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Fix leap-year Oct/Dec notch in the monthly FOD decay mask #39

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Merged
HughRunyan merged 1 commit into from
Aug 4, 2026
Merged

Fix leap-year Oct/Dec notch in the monthly FOD decay mask #39

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21 changes: 17 additions & 4 deletions SWEET_python/model_v2.py
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Original file line number Diff line number Diff line change
Expand Up @@ -264,10 +264,23 @@ def expand_to_months(annual_series_or_value):
half_year_offset = 0.5 * ks_monthly[None, :] # k at deposit month
integral_adjusted = integral - half_year_offset

# Mask out emissions where integral_adjusted <= 0
# This is analogous to the annual model's mask for years_back <= 0
# (no emissions until ~0.5 years after deposit)
mask_positive_integral = integral_adjusted > 0
# Mask out deposits younger than the ~half-year emission lag (analogous
# to the annual model's years_back <= 0 mask: no emission until ~0.5 yr
# after deposit). The cutoff is integral_adjusted == 0 (deposit exactly
# half a year old). In a LEAP year the monthly grid lands EXACTLY on that
# cutoff for the deposit six months before a 31-day emission month
# (Jun->Dec, Apr->Oct: 183/366 == 0.5), whereas in a common year it lands
# just past it (183/365 == 0.50137). integral_adjusted is a difference of
# two O(k) cumulative sums, so at the cutoff it is 0 +/- ~1e-15
# (catastrophic cancellation) and a bare `> 0` (or `>= 0`) includes/drops
# that whole deposit-month unpredictably -> a spurious ~1-2% Oct/Dec notch
# in leap years only. Use a tolerance well below any real k-time step
# (genuine "just past cutoff" values are ~1e-4..1e-3) but above FP noise,
# so the exact-half-year deposit is retained consistently in leap and
# common years. Within-year redistribution; leap-year annual rises by the
# physically-real extra-day mass (~+0.25%), common years unchanged.
_HALF_YEAR_TOL = 1e-9
mask_positive_integral = integral_adjusted >= -_HALF_YEAR_TOL
decay = np.exp(-integral_adjusted)
decay[~mask_valid] = 0
decay[~mask_positive_integral] = 0
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