好问题!问题是,对于大多数网格气候数据,经度坐标如下所示:
array([1.25, 3.75, 6.25, ..., 351.25, 353.75, 356.25, 358.75])
所以没有明确的longitude=0
点,这通常会在图中显示一条细白线。我有时也会在发表的论文(甚至是“自然”)中看到这个问题!
有很多方法可以解决这个问题,但最简单的方法是使用cartopy
包,其中有一个名为的实用程序add_cyclic_point
基本上是在两边插入数据longitude=0
观点。 (参考:https://scitools.org.uk/cartopy/docs/v0.15/cartopy/util/util.html https://scitools.org.uk/cartopy/docs/v0.15/cartopy/util/util.html)
该方法的唯一缺点是使用时xarray
这意味着您必须手动提取数据,然后就会丢失元数据,因此我编写了一个函数,以保持其良好且易于使用,同时维护元数据。
from cartopy.util import add_cyclic_point
import xarray as xr
def xr_add_cyclic_point(da):
"""
Inputs
da: xr.DataArray with dimensions (time,lat,lon)
"""
# Use add_cyclic_point to interpolate input data
lon_idx = da.dims.index('lon')
wrap_data, wrap_lon = add_cyclic_point(da.values, coord=da.lon, axis=lon_idx)
# Generate output DataArray with new data but same structure as input
outp_da = xr.DataArray(data=wrap_data,
coords = {'time': da.time, 'lat': da.lat, 'lon': wrap_lon},
dims=da.dims,
attrs=da.attrs)
return outp_da
Example
因此,例如,如果我的初始 DataArray 如下所示:
<xarray.DataArray 'tas' (time: 60, lat: 90, lon: 144)>
[777600 values with dtype=float32]
Coordinates:
* lat (lat) float64 -89.49 -87.98 -85.96 -83.93 ... 85.96 87.98 89.49
* lon (lon) float64 1.25 3.75 6.25 8.75 11.25 ... 351.3 353.8 356.2 358.8
* time (time) object 1901-01-16 12:00:00 ... 1905-12-16 12:00:00
Attributes:
long_name: Near-Surface Air Temperature
units: K
valid_range: [100. 400.]
cell_methods: time: mean
standard_name: air_temperature
original_units: deg_k
original_name: t_ref
cell_measures: area: areacella
associated_files: baseURL: http://cmip-pcmdi.llnl.gov/CMIP5/dataLocation...
当我绘制时间平均值时,给出:
tas.mean(dim='time').plot.contourf()
现在,我可以使用我的函数生成一个新的插值 DataArray,如下所示:
wrapped_tas = xr_add_cyclic_point(tas)
wrapped_tas
<xarray.DataArray (time: 60, lat: 90, lon: 145)>
array([[[251.19466, 251.19469, 251.19472, ..., 251.19226, 251.19073,
251.19466], ...
[250.39403, 250.39468, 250.39961, ..., 250.39429, 250.39409,
250.39403]]], dtype=float32)
Coordinates:
* time (time) object 1901-01-16 12:00:00 ... 1905-12-16 12:00:00
* lat (lat) float64 -89.49 -87.98 -85.96 -83.93 ... 85.96 87.98 89.49
* lon (lon) float64 1.25 3.75 6.25 8.75 11.25 ... 353.8 356.2 358.8 361.2
Attributes:
long_name: Near-Surface Air Temperature
units: K
valid_range: [100. 400.]
cell_methods: time: mean
standard_name: air_temperature
original_units: deg_k
original_name: t_ref
cell_measures: area: areacella
associated_files: baseURL: http://cmip-pcmdi.llnl.gov/CMIP5/dataLocation...
正如您所看到的,经度坐标已扩展了一个点,从 144->145 长度,这意味着它现在“环绕”了longitude=0
point.
这个新的 DataArray 在绘制时给出的图没有白线:)
wrapped_tas.mean(dim='time').plot.contour()
希望有帮助! :)