Surface Solar Radiation Downwards (ERA5)

Variable: ssrd

This parameter is accumulated over a particular time period which depends on 
the data extracted. The units are joules per square metre (J m-2). 
To convert to watts per square metre (W m-2), the accumulated values 
should be divided by the accumulation period expressed in seconds. 
The ECMWF convention for vertical fluxes is positive downwards.

Ex: ssrd/86400 (Wm-2/day)