You can do this easily using fread to read the data into ones string, textscan to conver the data to numeric values, then save it all again using fprintf:
% Read all of the data into one string:
fid = fopen('test.txt','rt');
S = fread(fid,'*char')';
fclose(fid);
% Locate the start of the numeric data:
idx = regexpi(S,'^<forcing>.+?$','lineanchors','end');
% Convert to numeric:
C = textscan(S(2+idx:end),'%u%u%u%u%u%f%f%f%f%f%f%f%f');
% Datestamps:
D = [C{:,1:5}];
% Other data:
E = [C{:,6:end}];
%
% Do your magic data processing here...
%
% Save the data:
fid = fopen('test_new.txt','wt');
fprintf(fid,'%s',S(1:idx));
for k = 1:size(D,1)
fprintf(fid,'\n%04d %02d %02d %02d %02d',D(k,:));
fprintf(fid,' %16.10f',E(k,:));
end
fclose(fid);
