^{Originally posted by gianlisi June 28, 2022}
Hello,

first of all, thank you for the great work. Metpy, and its integration with xarray, has been incredibly useful for my research.

I am writing about using metpy.calc.vorticity on a Lambert Azimuthal Equal Area grid around the north pole. As can be seen in the left panel of the attached picture, the sign of the computed vorticity is inverted in the upper half of the domain. The right panel shows what the vorticity should look like, as computed with xarray's differentiate.

The issue seems to originate in the azimuth dependent change of sign within the metpy.calc.lat_lon_grid_deltas function. Indeed, if I comment out line 848 and line 854 of metpy/calc/tools.py, the vorticity is returned with the correct sign over the entire domain. At the same time, I am afraid that changing the code will break the computation on other grids.

The definition of the grid I am using is

grid = { 'grid_mapping_name':'lambert_azimuthal_equal_area', 'longitude_of_projection_origin':0, 'latitude_of_projection_origin':90, 'false_easting':0, 'false_northing':0 }

Any help in understanding how to compute the correct vorticity would be greatly appreciated.

Thank you again for the great work!

Gianluca