Can you explain what you are trying to do? Why using the np.meshgrid?

Where are you obtaining the error? When calculating [phi, theta] or when obtainting the divergence ?

Are you trying to change the resolution from 0.25 to 0.5 and obtain the divergence (d variable) at a specific time?

I think should be better to access the data at the time=time with the function .isel as:

divergence = ds.isel(time=time).d.data

But still I'm not sure what you are doing, probably there is some `xarray` function that will do what you're expecting.