This file contains forecast, retrieved from era interim dataset.

ECMWF operationaly runs model 2 times each day (00 and 12 h), and forecast step is 3 hours.

year  mm  dd  hh       ssrd  fctst   ac.period
2013  05  10  03     0.0000     00     03-00  <== forecast start 2013-05-10 at 00h, valid for 00-03am period.
2013  05  10  06    53.9259     00     06-03  <== forecast start 2013-05-10 at 00h, valid for 03-06am period.

ssrd (=) W/(m2*h)

Notice that ssrd represents hourly average value for 3h interval.
 
It means that total radiation for 3 hour period is 3*ssrd.
Since the data are available at 3 hours time step, we couldn't provide you with hourly data.

----------header----------

year mm dd hh: VALID DATE
ssrd: surface solar radiation downwards (ecmwf grib indicatorOfParameter 169.128)
fctst: forecast start hour
ac.period: accumulation period