[firedrake] Changing a vector valued function in time loop

Dear Firedrakers, I'm trying to update a vector function in my time loop a.interpolate(Expression(["0.5*F0*cos(omega*t)*x[1]", "-0.5*F0*cos(omega*t)*x[0]"], F0=F0, omega=omega, t=t)) but if I use assign or interpolate, I get told the form does not have the respective property. How should I go about making this work, such that it'll change the value of a in my solver? Thanks will