Converting MERRA-2 Precipitation

[michaelwindsorcurtin]

Hello,

I am looking to use the monthly average of Total Precipitation (PRECTOT) from M2TMNXFLX. The current data is presented kg m^-2 s^-1 and I am wondering what the formula is for converting to other measurements of rainfall such as mm/hour or mm/day. From searching online I saw suggestions of multiplying the values by 86400 to get mm/day (although I understand this would be the average of the periods of the day rather than the cumulative amount for the day).

[GES DISC - emilyz]

The precipitation unit kg*m -2 *s -1 is the same as mm*s -1. Here are how to convert the MERRA-2 precipitation data unit :

mm/hour => precip*3600
mm/day => precip*3600*24

Note that MERRA-2 has two precipitation data:
PRECTOT - total precipitation from atm model physics
PRECTOTCORR - Bias corrected total precipitation

Please read the MERRA-2 file specification and relevant references for selecting the data.

https://gmao.gsfc.nasa.gov/pubs/docs/Bosilovich785.pdf

[mbosilov]

To be specific in the unit analysis:

kg/m2/s - Kilograms of water falling over an area in a second
divide this by 1000 kg/m3, the density of water to get m/s
multiply this by 1000 mm/m to get mm of water per second
Then
multiply by 3600 s/hr for mm/hr (or you can call that an accumulation of mm in an hour)
multiply by 86400 s/day for mm/day (or also the accumulated mm in a day)

I had a professor who would assume units of furlongs per fortnight to any number without a unit given, and you learn quickly to follow through on unit analysis details!