Summary
In the bulk-aerosol droplet activation code (ndrop_bam.F90), modes with dispersion_aer == 0 never get their f1, f2, eta parameters set, but maxsat reads all three for every mode with no guard — leading to nondeterministic, often near-total suppression of activation.
Location
src/physics/cam/ndrop_bam.F90:188-222, 321-357, 482-508
Problem
Modes with dispersion_aer == 0 (e.g. volcanic aerosol — the case the skip logic was added for) never get f1, f2, eta assigned, but maxsat reads all three for every mode with no guard. Garbage eta near zero triggers the sum = 1.e20 branch, forcing smax ≈ 1e-10 and suppressing essentially all activation; other garbage values perturb smax arbitrarily.
Impact
Nondeterministic (compiler/memory-dependent) droplet activation in bulk-aerosol (CAM4-physics) configurations containing any zero-dispersion climate aerosol. Affects prognostic state.
Suggested fix
Guard the maxsat accumulation so zero-dispersion modes are skipped consistently (matching the skip logic elsewhere), or initialize f1/f2/eta to inert values for those modes.
Found via a systematic line-by-line scientific code review of src/physics/cam/ at commit 21a78294 (≈cam6_4_180). This is a candidate bug identified by code inspection; it has not yet been confirmed by a model run. An issue-tracker search on 2026-06-11 found no existing report.
Summary
In the bulk-aerosol droplet activation code (
ndrop_bam.F90), modes withdispersion_aer == 0never get theirf1,f2,etaparameters set, butmaxsatreads all three for every mode with no guard — leading to nondeterministic, often near-total suppression of activation.Location
src/physics/cam/ndrop_bam.F90:188-222, 321-357, 482-508Problem
Modes with
dispersion_aer == 0(e.g. volcanic aerosol — the case the skip logic was added for) never getf1,f2,etaassigned, butmaxsatreads all three for every mode with no guard. Garbageetanear zero triggers thesum = 1.e20branch, forcingsmax ≈ 1e-10and suppressing essentially all activation; other garbage values perturbsmaxarbitrarily.Impact
Nondeterministic (compiler/memory-dependent) droplet activation in bulk-aerosol (CAM4-physics) configurations containing any zero-dispersion climate aerosol. Affects prognostic state.
Suggested fix
Guard the
maxsataccumulation so zero-dispersion modes are skipped consistently (matching the skip logic elsewhere), or initializef1/f2/etato inert values for those modes.Found via a systematic line-by-line scientific code review of
src/physics/cam/at commit21a78294(≈cam6_4_180). This is a candidate bug identified by code inspection; it has not yet been confirmed by a model run. An issue-tracker search on 2026-06-11 found no existing report.