求助：Metpy 计算的物理量与ERA5再分析资料中的物理量相差很大

Real-萧建仁

因为ERA5自己有散度和涡度可以下载，我自己初学也不太会用python，因此一些物理量计算需要验证下对错，结果发现ERA5再分析资料中的涡度散度和使用Metpy计算出来的相差很多。

比如这个是Metpy中关于涡度的定义，

1. ##定义
   
2. Definition : vorticity(*args: Any, **kwargs: Any)
   
3. Calculate the vertical vorticity of the horizontal wind.
   
4. Parameters
   
5. u(..., M, N) xarray.DataArray or pint.Quantity
   
6. x component of the wind
   
7. v(..., M, N) xarray.DataArray or pint.Quantity
   
8. y component of the wind
   
9. dxpint.Quantity, optional
   
10. The grid spacing(s) in the x-direction. If an array, there should be one item less than the size of u along the applicable axis. Optional if xarray.DataArray with latitude/longitude coordinates used as input. Keyword-only argument.
    
11. dypint.Quantity, optional
    
12. The grid spacing(s) in the y-direction. If an array, there should be one item less than the size of u along the applicable axis. Optional if xarray.DataArray with latitude/longitude coordinates used as input. Keyword-only argument.
    
13. x_dimint, optional
    
14. Axis number of x dimension. Defaults to -1 (implying [..., Y, X] order). Automatically parsed from input if using xarray.DataArray. Keyword-only argument.
    
15. y_dimint, optional
    
16. Axis number of y dimension. Defaults to -2 (implying [..., Y, X] order). Automatically parsed from input if using xarray.DataArray. Keyword-only argument.

##定义<br /> Definition : vorticity(*args: Any, **kwargs: Any)<br /> Calculate the vertical vorticity of the horizontal wind.<br /> Parameters<br /> u(..., M, N) xarray.DataArray or pint.Quantity<br /> x component of the wind<br /> v(..., M, N) xarray.DataArray or pint.Quantity<br /> y component of the wind<br /> dxpint.Quantity, optional<br /> The grid spacing(s) in the x-direction. If an array, there should be one item less than the size of u along the applicable axis. Optional if xarray.DataArray with latitude/longitude coordinates used as input. Keyword-only argument.<br /> dypint.Quantity, optional<br /> The grid spacing(s) in the y-direction. If an array, there should be one item less than the size of u along the applicable axis. Optional if xarray.DataArray with latitude/longitude coordinates used as input. Keyword-only argument.<br /> x_dimint, optional<br /> Axis number of x dimension. Defaults to -1 (implying [..., Y, X] order). Automatically parsed from input if using xarray.DataArray. Keyword-only argument.<br /> y_dimint, optional<br /> Axis number of y dimension. Defaults to -2 (implying [..., Y, X] order). Automatically parsed from input if using xarray.DataArray. Keyword-only argument.

下面是自己的部分：

1. dx, dy = mpcalc.lat_lon_grid_deltas(lon, lat)
   
2. #   计算出来的数据和原来的数据差很多，？？？？
   
3. wind_vor = mpcalc.vorticity(u,v,dx=dx,dy=dy,x_dim=-1,y_dim=-2)      
   
4. wind_div = mpcalc.divergence(u,v,dx=dx,dy=dy,x_dim=-1,y_dim=-2)     

dx, dy = mpcalc.lat_lon_grid_deltas(lon, lat)<br /> #   计算出来的数据和原来的数据差很多，？？？？<br /> wind_vor = mpcalc.vorticity(u<i>,v<i>,dx=dx,dy=dy,x_dim=-1,y_dim=-2)      <br /> wind_div = mpcalc.divergence(u<i>,v<i>,dx=dx,dy=dy,x_dim=-1,y_dim=-2)     

这个是Metpy算出来的相对涡度：
wind_vor
Out[19]:
array([[ 1.22826072e-05, -1.74879282e-06, -1.80048286e-05, ...,
        -1.22620830e-05, -1.35755185e-05, -1.83350494e-05],
       [ 2.63587489e-05,  1.48204562e-05,  6.34924628e-07, ...,
         9.65214860e-06,  1.33634245e-06, -8.16623809e-06],
       [ 2.67111189e-05,  1.33099878e-05, -2.92370330e-06, ...,
         3.26149484e-05,  2.52949367e-05,  1.80315283e-05],
       ...,
       [ 8.29784375e-05,  6.32179686e-05,  3.79085029e-05, ...,
         9.22821203e-06,  4.61023816e-06,  3.97450527e-07],
       [ 4.54908416e-05,  4.44313939e-05,  4.02905938e-05, ...,
         8.23294447e-06,  5.82363777e-06,  4.14160954e-06],
       [ 4.09520199e-05,  4.88505044e-05,  3.95765279e-05, ...,
         7.05006292e-06,  5.65144343e-06,  5.70478559e-06]]) <Unit('1 / second')>

这个是ERA5资料中的相对涡度：
masked_array(
  data=[[ 1.89363456e-06,  3.88504163e-06, -9.56801947e-06, ...,
         -2.28832029e-06, -2.48746100e-06, -9.65652645e-06],
        [ 2.47948159e-05,  1.28021200e-05, -8.37317523e-06, ...,
          4.43821026e-06, -2.99637614e-06, -1.11611451e-05],
        [ 3.08575441e-05,  1.62096387e-05,  1.67748434e-07, ...,
          4.09030864e-05,  3.31144721e-05,  2.21617332e-05],
        ...,
        [ 4.69436879e-05,  7.75892300e-05,  1.97056645e-05, ...,
          1.01026570e-05,  4.43821026e-06, -3.19039962e-07],
        [ 2.41752670e-05,  5.44667813e-05,  3.40216686e-05, ...,
          8.46527790e-06,  6.27473012e-06,  3.46463347e-06],
        [ 2.75385323e-05,  6.01091013e-05,  2.68083497e-05, ...,
          6.89427899e-06,  6.40749059e-06,  4.32757654e-06]],
  mask=False,
  fill_value=1e+20) <Unit('1 / second')>

Real-萧建仁

回填自己的坑，
问题由于边界处插值导致的，因为计算需要上下左右各有一个点，边界处缺失导致不一致。
处理办法，选要分析或出图略大的范围计算，再去分析或出图，结果基本一致。