GFS data Precipitation rate?

Question

Hello. I'm a programmer and I've been playing around with raw GFS data in order to try to forecasts rain fall. This is a two part question:

1. It gives me a number of bits of data - Precipitation rate, Total precipitation & Cloud water. Which of these is going to be of use in forecasting rain fall?

2. How do I convert from kg m-2 s-1 to mm/hr? (eg 6.00000021223e-06 kg m-2 s-1 which is a value Total precipitation )

Answer 1

1) Perhaps I don't understand your first question, because it seems straightforward to me.  If you need total rainfall, that is simply the total precipitation...if you want the rainfall rate, then you'll just use the precipitation rate.  I wouldn't worry about cloud water, because cloud water (which I assume is the equivalent of precipitable water) isn't necessarily proportional to rainfall...and you would need a number of other variables to determine how much of that cloud water is actually going fall as rain...it would get very complex.

2) I'm not familiar with kg m-2 s-1.  It seems to me that would be related to rainfall rate, not total precipitation...as the s-1 suggests it is a rate.  But I don't really understand what kg m-2 s-1 is supposed to represent.  The weight of water falling on an area per second?  In any case, I suppose density of water would need to factor into the conversion somewhere to get rid of the weight.  If you multiply by density of water (1000 kg/m^3), that would leave you with ms-1.  Then you just need to divide by 1000 (number of mm in a m) to get mm s-1, then multiply by 3600 (number of seconds in an hour) to get mm/hr.  So the result for your answer would be 0.0216 mm/hr....but again, this is rainfall rate, not a total precipitation.