Turbulent influence on the Tropical Tropopause Layer: global convection-resolving model study

Abstract

The Tropical Tropopause Layer (TTL) is the transition layer between the well-mixed, turbulent troposphere and the stably stratified stratosphere. It is fundamental to Earth’s climate, controlling stratospheric humidity and generating cirrus clouds. It is a long-standing question to what extent the TTL is dominated by turbulent tropospheric influence or slow stratospheric influence. Previous studies have used various methods relying on sparse observations, limited spatial domains, or coarse convection-parameterizing models. We employ a global convection-resolving (3km) model under real atmospheric conditions to access the inherently multi-scale question of turbulent influence on the heat budget, water budget, and transport timescales of theTTL. We find substantial turbulent cooling maximizing at the cold-point tropopause (∼.3 K/day).Resolved turbulence is associated with deep convection and unresolved turbulence is associated with shear instability generated by convectively-coupled equatorial Kelvin waves. Thus we find a dependence of the TTL thermal structure on turbulent influence. We find that ice sedimentation flux far exceeds vapor flux convergence throughout the TTL, indicating a significant role for turbulent influence on the TTL water budget. Finally, with a tracer experiment we estimate median age of tropopause-level air as 60 days (95% CI: 55–65), improving toward in-situ observational estimates (∼2 months) versus a standard GCM (∼256 days). Our results provide evidence from a novel modeling approach in support of the TTL as a turbulence-dominated entrainment layer. However, external constraints imposed on the model are significant, and indicate that a free-running configuration would be useful for further study.