  We converted the radiation part of the atmospheric model ECHAM to single
precision arithmetic. We analyzed different conversion strategies and finally
used a step by step change of all modules, subroutines and functions. We found
out that a small code portion still requires higher precision arithmetic. We
generated code that can be easily changed from double to single precision and
vice versa, basically using a simple switch in one module. We compared the
output of the single precision version in the coarse resolution with
observational data and with the original double precision code. The results of
both versions are comparable. We extensively tested different parallelization
options with respect to the possible performance gain, in both coarse and low
resolution. The single precision radiation itself was accelerated by about 40%,
whereas the speed-up for the whole ECHAM model using the converted radiation
achieved 18% in the best configuration. We further measured the energy
consumption, which could also be reduced.
