# Estimate EPW AMY parameters from HRRR Analysis

I am working to create Energy Plus Weather (EPW) Actual Meteorological Year (AMY) files from NOAA HRRR Model analysis. The provided links detail the 35 parameters required for the AMY model and variables provided by the HRRR analysis. Most of these parameters are easily found with minor unit conversions. However, the AMY file requires the following in units of Wh/m2:

• Cloud Forcing Net Solar Flux
• Visible Beam Downward Solar Flux
• Visible Diffuse Downward Solar Flux

I am struggling to convert the HRRR variables to the required AMY parameters. I understand from this post that beam and direct are synonymous, however I am unsure how to calculate the extraterrestrial and horizontal infrared radiation from this data as well as global position and time.

Are there any suggestions on how to approximate these values from the data I have and known approximations?

The HOMER documentation for solar radiance Describes how Extraterrestrial Horizontal Radiation and Extraterrestrial Direct Normal Radiation are calculated. These are theoretical quantities that describe radiation at the surface of the Earth's atmosphere before atmospheric losses and absorption. These can be estimated as a function of day of year, solar zenith angle, and solar constant.

Extraterrestrial Direct Normal Radiation ($$G_{on}$$) is the radiation that strikes a surface normal to the sun's rays at the edge of the atmosphere and is calculated as:

$$G_{o n}=G_{s c}\left(1+0.033 \cdot \cos \frac{360 n}{365}\right)$$

Where $$G_{sc}$$ is the solar constant which generally has a value of 1,367 $$W$$/$$m^2$$ and $$n$$ is the day of the year.

Extraterrestrial Horizontal Radiation ($$G_o$$) is the radiation that strikes a horizontal surface at the edge of the atmosphere and is calculated using the solar zenith angle $$\theta_z$$:

$$G_o = G_{on}\cos(\theta_z)$$

The Energy Plus Weather documentation provides a Heuristic to estimate Horizontal Infrared Radiation Intensity as a function of dry bulb temperature, dew point temperature, and opaque sky cover based on

• Walton, G.N. Thermal Analysis Research Program Reference Manual; US Department of Commerce, National Bureau of Standards: Washington, DC, USA, March 1983.
• Clark, G.; Allen, C. The estimation of atmospheric radiation for clear and cloudy skies. In Proceedings of the 2nd National Passive Solar Conference (AS/ISES), Philadelphia, PA, USA, 16–18 March 1978; pp. 675–678.