The Stratiform Radiation Effect on Tropical Cyclone Genesis

Moistening the environment of a precursor storm is a necessary step for tropical cyclone (TC) genesis. Cloud-radiative forcing (CRF) has been implicated in accelerating this process, but we do not fully understand how. Different cloud types are known to have distinct CRF, but their roles in driving upscale development through radiative feedback is largely unexplored. There is a growing consensus from observations that the areal growth of stratiform precipitation is a key precursor to TC genesis and intensification. This observational finding presents a conundrum, given that stratiform precipitation is directly tied to downdrafts, and hence ventilation, and the drying of the storm region. Therefore, we hypothesize that the cloud-radiative effect within stratiform cloud regions weakens downdrafts, allowing the environment to moisten more easily. To test this hypothesis, we analyzed output from convection-permitting ensemble Weather Research and Forecasting (WRF) model simulations of Super Typhoon Haiyan. We compared the control simulation (CTL) to sensitivity tests that exclude cloud-radiative feedback (NCRF) either everywhere or only in specific cloud types using a novel column-by-column cloud classification scheme.

These tests indicate that the upscale development of deep convection that manifests in TC genesis is accelerated by CRF, especially of that in stratiform and anvil regions. Analysis shows that CTL tests have less downward vertical mass flux in the mid to lower troposphere, indicating weaker (and/or fewer) downdrafts, than that of the NCRF tests, with the greatest differences being in stratiform regions. These findings support our hypothesis that cloud-radiative effects in the stratiform regions weaken downdrafts in developing TCs, in turn allowing the environment to moisten more easily. Further, we investigate the roles of CRF on different cloud types on the midlevel vortex. Understanding the importance of cloud type CRF, especially that of stratiform, could provide insight on the development of tropical convection and TC genesis.