Monthly average shortwave downward flux at the surface 
Monthly average downward photosynthetically active radiation at the surface
Monthly average surface albedo

Provided are month by month comparisons of the above parameters as obtained from:

A comparison of relevant snow products is also provided, using data from:

There was a problem with the snow information used originally with the NOAA/NESDIS operational product (came from Air Force). Note the gaps during February, March and April, which affected all derived surface parameters.  This product was replaced with the IMS product in December 1999.  During snow free months, the agreement between the various parameters is good, in particular, the downwelling SW. 

Please note:

The ISCCP products, both D1 and DX use sampled data, one pixel every 30 km.  In the D1 case, the pixels are gridded to 2.5 deg at GISS, and in the DX case, the gridding of the raw DX data to 0.5 deg is done at the University of Maryland.  Both the operational GCIP/GAPP product as well as the reprocessed one, use 4 km pixels.