TESTS ON COMBINED PROJECTION FORWARD DIFFERENCING INTEGRATION FOR STIFF PHOTOCHEMICAL FAMILY SYSTEMS AT LONG-TIME STEP

Accurate estimates of final tracer concentrations guide adjustment of individual species levels so that forward integration of stiff familie s can proceed at time steps an order of magnitude larger than inherent loss constants. Maintenance of a unified concentration vector in the solver ensures mass conservation. The sequence was perfected by raisin g the step size upward from 100 s in a standard 25 family 45 species s tratospheric box model with explicit Euler numerics. Projections were inserted as instabilities arose. Ultimately, a uniform DELTAt of 1 h w as achieved from 20-50 km at mid-latitudes. Results were compared over five equinoctial diurnal cycles with runs at 0.1 and 1 s steps in whi ch every constituent other than the individual atoms, CHO and CH3O, wa s handled separately. Agreement was better than 10(-2) for the major r eservoirs in almost all cases, and was equally close for radicals betw een 6 a.m. and 6 p.m., provided that photolysis constants were time av eraged. The few exceptions were due to experimentation with a lineariz ed implicit concentration predictor, or to the approximations underlyi ng family partitioning recipes. CPU timings extrapolated to a hypothet ical GCM grid suggest that 3-D modeling will be possible at the level of chemical resolution in the programs.