Abstract:moist physical processes play an important role in simulating phenomena of vario us scale,but it causes discontinuity of model equations with respect to model st ate.This discontinuity is expressed as “on-off” switches in the programs of p rediction model.In this paper,the four dimensional variational data assimilation sy stem of nonhydrostatic mesoscale model version 5(MM5) including Anthes-Kuo cumu lus parameterization scheme is studied.Real precipitation is simulated to study tangent linear and adjoint method of “on-off” switches.Analysis shows that if the“on-off” switches are kept the same as in the basic state,then the gradie nt,which is calculated by the adjoint model of a discretized model,can provide a good descent direction for the minimization procedure;switching on and off at e very other time step in the Kuo cumulus parameterization scheme doesn’t impact th e convergence rate of cost function;the existence of the switches doesn’t impac t improvement to the MM5 model rainfall prediction because not only specific humidity,but also wind,temperature and pressure are assimilated into the model.