Grid nudging assimilates reanalysis/analysis data in the model simulation, while observational nudging assimilates observations collected at particular stations into the model simulation.
Nudging occurs at every model time step. The data are interpolated to every model time step, and used to nudge. Please note that the gfdda_interval in the namelist is the input data interval, not the nudging interval.
It's possible to use both types of nudging at the same time; however, using FDDA over a fine grid domain may result in smooth fields, and without solid physical basis (in PBL). It is typically better to use analysis nudging over coarser domains (e.g., dx > 30km) and obs nudging for finer domains.
Nudging occurs at every model time step. The data are interpolated to every model time step, and used to nudge. Please note that the gfdda_interval in the namelist is the input data interval, not the nudging interval.
It's possible to use both types of nudging at the same time; however, using FDDA over a fine grid domain may result in smooth fields, and without solid physical basis (in PBL). It is typically better to use analysis nudging over coarser domains (e.g., dx > 30km) and obs nudging for finer domains.