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how to compute wind (uv) stream function?

Added by paolo de luca about 3 years ago

Hi,

I am struggling quite a lot for computing the stream function from u and v wind at 500hPa.

From what I understood I can do this with CDO as follows:

cdo merge u and v wind netcdfs

cdo remapbil to gaussian of the uv_ncfile

cdo uv2dv

and finally cdo dv2ps.

I tried all these passages but my output is not right. Maybe I am doing wrong the merging of the datasets...I don't know..

Would you be so kind to help a little bit?
I attach you here two ERA5 (0.25deg) ncfiles of u and v wind at 500hPa. Ideally I would like to retain the original ERA5 horizontal resolution for the stream function.

Thank you,
Paolo

Download all files
ERA5_v_500hPa_test.nc (29.7 MB) ERA5_v_500hPa_test.nc v_data.nc
ERA5_u_500hPa_test.nc (29.7 MB) ERA5_u_500hPa_test.nc u_data.nc

Replies (6)

RE: how to compute wind (uv) stream function? - Added by Ralf Mueller about 3 years ago

hi paolo!

I am not sure, but what have divergence and vorticity in common with the stream function? Maybe I miss the correct definition of the term stream function here, sry.

best wishes
ralf

RE: how to compute wind (uv) stream function? - Added by paolo de luca about 3 years ago

hi Ralf,

I referred to divergence and vorticity because cdo functions: uv2dv and dv2ps. to me these two steps were the only way I could get the stream function of u and v.

but yes, divergence and vorticity are not mentioned in the definition of the stream function :)

and still my question remains: (how) can I compute the stream function of u and v with cdo?

Thanks for your help,
P

RE: how to compute wind (uv) stream function? - Added by Ralf Mueller about 3 years ago

hi again!
I still miss your definition of the stream function of a velocity field.

cheers
ralf

RE: how to compute wind (uv) stream function? - Added by paolo de luca about 3 years ago

hello Ralf,

thanks again :)

see here for the definition of the stream function: https://en.wikipedia.org/wiki/Stream_function (Two-dimensional stream function, Definitions, Definition by use of a vector potential)

Best,
Paolo

RE: how to compute wind (uv) stream function? - Added by Uwe Schulzweida about 3 years ago

Hi Paolo,

Can you please describe what is wrong with your output?
We need the CDO version number (cdo -V) and the result of:

cdo -b F32 sp2gp -dv2ps -uv2dv -remapbil,F360 -seltimestep,1 -merge ERA5_u_500hPa_test.nc ERA5_v_500hPa_test.nc result
Cheers,
Uwe

RE: how to compute wind (uv) stream function? - Added by paolo de luca about 3 years ago

dear Uwe,

thanks!

here CDO -V:

(base) paolo@paolos-mbp Desktop % cdo -V
Climate Data Operators version 1.9.8 (https://mpimet.mpg.de/cdo)
System: x86_64-apple-darwin19.4.0
CXX Compiler: clang++ -std=gnu++11 -g -O2 -D_THREAD_SAFE -pthread
CXX version : Apple clang version 11.0.3 (clang-1103.0.32.59)
C Compiler: clang -g -O2 -D_THREAD_SAFE -pthread -D_THREAD_SAFE -D_THREAD_SAFE -pthread
C version : Apple clang version 11.0.3 (clang-1103.0.32.59)
F77 Compiler: gfortran -g -O2
F77 version : GNU Fortran (Homebrew GCC 9.3.0_1) 9.3.0
Features: 64GB 12threads C++11 Fortran DATA PTHREADS HDF5 NC4/HDF5 OPeNDAP SZ SSE4_2
Libraries: HDF5/1.12.0
Filetypes: srv ext ieg grb1 grb2 nc1 nc2 nc4 nc4c nc5
CDI library version : 1.9.8
cgribex library version : 1.9.4
ecCodes library version : 2.17.0
NetCDF library version : 4.7.4 of Jan 3 2021 17:05:10 $
hdf5 library version : 1.12.0
exse library version : 1.4.1
FILE library version : 1.8.3

and results attached:

(base) paolo@paolos-mbp Desktop % cdo -b F32 sp2gp -dv2ps -uv2dv -remapbil,F360 -seltimestep,1 -merge ERA5_u_500hPa_test.nc ERA5_v_500hPa_test.nc result
cdo(1) dv2ps: Process started
cdo(2) uv2dv: Process started
cdo(3) remapbil: Process started
cdo(4) seltimestep: Process started
cdo(5) merge: Process started
cdo(3) remapbil: Bilinear weights from lonlat (1440x721) to gaussian (1440x720) grid
cdo(5) merge: Processed 2 variables over 30 timesteps.
cdo(4) seltimestep: Processed 2076480 values from 2 variables over 2 timesteps.
cdo(3) remapbil: Processed 2076480 values from 2 variables over 1 timestep.
cdo(2) uv2dv: Processed 2073600 values from 2 variables over 1 timestep.
cdo(1) dv2ps: Processed 461760 values from 2 variables over 1 timestep.
cdo sp2gp: Processed 461760 values from 2 variables over 1 timestep [13.23s 4215MB].

looks like correct!

thanks,
Paolo

result (7.94 MB) result result stream
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