Improved degree-day calculations

[huard]

Addressing a Problem?

xclim is currently using a fairly crude approximation for HDD and CDD using daily average temperature. This neglects the daily cycle.

Potential Solution

The UK Met Office has developed an approximation that uses tasmin, tasmax and tas.
See https://rmets.onlinelibrary.wiley.com/doi/10.1002/joc.5362

Additional context

No response

Contribution

• I would be willing/able to open a Pull Request to contribute this feature.

Code of Conduct

• I agree to follow this project's Code of Conduct

[huard]

Case	Condition	HDD
1	Tmax ≤ Tbase (i.e. uniformly cold day)	HDD = Tbase − Tavg
2	Tavg ≤ Tbase < Tmax (i.e. mostly cold day)	HDD = [(Tbase − Tmin)/2] − [(Tmax − Tbase)/4]
3	Tmin < Tbase < Tavg (i.e. mostly warm day)	HDD = (Tbase − Tmin)/4
4	Tmin ≥ Tbase (i.e. uniformly warm day)	No heating is required, so HDD = 0

Case	Condition	CDD
1	Tmax ≤ Tbase (i.e. uniformly cold day)	No cooling is required so CDD = 0}
2	Tavg ≤ Tbase < Tmax (i.e. mostly cold day)	CDD = (Tmax − Tbase)/4
3	Tmin < Tbase < Tavg (i.e. mostly warm day)	CDD = [(Tmax − Tbase)/2) − [(Tbase − Tmin)/4]
4	Tmin ≥ Tbase (i.e. uniformly warm day)	CDD = Tavg − Tbase