Calculate precipitation from WRF model output
Use (1) Precipitation, as it is the sum of convective and non-convective precipitation, integrate it over one day, voilà.

converting [$kg/(m^2 s)$] (precip):
$$ precip [kg/(m^2 s)] \cdot dt [s] \cdot \frac{1}{ \rho_w [kg/(m^3)]} \cdot 1000 [mm/m] = total\_precip [mm]$$

where $\rho_w$ is the density of water.

Practically the last two factors cancel.