how to speed up the process of `getvar`

首先，使用 Python、wrf.getvar、netCDF4 以及列出的 SINALPHA、COSALPHA 和 uvmet_wspd_wdir 变量，复现 issue 中的计时比较。跟踪 getvar 入口点，并将其对 ALL_TIMES 和派生诊断的处理与直接读取进行比较；完成的标准是找出瓶颈，并证明一种不改变结果的更快方法。

I found that using wrf.getvar to get variables is quite slow.
For example

import time
import wrf
import netCDF4 as nc
wrfout = nc.Dataset(wrfout_file)
t0 = time.time()
SINALPHA_WRF = wrf.getvar(wrfout, 'SINALPHA', timeidx=wrf.ALL_TIMES, squeeze=False)
COSALPHA_WRF = wrf.getvar(wrfout, 'COSALPHA', timeidx=wrf.ALL_TIMES, squeeze=False)
t1 = time.time()
sinalpha = wrfout['SINALPHA'][:]
cosalpha = wrfout['COSALPHA'][:]
t2 = time.time()
print(f'getvar: {t1 - t0},directly: {t2 - t1}')

In my PC, the time of getvar is about 0.084 s, and the time of directly is about 0.015 s.
To be a little more complicated, to get the wind speed and the wind direction using wrf.getvar(wrfout, 'uvmet_wspd_wdir', timeidx=wrf.ALL_TIMES, squeeze=False) would take the time about 14.596 s, while we first get ua, va, SINALPHA, COSALPHA by wrf.getvarand then using the relation between these variables to get the wspd and wdir, whose values are not significant different from the result of getvar, but it only takes 2.263 s.

So, what reason make it seems that the calculation speed of getvar is slow and how to improve it?

In my case, I want to extract some variables from wrfout and backup, e.g, most of the variables may be the shape of (4,50,300,300), and I want to save some diagnostic variables to be (4, 10, 300, 300)(interpolated on certain pressure level or height). Any suggestion to efficiently accomplish this kind of task.

wrf.__version__='1.3.2'