met_em UU/VV and wrfinput U10/V10

[chunchih]

Hi,

I have a question about the UU and VV fields in WRF met_em files and the U10/V10 fields in the wrfinput file. All for the same domain.

I initialized my simulation at 12 UTC, using the met_em file at the same time. As far as I understand, the very first level in UU/VV is the 10-m u and v winds (and rotated to the model grid?) from the meteorological input data (I used GFS in this case). However, when I compared the de-staggered first-level UU (so that the x-dimension is e_we-1) to the U10 field in my wrfinput file, I found they are slightly different. Same for the lowest level VV in my met_em and V10 in my wrfinput. I wonder if U10 and V10 are re-interpolated somewhere in real.exe because I had thought the wind fields should be the same.

i.e.,
destaggered UU in met_em at the lowest level = U10 in wrfinput
destaggered VV in met_em at the lowest level = V10 in wrfinput

For the 2-m temperature, both the lowest level TT in met_em and T2 in wrfinput give the same values.

David

 

[Ming Chen]

U10 and V10 are diagnostic wind components at 10-m height , and they are not the same as the 1st level wind.

 

[chunchih]

Hi Ming,

Sorry for the late reply. Aren't the interpolated 10-m U and V from the input meteorological dataset appended to the UU and VV in met_em files as the lowest level winds? For example, for the older version of GFS inputs, there are 27 metgrid levels, with the 1st level for UU and VV representing the 10-m U and V winds. Please correct me if my interpretation is wrong.

After running real.exe, the wrfinput files have U10 and V10 fields, but they are slightly different from the lowest-level UU and VV in the met_em files at the same time. I had expected them to be the same (i.e, U10 in wrfinput = the lowest level UU in met_em, as with V10 and VV).

The attached figure shows what I meant. The first column shows the lowest level U and V from a met_em file, which I had thought should be the 10-m U and V winds from the meteorological input dataset. The middle column shows the U10 and V10 from the wrfinput file, initialized using the same met_em file. The difference between the lowest level UU and U10 (and the lowest level VV and V10) is nearly unnoticeable if comparing by their absolute magnitudes. However, calculating the difference (shown in the rightmost column) clearly shows that they don't match.

David