Calculating indicators for winter months

[TarekAMZ]

Setup Information

• Xclim version:

Context

Is there a quick way to calculate e.g., the annual number of dry spell days for winter only, season = ['DFJ']? However, that is for the correct winter season. For example, for the year 1960, that would be December 1959 until end of February 1960.

Steps To Reproduce

$ pip install foo --bar

[coxipi]

Hi! What you're looking for is:

• freq = QS-DEC for correct winter season
• some xclim functions have a **indexer argument. You can give arguments for xclim.core.calendar.select_time, in your case you want to specify month = [1,2,12].

Here is an example:

import xclim 
from xclim.testing import open_dataset 
pr = open_dataset("ERA5/daily_surface_cancities_1990-1993.nc").isel(location=0).pr
xclim.atmos.dry_spell_total_length(pr, freq="QS-DEC",  month=[1,2,12])

Incomplete winters on the edges (e.g. data starting on 1990-01-01 -> incomplete '89-'90 winter) and other seasons than winters are filled with NaNs. You can change the window for the number of days in a spell and thresh for the threshold defining dry days.

Cheers!

[TarekAMZ]

Hi,
Thanks for the quick reply. In fact, I'm working with the dry spell total length indicator. However, I would get to the number of dry days after.
I tried this solution and it worked fine. I also tried to drop the other months using 'drop = True' argument, but it did not make an effect, the other months remain as nan. I did a workaround by postprocessing the output by calculating the annual max, which is fine but not very efficient. If you have other ideas, that would be nice to know :)
Thanks again,
Tarek

[coxipi]

Hmm, it seems that resample in the indicator reintroduces times that were cut off, e.g. here is a minimal code showing this:

sub_pr  = xclim.core.calendar.select_time(pr, month=[1,2,12], drop=True)
sub_pr.resample(time="QS-DEC").mean().time

times outside the winters are also present.

Since all times are masked outside the winter season, you can also use YS-DEC, now I think about it, it's really what you asked for in the beginning haha, should have thought of that directly! :

xclim.atmos.dry_spell_total_length(pr, freq="YS-DEC", month=[1,2,12])
>>> [nan, 31., 22., 30., nan]

that gives the appropriate output.

Cheers!

[TarekAMZ]

Yes, this is exactly what I needed. Thanks a lot for the help :)

Cheers!