Slicing Iregular Grids

Slicing Iregular Grids#

Acknowledgement: The material was provided by Mrs. Kuan-Yun Wang, UC Davis.

非直線的剖面 (cross-section) 繪製#

本節學習如何使用xarray繪製「不是沿著相同經緯線」的cross section，這裡提供的範例是繪製渤海、黃海、東海到南海這個曲折線段上的 \(V\) 風垂直剖面。

首先引入需要的packages，以及讀取2016年1月份的V-wind資料，並取1月份平均。

import numpy as np
import xarray as xr
import matplotlib.pyplot as plt
import cartopy.crs as ccrs
import cmaps

v     = xr.open_dataset('data/vwnd.2016.nc').vwnd
v_jan = v.sel(time=(v.time.dt.month.isin([1]))).mean('time')

接下來讀取渤海、黃海、東海到南海這個曲折線段的經緯度，並選擇要繪製的經緯度範圍 (以文字檔儲存，其中第一欄為經度，第二欄為緯度)。

y_pos, x_pos= np.loadtxt('sellatlon_EA.txt', dtype=float, unpack=True)
x_pos = x_pos[y_pos <= 50]
y_pos = y_pos[y_pos <= 50]
x_pos = x_pos[y_pos >= 0]
y_pos = y_pos[y_pos >= 0]

---------------------------------------------------------------------------
FileNotFoundError                         Traceback (most recent call last)
Cell In[2], line 1
----> 1 y_pos, x_pos= np.loadtxt('sellatlon_EA.txt', dtype=float, unpack=True)
      2 x_pos = x_pos[y_pos <= 50]
      3 y_pos = y_pos[y_pos <= 50]

File /data/wtsai/micromamba/p3/lib/python3.10/site-packages/numpy/lib/npyio.py:1373, in loadtxt(fname, dtype, comments, delimiter, converters, skiprows, usecols, unpack, ndmin, encoding, max_rows, quotechar, like)
   1370 if isinstance(delimiter, bytes):
   1371     delimiter = delimiter.decode('latin1')
-> 1373 arr = _read(fname, dtype=dtype, comment=comment, delimiter=delimiter,
   1374             converters=converters, skiplines=skiprows, usecols=usecols,
   1375             unpack=unpack, ndmin=ndmin, encoding=encoding,
   1376             max_rows=max_rows, quote=quotechar)
   1378 return arr

File /data/wtsai/micromamba/p3/lib/python3.10/site-packages/numpy/lib/npyio.py:992, in _read(fname, delimiter, comment, quote, imaginary_unit, usecols, skiplines, max_rows, converters, ndmin, unpack, dtype, encoding)
    990     fname = os.fspath(fname)
    991 if isinstance(fname, str):
--> 992     fh = np.lib._datasource.open(fname, 'rt', encoding=encoding)
    993     if encoding is None:
    994         encoding = getattr(fh, 'encoding', 'latin1')

File /data/wtsai/micromamba/p3/lib/python3.10/site-packages/numpy/lib/_datasource.py:193, in open(path, mode, destpath, encoding, newline)
    156 """
    157 Open `path` with `mode` and return the file object.
    158 
   (...)
    189 
    190 """
    192 ds = DataSource(destpath)
--> 193 return ds.open(path, mode, encoding=encoding, newline=newline)

File /data/wtsai/micromamba/p3/lib/python3.10/site-packages/numpy/lib/_datasource.py:533, in DataSource.open(self, path, mode, encoding, newline)
    530     return _file_openers[ext](found, mode=mode,
    531                               encoding=encoding, newline=newline)
    532 else:
--> 533     raise FileNotFoundError(f"{path} not found.")

FileNotFoundError: sellatlon_EA.txt not found.

為x_pos、y_pos建立新的DataArray，並設定擁有相同的dimension。接著，使用xr.interp函數對 \(V\) 風場資料內插到所選的經緯度(x_pos, y_pos)上。

v_cro的維度是 \(N_{\text{lat}} \times N_{\text{level}}\) ；另外精度位置是lat緯度的函數。

最後就是用xarray.plot.contourf作圖，結果如下：

fig, ax = plt.subplots(nrows = 1, ncols = 1, figsize = (8,6))

v_plot   =  v_cro.plot.contourf(
            x='lat', y='level', ylim=(1000, 100),
            ax=ax,
            levels = np.arange(-20, 22, 2),
            cmap = cmaps.MPL_bwr, alpha = 1,
            add_colorbar=True,
            extend='both',
            cbar_kwargs={'orientation': 'vertical', 'aspect': 20, 'shrink': 0.9, 'extend':'both', 'label': ''})

ax.set_xlim([0, 50])
ax.set_xticks(np.arange(0, 50+1,5))
ax.set_xticks(np.arange(0, 50+1,1), minor = True)
ax.set_yticks([1000, 925, 850, 700, 600, 500, 400, 300, 200, 100])
ax.set_yticks(np.arange(1000, 100, -25), minor = True)
ax.xaxis.set_tick_params(labelsize=12)
ax.yaxis.set_tick_params(labelsize=12)
ax.set_xlabel('Latitude ($^{\circ }$N)')
ax.set_ylabel('Pressure levels')
ax.set_title('')
ax.grid()
ax.set_title('2016-01 V-wind along East Asia coastlines')

plt.show()

_images/c8e2b716b1ad075144c8e4fa370e00356f7505735d0c3c5db71e74fe61333433.png

Slicing Iregular Grids

Contents

Slicing Iregular Grids#

非直線的剖面 (cross-section) 繪製#