I suspect that the 20CR.v2 ensemble mean wind speeds provided publicly were calculated incorrectly, and I will briefly explain.
As confirmed in the attached plots, the ensemble mean speeds provided for 20CR were computed as:
wspd = [<u>^2 +<v>^2]0.5
where <> indicates the ensemble mean. The correct calculation would be
wspd = < [u2 + v2]0.5 >
As you can see in the attached, the provided ensemble mean speeds look nothing like the speeds in the individual ensemble members, and appear wrong.
After downloading all the individual ensemble members and recalculating the speeds using the correct formula above, I get an ensemble mean which is perfectly
consistent with the individual members (i.e. falls in the middle of them), as would be expected.
The ensemble mean wind speeds provided also look physically unrealistic (e.g. too low at extratropical latitudes prior to the 1990's), and exhibit unrealistically
large trends. When using the individual ensemble members, or the correctly re-calculated ensemble mean speed, these issues are corrected.
I suspect many users could be affected by this, and I hope that this info might be useful.
-Neil Swart
Note in the time-series plot attached the data are all smoothed with a 5-year wide boxcar.
Re: Land mask for 20CRV2c rectangular 2x2 grid
Re: 20CR: ensemble mean wind-speeds appear wrong
I also have a question:
Can you confirm whether this issue might affect the wind-stress fields too?
I.e.: Calculating the ensemble mean stress as
tauu = ^2
instead of
tauu = < u^2 >
Or are the stess fields computed online in the model from the instantaneous U and V and therefore okay?
Re: 20CR: ensemble mean wind-speeds appear wrong
**Should have read
tauu = \^2
instead of
tauu = < u^2 >
Re: 20CR: ensemble mean wind-speeds appear wrong
Re: 20CR: ensemble mean wind-speeds appear wrong
Hi Gil,
In these calculations using all the individual ensemble memebers I was only considering the sigma .995 level winds. I have also looked in some detail
at the 10 m winds - at least at the ensemble mean 10m winds downloaded from ESRL. My guess is that the same issue affects the 10 m wind
speeds, because they also exhibit the (unrealistically) large trends in time, as seen in the sigma .995 winds above.
That said, I cannot confirm this by performing the same calculation with the 10m winds, because the individual ensemble members for the 10 m winds
are not available on the nersc.gov site, as far as I can see.
-Neil
Re: 20CR: ensemble mean wind-speeds appear wrong
Re: 20CR: ensemble mean wind-speeds appear wrong
Re: 20CR: ensemble mean wind-speeds appear wrong
Hi Cathy,
Thats good to know, thanks. The resulting wind-speed errors are not small, but in fact are very large (>3m/s or 50% in some cases).
You can see that very clearly in my figure - but I'm having trouble getting that to show up in the post. SOrry but I'm a noob at editing these pages!
Land mask for 20CRV2c rectangular 2x2 grid
Is there somewhere a file with a land mask for the 20CRv2c 2x2 rectangular grid (91x180) ?
I only found the land mask for the gaussian grid (94x192).
Thanks in advance. Claude