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

Wed Jan 27 09:05:38 MST 2010

Jimy,

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. 

Mike

___________________________
Michael D. McAtee, Ph.D.
Senior Project Engineer, Meteorological Satellite Systems
The Aerospace Corporation
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email:  michael.mcatee at aero.org
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Fax: (402) 291-3137

From:
Jim Dudhia <dudhia at ucar.edu>
To:
Ligia Bernardet <Ligia.Bernardet at noaa.gov>
Cc:
wrf-users at ucar.edu, "Hacker, Joshua \(Josh\) \(CIV\)" <jphacker at nps.edu>
Date:
01/27/2010 09:21 AM
Subject:
Re: [Wrf-users] Comparing U,V 10 meters to observations
Sent by:
wrf-users-bounces at ucar.edu

All,
In cases of stronger winds the averaging time can be considered equivalent
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,
Jimy

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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