00Z 2-m Temps consistently colder in FV3 over higher terrain

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Timothy Barker, modified 7 Years ago.

00Z 2-m Temps consistently colder in FV3 over higher terrain

Youngling Posts: 3 Join Date: 9/20/12 Recent Posts

I'm seeing a consistent difference between the GFS and FV3 for 2-m temperature forecasts for 00Z times over higher terrain (GFS is warmer than FV3).

I actually find it kind of nice in Corey's comparison page to just step through with the arrow keys - and you'll see how the difference field jumps up every 8th frame over the western U.S. - on the forecast times that land on 00Z - and then jumps back down on the next frame.

I've attached the 21hr, 24hr, and 27hr comparisons from the 00Z run on the 19th - but other runs show very similar "jumps" at 00Z valid times. The other forecast differences move through space over time - but this warmer difference always pops back up to a certain extent over the western U.S. at the 00Z forecast times.

Corey Guastini, modified 7 Years ago.

RE: 00Z 2-m Temps consistently colder in FV3 over higher terrain

Youngling Posts: 6 Join Date: 5/5/15 Recent Posts

Wow, those are large differences. The site has an archive back to the end of August. It's hit-or-miss which cycles are available, but the 12Z 24 August cycle is there for example, and it does not display the same 2-m temperature behavior. The EMC FV3 parallel is continually changing, so it seems a change was recently introduced that is causing these differences. I've asked the developers if there is a change log that can be shared with the group.

I've also attached a plot of differences between the GFS and FV3-GFS terrain I made to investigate the temperature differences. Like other fields, the FV3 terrain appears smoother than the GFS terrain. It is unlikely the smoothness is causing large temperature differences, but it is still an oddity that I'd like to understand. I've directed a question about the terrain smoothness toward the FV3 developers.

Corey Guastini, modified 7 Years ago.

RE: 00Z 2-m Temps consistently colder in FV3 over higher terrain

Youngling Posts: 6 Join Date: 5/5/15 Recent Posts

A little bit of follow-up: there were some changes to EMC's FV3 runs on 7 September, but they affected primarily cloud water and stratospheric temperature and winds. I think this 00Z behavior may have been happening near sundown this entire time, but sundown is now closer to 00Z so we're noticing it more. I've attached an image from the FV3/GFS plume comparison for Boise 2-m temperature in the first 72-h of the 12Z 23 October cycle with some obs overlaid. Between 21Z and 00Z the temperature in the GFS (blue line) decreases but the temperature in the FV3 (orange line) either decreases less or even increases. The result is an approximately 3-degree difference each evening at 00Z. The two catch up again by 03Z. Historically, the GFS has decreased temperatures too quickly around 00Z; this appears to be better performance in the FV3. It's something to keep in mind as our evaluation continues.

As for the terrain, it's different because a different filter is applied.

Jack Settelmaier, modified 7 Years ago.

RE: 00Z 2-m Temps consistently colder in FV3 over higher terrain

Youngling Posts: 18 Join Date: 11/2/12 Recent Posts

Here's a case where the GFS is COLDER than the FV3, if you look at the forecast 2-meter Temps over the Appalachians for the strong cold front forecast to push through the mid Atlantic late this week.

Corey Guastini, modified 7 Years ago.

RE: 00Z 2-m Temps consistently colder in FV3 over higher terrain

Youngling Posts: 6 Join Date: 5/5/15 Recent Posts

The front pushing through is a neat case. Perhaps related to Jack's post showing colder temperatures pushing south in the GFS, the GFS also has more snow along the southern end of the boundary moving through OH and PA. The differences are subtle, but the GFS has historically forecast too much snow.

A note on the ptype plots. I assign an integer value to the 4 precipitation types, 1 corresponding to the generally coldest temperature profile (snow) and 4 corresponding to the warmest (rain). The difference plot is a subtraction of GFS from FV3, so red indicates where the FV3 had rain and the GFS had frozen precipitation. The difference field is only plotted at those points where both the GFS and FV3 forecast precipitation.

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