Impact of Cloud-radiative Processes on Predecessor Rain Events

A series of idealized numerical simulations is conducted to examine the effects of cloud-radiative processes on the formation of predecessor rain events (PREs) during the extratropical transition of a tropical cyclone. The simulations are performed using an aquaplanet version of the Weather Research and Forecasting v.3.5.1 model with a straight jet. Influenced by different scheme-dependent convective heating patterns and magnitudes, commonly employed radiation and microphysics parameterizations produce a variety of heavy rainfall patterns poleward and separate from the main tropical cyclone rain shield. It is found that cloud radiative forcing (CRF), the interaction of hydrometeors with longwave and shortwave radiation, produces a more robust PRE storm structure with stronger convective activity and wider bands of heavy rainfall. When CRF is neglected, PRE rainfall is reduced since the formation of non-convective precipitation associated with the microphysical scheme is delayed or vanishes altogether. It is hypothesized that cloud-radiative processes favorably modify the vertical thermal distribution of the PRE storm structure. A combination of longwave radiation warming in the middle atmosphere and radiative cooling at the cirrus cloud tops generates a potential vorticity (PV) anomaly couplet that increases low-level convergence and upward vertical motion, especially in conjunction with low-level frontogenetical forcing along a baroclinic zone. Furthermore, latent heat release associated with the PRE reinforces the favorable PV anomaly distribution, resulting in a positive feedback that both intensifies and prolongs the heavy rainfall. This study emphasizes that cloud-radiative processes cannot be neglected and motivates further research on how radiation parameterizations and microphysical schemes influence the the storm dynamics of high-impact weather phenomena such as predecessor rain events.