The Diurnal Cycle in Tropical Storm Erin (2007)

Abstract

The diurnal cycle in post-landfall tropical cyclones (TCs) is sparsely studied, despite its implications for the timing and behavior of associated impacts, particularly in rare cases of overland re-intensification when poorly forecast nocturnal hazards occur. A noteworthy example is Tropical Storm (TS) Erin in 2007, which reached its maximum TC intensity over Oklahoma. Observational analysis shows that Erin intensified and produced its most intense rainfall nocturnally. We use convection-permitting Weather Research and Forecasting (WRF) simulations, including sensitivity tests that shift the solar diurnal cycle by 12 h, to show that this nocturnal timing is tied to the diurnal cycle. Additional simulations that impose surface fluxes from the control simulation separate the roles of atmospheric radiative forcing from surface-flux forcing tied to the nocturnal low-level jet (NLLJ). These experiments reveal a nocturnal preference for rainfall-rate maxima independent of NLLJ timing. However, vortex intensification more closely follows the nocturnal surface-flux phase. This difference is explained by convective organization relative to the radius of maximum winds (RMW): rainfall peaks are followed by stronger vorticity growth when low-level convergence and ascent are organized near or within the RMW. In the imposed-flux experiment, the later rainfall peak has a shallower boundary layer, stronger low-level convergence near the RMW, and stronger inner-core ascent than the earlier radiatively timed peak. These results suggest that radiative forcing modulates rainfall timing, whereas the surface-flux response helps determine whether convection is organized near the RMW and able to contribute to vortex intensification.