Abstract: The obser vational fields in planet boundary layer and the model errorare modified here in variational assimilation.The gradients of the cost function with respect to control variables are calculated in the way of perturbation.Then the inference in theory and experiment analysis is produced to the method how to obtain the gradientin he way of perturbation.It is the results of numerical experiment using model data thatindicate a better modify to model errorin this way.It makes the model describing the physical process more correctly.Then the initial fields to the model after modifying can be fitthe "observational data" best.