Exploring the relationship between boundary layer clouds and hydrologic sensitivity

Abstract:

Previous studies have shown an anti-correlation between hydrologic sensitivity and equilibrium climate sensitivity(ECS) attributed to changes in boundary layer clouds. This anti-correlation was explained through an atmospheric energy budget framework where more longwave cooling towards the surface equated to a higher hydrologic sensitivity in models that had a lower ECS. However, from a mechanistic approach, it is still unclear how more low-clouds equate to more precipitation.

We hypothesize that we can explain this relationship from a mechanistic point of view through a surface evaporation lens. We run five model experiments using the Community Atmosphere Model Version 5 (CAM5) where we artificially tune the relative humidity threshold that controls low-cloud fraction across a range of 81 to 96.5 percent. Even in present-day climate simulations with fixed sea-surface temperatures, we find that a larger low-cloud fraction leads to more precipitation in the global mean. Additionally, more boundary layer clouds lead to stronger lower tropospheric mixing; a process quantified by the humidity gradient between 1000 hPa and 925 hPa. In CMIP5 and CMIP6 model ensembles, the lower tropospheric mixing explains as much as 49% and 21% of the global mean precipitation variance, respectively. We will present model experiments and diagnoses that explore the relationship between boundary layer clouds, lower tropospheric mixing and the global water cycle and focus on how mechanistically we can understand the relationship between hydrologic sensitivity and boundary layer clouds through lower tropospheric mixing.