Solving a coupled system of convection-diffusion equation

Hi all,

I am having trouble solving two coupled pdes, its described below

I have a 2d system (square) and from the top boundary a nutrient (C) flows in to the square. I have set the concentration S=S0 at the top boundary. The nutrient is defined as a convection diffusion equation

secondly I have cells (Pinit=0.1) inside the square modelled as a convection-diffusion equation.

\frac{\partial P }{\partial t}=D_p\triangledown P + K_bCP,
\frac{\partial C }{\partial t}=D_o\triangledown C + \lambda_oPC

I want to solve C using the initial value of P(Pinit) then to solve P using C.

I want to repeat the process till the system reaches steady state.

I don't know how to solve a coupled system in cosmos 5.1 or 4.4 (newer versions)

Please help me. attached is a screen shot of the equations