<html><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">I think the advective time scale idea falls apart when the wind is light.<div>There I would just compare it with long enough time averages to remove gusts,</div><div>because it is an area mean,</div><div>Jimy</div><div><br><div><div>On Jan 27, 2010, at 9:05 AM, Michael McAtee wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><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: <a href="mailto:michael.mcatee@aero.org">michael.mcatee@aero.org</a><br> Phone: (402) 292-1017<br> Fax: (402) 291-3137</font> <br> <br> <br> <table width="100%"> <tbody><tr valign="top"> <td><font size="1" color="#5f5f5f" face="sans-serif">From:</font> </td><td><font size="1" face="sans-serif">Jim Dudhia <<a href="mailto:dudhia@ucar.edu">dudhia@ucar.edu</a>></font> </td></tr><tr valign="top"> <td><font size="1" color="#5f5f5f" face="sans-serif">To:</font> </td><td><font size="1" face="sans-serif">Ligia Bernardet <<a href="mailto:Ligia.Bernardet@noaa.gov">Ligia.Bernardet@noaa.gov</a>></font> </td></tr><tr> <td valign="top"><font size="1" color="#5f5f5f" face="sans-serif">Cc:</font> </td><td><font size="1" face="sans-serif"><a href="mailto:wrf-users@ucar.edu">wrf-users@ucar.edu</a>, "Hacker, Joshua \(Josh\) \(CIV\)" <<a href="mailto:jphacker@nps.edu">jphacker@nps.edu</a>></font> </td></tr><tr valign="top"> <td><font size="1" color="#5f5f5f" face="sans-serif">Date:</font> </td><td><font size="1" face="sans-serif">01/27/2010 09:21 AM</font> </td></tr><tr valign="top"> <td><font size="1" color="#5f5f5f" face="sans-serif">Subject:</font> </td><td><font size="1" face="sans-serif">Re: [Wrf-users] Comparing U,V 10 meters to observations</font> </td></tr><tr valign="top"> <td><font size="1" color="#5f5f5f" face="sans-serif">Sent by:</font> </td><td><font size="1" face="sans-serif"><a href="mailto:wrf-users-bounces@ucar.edu">wrf-users-bounces@ucar.edu</a></font></td></tr></tbody></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> <br><font size="3">_______________________________________________<br> Wrf-users mailing list</font><font size="3" color="blue"><u><br> </u></font><a href="mailto:Wrf-users@ucar.edu"><font size="3" color="blue"><u>Wrf-users@ucar.edu</u></font></a><font size="3" color="blue"><u><br> </u></font><a href="http://mailman.ucar.edu/mailman/listinfo/wrf-users"><font size="3" color="blue"><u>http://mailman.ucar.edu/mailman/listinfo/wrf-users</u></font></a> <br> <br><font size="3">_______________________________________________<br> Wrf-users mailing list</font><font size="3" color="blue"><u><br> </u></font><a href="mailto:Wrf-users@ucar.edu"><font size="3" color="blue"><u>Wrf-users@ucar.edu</u></font></a><font size="3"><br> </font><a href="http://mailman.ucar.edu/mailman/listinfo/wrf-users"><font size="3">http://mailman.ucar.edu/mailman/listinfo/wrf-users</font></a> <br><tt><font size="2">_______________________________________________<br> Wrf-users mailing list<br> <a href="mailto:Wrf-users@ucar.edu">Wrf-users@ucar.edu</a><br> </font></tt><a href="http://mailman.ucar.edu/mailman/listinfo/wrf-users"><tt><font size="2">http://mailman.ucar.edu/mailman/listinfo/wrf-users</font></tt></a><tt><font size="2"><br> </font></tt> <br></blockquote></div><br></div></body></html>