Interpreting Ensemble Results

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mcwilliams@coj.net
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Interpreting Ensemble Results

Post by mcwilliams@coj.net »

I ran a forward trajectory ensemble at a starting height of 200 m AGL with HRRR from this morning. Here are the results. Is the divergence seen an indication that a single trajectory might not give reliable results this morning?
ensemble-06172021.GIF
Tianfeng.Chai
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Re: Interpreting Ensemble Results

Post by Tianfeng.Chai »

Yes, I agree. Because of the stochastic feature of atmospheric flow, it is always better to have multiple trajectory runs instead of only relying on a single trajectory.
mcwilliams@coj.net
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Re: Interpreting Ensemble Results

Post by mcwilliams@coj.net »

I ran a trajectory at 3 heights, 10 m, 100 m, and 200m (see attached), using the met file 20210617_06-11_hrrr, the same met file used in the ensemble trajectory above. My question now is, what does the ensemble say about the trajectories I just ran? Are they ok or of questionable accuracy?
traj-06172021.GIF
alicec
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Re: Interpreting Ensemble Results

Post by alicec »

Were you using the meteorological ensemble option in which it moves the meteorology around by 1 grid cell?

https://www.ready.noaa.gov/documents/Tu ... ensem.html
"Another multiple trajectory approach, an extension of a previous section on sources of trajectory error, is a variation of an old manual technique, where trajectories are started about the point of interest, and the divergence of those trajectories would provide a more quantitative estimate of the uncertainty associated with the center point trajectory. In HYSPLIT, this method has been incorporated directly into the code so that trajectories are automatically computed about a 3-dimensional cube centered about the starting point. However, the initial positions are not offset, just the meteorological data point associated with each particular trajectory, so that all trajectories start from the same point."

The spread in the trajectories gives you some idea of the uncertainty in the trajectory. If that uncertainty is larger than the accuracy you
want, then you probably want to be more careful and do some further investigation.
For instance you could run with other met data sets or see if there are any wind direction measurements to compare to.
mcwilliams@coj.net
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Re: Interpreting Ensemble Results

Post by mcwilliams@coj.net »

I used the online HYSPLIT, so whatever the default is. I assume 1 grid cell but it is not defined in the online version.
alicec
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Re: Interpreting Ensemble Results

Post by alicec »

Yes -it is always 1 grid cell.
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