<font size=2 face="sans-serif">Jimy,</font>
<br>
<br><font size=2 face="sans-serif">You indicated that "In cases of
stronger winds the averaging time can be considered equivalent to the advective
time-scale across a grid box..." Does this relationship hold
for light wind conditions or is there some different relationship that
applies. I ask because as you know the air quality community often
makes comparisons of model (WRF and MM5) 10-m wind to surface observations
for air pollution events that usually occur in stagnate, light wind
conditions. </font>
<br>
<br><font size=2 face="sans-serif">Mike</font>
<br><font size=2 face="sans-serif"> </font>
<br><font size=2 face="sans-serif">___________________________<br>
Michael D. McAtee, Ph.D.<br>
Senior Project Engineer, Meteorological Satellite Systems<br>
The Aerospace Corporation<br>
101 Nelson Drive<br>
Offutt, AFB, NE 68113-1023<br>
email: michael.mcatee@aero.org<br>
Phone: (402) 292-1017<br>
Fax: (402) 291-3137</font>
<br>
<br>
<br>
<table width=100%>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">From:</font>
<td><font size=1 face="sans-serif">Jim Dudhia <dudhia@ucar.edu></font>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">To:</font>
<td><font size=1 face="sans-serif">Ligia Bernardet <Ligia.Bernardet@noaa.gov></font>
<tr>
<td valign=top><font size=1 color=#5f5f5f face="sans-serif">Cc:</font>
<td><font size=1 face="sans-serif">wrf-users@ucar.edu, "Hacker, Joshua
\(Josh\) \(CIV\)" <jphacker@nps.edu></font>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">Date:</font>
<td><font size=1 face="sans-serif">01/27/2010 09:21 AM</font>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">Subject:</font>
<td><font size=1 face="sans-serif">Re: [Wrf-users] Comparing U,V 10 meters
to observations</font>
<tr valign=top>
<td><font size=1 color=#5f5f5f face="sans-serif">Sent by:</font>
<td><font size=1 face="sans-serif">wrf-users-bounces@ucar.edu</font></table>
<br>
<hr noshade>
<br>
<br>
<br><font size=3>All,</font>
<br><font size=3>In cases of stronger winds the averaging time can be considered
equivalent</font>
<br><font size=3>to the advective time-scale across a grid box when looking
at the U10 and V10.</font>
<br><font size=3>So for a 4 km hurricane run with 40 m/s, it would be 4000/40=100
seconds</font>
<br><font size=3>which may be comparable with 1-minute averages,</font>
<br><font size=3>Jimy</font>
<br>
<br><font size=3>On Jan 27, 2010, at 8:12 AM, Ligia Bernardet wrote:</font>
<br>
<br><font size=3>Josh and Juan,</font>
<br>
<br><font size=3>I ran into this issue while trying to compare 10-m model
wind forecasts against hurricane maximum wind reports, which are also an
average over a few minutes. There was no good solution. </font>
<br>
<br><font size=3>Having WRF output time series of variables at a grid point
or time series of statistical quantities (such as maximum winds) has been
a standing request to developers, and may be addressed at some point. </font>
<br>
<br><font size=3>I have talked to several hurricane modelers who, for research
purposes, output the model winds every time step to look at how much it
varies. Different authors got to different conclusions. Some modelers noted
that their model winds fluctuate a lot (and therefore averaging is really
necessary before comparing to obs), other noted that their winds are pretty
steady (and therefore direct comparison to observations is not a problem).</font>
<br>
<br><font size=3>So, I think this is an outstanding issue. If others have
experience with this, I would also like to know more.</font>
<br>
<br><font size=3>Ligia</font>
<br>
<br>
<br>
<br>
<br><font size=3>On Jan 26, 2010, at 5:06 PM, Hacker, Joshua (Josh) (CIV)
wrote:</font>
<br>
<br><font size=2 color=#000080 face="Arial">Apologies to those who know
this better than I do, but this is an interesting question…</font>
<br><font size=2 color=#000080 face="Arial"> </font>
<br><font size=2 color=#000080 face="Arial">Unless you do something fancy,
