We conduct a series of eight 45-day experiments with a global storm-resolving model (GSRM) to test the sensitivity of relative humidity urn:x-wiley:19422466:media:jame21889:jame21889-math-0001 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 urn:x-wiley:19422466:media:jame21889:jame21889-math-0002 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 urn:x-wiley:19422466:media:jame21889:jame21889-math-0003 changes ranging from 0.5% to 8% (absolute) in the mid troposphere. The generated urn:x-wiley:19422466:media:jame21889:jame21889-math-0004 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 urn:x-wiley:19422466:media:jame21889:jame21889-math-0005. The largest urn:x-wiley:19422466:media:jame21889:jame21889-math-0006 changes result from changes in parameterizations, suggesting that model physics represent a major source of humidity spread across GSRMs. The urn:x-wiley:19422466:media:jame21889:jame21889-math-0007 in the moist tropical regions is particularly sensitive to vertical mixing processes within the tropics, which impact urn:x-wiley:19422466:media:jame21889:jame21889-math-0008 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 urn:x-wiley:19422466:media:jame21889:jame21889-math-0009 of the dry tropical regions strongly depends on the exchange with the extratropics. The interaction between tropics and extratropics could change with warming and presage changes in the radiatively sensitive dry regions.
We conduct a series of eight 45‐day experiments with a global storm‐resolving model (GSRM) to test the sensitivity of relative humidity 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 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 changes ranging from 0.5% to 8% (absolute) in the mid troposphere. The generated 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 . The largest changes result from changes in parameterizations, suggesting that model physics represent a major source of humidity spread across GSRMs. The in the moist tropical regions is particularly sensitive to vertical mixing processes within the tropics, which impact 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 of the dry tropical regions strongly depends on the exchange with the extratropics. The interaction between tropics and extratropics could change with warming and presage changes in the radiatively sensitive dry regions. Water vapor is the most important greenhouse gas in the atmosphere. Therefore, for the prediction of future warming it is important that climate models capture the distribution of atmospheric humidity and its change under warming. However, climate models currently strongly disagree in their representation of humidity, causing uncertainty in climate predictions. A recent study has shown that, while there is better agreement among the newest generation of climate models, so called global storm‐resolving models, the remaining inter‐model differences are still relevant and therefore need to be better understood. To narrow down the causes of these differences, in this study we examine how much the humidity in a storm‐resolving model changes in response to changes in different model components, which are chosen to reflect the differences that currently exist between models. We find the largest humidity changes in response to changes in the model's representation of sub‐grid scale processes. In storm‐resolving models these are turbulent motions and cloud microphysics. Our results suggest that differences in the representation of these processes cause a major part of the humidity differences between storm‐resolving models. Sensitivity experiments suggest that parameterizations are the major source of relative humidity spread across global storm‐resolving models Vertical mixing processes strongly impact the humidity of the moist tropics by affecting last‐saturation statistics within the tropics Compared to the rest of the tropics, the humidity of the dry tropics is more sensitive to the pathways of exchange with the extratropics