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<pre class="moz-signature" cols="90">It's been pointed out to me that the GHI and DNI on the Legnica weather file were not switched, the appearance of which happened during
my postprocessing of the epw file.
Joe
Joe Huang
White Box Technologies, Inc.
346 Rheem Blvd., Suite 205A
Moraga CA 94556
<a class="moz-txt-link-abbreviated" href="mailto:yjhuang@whiteboxtechnologies.com">yjhuang@whiteboxtechnologies.com</a>
<a class="moz-txt-link-freetext" href="http://weather.whiteboxtechnologies.com">http://weather.whiteboxtechnologies.com</a> for simulation-ready weather data
(o) (925)388-0265
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</pre>
<div class="moz-cite-prefix">On 12/19/2017 9:44 AM, Joe Huang via
Bldg-sim wrote:<br>
</div>
<blockquote type="cite"
cite="mid:c8af49e2-ff99-474d-1701-12faa2c3dd71@whiteboxtechnologies.com">
<meta http-equiv="content-type" content="text/html; charset=utf-8">
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:part1.9BDB07E9.0A0EDB2D@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:part2.488C406C.F9153B4D@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>
</blockquote>
<br>
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