pyart.retrieve.vad_michelson#

pyart.retrieve.vad_michelson(radar, vel_field=None, z_want=None, gatefilter=None, valid_ray_min=16, max_speed_error=2.0)[source]#

Velocity azimuth display.

Creates a VAD object containing U Wind, V Wind and height that can then be used to plot and produce the velocity azimuth display.

Parameters:

  • radar (Radar) – Radar object used.

  • vel_field (string, optional) – Velocity field to use for VAD calculation.

  • z_want (array, optional) – Heights for where to sample vads from. None will result in np.linespace(0, 10000, 100).

  • gatefilter (GateFilter, optional) – A GateFilter indicating radar gates that should be excluded from the import vad calculation.

  • valid_ray_min (int, optional) – Minimum number of rays with valid velocity a gate needs to be used. Gates with fewer rays give noisy fits and are left out. Raising it smooths the profile but leaves more heights empty. Default is 16, the same as vad_browning.

  • max_speed_error (float, optional) – Largest standard error of the fitted wind speed, in m/s, for a gate to be used. The error grows when valid rays are few, bunched in one part of the circle, or noisy, so this rejects unreliable fits. Gates are then averaged into height bins weighted by 1 / error**2. None turns the check off. Default is 2.0.

Returns:

vad (HorizontalWindProfile) – A velocity azimuth display object containing height, speed, direction, u_wind, v_wind from a radar object.

Notes

At each gate, the radial velocities of the valid rays are fitted by least squares to v = c + a sin(az) + b cos(az), where az is the azimuth. The horizontal wind speed is sqrt(a**2 + b**2) / cos(elevation). Masked gates are left out of the fit.

The fit also gives the standard error of the speed. It is large when a gate has few valid rays, when they are bunched in one part of the circle, or when the data are noisy. Gates above max_speed_error are left out, and the rest are averaged into the z_want height bins weighted by 1 / error**2.

References

Michelson, D. B., Andersson, T., Koistinen, J., Collier, C. G., Riedl, J., Szturc, J., Gjertsen, U., Nielsen, A. and Overgaard, S. (2000) BALTEX Radar Data Centre Products and their Methodologies. In SMHI Reports. Meteorology and Climatology. Swedish Meteorological and Hydrological Institute, Norrkoping.