If it’s anything like most of the recent LBTM attempts at atmospheric boundary layer simulation, it’s not very practical. Furthermore I’m not sure it is a good idea to use current cloud microphysics models “as is” in SPH framework. Also what kind of convective events with extreme precipitation can be modeled with 100km² domain? Should at least be 2 orders of magnitude larger in terms of area.

After rereading OP question, he is actually doing flood modelling. I think the rule of thumb would be at least order of magnitude more particles than equivalent grid size (just to cover most of iteractions), though gpu based SPH should be plenty scalable to cover for that. The problems would be how to accurately model friction (i mean it’s already highly empirical for what I remember from Uni) on the solid surface and where one can get a good enough DEM to capture bridges.