The sensitivity of convective overshoots in the tropical tropopause layer in idealized simulations

Abstract:

It is still debated whether radiative heating observed in the tropical tropopause layer (TTL) is balanced primarily by cooling from convective overshoots, as in an entrainment layer, or by adiabatic cooling from large-scale eddy-driven upwelling. Previous numerical studies show discrepancy in the importance of the cooling associated with convective overshoots. In this study, three-dimensional cloud-resolving model simulations of radiative-convective equilibrium (RCE) were carried out with three different cloud microphysics schemes. We demonstrate that convective cooling in the TTL can be strongly modulated by cloud microphysics. Two of the schemes produce a hard-landing scenario in which convective overshoots reach the TTL with frequent large vertical velocity leading to strong cooling. The third scheme produces a soft-landing scenario in which convective overshoots rarely reach the TTL with large vertical velocity and produce little cooling in the TTL. The difference between the two scenarios is related to the different atmospheric cloud radiative effects (ACRE). The microphysics scheme that produces the soft-landing scenario has much stronger ACRE in the upper troposphere leading to a warmer and more stable layer which acts as a buffer zone to slow down the convective updrafts. We demonstrate the significant influence of cloud radiative effect on the TTL cooling by convective overshoots which can potentially be a cause for the disagreement between previous numerical studies on the role of convective cooling in the TTL. In addition to the RCE simulations, we also perform simulations including large-scale tropospheric overturning circulation to examine the relative contribution from diabatic and adiabatic processes associated with deep convection to the TTL heat budget.