Abstract. While state-of-the-art complex chemical mechanisms expand our understanding
of atmospheric chemistry, their sheer size and computational requirements
often limit simulations to short lengths or ensembles to only a few members.
Here we present and compare three 25-year present-day offline simulations
with chemical mechanisms of different levels of complexity using the
Community Earth System Model (CESM) Version 1.2 CAM-chem (CAM4): the Model
for Ozone and Related Chemical Tracers, version 4 (MOZART-4) mechanism, the
Reduced Hydrocarbon mechanism, and the Super-Fast mechanism. We show that,
for most regions and time periods, differences in simulated ozone chemistry
between these three mechanisms are smaller than the model–observation
differences themselves. The MOZART-4 mechanism and the Reduced Hydrocarbon
are in close agreement in their representation of ozone throughout the
troposphere during all time periods (annual, seasonal, and diurnal). While
the Super-Fast mechanism tends to have higher simulated ozone variability and
differs from the MOZART-4 mechanism over regions of high biogenic emissions,
it is surprisingly capable of simulating ozone adequately given its
simplicity. We explore the trade-offs between chemical mechanism complexity
and computational cost by identifying regions where the simpler mechanisms
are comparable to the MOZART-4 mechanism and regions where they are not. The
Super-Fast mechanism is 3 times as fast as the MOZART-4 mechanism, which
allows for longer simulations or ensembles with more members that may not be
feasible with the MOZART-4 mechanism given limited computational resources.
Final Report. Evaluating the Climate Sensitivity of Dissipative Subgrid-Scale Mixing Processes and Variable Resolution in NCAR's Community Earth System Model