[Wrfems] Comparing a 12 km run with 4.7 km run

Robert Rozumalski rozumal at ucar.edu
Tue Sep 15 14:39:49 MDT 2009




Good afternoon Henry,

I'm assuming that you ran 2 independent single domain experiments, i.e, 
no nesting.

What did you use for the convective & grid scale scheme configuration in 
each experiment?

Bob


Henry Steigerwaldt wrote:
> To All:
>
> We've seen some poor performances regarding simulated reflectivity 
> output by our WRF-ARW (4.7 km) model with what really
> happens. Also, I'd say many times the NSSL's WRF-ARW model (4 km) does 
> a much better job at predicting precip intensities and
> locations than our model does. We've tried to set up our local WRF 
> similar to NSSL's regarding the various options available in the
> setup routines (i.e. microphysics, radiation scheme, land surface 
> model, etc.).
>
> In correspondence with Dr. Bob, here's one thing he stated to improve 
> our model output...
> *I suspect the answer is to use a larger domain, which is my 
> recommendation  for most everything. The NSSL domain is
> 980x750 by 4km, which is probably much larger than yours and _allows 
> for small scale forcing to develop upstream of your_
> _region and outside of your current computational domain_. You can 
> cheat a bit with a nested simulation going from 12 to 4km
> with a large 12km domain. You can also start your run with a previous 
> 6hour forecast to get additional time.
> *___________________________________________________________________________________________
>
> So, as an experiment, today we ran the model twice for the 12Z run to 
> test out what Bob mentioned. The first run was the usual one with
> a 4.7 km resolution and the small domain. The second run was then run 
> with a much larger domain with a resolution of 12 km as Bob
> recommended above. Attached is the result.
>
> _/Focus on the differences within the area encompassed by the 
> square/_, which is our usual domain size for the 4.7 km runs we produce.
> Note that only part of the 12 km domain is visible in this screen 
> capture. On the left is the 12 km version and on the right is the 4.7 
> km version.
> Using this as an example, both show a 12-hour forecast valid at 00Z 16 
> SEP 2009 of the simulated Maximum Composite Reflectivity.
>
> I was expecting to see at the very least a bit of improvement in 
> forecast coverage of the precip in the bigger domain for reasons as 
> Bob stated
> above, than what was forecast in the much smaller domain we normally 
> use. But I was startled to see such a difference between the two runs
> in both areal coverage and intensities. They aren't even close! You 
> would think the areas where precip is forecast would be generally 
> similar,
> and with the 12 km version showing less distinctness of the actual 
> convective features of the echoes than the 4.7 km resolution version. For
> other hours I looked at, it makes one think you were looking at runs 
> for different time periods. They aren't similar at all.
>
> What's going on with this? This can't possibly be something one can 
> normally expect? I mean, the locations of the precip are so different,
> and the same for the intensities, yet the runs are based on the same 
> data and setup options for the most part. Only the size of the domains
> and the resolutions are different as far as we can tell.
>
> The purpose of this "experiment" was to determine whether perhaps 
> using a larger domain for our model, or perhaps nesting a smaller domain
> within a larger domain, would help improve our model precip forecasts.
>
> *Now I'm really confused! *So what do we do?
>
> Henry Steigerwaldt
> SOO, WFO Nashville, TN
> ------------------------------------------------------------------------
>
> ------------------------------------------------------------------------
>
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>   

-- 
Robert A. Rozumalski, PhD
NWS National SOO Science and Training Resource Coordinator

COMET/UCAR PO Box 3000   Phone:  303.497.8356
Boulder, CO 80307-3000





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