the WRF outputs instantaneous grid-point values, regardless of your output
interval. Those values are filtered non-trivially by some function
of implicit and explicit diffusion in the model (think physics, damping,
time and space discretization, etc). Therefore there is no one answer
to your question. </font>
<br><font size=2 color=#000080 face="Arial"> </font>
<br><font size=2 color=#000080 face="Arial">In practice (I believe) most
people just compare directly to 10-min averaged wind obs (WMO standard),
or 2-min averaged wind obs (USA) but in my opinion 2 minutes is usually
too short to be fair.</font>
<br><font size=2 color=#000080 face="Arial"> </font>
<br><font size=2 color=#000080 face="Arial">One approach to getting at
the averaging in the model is through data assimilation and statistical
consideration of observation error levels when the error may be dominated
by representativeness error (difference between scales represented in obs
and model). Others have looked in spectral space to get an idea of
the averaging scales.</font>
<br><font size=2 color=#000080 face="Arial"> </font>
<br><font size=2 color=#000080 face="Arial">The other thing that you might
consider, if you have the data to do it, is to compare WRF forecasts with
obs averaged over many different time lengths (or alternatively band-pass
filtered). Then you can get a more complete picture of what time
scales it can or cannot predict with skill, and decide whether it is doing
what you hope.</font>
<br><font size=2 color=#000080 face="Arial"> </font>
<br><font size=2 color=#000080 face="Arial">That said, I’d be interested
in hearing any other comments from the community and especially our verification
gurus – is there any “conventional” wisdom or rules of thumb that folks
are using or that have recently shown promise, empirically? Have
I missed something?</font>
<br><font size=2 color=#000080 face="Arial"> </font>
<br><font size=2 color=#000080 face="Arial">Sorry that there is no simple
answer.</font>
<br><font size=2 color=#000080 face="Arial"> </font>
<br><font size=2 color=#000080 face="Arial">Josh</font>
<br><font size=2 color=#000080 face="Arial"> </font>
<div align=center>
<br>
<hr></div>
<br><font size=2 face="Tahoma"><b>From:</b> </font><a href="mailto:wrf-users-bounces@ucar.edu"><font size=2 color=blue face="Tahoma"><u>wrf-users-bounces@ucar.edu</u></font></a><font size=2 face="Tahoma">
[</font><a href="mailto:wrf-users-bounces@ucar.edu"><font size=2 color=blue face="Tahoma"><u>mailto:wrf-users-bounces@ucar.edu</u></font></a><font size=2 face="Tahoma">]
<b>On Behalf Of </b>Juan Gonzalez<b><br>
Sent:</b> Tuesday, January 26, 2010 12:33 PM<b><br>
To:</b> </font><a href="mailto:wrf-users@ucar.edu"><font size=2 color=blue face="Tahoma"><u>wrf-users@ucar.edu</u></font></a><font size=2 face="Tahoma"><b><br>
Subject:</b> [Wrf-users] Comparing U,V 10 meters to observations</font>
<br><font size=3 face="Times New Roman"> </font>
<br><font size=3 face="Times New Roman">Hi everyone,</font>
<br><font size=3 face="Times New Roman"> </font>
<br><font size=3 face="Times New Roman">I am interested on validating the
WRF 10m wind forecast with observational data. I would like to know what
is the corresponding average scale for the WRF 10m winds, that is, do the
correspond to 1-minute, 10-minute average winds or what kind of average,
if any?</font>
<br><font size=3 face="Times New Roman"> </font>
<br><font size=3 face="Times New Roman">My WRF configuration outputs wind
every 3 hours.</font>
<br><font size=3 face="Times New Roman"> </font>
<br><font size=3 face="Times New Roman">Thanks for your help, </font>
<br><font size=3 face="Times New Roman">Juan Gonzalez <br>
<br>
-- <br>
Juan O. Gonzalez<br>
Research Assistant - Caribbean Integrated Coastal Ocean Observing System<br>
<br>
Graduate Student - Physical Oceanography<br>
Marine Sciences Department<br>
University of Puerto Rico - Mayaguez Campus</font>
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