How to get GHI data from MERRA-2? Help Requested

Does anyone know if MERRA2 contains hourly Global Horizontal Irradiance (GHI) data, or some other measurement which we can use to calculate GHI?, If so, could you please let me know how?

Background

I am attempting to use PVLib library in python to calculate the estimated solar generation of various solar farms across the world. PVLib can estimate GHI, Direct Normal Irradiance (DNI) and Diffuse Horizontal Irradiance (DHI), but I was hoping to bring in actual data.

Research and own findings

I've used MERRA-2 data previously to source hourly temperature and wind speeds. So I went looking here first for GHI data. 

I looked though GES DISC directory for various MERRA2 reanalysis files. I found M2T1NXRAD (found here) and figured since it contains radiation data that it would contain GHI, but I didn't see anything which looked like GHI in the field list (found here). I did find "surface incident shortwave flux (SWGDN)", but I am not a meteorologist and don't know how this relates to GHI, or if it can even be converted to GHI?

Browsing the internet, it does look like there is some relation between SWGDN and GHI. I've seen many publications allude to it (such as here and here) but I wasn't able to figure out how they are calculating it.

an extract from the above article states


  MERRA-2 uses the Goddard Earth Observing System version 5
  (GEOS-5) atmospheric model and data assimilation system (DAS). The
  dataset offers two-dimensional diagnostics of surface fluxes, single
  level meteorology, vertical integrals and land states, generated at 1
  hourly, 3 hourly, daily and monthly intervals. This paper utilises the
  surface incident shortwave flux (SWGDN) from the single-level
  diagnostic tavgM_2d_rad_Nx data product averaged over a monthly
  interval (NASA 2017). 


But it doesn't say how to convert SWGDN to GHI? if anyone knows how these two measures relate to one another, I would love an explanation!
Solved
After more investigation, and help from the PVLIB community, I learned that SWGDN is same as GHI.