Dear, I did not get your question completely, but I assume you asked the following-- In simple, simulation means to present computerized model for physical things. e.g. rainfall is a physical event and if you try to show rainfall in model format (computerized output) then you try to use physical and empirical equations together which can solve the physical phenomenon occurring during rainfall processes. of course, still we can not simulate (solve physically) all physical phenomenon but yet by using some assumptions and empirical equations we try to get more realistic output.  

Now you asked about the techniques of simulating rainfall (I could not understand what you mean by asking real rainfall data) you can either use the physical models like WRF etc or you can build some statistical model e.g. http://onlinelibrary.wiley.com/doi/10.1029/WR023i005p00885/abstract

Dear, I did not get your question completely, but I assume you asked the following-- In simple, simulation means to present computerized model for physical things. e.g. rainfall is a physical event and if you try to show rainfall in model format (computerized output) then you try to use physical and empirical equations together which can solve the physical phenomenon occurring during rainfall processes. of course, still we can not simulate (solve physically) all physical phenomenon but yet by using some assumptions and empirical equations we try to get more realistic output.  

Now you asked about the techniques of simulating rainfall (I could not understand what you mean by asking real rainfall data) you can either use the physical models like WRF etc or you can build some statistical model e.g. http://onlinelibrary.wiley.com/doi/10.1029/WR023i005p00885/abstract