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I only got two responses this time, which I attribute to either the
close proximity of the holidays or perhaps reader fatigue about
weather data <span class="moz-smiley-s1" title=":-)"></span> <br>
However, the answers I did receive, especially the second one, were
so much in line with what I saw on the weather file that it looks
copied which I must affirm was not the case (see the answers at the
bottom of this post).<br>
<br>
So, the three things I found unusual about this weather file are, in
order of ascending importance:<br>
1. The year is given as 2005 throughout, but the comment says that
it's a "typical year" file made from the time period 1970-2000. <br>
2. Line 8 says that the first day of the weather file, i.e., Jan.
1, is set as a Sunday, but January 1 2005 was a Saturday. <br>
3. The solar radiation appears to be shifted a half-hour ahead,
probably because the original file was created in Europe, which
tends to report the solar around the time step, e.g., -0:30 to
+0.30, whereas in North America it's reported for the preceding time
step, i.e., -1:00 to 0:00. Although that was the only criteria for
why I chose this weather file (Legnica, Poland), I later found other
more troubling aspects to the solar radiation that made me expand
the discussion to QC'ing the solar on weather files in general.<br>
<br>
I first became aware of the different conventions of reporting solar
back in 1993 when I was involved in an IEA project on Low-Energy
Cooling where one of the first items of business was to compare the
climate conditions in the participant countries. When I tried to
work with weather data from European colleagues, I was getting
impossibly large spikes of Direct Normal Irradiance (DNI) for many
sunrise hours due to this difference in convention, since I was
using a US simulation program that follows the US convention and
calculates the sun position at the midpoint of the preceding time
step.<br>
<br>
Although it might seem hard to tell which convention is being used
in a weather file, it's actually quite noticeable if one were to
compare the hourly profiles to those calculated by a Clear Sky
Model. For example, this plot shows July 1 - 4, where the forward
shifting of the solar on the weather file can be seen, particularly
if you look for the sunrise and sunset hours. What really surprised
me, though, was that the Global Horizontal Irradiance (GHI) and DNI
seem switched on the weather file. The GHI profile is generally more
smooth and the DNI more spiky, but here it's the reverse. Also,
the GHI greatly exceeds the Clear Sky GHI on the morning of Day 3,
which simply cannot happen.<br>
<img src="cid:part1.F497860D.439AA54E@whiteboxtechnologies.com"
alt="" class="" title="bodfnipgjcaipkjb.png" height="263"
width="1024"><br>
<br>
The data for Jan. 1 -4 reveals more indications that the GHI and DNI
on the weather file may be switched., with the GHI (?) again more
spiky and on Jan. 2 almost three times as much as the Clear Sky
GHI. The 30-minute shift is less visible owing to the small values,
but can still be detected by looking at the sunrise and sunset
hours.<br>
<img src="cid:part2.B69FA036.C53C70B7@whiteboxtechnologies.com"
alt="" class="" title="codmmoiielfdnbgj.png" height="263"
width="1024"><br>
<br>
So what does this say about QC'ing weather data? It seems that more
emphasis has gone into QC'ing excursions in temperature than solar
radiation. However, since the solar in almost all weather files is
not measured but calculated, there should be all the more reason to
regard it more carefully. Luckily, there are several simple, if not
simple-minded, facts that can be used as reality checks, e.g., (1)
the GHI should always be non-zero when the sun is above the horizon,
making it possible to determine the hours of sunrise and sunset,
(2) the GHI and DNI could never be greater than their Clear Sky
values (don't worry about ground reflectance or atmospheric
phenomena like cloud lensing since we're only dealing with
calculated data), and (3) the Direct Horizontal ( DHI = DNI *
arcsin of the solar angle) cannot be greater than the GHI. <br>
<br>
I hope this answer has provided some insight into the contents of
weather files. As in the previous contest, both respondents, Samuel
Letellier-Duchesne and Michael Kummert, are declared winners and
entitled to one free weather file of their choice from the WBT
archive. Just let me know. (Full disclosure: Samuel appears to be
Michael's graduate student :-))<br>
<br>
Holiday greetings to all!<br>
<br>
Joe<br>
-------- Forwarded Message --------
<table class="moz-email-headers-table">
<tbody>
<tr>
<th nowrap="nowrap">Subject: </th>
<td>Re: [Bldg-sim] Test your knowledge of simulation weather
file formats Part 2: the EnergyPlus *.epw format</td>
</tr>
<tr>
<th nowrap="nowrap">Date: </th>
<td>Fri, 15 Dec 2017 13:02:00 -0500</td>
</tr>
<tr>
<th nowrap="nowrap">From: </th>
<td>Samuel Letellier-Duchesne <a
class="moz-txt-link-rfc2396E"
href="mailto:samuel.letellier-duchesne@polymtl.ca"><samuel.letellier-duchesne@polymtl.ca></a></td>
</tr>
<tr>
<th nowrap="nowrap">To: </th>
<td>Joe Huang <a class="moz-txt-link-rfc2396E"
href="mailto:yjhuang@whiteboxtechnologies.com"><yjhuang@whiteboxtechnologies.com></a></td>
</tr>
</tbody>
</table>
<br>
<br>
Well, there is clearly something wrong with the sunup/sundown
portion of the data as there is no systematically no transition for
most days of the year. See image for comparison with us-based
weather file. Will try to find more!
