Abstract: 

Mounting evidence indicates the importance of cloud–radiation feedback in promoting the upscale development of tropical convection. This feedback is argued to be critical to maintaining the convective envelope of the Madden–Julian Oscillation (MJO) and to accelerating the genesis of tropical cyclones (TCs). Specifically, cloud–longwave radiative forcing (the “cloud greenhouse effect”) provides an additional diabatic heat source in regions where deep convection is already prolific relative to its surroundings, which in turn fosters more convection. Energy budgets and circulation balance assumptions can and have been leveraged to tell this story. Yet, the question of how radiative forcing acts to shepherd the clustering of deep convection, the sensible heating from which is much greater in magnitude, remains an enigma. In this presentation, I will tell a story of our efforts to pin this down by exploring the convective-scale response to cloud–radiative forcing (CRF) – that is, on short (e.g., one-hour) time scales. We will leverage convection-permitting WRF simulations of Typhoon Haiyan (2013) to conduct mechanism-denial experiments, wherein we will analyze the transient response of convection to the switch-off/on of CRF. Our results support a novel hypothesis that the CRF in widespread areas of stratiform precipitation is especially important. While stratiform precipitation is coupled to mesoscale downward motion in the lower troposphere, and hence midlevel convergence and enhanced ventilation of the column, CRF-warming suppresses these downdrafts by increasing buoyancy at and beneath cloud base. Observational analysis by others indicates that increasing stratiform rain fraction is a key signal prior to TCs undergoing rapid intensification (RI). Our results indicate that this relationship is helped (at the very least) by cloud–radiation feedback.