How model uncertainties influence tropical humidity in global
storm-resolving simulations
Abstract
We conduct a series of eight 45-day experiments with a global
storm-resolving model (GSRM) to test the sensitivity of relative
humidity R in the tropics to changes in model resolution and
parameterizations. These changes include changes in horizontal and
vertical grid spacing as well as in the parameterizations of
microphysics and turbulence, and are chosen to capture currently
existing differences among GSRMs. To link the R distribution in the
tropical free troposphere with processes in the deep convective regions,
we adopt a trajectory-based assessment of the last-saturation paradigm.
The perturbations we apply to the model result in tropical mean R
changes ranging from 0.5% to 8% (absolute) in the mid troposphere. The
generated R spread is similar to that in a multi-model ensemble of GSRMs
and smaller than the spread across conventional general circulation
models, supporting that an explicit representation of deep convection
reduces the uncertainty in tropical R. The largest R changes result from
changes in parameterizations, suggesting that model physics represent a
major source of humidity spread across GSRMs. The R in the moist
tropical regions is disproportionately sensitive to vertical mixing
processes within the tropics, which impact R through their effect on the
last-saturation temperature rather than their effect on the evolution of
the humidity since last-saturation. In our analysis the R of the dry
tropical regions strongly depends on the exchange with the
extra-tropics. The interaction between tropics and extratropics could
change with warming and presage changes in the radiatively sensitive dry
regions.