<div class=""><br class="">
</div>
<div class=""><img id="4FFE993D-F1F1-441F-A3EB-E7D78B5BAE23"
src="cid:part5.3FA75DAC.4B5039A4@whiteboxtechnologies.com"
class="" title="Unknown.png" alt="" height="768" width="1024"></div>
<br class="">
<div class="">
<div dir="auto" class="">Polytechnique Montréal <br class="">
Samuel Letellier-Duchesne<br class="">
Ph.D. Candidate<br class="">
Mechanical Engineering<br class="">
<div class="">2900 Edouard Montpetit Blvd,</div>
<div class="">Montreal, QC H3T 1J4 - Canada</div>
<div class=""><a
href="mailto:samuel.letellier-duchesne@polymtl.ca" class="">samuel.letellier-duchesne@polymtl.ca</a></div>
<div class="">Tel. +1-514-813-0056</div>
</div>
</div>
<br class="">
<div class="moz-cite-prefix">On 12/19/2017 4:27 AM, Michaël Kummert
wrote:<br>
</div>
<blockquote type="cite"
cite="mid:001301d378c4$c2d92450$488b6cf0$@polymtl.ca">
<div class="WordSection1">
<p class="MsoNormal">Joe,</p>
<p class="MsoNormal">Here are my comments on the file that you
asked us to examine.</p>
<p class="MsoNormal"><b> </b></p>
<p class="MsoNormal"><b>Small details:</b></p>
<p class="MsoNormal">The EPW file appears to be a typical
meteorological year but the year 2005 is given for all months,
which is probably not correct. The file also has zeros in the
minute field (5th field), while many typical EPW files have
60. I haven’t found a clear explanation of what that field is
supposed to contain.</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal"><b>The main “problem”: solar radiation</b></p>
<p class="MsoNormal">Solar radiation data in this EPW file seems
questionable:</p>
<ul type="disc">
<li class="MsoNormal">There is an apparent shift (roughly ½ h)
in solar radiation reported in the file. This is apparent
when comparing radiation in the file to calculated clear sky
radiation</li>
<li class="MsoNormal">The yearly average beam normal radiation
is much lower than in other data sources (Meteonorm), but
hourly values on some days are unrealistically high (in some
cases twice the clear sky radiation)</li>
<li class="MsoNormal">Calculated values for normal
extraterrestrial (12<sup>th</sup> column) are not correct.
That column probably shows another variable. On the other
hand, calculated values for extraterrestrial horizontal seem
correct</li>
</ul>
<p class="MsoListParagraph"> </p>
<ul type="disc">
<li class="MsoNormal">This indicates a problem in the original
data and/or in its interpretation and processing. If I had
to guess, I would propose the following explanation:</li>
<ul type="circle">
<li class="MsoNormal">Let’s assume only total horizontal is
available (estimated or measured). The shift in time
causes total radiation in the morning to be very high.
Some correlation similar to Erbs is used to estimate the
diffuse radiation, and that correlation gives a small
percentage of diffuse. Since the total radiation is very
high, the calculated normal radiation on the horizontal is
large, and the beam normal radiation is even larger. </li>
<li class="MsoNormal">This could explain the large spikes in
beam normal (mostly in the morning), but it does not
explain why the beam normal radiation is too low in
average.</li>
</ul>
</ul>
<p class="MsoNormal"> </p>
<p class="MsoNormal"><b>Other variables</b> (temperature,
humidity, wind speed) seem to be usable. I didn’t check
humidity calculations, I assumed the variables would be
consistent.</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">I have put some graphs in a presentation
which I may use one day in a class. I am attaching it in case
you want to take a look at some of the strange solar radiation
values vs clear sky radiation.</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Michaël</p>
</div>
</blockquote>
<br>
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