Global numerical weather model MERRA2 is used for this computation.
It has 72 terrain following levels from the surface to 1 Pa level,
0.67 deg resolution over longitude, 0.5 deg resolution over
latitude and 6 hour time resolution.

Reference that describes MERRA:

  Rienecker, M.M., M.J. Suarez, R. Gelaro, R. Todling, J. Bacmeister, 
  E. Liu, M.G. Bosilovich, S.D. Schubert, L. Takacs, G.-K. Kim, S. Bloom, 
  J. Chen, D. Collins, A. Conaty, A. da Silva, et al. (2011), 
  MERRA: NASA's Modern-Era Retrospective Analysis for Research and 
  Applications. J. Climate, 24, 3624-3648, doi:10.1175/JCLI-D-11-00015.1. 
  http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-11-00015.1

How to cite:

  (GMAO) (2015), inst3_3d_asm_Cp: MERRA-2 3D IAU State, Meteorology 
Instantaneous 3-hourly (p-coord, 0.625x0.5L42), version 5.12.4, Greenbelt, 
MD, USA: Goddard Space Flight Center Distributed Active Archive Center 
(GSFC DAAC), Accessed Enter User Data Access Date at doi: 10.5067/VJAFPLI1CSIV.
