# Single-layer water content variables

In different reanalysis products (ERA 20C and NOAA/OAR/ESRL/PSD 20th Century daily data specifically) one can find the following variables:

• Total column water (ERA)
• Total column rain water (ERA)
• Total column water vapor (ERA)
• Total column cloud liquid water (ERA)
• Cloud water content (NOAA)
• Precipitable water content (NOAA)

How are they calculated? What are the differences between them? Could you please cite a reference or book in which this is explained in details?

Which of these may serve as a best approximation to represent large-scale patterns of rainfall (monsoonal cycles)?

Which of these is the vertically integrated specific moisture?

Are any of these equivalent to the vertically integrated moisture flux divergence as an approximation for precipitation, as in Banacos and Schultz (2005)?

The ERA Interim reanalysis is made by ECMWF and the other reanalysis is made by NOAA. Reanalyzes of the past are made by running global climate models and these differ because NOAA and ECMWF use different models and perhaps even slightly different data. The models on the other hand can be very different for example on implementation of data assimilation, parametrization and even grid cell size.

All of these variables are vertically integrated. The ERA variables in the order you mention them contain:

-Total column water: all water

-Total column rain water: all liquid and solid water in droplets large enough not to be considered cloud droplets (my guess)

-Total column water vapor: all gas phase water

-Total column cloud liquid water: all liquid cloud water

The NOAA variables:

-Cloud water content: all liquid and solid cloud water

-Precipitable water content corresponds to Total column water

From these the Total column water vapor corresponds the Vertically integrated specific moisture. None of these is the divergence of the total column water vapor phase that you need. Although the paper you are referring approximates precipitation from moisture divergence, you could use just the Total column rain water and no approximation is needed (I didn't read the paper so I don't know if this is makes sense).

Edit, added sources: The ERA variable list can be found here and you can search variables there and press them to get the definitions too. For NOAA I couldn't find a source for the definitions of these variables. If someone finds please edit this answer

• Thank you very much. Could you please add some references to your answer? Jul 13, 2017 at 20:53
• Thank you! I have done some tests and Total Column Rain Water from ERA, from this list, is the variable that best resembles actual precipitation in both space and time (seasonal/interannual scale at least). But apparently there is not such an equivalent from NOAA. Any suggestion? Jul 14, 2017 at 9:11
• I found Precipitation Rate in NOAA variables that might be useful, but it seems to be in units of kg/((m^2)*s) for 3-hourly data, so you will have to change it a bit to make them comparable. Jul 14, 2017 at 9:33
• I noticed that on NCEP 20th Century, daily precipitation rate is availavle as a "forecast" variable, instead of "analysis". This is made very clear in their list of parameters. Any idea of I can use this prate normally? Thanks in advance (hoping to close this thread with this answer) Aug 29, 2017 at 23:25
• IMO you should accept this as an answer as it answers the question and ask another question about the differences of NCEP 'forecast' and 'analysis' parameters. Aug 30, 2017 at 7:04