# 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.

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" ""

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?

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

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.

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.

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.

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.

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！ I am using WRF-V4.5 with solar. But I found the SWDOWN is too high in Rainy days.
I have done a lot of experiments and the result is the same.
Is it because of the low cloud cover in the WRF simulation? My D02 is 2km×2km， and all the Cumulus Scheme was turned off in D02（I did not use Deng scheme）.
How to resolve this problem？
Thanks！

Last edited:
@haiqingsong
I agree that cloud cover could be a critical factor that affects SWDOWN. Which radiation scheme did you use?

@haiqingsong
I agree that cloud cover could be a critical factor that affects SWDOWN. Which radiation scheme did you use?
The radiation scheme is RRTMG. And When the MP was set to 38（Thompson 38）, WRF4.5 will crash.
mp_physics =10, 10,
write_thompson_mp38table = .false. ,
aer_opt = 3,
o3input = 2,
use_aero_icbc =.true.,
ra_lw_physics = 4, 4,
ra_sw_physics = 4, 4,
ra_sw_eclipse = 1,
swint_opt = 2,
sf_sfclay_physics = 5, 5,
sf_surface_physics = 4, 4,
bl_pbl_physics = 5, 5，
bldt = 0, 0,

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" ""