Processes controlling tropical tropopause temperature and stratospheric water vapour

The tropical tropopause is a region sensitive to many dynamical, physical, microphysical and radiative processes. The key processes influencing the tropical tropopause temperature and stratospheric water vapour concentrations are investigated using the Met Office HadGEM3 family of models. A series of sensitivity experiments are run to separate the effects of cirrus clouds, vertical advection, convection, ice optical and microphysical properties, and ozone, on tropical tropopause temperature and lower stratospheric water vapour concentrations. A warm bias in tropical tropopause temperature is common amongst CMIP5 models, and it has been shown in previous studies that this may have a direct radiative impact on surface climate, and a significant effect on the simulation of stratospheric chemical species. The above sensitivity experiments suggest means of significantly reducing these biases in a physical way, thereby potentially improving climate simulations.