LCL problem in v1.0

[stavrosdafis]

Hi all,

I am experiencing a problem with the calculation of LCL, related to the discussion in ticket #1200
c70ee01

I checked the thermo.py script and everything seems to be aligned with the closed discussions mentioned above.

Despite that, I keep receiving the following error when working with ARPEGE data and I checked that the problem appears over mountainous areas (if that helps). It is easy to skip the calculation of LCL by using other approaches but I also want to calculate other thermodynamic parameters in which the metpy LCL calculation is used (e.g. EL).

Any help will be very much appreciated.

"""
Traceback (most recent call last):
File "/home/dafis/anaconda3/envs/pyn_env/lib/python3.8/multiprocessing/pool.py", line 125, in worker
result = (True, func(*args, **kwds))
File "Plot.py", line 270, in METP
el, _ = EL(pressure[:, ind_lat, ind_lon] * units('Pa'),
File "/home/dafis/anaconda3/envs/pyn_env/lib/python3.8/site-packages/metpy/xarray.py", line 677, in wrapper
return func(*args, **kwargs)
File "/home/dafis/anaconda3/envs/pyn_env/lib/python3.8/site-packages/metpy/units.py", line 320, in wrapper
return func(*args, **kwargs)
File "/home/dafis/anaconda3/envs/pyn_env/lib/python3.8/site-packages/metpy/calc/thermo.py", line 599, in el
lcl_p, _ = lcl(pressure[0], temperature[0], dewpt[0])
File "/home/dafis/anaconda3/envs/pyn_env/lib/python3.8/site-packages/metpy/xarray.py", line 677, in wrapper
return func(*args, **kwargs)
File "/home/dafis/anaconda3/envs/pyn_env/lib/python3.8/site-packages/metpy/units.py", line 320, in wrapper
return func(*args, **kwargs)
File "/home/dafis/anaconda3/envs/pyn_env/lib/python3.8/site-packages/metpy/calc/thermo.py", line 357, in lcl
lcl_p = so.fixed_point(_lcl_iter, pressure.m, args=(pressure.m, w, temperature),
File "/home/dafis/anaconda3/envs/pyn_env/lib/python3.8/site-packages/scipy/optimize/minpack.py", line 916, in fixed_point
return _fixed_point_helper(func, x0, args, xtol, maxiter, use_accel)
File "/home/dafis/anaconda3/envs/pyn_env/lib/python3.8/site-packages/scipy/optimize/minpack.py", line 870, in _fixed_point_helper
raise RuntimeError(msg)
RuntimeError: Failed to converge after 50 iterations, value is nan
"""

[jthielen]

This looks rather similar to the initial issue discussed in #1705. Would you be able to find an example vertical profile from your data that exhibits this particular issue, and share the data for that example? It is known that NaNs in the pressure, temperature, and/or dewpoint fields will cause this particular error (see #1705 (comment)), and hopefully a fix will be coming soon in MetPy v1.0.1!

[stavrosdafis]

Thank you for indicating a similar comment on that. I don't have nan values in the model profiles and I managed to overcome this problem by using the parcel temperature profile.

I need the EL resulting from the most unstable parcel, so I first calculated this with the function "most_unstable_parcel" and then computed the Tparcel with the "parcel_profile" function, and I suppose that the following EL is a product of the most unstable parcel:

             el, _ = EL(pressure[:, ind_lat, ind_lon] * units('Pa'),
                                temperature[:, ind_lat, ind_lon] * units('K'),
                                dewpoint[:, ind_lat, ind_lon] * units('K'), Tparcel, which='most_cape')

I don't know why it works with a temperature profile of a parcel and it explodes without it, but I will try to provide you with the vertical profiles of the problematic grid point asap.

[stavrosdafis]

This is the vertical profile where the EL explodes with the aforementioned error:

pressure:
[100000. 95000. 92500. 90000. 85000. 80000. 75000. 70000. 65000.
60000. 55000. 50000. 45000. 40000. 35000. 30000. 27500. 25000.
22500. 20000. 17500. 15000. 10000.]

temperature:
[287.39636 284.3 282.07364 279.8432 275.30753 270.59286 265.7478
260.76923 255.72478 250.47887 245.03654 240.01895 234.76445 229.69646
225.47968 222.24866 221.53215 222.5039 223.74715 225.47037 226.60524
220.86287 215.51237]

Dew point:
[260.46606 257.65558 256.8046 256.17325 254.9643 253.88719 252.70776
250.59662 246.8971 243.48524 240.17911 233.28412 225.15358 219.00024
212.97299 208.93503 208.05182 204.53465 198.8836 195.10406 191.90439
191.28859 190.85577]