The original design of the NCEP RSM is a mathematical perturbation nesting into NCEP global spectral model (GSM). Even though the concept of the same sigma surface solves the regional nesting into global model with a simple implementation. But the method of the same sigma surface may result several problems, such as that the lateral boundary blending is required to smoothly connect regional high terrain with coarse resolution terrain of global model.

The recent modification of the NCEP RSM is to modify the mathematical perturbation into physical perturbation. In this modification, some of the previous treatment such as lateral boundary terrain blending is not necessary. In order to provide physical zero perturbation initially, the method used in the model to prepare base field is applied to the preparation of regional input. And base field uses the same resolution and same terrain as the regional model. After then, the initial condition will be zero perturbation if the initial condition is prepared from the global model without data assimilation, and the perturbation after integration is generated physically under the same model terrain between RSM and GSM. Then it will be easy to use for controlling physical perturbation.

An example to eliminate mean bias by zero out mean physical perturbation value at each time step results a corrected rainfall as compared with the run without mean bias correction. It is indicated that the mesoscale features are correctly downscaled under the mean bias correction through the implementation of physical perturbation. And in this mean bias correction, the RMSD between RSM and GSM is dramatically decreasing. It is implied that the physical perturbation of NCEP RSM can have climate integration without mean bias error.