low-pass filtered white noise time
c series for ramdomly forcing the shallow water model.
c dt = 7200. seconds
real*8 ddt,ddcoef,dranold,drannew,coef1,coef2
c
ddt=dt
ddcoef=dcoef
dranold=0.
coef1=(1.0-0.5*ddt*ddcoef)/(1.0+0.5*ddt*ddcoef)
coef2=ddt/(1.0+0.5*ddt*ddcoef)
iseed=9385729
c
return
c
entry randomwind (rannew)
c
drannew = coef1*dranold+coef2*(ran(iseed)-0.5)
c
dranold=drannew
rannew=drannew
c
return
ndays = 1;
ddcoef = 1/(7200.*12*ndays);
dranold=0.
coef1=(1.0-0.5*ddt*ddcoef)/(1.0+0.5*ddt*ddcoef)
coef2=ddt/(1.0+0.5*ddt*ddcoef)
drannew = zeros(1, 100000);
for i = 1:100000;
drannew(i) = coef1*dranold+coef2*(rand(1)-0.5);
dranold = drannew(i);
end
[h, f] = spectrum(drannew, 5000, 2500);
f = 6*f;
plot(log(f), f.*h(:,1));
xt = [128 64 32 16 8 4 2 1 0.5 0.25];
set(gca, 'XTick', log(1./xt), 'XTickLabel', xt);
title(['NDAYS = ' num2str(ndays)]);