3D Eulerian Calculation of water vapor age moments for climate change and atmospheric dynamics studies

The age of water vapor in the atmosphere is often invoked to explain a well known discrepancy in the change of the hydrologic cycle with global warming. Although moisture increases at a rate of 7% per degree of global warming, precipitation increases only at a rate of 2% per degree of global warming. The difference between these rates can be explained by a 5% increase in water vapor age per degree of global warming. Although this explanation works on a global scale, it does not explain the spatial distribution in the increase of the age, or the dynamical mechanisms which are responsible for this increase in age.

In this project, we demonstrate the potential of a 3D Eulerian age tracking system for the age of water vapor in a simplified atmospheric general circulation model. The age tracking system works by computing the moments of the age distribution, which form a recursive system. The moments themselves exist as passive tracers, so they can be transported with the water vapor using a consistent transport calculation. This method allows us to track the age of water vapor online in any configuration where the model can be run, including both control and climate change simulations. Our intial tests with an aquaplanet model show a relative increase with age with height and towards the poles, with a decrease over the midlatitude eddies and an increase in the updraft of the Hadley cell. Additionally by resolving the standard deviation of the age distribution we can calculate the shape parameter of the distribution (raio of mean to standard deviation), which shows which regions of the atmosphere are affected by transport from a single pathway and which regions are affected by transport from multiple pathways. We further demonstrate the ability of our age tracking system in more realistic model configurations and climate change scenarios.