Dear Rubinei Machado ,

The Surface solar radiation downwards in ERA5 is the accumulation, or time integral, over the last hour, with units of Wm-2s (or Jm-2), so to obtain the average, or mean, flux over the hour you should divide by 3600 seconds.

Dear Ghaieth,

For 2-3 years I have used the SSRD for agrometeorological computations and I have similar problem. I used, however, data from ERA-Interim. The problem is rooted in the clear understanding what this parameter actually means. Then most reasonable seems to compare your subset with other, if possible for closer point and time span, from independent source (in this case CM SAF or, more easily, NASA/POWER at https://power.larc.nasa.gov/, for example). Intending to help you I send you two files:

- the (semi-apocrific) guideline for the radiation components in MARS.

- illustrative figure from my data: the place is lat. 44.75, lon 20.75; the quantity is average Insolation Incident On A Horizontal Surface (unit: J/m^2/day).

Best regards: Hristo

Dear professor Hristo Chervenkov,

Thank you for your answer. Actually, we compared the data from ERA5 with the global solar radiation data of the same sites and the same period taken from the regional weather services database. The difference is very significant!! Please see attached an excel file of an example done with one station.

My question is whether SSRD is global solar radiation (RG) or do I have to subtract other components from SSRD to obtain RG? Or is it a problem with the ERA5 database?

Dear Ghaieth

I advise you to use MERRA-2 RE-ANALYSIS through the SoDa Service (www.soda-is.com)

http://www.soda-pro.com/web-services/meteo-data/merra

delivers time series of :

Temperature (K) Temperature at 2 m above ground

Relative humidity (%) Relative humidity at 2 m above ground

Pressure (hPa) Pressure at ground level

Wind speed (m/s) Wind speed at 10 m above ground

Wind direction (deg) Wind direction at 10 m above ground

Rainfall (kg/m2) Rainfall (= rain depth in mm)

Global irradiation (Wh/m2) Surface incoming shortwave irradiation (broadband)

Time steps range from 1 min up to 1 month (original data are hourly)

Spatial resolution of approx. 50 km.

The data are available since Jan. 1980.

Best regards

Kada.

Thank you Dr. Kada Bouchouicha

I am actually using ERA5 data because it is the regional weather services' recommendation, which is our research partner. if we could not solve the problem, we will definitely look into Merra-2 Reanalysis database.

Dear Ghaieth,

Such big difference could be explained with different dimensions of the quantities or, most probably, different radiation component. Check the guideline document again.

Hristo

Dear Dr. Chervenkov,

I am in discussion with the ERA5 help center. I will let you know their explanation for this problem.

Dear Ghaieth Ben Hamouda ,

Do you have any update about this problem?

Dear Rubinei Machado ,

The Surface solar radiation downwards in ERA5 is the accumulation, or time integral, over the last hour, with units of Wm-2s (or Jm-2), so to obtain the average, or mean, flux over the hour you should divide by 3600 seconds.

you have to divide by 3600. Read ERA5 documentation. But some values are very high!!!