# A question about SWDOWN (DOWNWARD SHORT WAVE FLUX AT GROUND SURFACE)

#### rhodria

##### New member
Hi all,
There is an output called SWDOWN (DOWNWARD SHORT WAVE FLUX AT GROUND SURFACE). Is this the same as Global Horizontal Solar Irradiance? How can I calculate the direct normal irradiance and the diffuse horizontal irradiance (which are needed for our building modeling) from it?

Thanks a lot.

#### Ming Chen

##### Moderator
Staff member
WRF can output the following variables that meet your need:
"SWDDIR" "Shortwave surface downward direct irradiance" "W m-2" ""
"SWDDIF" "Shortwave surface downward d iffuse irradiance" "W m-2" ""

#### rhodria

##### New member
WRF can output the following variables that meet your need:
"SWDDIR" "Shortwave surface downward direct irradiance" "W m-2" ""
"SWDDIF" "Shortwave surface downward d iffuse irradiance" "W m-2" ""
Hi Ming,

Thanks for replying. Is SWDOWN the same as Global Horizontal Solar Irradiance?

#### Ming Chen

##### Moderator
Staff member
I am not sure what Global Horizontal Solar Irradiance is. Is it from model simulation or from observations?

#### rhodria

##### New member
I am not sure what Global Horizontal Solar Irradiance is. Is it from model simulation or from observations?
Hi Ming,

It is from the TMY3 weather data. Global Horizontal Solar Irradiance is total amount of direct and diffuse solar radiation received on a horizontal surface. It is calculated by models.

#### Ming Chen

##### Moderator
Staff member
Then I guess it is equivalent to SWDOWN in WRF. Please compare the values of the two variables and make sure they are physically the same.

#### rhodria

##### New member
Then I guess it is equivalent to SWDOWN in WRF. Please compare the values of the two variables and make sure they are physically the same.
Hi Ming,

Do you have a document explaining how SWDOWN is calculated? I want to compare SWDOWN with the global horizontal irradiance calculated from the solar radiation model provided by ASHRAE Handbook.

#### Ming Chen

##### Moderator
Staff member
We don't have a specific document to describe how SWDOWN i calculated. You can find this information inside the WRF codes, which is dependent on what scheme you use. It is closely related to GSW and surface albedo.