Did the Low Level Jet or Diurnal Radiative Forcing Have the Greatest Impact on Tropical Storm Erin’s (2007) Overland Intensification?

In 2007, Tropical Storm Erin underwent a rare overland intensification over the state of Oklahoma. Previous studies have inspected how anomalously high moisture content, also known as the “brown ocean effect”, and the Great Plains Low Level Jet (GPLLJ) contributed to Erin’s re-intensification, but none have investigated the role of diurnal radiative forcing. Previous diurnal cycle studies established a nocturnal preference for over ocean convection and tropical cyclone (TC) intensification, which is driven by cloud-radiative processes. However, research into the effects of the diurnal cycle on TCs as they make landfall and beyond has been relatively scarce. Examining events like this is extremely important to further understand how radiative forcing impacts TCs as they move onshore and begin impacting population centers. To compare the influences of the GPLLJ and diurnal radiative forcing, we use the Weather Research and Forecast (WRF) model to reproduce Erin in a simulation. The diurnal cycle of incoming solar radiation also controls the nocturnal maxima of the GPLLJ, so we separate these mechanisms here by imposing the surface sensible and latent heat fluxes from a control simulation on a sensitivity test with a flipped solar clock (offset 12 hours). In this simulation, the peak of the GPLLJ aligns with solar noon, pitting the two forcing mechanisms against each other. We hypothesize that diurnal radiative forcing is the dominant control in the timing of Erin’s intensification and expect the storm to still reach peak strength at solar night, rather than when the GPLLJ is maximized. To test this, we inspect the simulated track and view time series of maximum wind speeds and rainfall rate in this sensitivity test to see how Erin’s intensification changes with these offsets. These results will also be compared to observations of rainfall and the GPLLJ to draw further conclusions of what caused Erin’s overland intensification.