Python3.8：MetPy 问题：grib2 文件的声明性绘图绘制空白地图

Question

自学 MetPy 中的声明式绘图功能，并不断遇到困难。这来自于在通过 xarray/cfgrib 打开后尝试将声明性绘图包应用于 grib2 文件。数据看起来健康，但实际温度数据似乎没有被传递和绘制。当我绘制它时，我得到一张空白地图。

我有下面的压缩代码，以及一些数据检查的打印输出，以表明数据似乎正常。

我错过了什么吗？ （我确定我是，但我想知道什么？）

谢谢！

import matplotlib.pyplot as plt
import xarray as xr
import numpy as np
import pygrib
import cartopy.crs as ccrs
import cartopy.feature as cfeat
import cartopy
from datetime import datetime, timedelta
import io
from metpy.units import units
from metpy.plots import ImagePlot, MapPanel, PanelContainer

##Open GFS grib2 file, initialized 2021060700, and pull data from hPa level designation.
ds = xr.open_dataset('/fewxops/Tom/learn_python/data/grib2/gfs.t00z.pgrb2.0p25.f024', engine='cfgrib', filter_by_keys={'typeOfLevel': 'isobaricInhPa'})

##Select individual level (from designated 'type of level')
ds_z500=ds.sel(isobaricInhPa=500)

##Create base image via MetPy
img = ImagePlot()
img.data = ds_z500
img.field = 't'
img.colormap = 'plasma'

#Create map panel (e.g. subplot in matplotlib)
panel = MapPanel()
panel.area = 'us'
#panel.layers = ['states']
panel.title = 'GFS 500mb Temp Forecast Example'
panel.plots = [img]

##Create panel container (e.g. figure in matplotlib)
pc = PanelContainer()
pc.size = (10,8)
pc.panels = [panel]
pc.show()

*******************************
home/fewx/anaconda3/lib/python3.8/site-packages/metpy/xarray.py:349: UserWarning: More than one time coordinate present for variable "t".
  warnings.warn('More than one ' + axis + ' coordinate present for variable'
Found valid latitude/longitude coordinates, assuming latitude_longitude for projection grid_mapping variable

print(ds_z500)
<xarray.Dataset>
Dimensions:        (latitude: 721, longitude: 1440)
Coordinates:
    time           datetime64[ns] ...
    step           timedelta64[ns] ...
    isobaricInhPa  float64 500.0
  * latitude       (latitude) float64 90.0 89.75 89.5 ... -89.5 -89.75 -90.0
  * longitude      (longitude) float64 0.0 0.25 0.5 0.75 ... 359.2 359.5 359.8
    valid_time     datetime64[ns] ...
Data variables:
    gh             (latitude, longitude) float32 ...
    t              (latitude, longitude) float32 ...
    r              (latitude, longitude) float32 ...
    q              (latitude, longitude) float32 ...
    w              (latitude, longitude) float32 ...
    wz             (latitude, longitude) float32 ...
    u              (latitude, longitude) float32 ...
    v              (latitude, longitude) float32 ...
    absv           (latitude, longitude) float32 ...
    o3mr           (latitude, longitude) float32 ...
Attributes:
    GRIB_edition:            2
    GRIB_centre:             kwbc
    GRIB_centreDescription:  US National Weather Service - NCEP
    GRIB_subCentre:          0
    Conventions:             CF-1.7
    institution:             US National Weather Service - NCEP
    history:                 2021-06-15T08:05 GRIB to CDM+CF via cfgrib-0.9.9...


print(ds_z500['t'].values)
[[245.93057 245.93057 245.93057 ... 245.93057 245.93057 245.93057]
 [245.85057 245.84056 245.84056 ... 245.85057 245.85057 245.85057]
 [245.72057 245.72057 245.71057 ... 245.73056 245.73056 245.72057]
 ...
 [226.94057 226.94057 226.94057 ... 226.94057 226.94057 226.94057]
 [226.96057 226.96057 226.96057 ... 226.96057 226.96057 226.96057]
 [226.88057 226.88057 226.88057 ... 226.88057 226.88057 226.88057]]

GFS Example

Answer 1

您没有遗漏任何内容，您只是在 ImagePlot 中的某些代码中遇到了限制，该代码试图帮助 CartoPy ——特别是关于使用 imshow 和纬度/经度数据进行绘图。您的示例代码应该可以正常工作，并且适用于ContourPlot。我已打开 an issue 以进一步调查如何在下一个版本中解决此问题。

同时，一种解决方法是在绘图之前对数据进行子集化，以消除 MetPy 有问题的包装“帮助”（大约 -180/+180 和 0/360 经度）：

# The order in these slices needs to match that of the data
ds_z500 = ds_z500.metpy.sel(longitude=slice(250, 315), latitude=slice(60, 10))