[Wrf-users] Comparing U,V 10 meters to observations

Jim Dudhia dudhia at ucar.edu
Wed Jan 27 08:21:41 MST 2010

In cases of stronger winds the averaging time can be considered  
to the advective time-scale across a grid box when looking at the U10  
and V10.
So for a 4 km hurricane run with 40 m/s, it would be 4000/40=100 seconds
which may be comparable with 1-minute averages,

On Jan 27, 2010, at 8:12 AM, Ligia Bernardet wrote:

> Josh and Juan,
> 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.
> 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.
> 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).
> So, I think this is an outstanding issue. If others have experience  
> with this, I would also like to know more.
> Ligia
> On Jan 26, 2010, at 5:06 PM, Hacker, Joshua (Josh) (CIV) wrote:
>> Apologies to those who know this better than I do, but this is an  
>> interesting question…
>> 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.
>> 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.
>> 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.
>> 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.
>> 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?
>> Sorry that there is no simple answer.
>> Josh
>> From: wrf-users-bounces at ucar.edu [mailto:wrf-users- 
>> bounces at ucar.edu] On Behalf Of Juan Gonzalez
>> Sent: Tuesday, January 26, 2010 12:33 PM
>> To: wrf-users at ucar.edu
>> Subject: [Wrf-users] Comparing U,V 10 meters to observations
>> Hi everyone,
>> 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?
>> My WRF configuration outputs wind every 3 hours.
>> Thanks for your help,
>> Juan Gonzalez
>> -- 
>> Juan O. Gonzalez
>> Research Assistant - Caribbean Integrated Coastal Ocean Observing  
>> System
>> Graduate Student - Physical Oceanography
>> Marine Sciences Department
>> University of Puerto Rico - Mayaguez Campus
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