<p>The <a href="https://www.homerenergy.com/products/pro/docs/3.11/how_homer_calculates_the_radiation_incident_on_the_pv_array.html" rel="nofollow noreferrer">HOMER documentation for solar radiance</a> Describes how Extraterrestrial Horizontal Radiation and Extraterrestrial Direct Normal Radiation are calculated. <strong>These are theoretical quantities</strong> that describe radiation at the surface of the Earth's atmosphere before atmospheric losses and absorption. These can be estimated as a function of <code>day of year</code>, <code>solar zenith angle</code>, and <code>solar constant</code>.</p>
<p><strong>Extraterrestrial Direct Normal Radiation</strong> (<span class="math-container">$G_{on}$</span>) is the radiation that strikes a surface <em>normal to the sun's rays</em> at the edge of the atmosphere and is calculated as:</p>
<p><span class="math-container">$$G_{o n}=G_{s c}\left(1+0.033 \cdot \cos \frac{360 n}{365}\right)$$</span></p>
<p>Where <span class="math-container">$G_{sc}$</span> is the solar constant which generally has a value of 1,367 <span class="math-container">$W$</span>/<span class="math-container">$m^2$</span> and <span class="math-container">$n$</span> is the day of the year.</p>
<p><strong>Extraterrestrial Horizontal Radiation</strong> (<span class="math-container">$G_o$</span>) is the radiation that strikes a horizontal surface at the edge of the atmosphere and is calculated using the solar zenith angle <span class="math-container">$\theta_z$</span>:</p>
<p><span class="math-container">$$G_o = G_{on}\cos(\theta_z)$$</span></p>
<p>The <a href="https://bigladdersoftware.com/epx/docs/8-3/auxiliary-programs/energyplus-weather-file-epw-data-dictionary.html#field-horizontal-infrared-radiation-intensity" rel="nofollow noreferrer">Energy Plus Weather documentation</a> provides a Heuristic to estimate <strong>Horizontal Infrared Radiation Intensity</strong> as a function of <code>dry bulb temperature</code>, <code>dew point temperature</code>, and <code>opaque sky cover</code> based on</p>
<ul>
<li>Walton, G.N. Thermal Analysis Research Program Reference Manual; US Department of Commerce, National Bureau of Standards: Washington, DC, USA, March 1983.</li>
<li>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.</li>
</ul>

CC BY-SA 4.0
2023-07-26T14:41:34.207Z
2023-07-27T20:20:45.873Z
2023-07-27T20:20:45.873Z
J Edward Hammond
<p>I am working to create <a href="https://bigladdersoftware.com/epx/docs/8-3/auxiliary-programs/energyplus-weather-file-epw-data-dictionary.html" rel="nofollow noreferrer">Energy Plus Weather (EPW) Actual Meteorological Year (AMY) files</a> from <a href="https://www.nco.ncep.noaa.gov/pmb/products/hrrr/hrrr.t00z.wrfsfcf00.grib2.shtml" rel="nofollow noreferrer">NOAA HRRR Model analysis</a>. 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/m<sup>2</sup>:</p>
<ul>
<li>Extraterrestrial Horizontal Radiation</li>
<li>Extraterrestrial Direct Normal Radiation</li>
<li>Horizontal Infrared Radiation Intensity</li>
<li>Global Horizontal Radiation</li>
<li>Direct Normal Radiation</li>
<li>Diffuse Horizontal Irradiance</li>
</ul>
<p>Unfortunately, HRRR radiation-related variables are:</p>
<ul>
<li>Upward Long-Wave Rad. Flux</li>
<li>Downward Short-Wave Radiation Flux</li>
<li>Downward Long-Wave Rad. Flux</li>
<li>Upward Short-Wave Radiation Flux</li>
<li>Upward Long-Wave Rad. Flux</li>
<li>Cloud Forcing Net Solar Flux</li>
<li>Visible Beam Downward Solar Flux</li>
<li>Visible Diffuse Downward Solar Flux</li>
<li>Upward Short-Wave Radiation Flux</li>
</ul>
<p>I am struggling to convert the HRRR variables to the required AMY parameters. I understand from <a href="https://earthscience.stackexchange.com/questions/5439/shortwave-and-longwave-radiation-terminology-direct-beam-par-flux">this post</a> 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.</p>
<p>Are there any suggestions on how to approximate these values from the data I have and known approximations?</p>

<atmosphere-modelling><measurements><numerical-modelling><atmospheric-radiation><radiation-balance>
Estimate EPW AMY parameters from HRRR Analysis
J Edward Hammond
# Estimate EPW AMY parameters from HRRR Analysis

(Asked by: username_0 on 2023-07-26)

I am working to create [Energy Plus Weather (EPW) Actual Meteorological Year (AMY) files](https://bigladdersoftware.com/epx/docs/8-3/auxiliary-programs/energyplus-weather-file-epw-data-dictionary.html) from [NOAA HRRR Model analysis](https://www.nco.ncep.noaa.gov/pmb/products/hrrr/hrrr.t00z.wrfsfcf00.grib2.shtml). 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:

* Extraterrestrial Horizontal Radiation
* Extraterrestrial Direct Normal Radiation
* Horizontal Infrared Radiation Intensity
* Global Horizontal Radiation
* Direct Normal Radiation
* Diffuse Horizontal Irradiance

Unfortunately, HRRR radiation-related variables are:

* Upward Long-Wave Rad. Flux
* Downward Short-Wave Radiation Flux
* Downward Long-Wave Rad. Flux
* Upward Short-Wave Radiation Flux
* Upward Long-Wave Rad. Flux
* Cloud Forcing Net Solar Flux
* Visible Beam Downward Solar Flux
* Visible Diffuse Downward Solar Flux
* Upward Short-Wave Radiation Flux

I am struggling to convert the HRRR variables to the required AMY parameters. I understand from [this post](https://earthscience.stackexchange.com/questions/5439/shortwave-and-longwave-radiation-terminology-direct-beam-par-flux) 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?

**Tags:** atmosphere-modelling, measurements, numerical-modelling, atmospheric-radiation, radiation-balance


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## 1 answers

### Answer by username_0 on 2023-07-26. Score: 1 | Accepted answer

The [HOMER documentation for solar radiance](https://www.homerenergy.com/products/pro/docs/3.11/how_homer_calculates_the_radiation_incident_on_the_pv_array.html) 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](https://bigladdersoftware.com/epx/docs/8-3/auxiliary-programs/energyplus-weather-file-epw-data-dictionary.html#field-horizontal-infrared-radiation-intensity) 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.


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