Impact of parameter estimation on the performance of the FSU Global Spectral Model using its full physics adjoint
Zhu Yanqiu, I.M.Navon
The full-physics adjoint of the FSU Global Spectral Model of version T42L12 was applied to carry out and assess the impact of key parameters estimation using an initialized analysis data set. The three key parameters, i.e., the bi-harmonic horizontal diffusion coefficient, the ratio of the transfer coefficient of moisture to the transfer coefficient of sensible heat and the Asselin filter coefficient as well as the initial conditions were optimally recovered from the observations using the adjoint parameter estimation.
The 6-hr forecasts at the end of the assimilation window starting from the retrieved optimal initial conditions and the optimally identified parameter values successfully captured the main features of the analysis fields. We conducted a number of experiments separately to assess the effect of carrying out 4-D VAR assimilation on both the initial conditions and key parameters versus the effect of optimally estimating the key parameters alone. A positive impact on the ensuing forecasts due to each optimally identified parameter value was observed and the maximum benefit was obtained by the combined effect of the three optimal parameter values. The results also showed that in the ensuing forecasts the model tended to ``forget" the impact of optimal initial condition first while the impact of the optimally identified parameter values persisted beyond 72 hours.