The Role of Diurnal Cloud-Radiative Forcing on the Overland Intensification of Tropical Storm Erin (2007)

Tropical Storm Erin (2007) was an unusual tropical cyclone (TC) that re-intensified over the state of Oklahoma. Prior research uncovered multiple factors that were potentially responsible for Erin’s strange re-intensification, such as abundant surface moisture producing a ”brown ocean” effect. The rare case of an overland intensification renders this a suitable case to test diurnal variability in a post-landfall TC. Previous studies on the diurnal cycle in TCs have established a nocturnal preference for rapid intensification and deeper convection. However, research into the effects of the diurnal cycle on TCs as they make landfall and beyond has been relatively scarce in comparison, especially through the lens of cloud-radiation feedback. Even further, the necessity for understanding the behavior of TCs once they make landfall is extremely important for predictability due to impacts on humans and population centers on land with regards to major rainfall. As a result, we chose to investigate the diurnal cycle in Tropical Storm Erin (2007) using the Weather Research and Forecasting (WRF) model through sensitivity tests by modifying the diurnal cycle and cloud radiative processes. Preliminary sensitivity tests on an observationally-validated control run have shown that offsetting the time of the diurnal clock by 12 hours results in an intensification period offset by 12 hours, confirmed through analysis of rainfall and storm track. However, questions still remain about the relative role of diurnal radiative forcing through cloud-radiative feedback. Our hypothesis is that the effects of shortwave radiation and its interactions with the TC’s deep, high clouds stifle convective intensity during daylight hours and, as such, the absence of shortwave radiation allows convection to intensify, making shortwave cloud radiative feedback a dominating component in the behavior of TC Erin’s intensification. Ongoing WRF sensitivity tests will make clouds fully transparent to investigate how the storm develops when only clear-sky radiative forcing is included to assess how storm intensification changes as a result of different radiative forcing mechanisms.