Improvements in Cloud and Water Vapor Simulations Over the Tropical Oceans in CMIP6 Compared to CMIP5
Cloud and water vapor play important roles in modifying the surface warming caused by increasing greenhouse gases. However, their spatial and temporal variations in the atmosphere are driven by processes that occur over multiple scales, some of which are smaller than the grid sizes of global climate models (GCMs) and thus are difficult to simulate. Accurate representation of cloud and water vapor in the GCMs is critical to reducing the uncertainties of climate change predictions.
This study highlights areas of progress in simulating cloud and water vapor by GCMs and could help inform future improvements to climate models.
We use NASA satellite observations (CloudSat, AIRS, and MLS) to assess the representation of cloud and water vapor structures in 28 climate models that participate in the Coupled Model Intercomparison Project Phase 6 (CMIP6). We assign each model a numerical score based on its performance in simulating observed spatial mean and variance and the correlation between the observed and modeled multi-year mean cloud and water vapor structures in lower, middle, and upper troposphere, and near the tropopause over tropical oceans. Measurable improvements in CMIP6 models are found relative to CMIP5 models for both cloud and water vapor. The differences between models and satellite observations and the spread across the models are reduced. In addition, we find that the models’ equilibrium climate sensitivity (ECS) is correlated with overall performance scores for both CMIP5 and CMIP6 models, with a weaker correlation in CMIP6 than in CMIP5.