bug in ERA5 polar trajectories?

[lars2015]

• The CLaMS group identified a bug in polar trajectories using ERA5 data. Air parcels sometimes stick to the pole (see plot below):

• The issue seems to be related to a new interpolation scheme of the meteo data on MARS implemented in 2019.

• More information can be found in the meteocloud issue tracker: https://jugit.fz-juelich.de/slcs/meteocloud/-/issues/66

• Identify and further investigate a test case for MPTRAC

• Add a fix to the meteo data during meteo data preprocessing?

[lars2015]

• Received a python code from Jens-Uwe used to fix the CLaMS meteo data files:

for ihem in range(2):
    if ihem == 0:
        i89 = 1
        i90 = 0
        lon = era5.variables['lon'][:] * np.pi/180
    else:
        i89 = nlats-2
        i90 = nlats-1
        # correct sign of longitude cycle that is opposite at the south pole
        lon = -era5.variables['lon'][:] * np.pi/180

    for ilev in range(nlevs):
        # transform 89 degree u and v winds into cartesian coordinates and take the mean
        u89 = era5.variables['U'][0,ilev,i89,:]
        v89 = era5.variables['V'][0,ilev,i89,:]
        u89x = u89 * np.cos(lon)
        u89y = u89 * np.sin(lon)
        v89x = -v89 * np.sin(lon)
        v89y = v89 * np.cos(lon)
        vel89x = np.mean(u89x + v89x)
        vel89y = np.mean(u89y + v89y)

        u90 = vel89x * np.cos(lon) + vel89y * np.sin(lon)
        v90 = -vel89x * np.sin(lon) + vel89y * np.cos(lon)

        era5.variables['U'][0,ilev,i90,:] = u90
        era5.variables['V'][0,ilev,i90,:] = v90

• Replaces the winds at the poles by mean winds at 89 degrees N/S (in Cartesian coordinates).

• Related discussion: https://www.researchgate.net/post/How_do_I_compute_the_wind_speed_at_the_pole_90N_or_90S_by_ncep_ncar_reanalysis_u_and_v_wind_data

• Tested this fix with the existing test cases of MPTRAC and by means of synthetic wind fields created by the wind tool.