Source for current Number of Chilling Days "clock reset" dates?

[mpaiao]

I think this is a rather minor one, but in refactoring some of the code from phenology, and I came across these dates to "reset" the clocks for Growing Degree Days (GDD) and Number of Chilling Days (NCD).

fates/biogeochem/EDPhysiologyMod.F90

Lines 1027 to 1034 in 626f5e2

	!Zero growing degree and chilling day counters
	if (currentSite%lat > 0)then
	ncdstart = 270 !Northern Hemisphere begining November
	gddstart = 1 !Northern Hemisphere begining January
	else
	ncdstart = 120 !Southern Hemisphere beginning May
	gddstart = 181 !Northern Hemisphere begining July
	endif

The values on the comments for the Northern Hemisphere, Nov 1 for NCD and Jan 1 for GDD, correspond to the Botta et al. (2000) phenology algorithm, however the coded value for NCD (ncdstart ) is Sept 27. The values for the Southern Hemisphere are consistent with Botta et al. (2000).

Presumably the earlier date is useful for when running FATES at very high latitudes, but I wanted to check that this was the intention for switching the dates, and adjust the comments accordingly. If we keep the coded value, I am also wondering if we should make the Southern Hemisphere values mirror those for the Northern Hemisphere (i.e., Mar 27 for ncdstart). Probably we could use the equinox dates or Oct 1 / Apr 1 because the current date seems a bit arbitrary.