# Geostrophic wind for WRF SCM

#### MartinHagman

##### New member
Hi all!

I am running WRF SCM and to force it by tendencies from the WRF 3D model. The 3D model uses IFS HRES as host model for the analysis and boundary values. Before I ran ideal cases with a constant geostrophic wind. Now I am forcing it with values from WRF 3D. Everything works fine, but I have problems to calculate the geostrophic wind from WRF 3D, which is needed to run real-time SCM cases. The WRF 3D analysis is just an interpolation from the IFS field at that time. In this case 06 UTC.

I am running WRF4.4 and WRFSCM4.3.3 with a Lambert coformal projection over Scandinavia. Horizontal resolution is 3000 m. Raa is the density.

I am calculating the geostrophic wind according to the following formulas, where I can choose longitude and latitude:

U_G_WRF = -1/(Raa*f) * (P_TOT[0, :, LATITUDE+1, LONGITUDE] - P_TOT[0, :, LATITUDE-1, LONGITUDE])/(2 * 3000)
V_G_WRF = 1/(Raa*f) * (P_TOT[0, :, LATITUDE, LONGITUDE+1] - P_TOT[0, :, LATITUDE, LONGITUDE-1])/(2 * 3000)

I have attached 2 figures from a gridpoint close to Sodankylä in northern Finland.

Figure 1: WRF and IFS wind, and geostrophic wind, at time zero, where I have rotated the WRF winds according to:

Uearth = U*cosalpha - V*sinalpha
Vearth = V*cosalpha + U*sinalpha

The total wind in WRF and IFS HRES is equal after interpolation, so the WRF winds seems properly rotated. The geostrophic wind, on the other hand, is displaced along the x-axis but have the same shape as in the IFS HRES. The geostrophic wind is also rotated according to the formulas above after calculation of U_G and V_G.

Figure 2:

The same plot in a 9-hour simulation where I have made the calculations in the same way. Now the geostrophic wind looks better, except for altitudes above 10 km. I guess it depends on small pressure gradients at higher altitudes. It looks the same at high altitudes in IFS HRES, but the oscillations are smaller. I can skip the geostrophic wind at higher altitudes, but WHAT IS WRONG WITH THE GEOSTROPHIC WIND AT TIME 0?

Thankful for help!

Best,

Martin Hagman, Stockholm, Sweden

#### Attachments

• Skärmavbild 2023-03-16 kl. 15.44.02.png
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• Skärmavbild 2023-03-16 kl. 15.47.32.png
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Hi Martin,
Apologies for the long delay in response to your inquiry. Since it's been a while, I first wanted to ask if you're still having trouble with this. If so, can you print out the values of the variables that go into the geostrophic wind calculation at hour 0 and attach that for us? Thanks!

Hi Martin,
Apologies for the long delay in response to your inquiry. Since it's been a while, I first wanted to ask if you're still having trouble with this. If so, can you print out the values of the variables that go into the geostrophic wind calculation at hour 0 and attach that for us? Thanks!
Hi Kelly!

Yes, still having the same problem, but havn't seen your reply until today, because I thought it wouldn't be answered.

I have attached 2 files, where Geostropohic_wind.txt is a textfile consisting of all the values you asked for at time zero. The other file is a Python script that creates the force file for the SCM, but for some reason I couldn't attach a .py file here, so I changed the last part of the file name to .txt instead. It is still a Python file, though, so maybe you just can change .txt to .py again. Is this feasible?

In the Python script, look for the "Geostrophic wind block"! There you can see how the calculations of the geostrophic wind and the variables in the "Geostrophic_wind.txt"-file are made.

Best,
Martin Hagman, Stockholm, Sweden

#### Attachments

• Geostrophic_wind.txt
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• Create_Force_File_VALUES.txt
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Hi Martin,

I discussed your issue with a colleague and it looks like the input data to the equation is correct (confirmed by your calculation at a later time). So we think this discrepancy is likely due to the imbalance at the model start time due to interpolation, both horizontal and vertical in a cold start mode, especially if the station is located in places of complex terrain. The imbalance is removed as the model integrates in time and becomes more balanced between atmospheric variables, model terrain and model equations. Therefore, it should not be an issue and it should be okay to proceed.

Hi Kelly!

THANKS!

OK, I understand. Have you in some way discussed why only the geostrophic u-wind is affected, while the geostrophic v-wind seems good at time zero? Can it have something to do with the interpolation of the IFS HRES grid, where distances between longitudes shrink towards the pole...?

Best,
Martin

Last edited:
Apologies for the delay. Unfortunately we aren't really sure about that at this time. Have you looked at other locations? Do the errors or biases have the same characteristics?