Experiences with Gravity Wave Drag turned on?

Forum dedicated to older versions of EMS package (WRFEMS v3.2, v3.1 or older). Support is user-to-user based, so please help others if you can.
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j0nes2k
Posts: 56
Joined: Wed Feb 03, 2010 5:06 pm

Experiences with Gravity Wave Drag turned on?

Post by j0nes2k » Wed Jun 09, 2010 2:14 pm

Hello,

I just encountered a problem which is perhaps related to the new gravity wave drag option. My main interest is winds over Europe, so I set up a domain with 12km resolution over Europe. While browsing through the configuration files, I found that gravity wave drag could probably be a good option for me (DX > 10km, mountain ranges in the domain), so I turned it on.

Now I found the following problem: this option seems to have a very large effect on the 10m wind speeds. As an example, the wind for Crete / Greece gets messed up quite hard. There is a strong northerly wind over the Aegean Sea, and in my simulation the mountain ranges over Greece (more or less three hills which go up to 900m height) block the wind very hard so that the wind shadow behind these mountains stretches over 70km over the open sea.

What are your experiences with Gravity Wave Drag turned on? Are there any additional configuration options for Gravity Wave Drag besides turning it on and off?

Best regards,
Jonas Kaufmann

j0nes2k
Posts: 56
Joined: Wed Feb 03, 2010 5:06 pm

Re: Experiences with Gravity Wave Drag turned on?

Post by j0nes2k » Fri Jun 11, 2010 5:47 pm

Hello,

did some more research on that, and first thing I noticed was that this phenomenon does not have anything to do with GWD. I switched it off and the results stayed more or less the same. So I compared the results to another self-compiled WRF model I am running (v3.1, 21km, Europe). This one does also show a slight wind shadow effect, but not in the amount as the WRF-EMS model does. The differences in physics and dynamics are as follows:

mp_physics: 3 (old) vs. 2 (ems)
sf_surface_physics: 1 (old) vs. 2 (ems)
num_soil_layers: 5 (old) vs. 4 (ems)
diff_6th_opt: 0 (old) vs. 2 (ems)
damp_opt: 0 (old) vs. 3 (ems)
damp_coef: 0.01 (old) vs. 0.2 (ems)
There are also a lot of parameters that are only present in theWRF-EMS namelist, not in the WRF one.

Has anyone got an idea where this might come from? Thank you in advance!

Best regards,
Jonas Kaufmann

meteoadriatic
Posts: 1602
Joined: Wed Aug 19, 2009 10:05 am

Re: Experiences with Gravity Wave Drag turned on?

Post by meteoadriatic » Fri Jun 11, 2010 7:53 pm

Can we see an example (map)?

j0nes2k
Posts: 56
Joined: Wed Feb 03, 2010 5:06 pm

Re: Experiences with Gravity Wave Drag turned on?

Post by j0nes2k » Mon Jun 14, 2010 9:21 am

Hello,

here are some examples:

Example 1 - WRF:
Image

Example 1 - WRF-EMS:
Image

Example 2 - WRF:
Image

Example 2 - WRF-EMS:
Image

As you can see, there is a large difference between the two models (the corresponding maps are from the same initialization). I think it is very unlikely that in a wind shadow area about 70km behind the mountain there will be no wind. What can be the cause for this?

Best regards,
Jonas Kaufmann

meteoadriatic
Posts: 1602
Joined: Wed Aug 19, 2009 10:05 am

Re: Experiences with Gravity Wave Drag turned on?

Post by meteoadriatic » Mon Jun 14, 2010 12:35 pm

I would say that differences here are due to different horizontal resolution between two models. 21km resolution model is not capable of creating orografic effects in the lee side of mountain. It looks to me that your 12km model see some hydraulic jump and creation of gravitational waves on lee side when airflow passes top of mountain and thus there are areas where wind speed is larger, and also some areas where airflow does not reach ground because of different shaped gravitational lee side wave.

I think, that your 12km forecast could be very likely more correct than 21km one. It would be essential to proove that by going to observe real situation on the field, but... I live on eastern Adriatic coast, close to mountain Velebit. In northeastern wind flow this mountain create impressive hydraulic jump and then we have most powerful wind in Europe (called Bora - Bura) that can easy exceed in gusts 50m/s or even more (maximum caught with anemometer was 85m/s gust). What I mean by this is that in such situations when somewhere down the mountain you have hurricane force wind, in same time, only few kilometers away there are areas with very light winds that exist with no apparent reason (eg. no barrier to block airflow or similar). And my models see these differences in wind speed often very well (especially ARW model when run at 4km resolution or better).

j0nes2k
Posts: 56
Joined: Wed Feb 03, 2010 5:06 pm

Re: Experiences with Gravity Wave Drag turned on?

Post by j0nes2k » Mon Jun 14, 2010 7:24 pm

Hello meteoadriatic,

interesting points, thank you! I have found a forum about Crete and began to look for first-hand knowledge from locals like sailors or fishermen in that area. The first responses are already coming in, and it seems that my 12km model could be right. I got responses about particular windy and windless places just as predicted by the model. The only thing that remained unanswered (and where I am still uncertain) is the amount of stretch of the windless zone into the sea.

Again, thank you for your input, much appreciated!

Best regards,
Jonas Kaufmann

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