Stratiform–Radiation Feedback: A Dynamic Pathway of Upscale Convective Growth through Cloud–Radiation Feedback

Mounting evidence suggests 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 tropical cyclone (TC) genesis. Specifically, cloud–longwave radiative forcing provides a differential diabatic heat forcing in areas of enhanced convective clustering, which in turn promotes further convective upscale development. Energy budgets and circulation balance assumptions have been leveraged to tell this story. Yet, the question of how this feedback manifests in convective-scale dynamics remains an enigma. Here we tell a story of our efforts to pin this down by exploring the convective-scale response to cloud–radiative forcing on short (e.g., one-hour) time scales. We will leverage convection-permitting WRF simulations of Typhoon Haiyan (2013) and Hurricane Maria (2017) 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 imply that this relationship is helped (at the very least) by cloud–radiation feedback.